Sonja Aulbach
email: s.aulbach@em.uni-frankfurt.de

Malcolm Massuyeau

Fabrice Gaillard

Gumer Galán

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

Table of Contents

Observed and inferred accumulations of metasomatic minerals in cratonic mantle

Phase equilibria and thermodynamics: Can metasomatism explain MLDs?

Summary and conclusions

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M A N U S C R I P T

Introduction

Geophysically detectible mid-lithospheric discontinuities (MLD) and lithosphereasthenosphere boundaries (LAB) in Earth's ancient continental masses (cratons; Fig. 1) have received much attention over recent years. A plethora of papers has been published on the topic of both, with a focus on geophysical observables (e.g. [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF][START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF], and on petrologic-geochemical data (e.g. O'Reilly and [START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF]. The overarching goal of the various lines of investigating cratonic lithosphere, including current efforts to find and explain cratonic mantle discontinuities, is to elucidate the origin and evolution of continents, which bears on early terrestrial dynamics, such as the nature of crust and mantle differentiation or the viability of plate tectonics [START_REF] Gerya | Precambrian geodynamics: Concepts and models[END_REF].

Xenoliths, xenocrysts and magmas forming in or passing through cratonic mantle provide temporally and spatially punctuated information on the composition and fabric of cratonic roots at the time of magmatism, and on the nature and timing of events that formed them.

Geophysical observations can provide regional or craton-scale insights, but require validation from actual sample properties, and must rely on background information afforded by mantle samples for correct interpretation. Although these different modes of lithosphere interrogation do not operate on the same spatial or temporal scales, they offer the opportunity to investigate lithospheric discontinuities through the joint investigation of all available physical, dynamic, chemical and petrologic constraints.

Perhaps not surprisingly, there is little agreement on the origins of cratonic mantle discontinuities, and the increasing integration of multiple geophysical observables with the mineralogy and composition of mantle samples has led to the recognition that universally applicable mechanisms are unlikely to explain all observations (e.g. [START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF] A C C E P T E D M A N U S C R I P T

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4 [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. It is clear, however, that the peculiarities of cratonic lithosphere composition and construction -cool geothermal gradients, compositional and rheological stratification, multiple metasomatic overprints in the course of their long existence -offer myriad possibilities for the generation, modification, translation and even obliteration of such discontinuities in ancient lithospheric keels.

In this review, we first provide a brief introduction into the cratonic lithosphere, its uniqueness compared to younger lithosphere, and the nature and shortcomings of the cratonic mantle sample (section 2). We then present the different lines of investigation that can be combined to make inferences on the nature and origin of cratonic lithosphere discontinuities (section 3): Petrographic-geochemical observations (microstructures, mineral modes, elemental and isotopic composition), geothermobarometry (to locate mantle samples in the lithosphere column), thermodynamic modelling and geophysical observations (seismic velocity and anisotropy, resistivity, reflectivity). We subsequently discuss possible origins of the two major mantle lithospheric discontinuities -LABs and MLDs (section 4). We conclude with a summary of the most important and, in some cases, novel insights gained from this exercise. In particular, we would like to highlight that the combined perspective from petrology, geochemistry and thermodynamic modelling indicates that most MLDs may be best explained by accumulations of seismically slow minerals deposited during past magmatic-metasomatic activity. Finally, we suggest that the sparseness of seismic LAB observation beneath stable cratons relate to the difficulty to form and retain melt beneath these typically thick (200-250 km) lithospheres.

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Craton definition, craton types and nature of the cratonic mantle sample

Cratons are often defined as the cores of tectonomagmatically quiescent continents where the last major tectonothermal event occurred in the Archaean or Palaeoproterozoic and which were amalgamated by Neoarchaean time (≥ 2.5 Ga); larger continental regions that arose by later collisions, as evidenced by Proterozoic mobile belts, are also sometimes referred to as cratons (Fig. 1). These mobile belts shielded the cratonic cores from later tectonomagmatic activity, ensuring their long-term stability [START_REF] Lenardic | The role of mobile belts for the longevity of deep cratonic lithosphere: The crumple zone model[END_REF], although in some cases later reactivation with minor to severe loss of cratonic lithosphere has been documented (e.g.

O'Reilly and [START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF]. Compared to younger continental lithosphere, cratonic mantle is characterised by exceptionally high degrees of melt extraction, which resulted in strong Feand H 2 O-depletion (hence buoyancy and viscosity) that allowed the formation of exceptionally stable and thick lithosphere (sometimes referred to as "roots"; O' Reilly et al., 2001;[START_REF] Griffin | The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications[END_REF]; for a compilation of cratonic crustal and cratonisation ages as well as thermal thicknesses, see Table 1 in Artemieva, 2006) and that can be linked to warmer Archaean mantle potential temperatures T P [START_REF] Davies | Effect of plate bending on the Urey ratio and the thermal evolution of the mantle[END_REF][START_REF] Herzberg | Thermal history of the Earth and its petrological expression[END_REF]. These properties play a pivotal role in the longevity of cratons [START_REF] Jordan | Structure and formation of the continental tectosphere[END_REF]Abbott et al., 1997;[START_REF] Lenardic | Longevity and stability of cratonic lithosphere: Insights from numerical simulations of coupled mantle convection and continental tectonics[END_REF]Afonso and Schutt, 2012;Wang et al., 2014).

In the context of this review, how cratons formed and evolved is important in that it affects the composition and fabric of the lithospheric mantle that may today be visible as discontinuities. Processes that have been put forward for the formation of diamondiferoustherefore thickcratonic roots range from subduction-accretion or collisional settings to plume tectonics [START_REF] Lee | Geochemical/petrologic constraints on the origin of cratonic mantle[END_REF]Pearson andWittig, 2008, 2014;[START_REF] Lee | Building and Destroying Continental Mantle[END_REF]Aulbach, 2012) (Fig. 2). The depth of the LAB may vary as a function of T P and composition (Fig. 3A), and

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6 may increase due to thickening during the aforementioned accretionary and plume subcretion processes, or decrease due to a variety of destructive processes described in the next section.

Intact vs disturbed lithospheric roots and associated magmatism

Intact cratonic roots extend to ~200-250 km and are characterised by high seismic velocities, low electrical conductivities and low surface heat flow (Artemieva and Mooney, 2001;[START_REF] Priestley | The thermal structure of the lithosphere from shear wave velocities[END_REF][START_REF] Eaton | The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons[END_REF][START_REF] Gardès | Toward a unified hydrous olivine electrical conductivity law[END_REF], consistent with a cold and thick lithospheric lid. Because they are thicker than the depths where dry peridotite crosses its solidus at ambient mantle potential temperatures, only small-volume, volatile-rich melts, such as kimberlites, lamproites and alkali basalts, are typically able to form beneath such lids in hot and/or oxidising, carbonated(-hydrous) mantle [START_REF] Rohrbach | Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling[END_REF][START_REF] Dasgupta | Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time[END_REF] (Fig. 3A). Occasionally, anomalously hot and/or fertile mantle can lead to the emplacement of higher-volume melts, despite the presence of a thick lithosphere (e.g. the 2.05 Ga Bushveld event; [START_REF] Wilson | A Chill Sequence to the Bushveld Complex: Insight into the First Stage of Emplacement and Implications for the Parental Magmas[END_REF]. Beneath some cratonic areas, investigation of mantle xenoliths and geophysical observations has revealed much thinner or strongly modified lithospheric roots. A number of processes can conspire to erode or destroy cratonic lithosphere. Heating, for example by impinging plumes, addition of water, for example by dehydration of seawater-altered slabs beneath cratons or in the transition zone, injection of wet melts at the LAB, or addition of Fe-rich melts during metasomatism (Fig. 4), lower the viscosity and density contrast between cratonic root and convecting mantle, imperilling its stability [START_REF] Foley | Rejuvenation and erosion of the cratonic lithosphere[END_REF][START_REF] Li | Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the mantle rheology and hydration-induced thinning of continental lithosphere[END_REF][START_REF] Peslier | Olivine water contents in the continental lithosphere and the longevity of cratons[END_REF][START_REF] Windley | Delamination/thinning of sub-continental lithospheric mantle under Eastern China: The role of water and multiple subduction[END_REF][START_REF] Selway | Water content of the Tanzanian lithosphere from magnetotelluric data: Implications for cratonic growth and stability[END_REF]Wang et al., 2014[START_REF] Wang | The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening[END_REF]Pearson et al., 2014;[START_REF] Zheng | Refertilization-driven destabilization of subcontinental mantle and the importance of initial lithospheric thickness for the fate of continents[END_REF]. The consequences are (1) foundering of gravitationally instable, deep, little depleted mantle residues of polybaric melting (Arndt et al., 2009), (2) foundering due to an inverted temperature gradient developing near the base of the lithosphere (Michaut et al., 2009), (3) the establishment of melt-percolation fronts atop anomalously hot mantle generating MLDs [START_REF] Wölbern | Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions[END_REF] and (4) densification and rheological weakening due to melt infiltration [START_REF] Wang | The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening[END_REF][START_REF] Zheng | Refertilization-driven destabilization of subcontinental mantle and the importance of initial lithospheric thickness for the fate of continents[END_REF] (Fig. 4). Such processes may also act at modern LABs, leading to thinning and, in cases, even complete past (or future) loss of deep cratonic roots.

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Thinning cratons are characterised by a record of progressively more voluminous magmas, enabling increasing melting intervals to be achieved in upwelling convective mantle, starting with volatile-rich melts, such as carbonatites and kimberlites, and ending with silicate melts, such as melilitites and alkali basalts (e.g. North Atlantic craton: [START_REF] Tappe | Craton reactivation on the Labrador Sea margins: Ar-40/Ar-39 age and Sr-Nd-Hf-Pb isotope constraints from alkaline and carbonatite intrusives[END_REF] western edge of the Tanzanian craton: [START_REF] Rosenthal | Petrogenesis of strongly alkaline primitive volcanic rocks at the propagating tip of the western branch of the East African Rift[END_REF] (Fig. 5A). Interaction of the lowermost 40 km of the Kaapvaal root with, likely plume-derived, asthenospheric melts linked to group I kimberlite magmatism led to severe refertilisation (O'Reilly and [START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF]. Rift-related thinning, accompanied by melt infiltration and severe Fe enrichment, is evident for the margins of the Tanzanian craton [START_REF] Ritsema | Upper mantle seismic velocity structure beneath Tanzania, east Africa: Implications for the stability of cratonic lithosphere[END_REF][START_REF] Lee | Compositionally stratified cratonic lithosphere: Petrology and geochemistry of peridotite xenoliths from the Labait Voclcano, Tanzania[END_REF] and suggested for Sao Francisco craton [START_REF] Pinto | Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating[END_REF]. The Moroccan Atlas, part of the Saharan metacraton (Abdelsalam et al., 2002), may once have been underlain by a cratonic root prior to rifting and orogenesis, enabled by craton-edge driven convection currents (Kaislaniemi and van Hunen 2014). The most striking example of cratonic lithosphere destruction may be the North China craton, where a diamondiferous root that was still present in the Palaeozoic, as sampled by kimberlites, was replaced by hotter and thinner mantle, as sampled by Cenozoic basaltic magmatism [START_REF] Tang | Differential destruction of the North China Craton: A tectonic perspective[END_REF]. Other examples include the Mojavia and Wyoming cratons, where interaction with the Farallon plate is evidenced by the presence of unusually fertile, thin and wet remaining root [START_REF] Lee | Preservation of ancient and fertile lithospheric mantle beneath the southwestern United States[END_REF][START_REF] Lee | Trace element evidence for hydrous metasomatism at the base of the North American lithosphere and possible association with Laramide low-angle subduction[END_REF][START_REF] Li | Water contents in mantle xenoliths from the Colorado Plateau and vicinity: Implications for the mantle rheology and hydration-induced thinning of continental lithosphere[END_REF]). Thus, emplacement of higher-volume, alkaline basaltic magmas betrays the presence of disrupted

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8 cratonic roots, which allows longer melting columns to develop in decompressing convecting mantle.

Accidental mantle samples: Xenoliths and xenocrysts

Asthenophere-derived magmas penetrating intact or disturbed cratonic roots often entrain fragments (xenoliths) of the wall-rock they traverse, which may disintegrate to single grains (xenocrysts, including diamond) that still bear the geochemical fingerprint of their source rocks (Griffin et al., 1999;[START_REF] Grütter | An updated classification scheme for mantle-derived garnet, for use by diamond explorers[END_REF]. Combined with geothermobarometry, these samples provide invaluable information on the depth-dependent composition and fabric of the cratonic mantle keel, from which they were entrained, although constraints on structural relationships afforded by outcrops are precluded. Xenoliths and xenocrysts inevitably provide snapshots of the lithospheric mantle state at the time of magmatism that cannot be unquestioningly compared to present-day geophysical observation: Heating associated with metasomatism may dissipate, melts may freeze and fabric may be destroyed due to recrystallisation. In addition, the volume of cratonic mantle sampled by magmas is unlikely to be representative of the whole craton because magmas preferentially utilise multiply reactivated lithospheric weak zones as pathways, and the conduit walls are accordingly highly overprinted [START_REF] Poudjom-Djomani | Variations of the effective elastic thickness (Te) and structure of the lithosphere beneath the Slave Province, Canada[END_REF]Afonso et al., 2008;Giuliani et al., 2015a).

Finally, the size of the mantle sample is limited by dike width and loading capacity of the host magma [START_REF] Sparks | Dynamical constraints on kimberlite volcanism[END_REF], which restricts the accuracy, with which bulk chemical and modal compositions can be determined. This accidental and, in particular, unrepresentative nature of the cratonic mantle sample also implies that direct matching between samplederived 1D lithosphere profiles and regional geophysical surveys is fraught with uncertainty.

Modes of lithosphere interrogation

3.1 Petrology, fabric, geochemistry and thermodynamic modelling

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Over its long existence and subsequent to the partial melting event leading to its depletion, the cratonic mantle has been multiply refertilised and metasomatised (e.g. O'Reilly and [START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF][START_REF] Pearson | The Formation and Evolution of Cratonic Mantle Lithosphere -Evidence from Mantle Xenoliths[END_REF]. It is known that fertile peridotite changes its modal composition with increasing degree of melt extraction from lherzolite (olivine, orthopyroxene, clinopyroxene plus feldspar or Al-spinel or garnet, depending on pressure) to harzburgite (≤ 5 vol.% clinopyroxene) and dunite (≤ 10 vol.% pyroxenes) [START_REF] Streckeisen | To each plutonic rock its proper name[END_REF], while refertilisation by melts reverses this process [START_REF] Griffin | The Composition and Evolution of Lithospheric Mantle: a Re-evaluation and its Tectonic Implications[END_REF] (although note that some dunites and harzburgites result from melt-peridotite interaction; [START_REF] Kelemen | A review of melt migration processes in the adiabatically upwelling mantle beneath oceanic spreading ridges[END_REF].

In particular the presence of atypical phases in primitive mantle, such as amphibole, phlogopite, carbonates, Ti minerals and others ("modal metasomatism"; [START_REF] Harte | Mantle peridotites and processes -the kimberlite sample[END_REF] attests to the former presence of fluids or melts that significantly affected the mantle lithosphere and possibly changed its thermophysical properties, such as density and seismic velocity (Afonso and Schutt, 2012;[START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF][START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF]. In addition, peridotite fabric generates anisotropy and therefore affects the seismic response of the mantle [START_REF] Savage | Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?[END_REF].

The most common process producing both shape-and crystal-preferred orientations associated with fabric is ductile deformation by dislocation creep [START_REF] Tommasi | Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia[END_REF]. For example, foliation and lineation defined by elongate crystals and the sometimes associated crystallographic-preferred orientation (CPO), generated during mineral dissolutionreprecipitation in response to reactive melt migration, are detectible microscopically or using modern analytical techniques (e.g. Electron Back-Scattered Diffraction; [START_REF] Tommasi | Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia[END_REF][START_REF] Bascou | Seismic velocities, anisotropy and deformation in Siberian cratonic mantle: EBSD data on xenoliths from the Udachnaya kimberlite[END_REF][START_REF] Baptiste | Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa[END_REF]. However, a straightforward relationship between fabric and CPO is not always evident (e.g. [START_REF] Tommasi | Deformation, reactive melt percolation, and seismic anisotropy in the lithospheric mantle beneath the southern Ethiopian rift: constraints from mantle xenoliths from Mega[END_REF]. While crystal growth during static recrystallisation tends to destroy the shape-preferred orientation, the CPO can be preserved [START_REF] Baptiste | Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa[END_REF][START_REF] Tommasi | Heterogeneity and anisotropy in the lithospheric mantle[END_REF]. In contrast, CPO is weakened by dynamic recrystallisation or melt-/fluid-rock reactions leading to neo-crystallisation of

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ACCEPTED MANUSCRIPT 10 olivine [START_REF] Tommasi | Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia[END_REF]2008), of pyroxenes (Le Roux et al., 2008;[START_REF] Soustelle | Deformation and Fluid-Rock Interaction in the Supra-subduction Mantle: Microstructures and Water Contents in Peridotite Xenoliths from the Avacha Volcano, Kamchatka[END_REF] or of both [START_REF] Hidas | Fluid-assisted strain localization in the shallow lithospheric mantle[END_REF].

Partial melt extraction and modal metasomatism are usually accompanied by changes in both major-and trace-element composition, while cryptic metasomatism only affects the traceelement budget [START_REF] Dawson | Contrasting types of upper mantle metasomatism[END_REF]. Metasomatic agents range from carbonated melts imparting strongly sinusoidal REE patterns in garnet (commonly in clinopyroxene-free harzburgites and in inclusions in diamond), to silico-carbonatite melts resulting in mildly sinusoidal REE patterns in garnet. Interaction with silicate melts leads to "normal" LREE-depleted patterns, with smoothly increasing abundances towards the HREE, which are indistinguishable from those expected in garnet from fertile (undepleted) lherzolites (Fig. 6A). The trace element composition of the lithosphere does not directly influence its thermophysical properties, with the possible exception of the effect of heat-producing elements on the geothermal gradient and of H + , which significantly affects rheology and other mantle properties [START_REF] Hirth | Water in the oceanic upper mantle: Implications for rheology, melt extraction and the evolution of the lithosphere[END_REF]. The distinctive trace-element signatures imparted by the passage of different melts and fluids can alert us to changes of dynamic or tectonic regimes, which may in turn have affected lithospheric fabric and mineralogy (e.g. [START_REF] Simon | The origin and evolution of the Kaapvaal cratonic lithospheric mantle[END_REF]Agashev et al., 2013;[START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF], providing important context for the interpretation of geophysical observations. Even when precise ages prove elusive, radiogenic isotope compositions constrain time-integrated enrichment and depletion in the cratonic lithosphere that can be correlated with tectonothermal events evident in the crustal record (e.g. [START_REF] Carlson | Isotopic inferences on the chemical structure of the mantle[END_REF][START_REF] Pearson | Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds in[END_REF].

The chemical composition of mantle xenoliths, predominantly peridotite and subordinately eclogites and pyroxenites [START_REF] Schulze | Constraints on the abundance of eclogite in the upper mantle[END_REF], can be combined with experimentally calibrated

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geothermobarometer combinations to locate them in the mantle column [START_REF] Smith | Water-rock interactions, orthopyroxene growth, and Si-enrichment in the mantle: evidence in xenoliths from the Colorado Plateau, southwestern United States[END_REF][START_REF] Grütter | Pyroxene xenocryst geotherms: Techniques and application[END_REF][START_REF] Nimis | Internally consistent geothermometers for garnet peridotites and pyroxenites[END_REF]. Geothermobarometry allows one-dimensional mantle stratigraphies to be constructed for each xenolith locality, which may be combined with other localities to map the lithosphere laterally [START_REF] Griffin | Lithosphere mapping beneath the north American plate[END_REF]. Some lithologies implicated in the generation of MLDs (garnet-or orthopyroxene-free pyroxenites, phlogopitites; [START_REF] Wölbern | Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions[END_REF], but also spinel peridotites and eclogites, lack mineral assemblages required for pressure calculation, although appropriate thermometers may exist. Their position in the lithosphere column is then inferred by assuming that they equilibrated to a known pressuretemperature array, typically obtained from garnet-bearing peridotites and pyroxenites from the same kimberlite, and by projecting calculated temperatures onto this geotherm (e.g. [START_REF] Kopylova | Spatial distribution of eclogite in the Slave cratonic mantle: The role of subduction[END_REF]. Uncertainties on temperatures and pressures obtained from thermobarometry of ~50º C and ~0.4 GPa, respectively, have been cited [START_REF] Smith | Water-rock interactions, orthopyroxene growth, and Si-enrichment in the mantle: evidence in xenoliths from the Colorado Plateau, southwestern United States[END_REF][START_REF] Nimis | Internally consistent geothermometers for garnet peridotites and pyroxenites[END_REF]. Thus, uncertainties on the corresponding depths lie within the narrow depth interval over which a chemical, thermal or microstructural change has to occur to be geophysically detectible as a discontinuity (30-40 km; [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF].

Pressure-temperature arrays defined by appropriate samples also provide constraints on the thermal state of the mantle at the time of xenolith or xenocryst entrainment. Thus, stable cratons with thick keels typically have lower geothermal gradients than disrupted cratons with thinner keels, and older kimberlites sample a warmer cratonic lithosphere than younger ones (Artemieva, 2009;[START_REF] Grütter | Pyroxene xenocryst geotherms: Techniques and application[END_REF]. Kinked geotherms (Lesotho, Kaapvaal craton: [START_REF] Boyd | Origin of the ilmenite-siicate nodules in kimberlites from Lesotho and South Africa[END_REF], stepped geotherms (Slave craton: Griffin et al., 1999) 5B). In contrast, differences in the initial thermal state, buoyancy and viscosity resulting from the formation of cratons in hot plumes vs. cool slab environments have dissipated by today [START_REF] Eaton | Ephemeral isopycnicity of cratonic mantle keels[END_REF].

While thermodynamic modelling has long been employed to investigate the composition and quantity of melts produced as a function of temperature and volatile (H 2 O) content in chemical equilibration with the convective mantle [START_REF] Ghiorso | Chemical mass-transfer in magmatic processes. 4. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilbria in magmatic systems at elevated tempeartures and pressures[END_REF][START_REF] Ghiorso | The pMELTS: A revision of MELTS for improved calculation of phase relations and major element partitioning related to partial melting of the mantle to 3 GPa[END_REF], the presence of carbonates in peridotite and their effects on the mantle solidus has until recently only been parameterised in empirical models [START_REF] Dasgupta | Carbondioxide-rich silicate melt in the Earth's upper mantle[END_REF]. A new thermodynamic model [START_REF] Massuyeau | A model for the activity of silica along the carbonatite-kimberlite-mellilitite-basanite melt compositional joint[END_REF] improves on this, based on the silica activity in the melt in presence of both CO 2 and H 2 O. This approach is here for the first time applied to investigation of the stability of melts at the LAB (section 4.1) and of phlogopite at MLD depths (section 4.2).

Geophysical observations

A variety of geophysical tools are available to study the physical properties of the lithosphere, including discontinuities, and these have been reviewed in detail in several recent papers [START_REF] Selway | Water content of the Tanzanian lithosphere from magnetotelluric data: Implications for cratonic growth and stability[END_REF][START_REF] Fischer | Crust and Lithospheric Structure -Seismological Constraints on the Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF][START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF][START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF]. Discontinuities in seismic velocity and anisotropy in the crust and upper mantle are imaged using receiver functions (RF), based on different arrival times for primary and converted P-and S-waves (termed PRF and SRF, respectively) and back-azimuthal differences in amplitudes [START_REF] Kind | Seismic receiver functions and the lithosphere-asthenosphere boundary[END_REF][START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF][START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF].

These high-frequency body waves are sensitive to small structures, with a depth resolution of 10-15 km for velocity contrasts at mantle depth. In constrast, body wave tomography has a lower depth resolution, but the best lateral resolution [START_REF] Leveque | From ACH tomographic models to absolute velocity models[END_REF][START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. Because the depth interval of the transition between seismically slow and fast

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13 regions is poorly resolved, trade-offs between the depth range and the sharpness of the transition imply that seismic velocity reductions can be modelled both as a discontinuity (large contrast) or a gradient [START_REF] Lebedev | Seismic structure of Precambrian lithosphere: New constraints from broad-band surface-wave dispersion[END_REF].

A common tool to study anisotropy is shear-wave splitting analysis of various teleseismic phases, typically originating as S-waves that are converted to P-waves in the core and reconverted to S-waves (splitting of core phases) [START_REF] Savage | Seismic anisotropy and mantle deformation: What have we learned from shear wave splitting?[END_REF]. This type of analysis has good lateral but poor depth resolution. Surface waves can also be used to image anisotropy in wave speeds with better depth, but poorer lateral resolution, whereby the phase velocities of surface waves can vary in the direction of propagation (azimuthal anisotropy) and in vertically polarised Rayleigh and horizontally polarised Love waves (radial anisotropy) [START_REF] Montagner | Where can seismic anisotropy be detected in the earth's mantle? In boundary layers[END_REF][START_REF] Becker | On the relationship between azimuthal anisotropy from shear wave splitting and surface wave tomography[END_REF]. The juxtaposition of mantle layers with isotropic and anisotropic seismic wave speeds or with contrasting geometries of azimuthal or radial seismic anisotropies can produce conversions of S-and P-waves that appear as MLDs. Assuming radial anisotropy stratification, radial anisotropy could cause velocity reductions (negative phases) in SRF, whereas azimuthal anisotropy should cause both positive, negative or no phases, depending on the direction of wave propagation relative to the orientation of the anisotropy and the location of the seismic receiver [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF].

Finally, magnetotelluric data provide a means to interrogate the conductivity in lithospheric mantle, where high resistivity (low conductivity) is expected for dry and depleted mantle and low resistivity may be caused by a melt, an interconnected film or layer of conductive minerals, or of water stored in nominally anhydrous minerals (NAMs) [START_REF] Selway | Water content of the Tanzanian lithosphere from magnetotelluric data: Implications for cratonic growth and stability[END_REF].

Discussion

4.1 Lithosphere-asthenosphere boundary (LAB)

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Various definitions exist for the continental LAB (chemical, thermal, mechanical, seismic, electrical;Artemieva and Mooney, 2001;[START_REF] Eaton | The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons[END_REF], implying that the thickness of cratonic roots can be measured by a variety of geochemical, petrologic and geophysical means. Compositionally, the LAB is typified by a transition from depleted to fertile (or refertilised) peridotite that has been mapped using xenoliths and xenocrysts [START_REF] Gaul | Mapping olivine composition in the lithospheric mantle[END_REF][START_REF] Kopylova | Chemical stratification of cratonic lithosphere: constraints from the Northern Slave craton, Canada[END_REF][START_REF] Griffin | Cr-pyrope garnets in the lithospheric mantle 2. Compositional populations and their distribution in time and space[END_REF][START_REF] Griffin | Lithosphere mapping beneath the north American plate[END_REF]. This approach may, however, underestimate the true base of the cratonic mantle root (O'Reilly and [START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF].

Thermally, ancient thick roots are characterised by low surface heat flow [START_REF] Jaupart | Heat flow and thickness of the lithosphere in the Canadian Shield[END_REF]Artemieva and Mooney, 2001). Physically, reduced shear (S)-and compressional (P)-wave speeds at ~170-250 km depth depict the transition from the cold and viscous lithosphere to the asthenosphere (e.g. [START_REF] Romanowicz | The Thickness of Tectonic Plates[END_REF][START_REF] Eaton | The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons[END_REF][START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Ford | The lithosphereasthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging[END_REF]. This may be accompanied by a reduction in electrical resistivity [START_REF] Jones | Velocity-conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin-Shtrikman extremal bounds[END_REF][START_REF] Eaton | The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons[END_REF] and an increase in seismic velocity anisotropy [START_REF] Foster | A sharp cratonic lithosphere-asthenosphere boundary beneath the American Midwest and its relation to mantle flow[END_REF].

These changes in the physical and chemical properties at the base of the cratonic lithosphere do not always result in a geophysically detectible discontinuity. In particular the cratonic LAB is known to produce a much weaker seismic signature than the oceanic LAB, and is often characterised by a small and gradual velocity change with a weak and intermittent seismic signal [START_REF] Eaton | The elusive lithosphere-asthenosphere boundary (LAB) beneath cratons[END_REF]Abt et al. 2010;[START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Hopper | The lithosphere-asthenosphere boundary and the tectonic and magmatic history of the northwestern United States[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF]. Here, we discuss the origin of the variable signal from cratonic LABs with respect to the stability of small volumes of melt, which can satisfy both seismic and electrical anomalies, and in particular explore the conditions required for generating melt in the sublithosphere and maintaining it near LAB depth.

Strong vs. weak or absent cratonic seismic LAB signals: The role of reworking and rejuvenation

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When present, seismically imaged discontinuities (localised in a <30 km depth interval) beneath cratons (e.g. Baltic shield, Canadian shield and parts of the North American craton; [START_REF] Rychert | A sharp lithosphere-asthenosphere boundary imaged beneath eastern North America[END_REF][START_REF] Ford | The lithosphereasthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging[END_REF][START_REF] Foster | A sharp cratonic lithosphere-asthenosphere boundary beneath the American Midwest and its relation to mantle flow[END_REF] cannot be produced by thermal effects alone and instead require contrasts in composition, fabric, water content or the presence of partial melt or volatiles [START_REF] Rychert | A Global View of the Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF]Ford et al., 2010). For example, ponding of plume-derived hot material beneath the lithosphere, thermal insulation of the mantle beneath large continental masses or a contrast in anisotropy or grain size, enhanced by mantle flow at lithospheric edges, have been invoked [START_REF] Lebedev | Global upper-mantle tomography with the automated multimode inversion of surface and S-wave forms[END_REF][START_REF] Foster | A sharp cratonic lithosphere-asthenosphere boundary beneath the American Midwest and its relation to mantle flow[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF]. Where discrete LABs are not detected at depths commensurate with the extent of seismically fast roots, the boundary is characterised by a velocity gradient and hence more accurately described as a transition zone that is spread out over a large depth interval (~50 km; [START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Ford | The lithosphereasthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging[END_REF][START_REF] Yuan | 3-D shear wave radially and azimuthally anisotropic velocity model of the North American upper mantle[END_REF].

Oftentimes, highly disparate depths to the LAB are determined. For example, results for the Tanzanian lithosphere thickness vary from 150 to 200 km [START_REF] Ritsema | Upper mantle seismic velocity structure beneath Tanzania, east Africa: Implications for the stability of cratonic lithosphere[END_REF][START_REF] Weeraratne | Evidence for an upper mantle plume beneath the Tanzanian craton from Rayleigh wave tomography[END_REF][START_REF] Fishwick | Surface wave tomography Imaging of the lithosphere-asthenosphere boundary beneath central and southern Africa?[END_REF]. The shallower range of these estimates may represent the tops of live melt percolation fronts that may manifest themselves as MLDs (section 4.2).

Topography (that is, a non-flat LAB with variable depths) due to thermochemical erosion (Fig. 4) will further prevent coherent seismic signals from the LAB as melt may accumulate in depressions in the LAB, or may be produced due to additional decompression in ascending mantle at these depressions. Accordingly, weak seismic velocity gradients near the LAB have been attributed to thermal and melt-related disruptions [START_REF] Hopper | The lithosphere-asthenosphere boundary and the tectonic and magmatic history of the northwestern United States[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF]. These disruptions eventually lead to lithosphere rejuvenation accompanied by Fe-enrichment, which may explain low resistivity in the deep lithosphere along with observed

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16 positive geoid and Bouguer gravity anomalies, as applies to the São Francisco craton [START_REF] Pinto | Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating[END_REF].

The Kaapvaal craton is a particularly well-studied example. Plume impingement beneath the craton in the Cretaceous is thought to be responsible for kimberlite magmatism at that time [START_REF] Roex | Petrogenesis of group I kimberlites from Kimberley, South Africa: Evidence from bulk-rock geochemistry[END_REF][START_REF] Becker | Geochemistry of South African on-and off-craton, Group I and Group II kimberlites: Petrogenesis and source region evolution[END_REF]. It may also have resulted in the deep intersection of the volatile-rich peridotite solidus and production of partial melts that have pervasively metasomatised and refertilised the deep mantle root, leading to "loss" of 40 km of the Kaapvaal root by asthenospherisation. However, unradiogenic Os retained in some of the deepest and hottest samples suggests that the root has not been completely replaced [START_REF] Kobussen | Ghosts of lithospheres past: Imaging an evolving lithospheric mantle in southern Africa[END_REF][START_REF] Begg | The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution[END_REF][START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF]. The deep metasomatised root has warmed substantially [START_REF] Bell | Mesozoic thermal evolution of the southern African mantle lithosphere[END_REF] and become in parts almost indistinguishable from asthenospheric mantle. The thinner "LAB" corresponds to a depth where the amplitude of non-thermal seismic variations beneath the Kaapvaal craton is sharply reduced, which has been interpreted as the base of the chemical boundary layer (Artemieva, 2009). In contrast, teleseismic tomography and S receiver functions have revealed a lithosphere thickness of 250-300 km [START_REF] Zhang | High-resolution global upper-mantle structure and plate-tectonics[END_REF][START_REF] Sodoudi | Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton[END_REF].

In summary, it appears that intact and undisturbed cratonic roots will not produce a geophysically detectible LAB signal, whereas those undergoing melt-related reworking and rejuvenation do. Therefore, we next investigate the conditions that must be met in order to generate melt beneath the cratonic LAB and to keep it there long enough to produce a discontinuity, as well as the nature of the melt that is most likely to be produced.

Nature of melt near cratonic LABs: Evidence from mantle samples

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Convecting mantle will only melt beneath cratonic LABs in exceptional circumstances and if so, small melt fractions usually prevail because the lithospheric lid impedes further decompression melting of upwelling mantle [START_REF] Foley | Rejuvenation and erosion of the cratonic lithosphere[END_REF]. Nevertheless, kimberlites and related magmas were undoubtedly repeatedly produced beneath cratonic lithospheres, at least from the Proterozoic onward [START_REF] Becker | Geochemistry of South African on-and off-craton, Group I and Group II kimberlites: Petrogenesis and source region evolution[END_REF][START_REF] Tappe | Petrology and Nd-Hf Isotope Geochemistry of the Neoproterozoic Amon Kimberlite Sills, Baffin Island (Canada): Evidence for Deep Mantle Magmatic Activity Linked to Supercontinent Cycles[END_REF]. Mantle samples provide evidence for the oxidising nature of the melts interacting with cratonic roots [START_REF] Frost | The redox state of Earth's mantle[END_REF][START_REF] Foley | A Reappraisal of Redox Melting in the Earth's Mantle as a Function of Tectonic Setting and Time[END_REF], as they record more oxidising conditions at their depth of entrainment than would be expected in a fertile or depleted mantle (Fig. 3C). Indeed, a general correspondence between metasomatism and oxidation has been noted [START_REF] Woodland | Metasomatic interactions in the lithospheric mantle: Petrologic evidence from the Lherz massif, French Pyrenees[END_REF][START_REF] Creighton | Oxidation state of the lithospheric mantle beneath Diavik diamond mine, central Slave craton, NWT, Canada[END_REF][START_REF] Yaxley | An oxygen fugacity profile through the Siberian Craton -Fe K-edge XANES determinations of Fe3+/Sigma Fe in garnets in peridotite xenoliths from the Udachnaya East kimberlite[END_REF].

Carbonatites, which are only stable at temperatures far below the dry peridotite solidus (Fig. 3A) [START_REF] Dasgupta | The deep carbon cycle and melting in Earth's interior[END_REF][START_REF] Dasgupta | Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time[END_REF], may form in a volatile-bearing peridotite due to decompression in sublithospheric convection cells established as a consequence of topography beneath cratons (e.g. [START_REF] King | African hot spot volcanism: Small-scale convection in the upper mantle beneath cratons[END_REF], in a chemically buoyant plume (e.g. [START_REF] Simmons | Thermochemical structure and dynamics of the African superplume[END_REF][START_REF] Dannberg | Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept[END_REF], or due to thermal insulation beneath large continental plates (Andersen and King, 2014;[START_REF] Porritt | Lithospheric architecture beneath Hudson Bay[END_REF]. In contrast, kimberlite formation may additionally require excess T P (thermal plume) enabling upwelling mantle to be above the carbonated peridotite solidus over a large enough pressure interval to dissolve SiO 2 [START_REF] Dasgupta | Ingassing, Storage, and Outgassing of Terrestrial Carbon through Geologic Time[END_REF], and/or enriched conditions (high CO 2 -H 2 O contents; section 4.1.3). Indeed, kimberlite magmatism has been linked to episodic impingement of mantle plumes beneath cratons [START_REF] Heaman | The temporal evolution of North American kimberlites[END_REF][START_REF] Heaman | U-Pb geochronology and Sr/Nd isotope compositions of groundmass perovskite from the newly discovered Jurassic Chidliak kimberlite field, Baffin Island, Canada[END_REF]. In addition, the vast majority of mantle peridotites have trace-element patterns consistent with having equilibrated with carbonated silicate melts rather than carbonatites (e.g. [START_REF] Simon | The origin and evolution of the Kaapvaal cratonic lithospheric mantle[END_REF];

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18 [START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF]. This may also be supported by f O2 measurements [START_REF] Stagno | The oxidation state of the mantle and the extraction of carbon from Earth's interior[END_REF], indicating that enough melting has occurred to dilute incipient carbonatite melts with silicates.

Conditions for melt production below intact craton roots

While there is no present-day magmatic surface expression of kimberlite or other magmatism in intact cratons, as discussed in section 4.1.1, there is no a priori reason why melt should not currently be present beneath some cratonic LABs. Several conditions must be met to cause a seismic or electromagnetic discontinuity by the presence of melt below intact cratons: (1) The melt must be of a composition and volume sufficient to produce the discontinuity, (2) it must be interconnected and (3) it must not drain, or be continuously replenished [START_REF] Karato | On the origin of the asthenosphere[END_REF]. In general, in an upwelling mantle package, the fraction and nature of partial melts is determined by the composition-dependent melting interval and melt productivity. The depth of onset of partial melting is a function of temperature (hotter = deeper), composition (such as H 2 O content where wetter = deeper), and f O2 (metal saturation, onset of redox melting), while the depth where partial melting ends is the cratonic LAB (e.g [START_REF] Novella | The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites[END_REF][START_REF] Green | Experimental Study of the Influence of Water on Melting and Phase Assemblages in the Upper Mantle[END_REF]. The effect on the mantle solidus of adding H 2 O to the peridotite source is much stronger than that of CO 2 , which, being more incompatible, nevertheless dominates in the melt composition for small melt fractions [START_REF] Hirschmann | Partial melt in the oceanic low velocity zone[END_REF].

The nature of the melt produced beneath the LAB has a strong effect on the registered seismic and electrical signature it can produce. For example, carbonated melts are more conductive and less viscous than basaltic melts [START_REF] Gaillard | Carbonatite Melts and Electrical Conductivity in the Asthenosphere[END_REF], therefore having good grainwetting properties [START_REF] Minarik | Interconnectivity of carbonate melt at low melt fraction[END_REF][START_REF] Hunter | The equilibrium geometry of carbonate melts in rocks of mantle composition[END_REF]. Very small amounts (0.03-0.2%) are required to produce an electrical anomaly, such as that observed beneath the São Francisco craton [START_REF] Pinto | Magnetotelluric deep soundings, gravity and geoid in the south Sao Francisco craton: Geophysical indicators of cratonic lithosphere rejuvenation and crustal underplating[END_REF][START_REF] Sifré | Effects of temperature, pressure and chemical compositions on the electrical conductivity of carbonated melts and its relationship with viscosity[END_REF]. The melt fraction required to produce a 6 to 9% reduction in shear wave velocity, as is typically observed at

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oceanic LABs, is more debated since according to conventional wisdom 3.5 to 4% are required for isotropically distributed melt, which decreases to 0.25 to 1.25% for melt desegregating into bands due to deformation [START_REF] Kawakatsu | Seismic Evidence for Sharp Lithosphere-Asthenosphere Boundaries of Oceanic Plates[END_REF]. It must be noted, however, that recent experimental data indicate that less than 1% of melt would explain most shear wave velocity reductions at the LAB [START_REF] Chantel | Experimental evidence supports mantle partial melting in the asthenosphere[END_REF]. Such melt fractions are associated with the formation of silico-carbonatite or small-volume basaltic melts [START_REF] Hirschmann | Partial melt in the oceanic low velocity zone[END_REF][START_REF] Dasgupta | Carbondioxide-rich silicate melt in the Earth's upper mantle[END_REF], but could also be reached for incipient melts if buoyancy-driven melt migration is taken into account [START_REF] Grégoire | Mantle mush compaction: A key to understand the mechanisms of concentration of kimberlite melts and initiation of swarms of kimberlite dykes[END_REF]. Since only up to 1.5 wt% of melt can be produced at 100 km depth in a typically enriched source with 300 ppm CO 2 and 300-500 ppm H 2 O, or 1% of melt at 180 km depth in a source with 1600 ppm H 2 O at ambient T P [START_REF] Dasgupta | Carbondioxide-rich silicate melt in the Earth's upper mantle[END_REF][START_REF] Novella | The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites[END_REF], higher water or alkali contents and/or excess T P are required to produce significant melt volume beneath cratonic roots at 250 km depth.

We use a predictive approach by employing an improved version of the recent thermodynamic parameterisation considering the presence of volatiles, i.e. CO 2 and H 2 O [START_REF] Massuyeau | A model for the activity of silica along the carbonatite-kimberlite-mellilitite-basanite melt compositional joint[END_REF]. Our calculations show that, assuming sufficiently oxidising conditions to stabilise carbonates [START_REF] Rohrbach | Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling[END_REF][START_REF] Stagno | The oxidation state of the mantle and the extraction of carbon from Earth's interior[END_REF]

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ACCEPTED MANUSCRIPT 20 depends on hydrogen solubility in the minerals, which may be minimum near cratonic LAB depth [START_REF] Mierdel | Water solubility in aluminous orthopyroxene and the origin of Earth's asthenosphere[END_REF]). An interconnected melt may form from smaller individual melt increments if these accumulate at the rheological boundary between lithosphere and asthenosphere [START_REF] Hirschmann | Partial melt in the oceanic low velocity zone[END_REF].

Melt retention and resupply at the LAB

The geodynamic and compositional requirements to generate sufficient melt at the LAB have likely been met many times throughout cratonic histories, but in order to produce a geophysically detectible anomaly today, it is also necessary to either retain or replenish this melt. Once formed, compaction and topography at the LAB may cause the melt to drain even at small melt fractions [START_REF] Mckenzie | Some remarks on the movement of small melt fractions in the mantle[END_REF][START_REF] Faul | Melt retention and segregation beneath mid-ocean ridges[END_REF], and in particular the low viscosity of carbonatite melt militates against retention at the LAB (e.g. [START_REF] Hirschmann | Partial melt in the oceanic low velocity zone[END_REF]. Because the LAB represents a permeability barrier, melts are expected to migrate upward buoyantly and focus at depressions in the LAB if the slope towards that dip is sufficiently high, until they are extracted in veins [START_REF] Gregg | Geodynamic Models of Melt Generation and Extraction at Mid-Ocean Ridges[END_REF]. In fact, craton edges and other zones with weaker cratonic signatures (slower velocities, lower elastic thickness and resistivity compared to intact roots), such as intra-cratonic boundaries, have been identified as the locus of preferred kimberlite magmatism [START_REF] Poudjom-Djomani | Variations of the effective elastic thickness (Te) and structure of the lithosphere beneath the Slave Province, Canada[END_REF][START_REF] Malkovets | Diamond, subcalcic garnet, and mantle metasomatism: Kimberlite sampling patterns define the link[END_REF]Artemieva, 2009;[START_REF] Begg | The lithospheric architecture of Africa: Seismic tomography, mantle petrology, and tectonic evolution[END_REF][START_REF] Faure | Seismic Architecture of the Archean North American Mantle and Its Relationship to Diamondiferous Kimberlite Fields[END_REF]. In this context, it is worth highlighting that the sharp seismic signature for oceanic LAB may be related to the fact that it overlaps the melt freezing boundary (Sifré et al 2014), which effectively represents a thermodynamic permeability boundary, but that this model cannot be extended to much deeper cratonic LABs.

Drainage might be retarded under some circumstances. Melt mobility at cratonic LAB depths may be counteracted by the small ratio of density contrast between basaltic melts and mantle

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ACCEPTED MANUSCRIPT 21 at 6 to 7 GPa [START_REF] Sakamaki | Ponded melt at the boundary between the lithosphere and asthenosphere[END_REF]. Volatile-bearing alkali basalts may even be neutrally buoyant at cratonic LAB depths [START_REF] Crepisson | Magmas trapped at the continental lithosphere-asthenosphere boundary[END_REF]. However, it must be emphasised that more data on the physical properties of CO 2 -and H 2 O-rich melts that are expected at cratonic LABs are required to fully explore their stability at the LAB and thus their potential to explain seismic and electrical anomalies when present. Finally, several processes may facilitate melt replenishment below cratonic LABs. Long-lived thermochemical mantle upwellings (e.g. [START_REF] Zhao | Seismic evidence for a chemically distinct thermochemical reservoir in Earth's deep mantle beneath Hawaii[END_REF] could continuously advect peridotite that is oxidising enough to enable redox melting at 250 km depth. Alternatively, low-density methane-rich fluids continually rising from the deep upper mantle and transition zone could be oxidised at the depth of metal saturation, providing a continuous supply of reduced carbon from which carbonated melt could be generated near the base of thick lithospheres [START_REF] Rohrbach | Metal saturation in the upper mantle[END_REF][START_REF] Frost | The redox state of Earth's mantle[END_REF][START_REF] Rohrbach | Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling[END_REF]) (Fig. 4).

Mid-lithospheric discontinuities (MLDs): Geophysical expression of metasomatism,

Palaeo-LABs or future LABs ?

While negative velocity gradients beneath intact cratons at ~170-250 km depth are generally interpreted as the LAB (section 4.1), seismic discontinuities, with a thickness of ~30-40 km and occurring at 60-160 km depth (predominantly between 80-120 km depth) beneath continental lithosphere [START_REF] Thybo | The heterogeneous upper mantle low velocity zone[END_REF][START_REF] Rychert | A Global View of the Lithosphere-Asthenosphere Boundary[END_REF] have been interpreted as MLDs [START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Yuan | Lithospheric layering in the North American craton[END_REF][START_REF] Ford | The lithosphereasthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging[END_REF][START_REF] Foster | A sharp cratonic lithosphere-asthenosphere boundary beneath the American Midwest and its relation to mantle flow[END_REF][START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF][START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]; for a compilation of MLD locations, depths and associated velocity decreases see [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] (Fig. 3D). In some cases, cratonic

MLDs are continuous with neighbouring, younger tectonic areas (Western Canada, USA, Australia; [START_REF] Mercier | The teleseismic signature of fossil subduction: Northwestern Canada[END_REF][START_REF] Ford | The lithosphereasthenosphere boundary and cratonic lithospheric layering beneath Australia from Sp wave imaging[END_REF][START_REF] Foster | A sharp cratonic lithosphere-asthenosphere boundary beneath the American Midwest and its relation to mantle flow[END_REF], whereas in others, they are truncated, indicating translithospheric boundaries that depict terrane sutures at depth [START_REF] Courtier | Seismic anisotropy associated with continental lithosphere accretion beneath the CANOE array, northwestern Canada[END_REF] (Fig. 2C).

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22 Discontinuities within the lithospheric mantle typically have negative gradients, but both negative and positive phases are observed [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF][START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF].

Negative phases are generated by waves travelling upwards from a seismically slower to a seismically faster layer, whereby the amplitude of the phase correlates positively with the size of the velocity contrast and sharpness of the velocity gradient [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. MLDs are also detected when adjacent layers show seismic anisotropy contrasts with regard to strength and orientation [START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF] and in some instances they correlate with anomalous conductivity (e.g. [START_REF] Jones | Electric lithosphere of the Slave craton[END_REF]. An important point that has been emphasised in recent studies is that the results of seismic studies depend critically on the energy spectrum used to filter the data. Thus, low frequencies favour the appearance of single coherent large phases, whereas higher frequencies allow the detection of multiple conversions with lower amplitudes that appear as multiple MLDs [START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF]. As a consequence, single larger MLDs may be an artefact of lower frequency filtering [START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF].

MLDs related to fabric

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Anisotropy contrasts at the pervasive-to-focussed melt flow transition

Asthenosphere-derived partial melts are thought to penetrate into and react with the deep cratonic lithosphere diffusively and pervasively, and the effects of this melt percolation are evident in all cratonic mantle sections [START_REF] Gaul | Mapping olivine composition in the lithospheric mantle[END_REF][START_REF] Griffin | Cr-pyrope garnets in the lithospheric mantle 2. Compositional populations and their distribution in time and space[END_REF][START_REF] Griffin | Lithosphere mapping beneath the north American plate[END_REF].

Variations in the degree of interaction of multiple batches of melt with the mantle lithosphere, with which the melt is initially out of chemical and thermal equilibrium, may lead to variable depths of freezing due to cooling and reactions with pre-existing mantle. After each "conditioning" of the lithosphere, subsequent batches of melt may percolate upwards further before experiencing chemical or thermal death, until the melt finally focuses into veins and channels and is extracted to the surface [START_REF] Gregg | Geodynamic Models of Melt Generation and Extraction at Mid-Ocean Ridges[END_REF] (Fig. 7A). This melt focussing into veins and deformation associated with it may cause radial anisotropy (Fig. 7B). A discontinuity with a negative velocity gradient may then ensue if radial anisotropy is associated with an abrupt transition from a layer with strong to a layer with weak anisotropy with depth, depending on the sign of the radial anisotropy [START_REF] Rychert | A Global View of the Lithosphere-Asthenosphere Boundary[END_REF]. This is observed in the Australian continent [START_REF] Yoshizawa | The lithosphere-asthenosphere transition and radial anisotropy beneath the Australian continent[END_REF]. A scenario involving ~10% pyroxenite and wehrlite dykes (tectonic stockworks) has been proposed by [START_REF] Snyder | Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities[END_REF][START_REF] Snyder | Imaging Archaean-age whole mineral systems[END_REF] and [START_REF] Snyder | Lithospheric architecture of the Slave craton, northwest Canada, as determined from an interdisciplinary 3-D model[END_REF] to explain seismic anisotropy observed below the Canadian Shield, although calculations suggest that the contrast in density and seismic velocity is small [START_REF] Tommasi | Heterogeneity and anisotropy in the lithospheric mantle[END_REF].

In the Kaapvaal craton, annealing in coarse peridotites weakened the fabric related to prior deformation in response to melt percolation, but 12% of the samples, possibly from the shallower, cool and little metasomatised part of the mantle, retained foliations and lineations [START_REF] Baptiste | Deformation and hydration of the lithospheric mantle beneath the Kaapvaal craton, South Africa[END_REF]. Since the dispersion of crystal-preferred orientation (CPO) and associated weakening of seismic anisotropy may be assisted by melt-rock interaction A C C E P T E D M A N U S C R I P T [START_REF] Tommasi | Seismic anisotropy and compositionally induced velocity anomalies in the lithosphere above mantle plumes: a petrological and microstructural study of mantle xenoliths from French Polynesia[END_REF], the products of which may be oversampled in magma-entrained xenoliths (section 2.2), the presence of seismic anisotropy may be underestimated based on the xenolith record.

Frozen rock fabrics: MLDs as palaeo-LABs

The frequent association of MLDs with azimuthal anisotropy has been ascribed to frozen rock fabric and can produce a discontinuity if the anisotropy contrast between adjacent layers is sufficiently large (Wang et al., 2014;[START_REF] Beghein | Changes in Seismic Anisotropy Shed Light on the Nature of the Gutenberg Discontinuity[END_REF]. Azimuthal anisotropy is unlikely to result in consistently negative phases independent of the back-azimuth, which is how the majority of MLDs appear in SRF studies (Table 1 in [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF], and it can therefore not globally cause MLDs having negative phases [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. Nevertheless, some

MLDs have been associated with formation and thickening of cratonic nuclei (Abt et al., 2010;[START_REF] Fischer | The Lithosphere-Asthenosphere Boundary[END_REF][START_REF] Miller | Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions[END_REF][START_REF] Yuan | Lithospheric layering in the North American craton[END_REF][START_REF] Snyder | Imaging Archaean-age whole mineral systems[END_REF][START_REF] Cooper | Craton formation: Internal structure inherited from closing of the early oceans[END_REF][START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF]. If they represent palaeo-LABs prior to root thickening, azimuthal anisotropy could result by the same mechanisms as proposed for modern LABs, i.e. CPO of minerals deforming at the interface of the protocratonic nucleus and the palaeo-convecting mantle, provided that a sharp change in orientation or intensity of the CPO is involved [START_REF] Montagner | Where can seismic anisotropy be detected in the earth's mantle? In boundary layers[END_REF][START_REF] Lebedev | Seismic structure of Precambrian lithosphere: New constraints from broad-band surface-wave dispersion[END_REF].

Interestingly, initial lithosphere thicknesses estimated by [START_REF] Lee | Calculating melting temperatures and pressures of peridotite protoliths: Implications for the origin of cratonic mantle[END_REF] for peridotite xenoliths (Siberian, Slave, Kaapvaal, Tanzanian cratons) range from 30-150 km depth, which overlaps the current depths of MLDs of ~60 to 160 km [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF].

Dipping or tapered fabrics associated with some MLDs in cratonic lithosphere may relate to craton root thickening involving plate tectonics [START_REF] Bostock | Anisotropic upper-mantle stratigraphy and architecture of the Slave craton[END_REF][START_REF] Cook | Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian shield[END_REF][START_REF] Chen | New constraints on the upper mantle structure of the Slave craton from Rayleigh wave inversion[END_REF][START_REF] Snyder | Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities[END_REF][START_REF] Miller | Formation of cratonic mantle keels by arc accretion: Evidence from S receiver functions[END_REF]Ashchepkov et al., 2014;[START_REF] Cooper | Craton formation: Internal structure inherited from closing of the early oceans[END_REF]

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25 2014; Wirth and Long, 2014) (Fig. 2C). Alternatively, upwelling mantle underplating a palaeo-LAB, with material flowing laterally towards thinner craton margins after impinging upon the lithospheric lid [START_REF] Sleep | Local lithospheric relief associated with fracture zones and ponded plume material[END_REF][START_REF] Shervais | Lithospheric topography, tilted plumes, and the track of the Snake River-Yellowstone hot spot[END_REF] may play a role (Fig. 2A).

Topography developed at the palaeo-LAB prior to root thickening (Fig. 4) helps explain the seismic complexity of cratonic lithospheric mantle and the occurrence of multiple, spatially distributed MLDs with both negative and positive velocity phases that appear only when short-period filtering is applied [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. A major argument in favour of MLDs being caused by processes dating back to cratonic lithosphere construction is that these discontinuities sometimes appear to be correlated with tectonic units and truncated at craton margins [START_REF] Porritt | Lithospheric architecture beneath Hudson Bay[END_REF]. They show steep-sided seismic velocity offsets that therefore have persisted for billions of years [START_REF] Poupinet | Seismic tomography beneath stable tectonic regions and the origin and composition of the continental lithospheric mantle[END_REF].

Present-day ponded melts or fluids

The global existence of a discontinuity at cratonic MLD depths has been ascribed to the presence of melt [START_REF] Thybo | The heterogeneous upper mantle low velocity zone[END_REF]. The solidi of various mantle source rocks, combined with the thermal state of cratonic lithosphere, determine at which temperature and depth melt can be present (Fig. 3A). In disrupted cratons currently overlying anomalously hot mantle, MLDs plausibly represent the top of a live melt percolation front. For example, reduced S-wave velocities at depths of 50-100 km beneath the Tanzanian craton and adjacent rift have been correlated to the presence of phlogopite-bearing rocks and pyroxenites in mantle xenolith suites [START_REF] Wölbern | Melt infiltration of the lower lithosphere beneath the Tanzania craton and the Albertine rift inferred from S receiver functions[END_REF]. In contrast, the cool geothermal gradients of intact cratonic mantle preclude the presence of melt at the LAB that could percolate to MLD depths, except in cases of mantle containing abnormal amounts of H 2 O and/or CO 2 or at exceptionally high T P (Fig. 3A), as discussed in section 4.1.

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The effects of melt percolation on the mantle immediately prior to kimberlite eruption are evident in chemically zoned minerals and unequilibrated microstructures in mantle xenoliths that could not survive in the mantle for extended periods of time (>10s of million years) (O'Reilly and [START_REF] O'reilly | The continental lithosphere-asthenosphere boundary Can we sample it?[END_REF][START_REF] Pearson | The Formation and Evolution of Cratonic Mantle Lithosphere -Evidence from Mantle Xenoliths[END_REF]. The trace-element and isotopic signatures in mantle xenoliths from various cratons identify these melts as silico-carbonatites similar to kimberlites (Simon et al., 2003;Agashev et al., 2013;[START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF]) (Figs.

3C, 6). Saline, CO 2 -poor fluids (brines) have also been implicated in the late (around the time of kimberlite magmatism) formation of fibrous diamonds, for which a genetic connection to kimberlites has been established (e.g. Klein [START_REF] Klein-Bendavid | Fluid inclusions in diamonds from the Diavik mine, Canada and the evolution of diamond-forming fluids[END_REF][START_REF] Klein-Bendavid | The sources and time-integrated evolution of diamond-forming fluids -Trace elements and isotopic evidence[END_REF][START_REF] Weiss | Highly saline fluids from a subducting slab as the source for fluid-rich diamonds[END_REF].

Given the extended time before perturbed gradients relax back to the conductive geotherm [START_REF] Michaut | Secular cooling and thermal structure of continental lithosphere[END_REF]) and the young age of kimberlite magmatism in many cratons (<500 Ma, see Table 2 in [START_REF] Grütter | Pyroxene xenocryst geotherms: Techniques and application[END_REF], the effects of melt infiltration may still contribute to abnormal present-day heat flows (e.g. Kaapvaal craton; [START_REF] Bell | Mesozoic thermal evolution of the southern African mantle lithosphere[END_REF].

Partial melting in the cratonic lithosphere may result from sources with considerably lower solidi than fertile peridotite, such as pyroxene-and/or hydrous mineral-rich domains, the generation of which then requires some prior tectonothermal activity, during which they were emplaced from melt produced either in or below the lithosphere (decompression, influx of volatiles, heating) (Fig. 5). The most noted examples are assemblages of mica-amphibolerutile-ilmenite-diopside (MARID) and related rocks, which form part of xenolith suites in the Kaapvaal craton and which have been linked to Cretaceous kimberlite activity [START_REF] Dawson | MARID (mica-amphibole-rutile-ilmenite-diopside) suite of xenoliths in kimberlite[END_REF][START_REF] Erlank | Evidence for mantle metasomatism in peridotite nodules from the Kimberley pipes, South Africa[END_REF][START_REF] Konzett | The timing of MARID metasomatism in the Kaapvaal mantle: An ion probe study of zircons from MARID xenoliths[END_REF][START_REF] Konzett | Modal metasomatism in the Kaapvaal craton lithosphere: constraints on timing and genesis from U-Pb zircon dating of metasomatized peridotites and MARID-type xenoliths[END_REF][START_REF] Grégoire | Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited[END_REF]. The remobilisation of such metasomes as low-volume melts requires only a modest amount of heat, for example during plume impingement, or decompression during stretching of the continental lithosphere (Mackenzie, 1989;[START_REF] Niu | Geochemistry -The origin of alkaline lavas[END_REF]. However, if such melts or fluids were

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ACCEPTED MANUSCRIPT 27 present at MLD depth, they should be visible as conductive layers in MT studies, which is not generally the case [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF].

Deposition of metasomatic minerals during past metasomatism

In light of the arguments presented in the previous section, it seems unlikely that globally detected cratonic MLDs are the product of the current presence of melt or fluid. In contrast, seismically slow hydrous minerals and pyroxenes may have been deposited in the cratonic lithosphere at some time in the past and may be stable over aeons in cool cratonic roots [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. The amount of metasomatic minerals necessary to produce the measured 2-7% velocity reductions over 10-20 km are 5-10% for phlogopite or 10-15% for carbonates or >15% for amphiboles or 45-100% for pyroxenite [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF]. Density contrasts of such metasomatic layers with the surrounding mantle may lead to long-term gravitational instability [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF]. Rather than precluding their current presence at MLDs, such metasomes may imply that such accumulations are not likely to be old and that future redistributions to density-stable conditions are possible. A major argument in favour of a secondary origin of at least some MLDs, unrelated to craton construction, is their continuity across distinct tectonic units, as is the case for the Kalahari craton [START_REF] Savage | Evidence for a compositional boundary within the lithospheric mantle beneath the Kalahari craton from S receiver functions[END_REF] and parts of the North American continent [START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF]. The main modes of introduction of metasomatic minerals would have involved metamorphic (dehydration, partial melting) processes in underthrusting oceanic slabs or infiltration of small-volume asthenosphere-derived partial melts. Archaean geothermal gradients would have been twice as high (corresponding to surface heat flows of 45-90 mW/m 2 ) as those recorded in xenolith entrained in kimberlites over the last Ga [START_REF] Michaut | Thermal evolution of cratonic roots[END_REF] (Fig. 3B). Nevertheless, the old ages of diamonds (summarised in [START_REF] Gurney | Diamonds through Time[END_REF]Helmstadt, 2013) indicate that the roots at depth >~150 km must have cooled rapidly to near-present conditions after craton stabilisation.

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Metasomatic minerals accumulated during subduction

Subduction-accretion of oceanic slabs has been a popular model for the construction of cratonic roots [START_REF] Helmstaedt | Southern African kimberlites and their mantle sample: implications for Archean tectonics and lithosphere evolution[END_REF][START_REF] Canil | Mildly incompatible elements in peridotites and the origins of mantle lithosphere[END_REF]Pearson andWittig, 2008, 2014) and has been invoked to explain dipping seismic discontinuities, and in some cases spatially coincident seismic and conductive anomalies, in some cratonic mantle samples [START_REF] Cook | Frozen subduction in Canada's Northwest Territories: Lithoprobe deep lithospheric reflection profiling of the western Canadian shield[END_REF][START_REF] Snyder | Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities[END_REF][START_REF] Chen | Geophysical Detection of Relict Metasomatism from an Archean (similar to 3.5 Ga) Subduction Zone[END_REF][START_REF] Cooper | Craton formation: Internal structure inherited from closing of the early oceans[END_REF]. Copious amounts of water may have been introduced from seawater-altered sediment, oceanic crust and mantle into mantle overlying subduction zones during dehydration reactions [START_REF] Rüpke | Serpentine and the subduction zone water cycle[END_REF][START_REF] Schmidt | Devolatilization During Subduction[END_REF], leading to formation of phlogopite and orthopyroxene by reactions of the mantle with siliceous hydrous fluids or melts [START_REF] Wyllie | The formation of mantle phlogopite in subduction zone hybridisation[END_REF][START_REF] Sato | Phase relations of natural phlogopite with and without enstatite up to 8 GPa: Implication for mantle metasomatism[END_REF][START_REF] Konzett | The stability of hydrous potassic phases in lherzolitic mantle -An experimental study to 9 center dot 5 GPa in simplified and natural bulk compositions[END_REF][START_REF] Vielzeuf | Melting relations in hydrous systems revisited: application to metapelites, metagreywackes and metabasalts[END_REF]. Such hydrous mineral-rich lithologies may persist for Ga timescales. For example, phlogopite-rich xenoliths, though very few, with 2.7 Ga minimum ages coincident with the age of craton amalgamation, have been reported from the central Slave craton (Aulbach et al., 2007). Phlogopite in mantle xenoliths from the Kaapvaal craton have been dated to 1.25 -1.0 Ga, coincident with and linked to metasomatism in the course of the formation of the Namaqua-Natal belt at the southern craton margin [START_REF] Hopp | Ar-40/Ar-39-ages of phlogopite in mantle xenoliths from South African kimberlites: Evidence for metasomatic mantle impregnation during the Kibaran orogenic cycle[END_REF]. The deposition of hydrous metasomatic mineral assemblages near dipping slabs may reinforce seismic velocity anomalies produced by associated shear zones [START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF] (Fig. 5A).

Mantle metasomatism

Seismically slow hydrous minerals, such as amphibole (at pressures <3 GPa; [START_REF] Fumagalli | Alkali in phlogopite and amphibole and their effects on phase relations in metasomatized peridotites: a high-pressure study[END_REF]) or phlogopite at temperature <1150º C (Sekine and [START_REF] Wyllie | The formation of mantle phlogopite in subduction zone hybridisation[END_REF][START_REF] Hansen | Thermal classification of lithospheric discontinuities beneath USArray[END_REF][START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF] also precipitate from OIB-like potassic, hydrous magmas in within-plate

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ACCEPTED MANUSCRIPT 29 settings [START_REF] Pilet | Liquid line of descent of a basanitic liquid at 1.5 Gpa: constraints on the formation of metasomatic veins[END_REF][START_REF] Pilet | Monte Carlo Simulations of Metasomatic Enrichment in the Lithosphere and Implications for the Source of Alkaline Basalts[END_REF][START_REF] Grant | Experimental reactions between olivine and orthopyroxene with phonolite melt: implications for the origins of hydrous amphibole plus phlogopite plus diopside bearing metasomatic veins[END_REF]. The composition of most kimberlites demands that they interacted with (or even formed in) metasomatised refractory cratonic mantle sources [START_REF] Becker | Geochemistry of South African on-and off-craton, Group I and Group II kimberlites: Petrogenesis and source region evolution[END_REF][START_REF] Sokol | The role of water in generation of group II kimberlite magmas: Constraints from multiple saturation experiments[END_REF] and that, conversely, the cratonic mantle is modified by their passage. Because small-volume melts do not transport heat, complete or partial crystallisation in the cool cratonic lithosphere is expected [START_REF] Mckenzie | Some remarks on the movement of small melt fractions in the mantle[END_REF]. [START_REF] Sleep | Stagnant lid convection and carbonate metasomatism of the deep continental lithosphere[END_REF] suggests that kimberlites are rarely able to penetrate beyond mid-lithospheric depths, where the cratonic mantle is under compression and becomes cold and viscous, and instead form sills that crystallise in situ as they cool. The kimberlite production rate over the last 2.5 Ga would correspond to 0.4 km 3 /year, amounting to a total thickness of 2 km if it were concentrated in a layer beneath the continents, which would sustain a kimberlite eruption every 10 to 100 years; consequently, the much lower observed frequency indicates that the majority of kimberlite melts are trapped at depth [START_REF] Sleep | Stagnant lid convection and carbonate metasomatism of the deep continental lithosphere[END_REF].

Since low-volume melts and fluids are enriched in incompatible elements, among them the heat-producing elements (HPE: K, Th and U), they may have significant thermal effects (O'Reilly and [START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF]. Estimates of the heat produced by radioactive decay in a several km thick kimberlite layer suggest that it should result in a strong curvature in the regional geotherm around that depth that would be resolvable with xenolith data [START_REF] Sleep | Stagnant lid convection and carbonate metasomatism of the deep continental lithosphere[END_REF], which is not observed [START_REF] Grütter | Pyroxene xenocryst geotherms: Techniques and application[END_REF]. Large accumulations of amphiboles (15% over a depth interval of 20 km) are also precluded based on heat flow arguments, as modelled by [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. However, smaller volumes of hydrous minerals than those required to affect cratonic heatflow may explain MLDs which have smaller and more distributed velocity reductions than it appears in studies using low-frequency filtering [START_REF] Wirth | A contrast in anisotropy across mid-lithospheric discontinuities beneath the central United States-A relic of craton formation[END_REF]. [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] suggest that a metasomatic phlogopite layer, now at MLD depth, was precipitated at the pre-thickening palaeo-LAB. This appears unlikely, given that the

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temperature at the LAB, where the lithospheric mantle temperature approaches that of the convecting mantle, precludes the stability of phlogopite (Fig. 3), even more so in the warmer Archaean. It is also doubtful that subsequent cratonic root thickening and gradual displacement of the LAB to greater depth is due to conductive cooling [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] because this process confers neither the dryness and viscosity nor the chemical depletion and buoyancy required for craton survival (Lendardic et al., 2003;[START_REF] King | Archean cratons and mantle dynamics[END_REF]Wang et al., 2014).

Observed and inferred accumulations of metasomatic minerals in cratonic mantle

A major possible problem with calling for volumes of hydrous minerals large enough to produce a seismically visible anomaly is that direct evidence for the presence of carbonate-, phlogopite-or amphibole-rich rocks in cratonic xenolith suites appears sparse, that they are reported at depths different from MLD depths, or that they are distributed over depth intervals too large to produce a discontinuity [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF][START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF]. However, this paucity may relate to: (1) Kimberlites sampling a limited and anomalous volume of the mantle that may be unusually metasomatised, but may also have disproportionately contributed to the generation of small-volume, volatile-rich magmas, with preferential consumption of assemblages rich in hydrous minerals and pyroxenes (e.g. [START_REF] Grégoire | Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited[END_REF][START_REF] Becker | Geochemistry of South African on-and off-craton, Group I and Group II kimberlites: Petrogenesis and source region evolution[END_REF][START_REF] Konzett | Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: Evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases[END_REF]. However, since kimberlites form at high pressures, metasomatic minerals deposited at lower pressures, where MLDs occur, should not be tapped by episodic kimberlite magmatism.

(2) Amphibole-and phlogopite-rich rocks being underreported in the literature because many studies focus on obtaining constraints on the age and mode of cratonic lithosphere formation, which therefore explicitly target peridotite devoid of visibly metasomatic minerals. (3) Carbonates, moderate accumulations (10-15%) of which can produce a strong negative velocity gradient [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF], rapidly decomposing upon decompression, implying that their rare occurrence in

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mantle xenoliths has no bearing on their presence at mantle depth [START_REF] Canil | Experimental study bearing on the absence of carbonate in mantle-derived xenoiths[END_REF]. Similarly, hydrous minerals may be preferentially disrupted during transport in the kimberlite or alteration at the surface. Indeed, even when metasomatic minerals are not directly observed, evidence for the passage of low-volume hydrous or carbonated melts, from which they would have plausibly precipitated in the cratonic lithosphere, is wide-spread (Fig. 6).

Direct evidence for the presence of metasomatic minerals in cratonic mantle comes from mantle xenoliths and xenocrysts. A tabulated summary of observations is given in [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] and [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF]. As mentioned in section 4.2.2, some of the most prominent examples of such metasomes occur in the Kaapvaal craton (MARID and related rocks).

Pressure-temperature estimates locate these metasomes in the shallower lithosphere (1.5-3.7

GPa), while age dating demonstrates a close temporal link to the older Group II and subsequent Group I kimberlite magmatism (80-90 Ma) or to the preceding Karroo magmatic event [START_REF] Konzett | The timing of MARID metasomatism in the Kaapvaal mantle: An ion probe study of zircons from MARID xenoliths[END_REF][START_REF] Konzett | Modal metasomatism in the Kaapvaal craton lithosphere: constraints on timing and genesis from U-Pb zircon dating of metasomatized peridotites and MARID-type xenoliths[END_REF][START_REF] Konzett | Two episodes of fluid migration in the Kaapvaal Craton lithospheric mantle associated with Cretaceous kimberlite activity: Evidence from a harzburgite containing a unique assemblage of metasomatic zirconium-phases[END_REF][START_REF] Grégoire | Trace element geochemistry of phlogopite-rich mafic mantle xenoliths: their classification and their relationship to phlogopite-bearing peridotites and kimberlites revisited[END_REF]Bell et al., 2005;[START_REF] Kobussen | Ghosts of lithospheres past: Imaging an evolving lithospheric mantle in southern Africa[END_REF][START_REF] Giuliani | Oxide, sulphide and carbonate minerals in a mantle polymict breccia: Metasomatism by proto-kimberlite magmas, and relationship to the kimberlite megacrystic suite[END_REF][START_REF] Giuliani | Petrogenesis of Mantle Polymict Breccias: Insights into Mantle Processes Coeval with Kimberlite Magmatism[END_REF][START_REF] Giuliani | Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?[END_REF]. Phlogopite-bearing polymict breccias from 110-115 km depth have also been linked to group I kimberlite magmatism and were interpreted as failed kimberlite intrusions that paved the way for subsequent kimberlite eruption [START_REF] Giuliani | Petrogenesis of Mantle Polymict Breccias: Insights into Mantle Processes Coeval with Kimberlite Magmatism[END_REF]. Although the Kaapvaal MARID and related rocks are carbonate-poor, xenoliths composed of phlogopite and carbonate in lamproites from the Churchill province attest to the co-precipitation of these two minerals [START_REF] Peterson | Glmmerite xenoliths in early Proterozoic ultrapotassic rocks from the Churchill province[END_REF]. In contrast, nominally anhydrous, pyroxene-rich metasomes, perhaps similar to those in western branch of the East African Rift [START_REF] Davies | Pb-Sr-Nd isotope and trace element data bearing on the origin of the potassic subcontinental lithosphere beneath south-west Uganda, Kimberlites and related rocks[END_REF], are expected at higher temperatures (greater depths).

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32 Intense low-temperature phlogopite-related metasomatism, inferred from the TiO 2 -Zr systematics of garnet xenocrysts, is recorded at depths of 100-120 km in the Kaapvaal craton, consistent with the aforementioned presence of MARID and similar rocks in Kaapvaal xenolith suites (O'Reilly and [START_REF] O'reilly | Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: Geophysical implications[END_REF], and is commonly observed in shallow garnet peridotites (Fig. 6). Interaction with carbonatites is fingerprinted by characteristic strong LREE-enrichment and relative depletions in the HFSE [START_REF] Rudnick | Carbonatite metasomatism in the northern Tanzanian mantle -petrographic and geochemical characteristics[END_REF][START_REF] Ionov | Carbonated peridotite xenoliths from Spitsbergen -implications for trace-element signature of mantle carbonate metasomatism[END_REF] (Fig. 6). Moreover, the chemical composition of typically cratonic magmas can be used to infer the mineralogy of their mantle source. Numerous experimental studies address the relationships between phlogopite and potassic carbonated melts, such as Group II kimberlites (e.g. [START_REF] Sweeney | Phase relations of a natural MARID composition and implications for MARID genesis, lithospheric melting and mantle[END_REF][START_REF] Ulmer | Generation and differentiation of group II kimberlites: Constraints from a high-pressure experimental study to 10 GPa[END_REF][START_REF] Enggist | Phase relations of phlogopite and pyroxene with magnesite from 4 to 8 GPa: KCMAS-H 2 O and KCMAS-H 2 O-CO 2[END_REF]. These exotic rocks have been emplaced over a considerable time span -between 65 Ma in the Indian Bastar craton (Chalapathi [START_REF] Rao | Petrogenesis of the end-Cretaceous diamondiferous Behradih orangeite pipe: implication for mantle plume-lithosphere interaction in the Bastar craton, Central India[END_REF] and 1.92 Ga in the Karelian craton in NW Russia [START_REF] Priyatkina | 1.92 Ga kimberlitic rocks from Kimozero, NW Russia: Their geochemistry, tectonic setting and unusual field occurrence[END_REF] and occur not only in Southern Africa, but also in Australia, India, Russia and Finland [START_REF] Giuliani | Did diamond-bearing orangeites originate from MARID-veined peridotites in the lithospheric mantle?[END_REF]. Multiple generations of metasomes, in part refilling older melt pathways and ranging from isotopically young (asthenosphere-derived) carbonate-phlogopite assemblages to isotopically aged (lithosphere-derived) MARID-type assemblages, may be present in the deeper reaches of a single craton. This has been deduced from the chemical and isotopic composition of alkaline ultramafic melts in the North Atlantic and Congo-Tanzanian cratons [START_REF] Tappe | Genesis of ultramafic lamprophyres and carbonatites at Aillik Bay, Labrador: A consequence of incipient lithospheric thinning beneath the North Atlantic craton[END_REF][START_REF] Tappe | Craton reactivation on the Labrador Sea margins: Ar-40/Ar-39 age and Sr-Nd-Hf-Pb isotope constraints from alkaline and carbonatite intrusives[END_REF][START_REF] Tappe | Between carbonatite and lamproite -Diamondiferous Torngat ultramafic lamprophyres formed by carbonate-fluxed melting of cratonic MARID-type metasomes[END_REF][START_REF] Tappe | The newly discovered Jurassic Tikiusaaq carbonatite-aillikite occurrence, West Greenland, and some remarks on carbonatitekimberlite relationships[END_REF][START_REF] Rosenthal | Petrogenesis of strongly alkaline primitive volcanic rocks at the propagating tip of the western branch of the East African Rift[END_REF].

Thus, there is ample indirect evidence for the presence of seismically slow minerals in the cratonic lithosphere.

Phase equilibria and thermodynamics: Can metasomatism explain MLDs?

In this section, we discuss the nature of the melts responsible for metasomatism of the cratonic lithosphere following two distinct steps: (1) A review of the experimental work

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carried out over the last decades, and (2) a predictive approach using the recent thermodynamic parameterisation from [START_REF] Massuyeau | A model for the activity of silica along the carbonatite-kimberlite-mellilitite-basanite melt compositional joint[END_REF]. Because of the limited stability of amphibole (<1070°C, <3 GPa, depending on water fugacity, Fig. 3B; Green et al., 2010), its precipitation in the cratonic lithosphere can only explain MLDs observed at shallower levels, to approximately 90 km. Thus, a different mechanism is needed to explain the presence of MLD at greater depths. As reviewed above, there is both direct and indirect evidence for the existence of phlogopite (±clinopyroxene, K-richterite)-rich veins. Here, we focus on the potential "metasomatising" role of CO 2 -H 2 O-alkali-rich liquids (such as carbonatites, kimberlites, nephelinites and basanites) in the introduction of phlogopite in the cratonic mantle. We also probe the depth range in which such melts can be produced relative to the field of phlogopite stability and the presence of MLDs.

Numerous experimental studies have focused on the formation of melts under cratonic pressure-temperature-composition conditions, with the emerging consensus that both CO 2 and H 2 O are necessary for melting to occur in cratons [START_REF] Foley | Experimental constraints on phlogopite chemistry in lamproites. 1. The effect of water activity and oxygen fugacity[END_REF][START_REF] Foley | The composition of near-solidus melts of peridotite in the presence of CO2 and H2O between 40 and 60 kbar[END_REF]Brey et al., 2008[START_REF] Brey | Influence of water and fluorine on melting of carbonated peridotite at 6 and 10 GPa[END_REF][START_REF] Girnis | Formation of primary kimberlite melts -Constraints from experiments at 6-12 GPa and variable CO2/H2O[END_REF][START_REF] Novella | The Composition of Hydrous Partial Melts of Garnet Peridotite at 6 GPa: Implications for the Origin of Group II Kimberlites[END_REF][START_REF] Sokol | Conditions of carbonation and wehrlitization of lithospheric peridotite upon interaction with carbonatitic melts[END_REF][START_REF] Sokol | Melting experiments on the Udachnaya kimberlite at 6.3-7.5 GPa: Implications for the role of H2O in magma generation and formation of hydrous olivine[END_REF][START_REF] Sokol | The role of water in generation of group II kimberlite magmas: Constraints from multiple saturation experiments[END_REF]Sokol et al., , 2015;;[START_REF] Sharygin | Melting phase relations of the Udachnaya-East Group-I kimberlite at 3.0-6.5 GPa: Experimental evidence for alkali-carbonatite composition of primary kimberlite melts and implications for mantle plumes[END_REF]. In most cases, experimental melts are low to very low in silica, spanning the compositional range from carbonatites to kimberlites.

Considering the present-day geotherm of intact cratons (Fig. 8A), it is clear that melting is only possible at depth exceeding 150-200 km depth (Fig. 3A). In that case, melt composition is only carbonatitic (Fig. 9). Significantly higher lithospheric heat production in the Archaean implies that geothermal gradients were elevated and that cratonic lithospheres have cooled with time [START_REF] Michaut | Thermal evolution of cratonic roots[END_REF] (Fig. 8B). Consequently, for a cratonic geotherm close to formation (3000 Ma), kimberlitic melts are stabilised from about 150 to 300 km depth, whereas carbonatitic melts are predicted above 150 km (Fig. 9). From 2000 Myrs to now, the

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34 melt is mostly carbonatitic at any depth. These calculations, performed for different geotherms according to the age of the craton, show that silica-poor alkali-volatile-rich melts, from carbonatites to kimberlites, can be stabilised in cratons and can introduce the "metasomatising" agents necessary for the formation of phlogopite.

More formally, phlogopite in CO 2 -bearing peridotite is stabilised at pressures lower than about 4-5 GPa (Figs. 3B,8) according to the following reaction [START_REF] Ulmer | Generation and differentiation of group II kimberlites: Constraints from a high-pressure experimental study to 10 GPa[END_REF]:

with the metasomatic agents K 2 CO 3 (l) and H 2 O(l) added by silica-poor alkali-volatile-rich melts previously described. As noted above, the typical shear velocity reduction of between 2

and 7 % at 60-160 km depth [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] can be obtained by accumulation of 5-15 % of phlogopite (Fig. 10) in this depth range. If we compare the cooling of the cratonic lithosphere with time and the stability domain of phlogopite (Fig. 8B), several interesting observations can be made: (1) Phlogopite can be stabilised and persist at pressures relevant to typical cratonic settings. (2) Phlogopite cannot precipitate from melts leaving a fertile mantle source that contains water as the only significant volatile, except for very hot conditions (>1200º C), shallow pressures (2 GPa) and elevated water contents (460 ppm H 2 O).

Conversely, a carbonated and hydrated convecting mantle intersects its solidus at depths overlapping the stability field of phlogopite and the reported presence of MLDs. However, in modern cool cratonic lithosphere, melts form in this source at depths greater than the stability field of phlogopite, and upward percolation is required in order for phlogopite to precipitate under these conditions. (3) As suggested by [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF], the cooling of lithosphere with time leads to displacement of the intersection of the carbonated peridotite solidus and to the development of a "permeability cap" where subsequent melt batches are stalled. The cooling not only triggers a deepening of the solidus intersection, but also a decrease in the thickness of the phlogopite layer, with an increase in the maximal pressure for the precipitation of phlogopite. These different observations are illustrated in a cartoon (Fig. 8C).

Summary and conclusions

We have integrated the physical properties, chemical composition, mineralogy and fabric of cratonic mantle samples (xenoliths, xenocrysts, melts) with experimental and thermodynamic constraints on the formation and migration of melts below and within cratons in order to isolate the petrologically most viable mechanisms for geophysically observed cratonic mantle discontinuities, namely the LAB and MLDs. Bearing in mind that geophysical and petrologic modes of lithosphere interrogation do not operate on the same spatial or temporal scales, our main summary and conclusions are as follows:

(1) In cratons, the LAB is typified by a transition from depleted to fertile (or refertilised) peridotite, by the preponderance of sheared mantle xenoliths lying on the high-temperature limb of kinked geotherms and by reduced shear (S)-and compressional (P)-wave speeds at ~170-250 km depth, in some cases accompanied by a reduction in electrical resistivity.

Because the presence of a deep lithospheric lid beneath intact cratons requires deep intersection of the solidus, only small degrees of interconnected partial melts (carbonatite, kimberlite, alkaline basalt) are expected the LAB, possibly linked to upwelling of thermochemically anomalous convecting mantle. This may explain the paucity of seismic cratonic LAB observations. Rapid drainage of melt may be counteracted by pervasive shearing and small crystal sizes in the deepest cratonic mantle, by small density contrasts between melt and lithosphere and by replenishment in long-lived mantle upwellings.

Seismically detectible LABs are therefore sites of melt-lithosphere interaction that may in the future lead to severe disruption of the cratonic root. Archaean to immediately preceding kimberlite eruption and are the expected consequence of the interaction of cratonic lithosphere with hydrous siliceous melts/fluids during subduction beneath (proto)cratons, or with asthenosphere-derived small-volume melts, such as the kimberlites that eventually transport cratonic mantle samples to the surface. The continuity of some MLDs across distinct tectonic units suggests a secondary origin of at least some MLDs, unrelated to craton construction. Thermal and density considerations imply that some of these buoyant metasomes, which are rich in heat producing elements, are young and may be the sites of future reorganisation of cratonic mantle roots.

(5) Amphibole stability being limited to <~90 km depth, our thermodynamic approach focuses on the depth range in which CO 2 -H 2 O-alkali-rich liquids (carbonatites, kimberlites, nephelinites and basanites) can be produced relative to the field of phlogopite stability and the presence of MLDs. Our calculations show that such melts form in the asthenosphere and penetrate upwards, possibly conditioning the lithosphere during multiple intrusions.

Phlogopite is precipitated as a function of age-dependent thermal state, thus explaining variable MLD depths. Even if not directly observed, such isotopically aged metasomes have been shown to contribute to small-volume volatile-rich melts typically penetrating cratonic lithospheres. Finally, we suggest that the apparent paucity of evidence for phlogopite-or amphibole-rich assemblages in the mantle xenolith record at geophysically imaged MLD depths, if not due to preferential disaggregation or alteration, relates to undersampling by both kimberlites and humans.

Figure Captions

Fig. 1 Global map of cratons (modified after [START_REF] Bleeker | The late Archean record: a puzzle in ca. 35 pieces[END_REF]Pearson andWittig, 2008, 2014).

Grey: exposed Archaean crustal rocks; white dotted lines: other fragments of composite cratons; yellow dashed lines: Archaean cratonic bocks amalgamated during the Proterozoic; stars: intact (thick) cratonic lithosphere sampled by kimberlites and related rocks; circles: disturbed (thinned) cratonic lithosphere sampled by basalt-borne xenoliths (an extensive list of mantle xenolith localities is provided by [START_REF] Nixon | Mantle Xenoliths[END_REF]. Uncertain boundaries are denoted with question marks. nucleus is subcreted by upwelling plume mantle (e.g. Griffin et al., 1999[START_REF] Griffin | Lithosphere mapping beneath the north American plate[END_REF]Arndt et al., 2009). Vertical lineation in the plume stem gives way to horizontal lineation when the plume impinges upon the LAB [START_REF] Ito | Seismic anisotropy and shear wave splitting associated with mantle plume-plate interaction[END_REF]. Dipping lineations may result when material flows laterally towards thinner craton margins [START_REF] Sleep | Local lithospheric relief associated with fracture zones and ponded plume material[END_REF][START_REF] Shervais | Lithospheric topography, tilted plumes, and the track of the Snake River-Yellowstone hot spot[END_REF]. Additional mW/m 2 ). The red line on the carbonated peridotite solidus highlights the ledge above which upward percolating melts would freeze; it would lie at greater depth in water-bearing or depleted mantle [START_REF] Hopper | The meaning of midlithospheric discontinuities: A case study in the northern US craton[END_REF]. Shown for comparison are mantle potential temperatures (T P ) at modern ridges (~1350º C; [START_REF] Herzberg | Thermal history of the Earth and its petrological expression[END_REF], Archaean ridges (~1450º C; [START_REF] Davies | Effect of plate bending on the Urey ratio and the thermal evolution of the mantle[END_REF] and at excess T P (plumes); black stars illustrate how the dry peridotite solidus is intersected at increasing depth with increasing T P , allowing the formation of increasingly thick melt-depleted residue (= lithosphere) due to larger melting intervals (e.g. [START_REF] Herzberg | Thermal history of the Earth and its petrological expression[END_REF]; thickness variations may further result from composition-dependent location of the mantle solidus [START_REF] Hirschmann | Mantle solidus: Experimental constraints and the effects of peridotite composition[END_REF]; purple star shows intersection of the Stability of amphibole [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF], (2) stability of MARID-type K-richterite and (3) solidus of dry MARID [START_REF] Sweeney | Phase relations of a natural MARID composition and implications for MARID genesis, lithospheric melting and mantle[END_REF]; (4) solidus of dry MARID, (5) stability of MARID-type K-richterite and (6) of K-richterite in the system KCNMASH [START_REF] Konzett | Potassium amphibole stability in the upper mantle: An experimental study in a peralkaline KNCMASH system to 8.5 GPa[END_REF]); (7) stability of phlogopite [START_REF] Ulmer | Generation and differentiation of group II kimberlites: Constraints from a high-pressure experimental study to 10 GPa[END_REF]. Shown for comparison are geothermal gradients expected in intact cratonic roots today and 3 Ga ago [START_REF] Michaut | Secular cooling and thermal structure of continental lithosphere[END_REF]. (C) Oxygen fugacity relative to the fayalite-magnetite-quartz (FMQ) oxygen buffer determined for cratonic mantle samples. Also shown are the fields for graphite, diamond and carbonate melt stability, as well as the carbonate concentration in the melt as a function of f O2 , with increasing f O2 stabilising increasing amounts of CO 3 2-in the melt (after [START_REF] Stagno | The oxidation state of the mantle and the extraction of carbon from Earth's interior[END_REF]. Thick red stippled line shows f O2 as a function of pressure for pre- Fig. 4 Establishment of a palaeo-LAB with topography due to winnowing at the base of the lithosphere. Rifting and stretching due to impingement of a thermo-chemical plume, passive mantle upwelling, the establishment of small-scale convection cells in the sub-lithosphere due to continental insulation, or convection at the craton edge due to a step in in the LAB may lead to infiltration by low-degree melts and consequent thermochemical erosion [START_REF] Lee | Building and Destroying Continental Mantle[END_REF], densification and destabilisation [START_REF] Wang | The thinning of subcontinental lithosphere: The roles of plume impact and metasomatic weakening[END_REF] and foundering of denser, less depleted and potentially cooler deep residues (Arndt et al., 2009;[START_REF] Michaut | Thermal evolution of cratonic roots[END_REF]. Lowdensity methane-rich fluids below the depth of metal saturation may provide a steady supply

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55 of carbon that is oxidised just below the LAB, escaping as carbonatite [START_REF] Frost | The redox state of Earth's mantle[END_REF][START_REF] Rohrbach | Redox freezing and melting in the Earth's deep mantle resulting from carbon-iron redox coupling[END_REF]. Melt may form and/or be focussed into depressions at the LAB. Siberian and Slave cratons (PetDB; http://www.earthchem.org/petdb/) betraying the passage of increasingly volatile-rich melts (decreasing melt volumes) after interaction with garnetbearing mantle [START_REF] Aulbach | Nature and timing of metasomatism in the stratified mantle lithosphere beneath the central Slave craton (Canada)[END_REF]. Low Zr/Y has been identified as typical of phlogopitebearing xenoliths and can be recognised in garnet even when phlogopite is not sampled in the xenolith volume (Griffin et al., 1999).

Fig. 9

Approximate melt composition is calculated for a cratonic lithosphere using a revised thermodynamic model of [START_REF] Massuyeau | A model for the activity of silica along the carbonatite-kimberlite-mellilitite-basanite melt compositional joint[END_REF], calibrated on an expanded set of experimental data (197 new experimental points) to increase the accuracy on peridotite melting under CO 2 -poor conditions at pressures > 1-2 GPa. Geotherms for the cratonic lithosphere are a function of its age [START_REF] Michaut | Thermal evolution of cratonic roots[END_REF], and we consider a mantle with 500 ppm H 2 O -500 ppm CO 2 . Cratonic lithosphere is described as a two-layer structure: (1) a depleted layer that extends from 60 km to 160 km, (2) a less depleted layer that extends from 160 to 250 km depth (Afonso et al., 2008). The mineral proportions and chemical compositions of these layers are approximated from [START_REF] Walter | Melt extraction and compositional variability in mantle lithosphere[END_REF].

Fig. 10

Shear velocities (Vs) of different peridotite models relevant to cratons, from a Voigt-Reuss-Hill average [START_REF] Hacker | Subduction factory -1. Theoretical mineralogy, densities, seismic wave speeds, and H2O contents[END_REF]. Calculations are done for 2 to 5 GPa, following the present-day geotherm [START_REF] Michaut | Thermal evolution of cratonic roots[END_REF]Fig. 8) and using an Excel spreadsheet developed

  definition, craton types and nature of the cratonic mantle sample 2.1 Intact vs disturbed lithospheric roots and associated magmatism 2.2 Accidental mantle samples: Xenoliths and xenocrysts 3. Modes of lithosphere interrogation 3.1 Petrology, fabrics, geochemistry and thermodynamic modelling 3.2 Geophysical observations 4. Discussion 4.1 Lithosphere-asthenosphere boundary (LAB) 4.1.1 Strong vs. weak or absent cratonic seismic LAB signals: The role of reworking and rejuvenation 4.1.2 Nature of melt near cratonic LABs: Evidence from mantle samples 4.1.3 Conditions for melt production below intact craton roots 4.1.4 Melt retention and resupply at the LAB 4.2 Mid-lithospheric discontinuities (MLDs): Geophysical expression of metasomatism, Palaeo-LABs or future LABs? 4.2.1 MLDs related to fabric Radial anisotropy contrasts at the pervasive-to-focussed melt flow transition Frozen rock fabrics: MLDs as palaeo-LABs 4.2.2 Present-day ponded melts or fluids 4.2.3 Deposition of metasomatic minerals during past metasomatism Metasomatic minerals accumulated during subduction Mantle metasomatism

  [START_REF] Selway | The seismic mid-lithosphere discontinuity[END_REF] present a comprehensive review of the various proposed origins of MLDs with regard to physical and geological plausibility, based on the evaluation of calculated S-wave velocities and of forward modelled RF, using various compositions and mineral modal abundances. In keeping with their work, we focus here on mechanisms involving the presence (1) of contrasting fabrics, such as changes in orientation of anisotropy,(2) of melts or fluids in the present-day lithosphere and (3) of seismically slow minerals (hydrous minerals, pyroxenes) deposited from melts or fluids during past tectonothermal events.

  LABs are mutable boundaries that are affected by thermochemical erosion from melting in upwelling convecting mantle, thermal insulation beneath large continental masses, craton-edge driven convection and mechanical erosion or hydration from subduction at craton margins. This leads to various states of LAB disturbance ranging from melt infiltration and densification of the deep cratonic root, which is isotopically still distinguishable from the sublithospheric mantle (Kaapvaal craton), to true asthenospherisation and partial or complete loss of deep cratonic lithosphere (Tanzania, North China craton), and finally complete destruction in the course of rifting (North Atlantic craton). Melt percolation and attendant rejuvenation explain weak, intermittent or discordant LAB signals.(3) MLDs are recorded between 60 and 160 km depth (predominantly 80-120 km) over a range of ~30-40 km and are characterized by both positive and negative seismic velocity anomalies, often accompanied by anisotropy. Because of the cool thermal state of intact cratonic roots, the present-day global presence of melts at MDL depths is unlikely. Preferred explanations centre on the accumulation of seismically slow metasomatic minerals (metasomes rich in hydrous minerals, pyroxenes and/or carbonates) and the acquisition of fabric due to deformation during craton construction involving plume subcretion or collisional tectonics. The latter is consistent with the observation of those MLDs that appear truncated at intracratonic sutures.(4) Azimuthal and radial anisotropy accompanied by seismic velocity reduction may result from the accumulation of metasomes as layers at or as subvertical veins above the depth at which melt flow transitions from pervasive to focussed flow at the mechanical boundary layer, above which cratonic mantle is cold and viscous. They have ages ranging from

Fig. 2

 2 Fig. 2 Models for craton thickening and associated development of fabric. (A) A cratonic

  Fig.3(A) Solidi of dry and carbonated peridotite[START_REF] Dasgupta | Carbondioxide-rich silicate melt in the Earth's upper mantle[END_REF] (solid blue lines),

  at much greater depth, relevant to the generation of small degrees of melt at modern LABs. (B) Stability of phlogopite and amphibole (pargasite): (1)

  metasomatic mantle from Creighton et al. (2009) for comparison, highlighting that many cratonic mantle xenoliths are more oxidised. (D) Histogram shows number of MLD observations per 2º x 2º bin across the globe (from Rader et al., 2015); black bar below shows depths of intact cratonic LABs (O'Reilly and Griffin, 2010).

Fig. 5

 5 Fig. 5 (A) Intact craton with initial disturbance at the LAB and craton margins (left) -

Fig. 6

 6 Fig. 6 (A) Schematic evolution of REE patterns in garnet: "Normal" chondrite-normalised

Fig. 7

 7 Fig. 7 Melt migration and solidification in cratonic mantle where a thermal boundary layer

Fig. 8

 8 Fig.8(A) Pressure-temperature conditions prevailing in intact and disrupted cratons (having

Table 1

 1 SiO 2 content of melts and melt fractions as a function of source composition and T P At 150 km depth, for the depleted mantle, immiscibility may occur as predicted by the model. Values in brackets represent the SiO2 content and melt fraction of the immiscible carbonate liquid coexisting with the silicate-rich melt. 1 Primitive mantle (PM) at a mantle TP of 1350 o C and with 500 ppm H2O -500 ppm CO2 2 Depleted mantle (DM, MORB source) at a mantle TP of 1350 o C and with 200 ppm H2O and 200 ppm CO2 3 Enriched mantle (EM, OIB source) at a mantle TP of 1350 o C and with 800 ppm H2O and 800 ppm CO2 4 Enriched mantle (EM) at a mantle TP of 1530 o C and with 800 ppm H2O and 800 ppm CO2 EM and DM are from Hirschman (2010), PM represents an intermediate composition

	LAB depth (km)	100	150	200	250
	%melt in PM	1		0.65	0.41	0.27	0.17
	SiO 2 content (wt.%)	39.0	32.8	24.7	12.5
	%melt in DM 2		0.21	0.13 (0.05)*	0.05	0.05
	SiO 2 content			37.7	30.3 (6.65)*	5.53	4.98
	%melt in EM	3		1.15	0.72	0.52	0.40
	SiO 2 content			39.5	33.7	27.5	21.2
	%melt in hot EM	4	3.13	1.66	1.26	1.11
	SiO 2 content			45.6	42.1	39.1	36.7

*

Acknowledgments

SA gratefully acknowledges funding from the Deutsche Forschungsgemeinschaft under grant DFG AU386/8. Andrea Tommasi and Ingo Wölbern are thanked for providing invaluable

by [START_REF] Hacker | Subduction Factory 3: An Excel worksheet and macro for calculating the densities, seismic wave speeds, and H2O contents of minerals and rocks at pressure and temperature[END_REF]. Velocity for a reference model depleted cratonic lithosphere peridotite (Afonso et al., 2008), consisting of 85 % olivine (75.2 % forsterite, 9.8 % fayalite) and 15 % orthopyroxene (13.5 % enstatite, 1.5 % ferrosilite) is shown. The dark-shaded area around the reference model represents variations due to adding 6 % garnet: almandine, grossular or pyrope, for which we can observe a small variation of Vs, principally characterized by an increase. Different thin maroon curves are the reference peridotite model modified by adding different proportions of phlogopite: 2, 5, 10 and 15 %. Phlogopite is added by replacing orthopyroxene (principally enstatite followed by ferrosilite component) [START_REF] Ulmer | Generation and differentiation of group II kimberlites: Constraints from a high-pressure experimental study to 10 GPa[END_REF]. The presence of phlogopite, between 2 and 15 % in proportion, is a good candidate to explain the decrease of Vs, between 2 and 7% [START_REF] Rader | Characterization and Petrological Constraints of the Midlithospheric Discontinuity[END_REF] of the reference state as represented by the blue-shaded area.