Florent Hodel
email: florent.hodel.geo@gmail.com

Mélina Macouin

Ricardo Ivan

Ferrreira Trindade

Jefferson F D F Araujo

Marc Respaud

Jean-François F Meunier

Laurent Cassayre

Sonia Rousse

Loïc Drigo

Juliano Schorne-Pinto

Macouin

R I F Trindade

J F D F Ara

Magnetic Properties of Ferritchromite and Cr-Magnetite and Monitoring of Cr-Spinels Alteration in Ultramafic and Mafic Rocks

Keywords: Data Set S1, Data Set S2, Data Set S3, Data Set

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

are less well constrained such as those of the Cr-Fe-spinel series resulting from spinel alteration, which are important phases in igneous petrology studies.

Spinels are an abundant accessory mineral in mafic and ultramafic rocks of both terrestrial and extraterrestrial origin, from which they constitute the main aluminum phase (e.g., [START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] Gattacceca | Low temperature magnetic transition of chromite in ordinary chondrites[END_REF]. Cr-spinels in turn, besides being the main ore mineral for chromium, have been extensively used to constrain both the magmatic and metamorphic history of ultramafic rocks (e.g., [START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] Barnes | The range of spinel compositions in terrestrial mafic and ultramafic rocks[END_REF][START_REF] Dick | Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas[END_REF]. Indeed, the chemistry of spinel core reflects partial melting and refertilization processes while spinel alteration rims (when present) testify the metamorphic conditions endured by their host rock (e.g., [START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] Barnes | The range of spinel compositions in terrestrial mafic and ultramafic rocks[END_REF][START_REF] Dick | Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas[END_REF]. Spinels are commonly the only pristine mineral preserved in fully serpentinized peridotites, making them one of the most investigated minerals in these rocks (e.g., [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF][START_REF] Arai | Compositional variation of olivine-chromian spinel in Mg-rich magmas as a guide to their residual spinel peridotites[END_REF][START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] Barra | Alteration patterns of chromian spinels from La Cabaña peridotite, south-central Chile[END_REF][START_REF] Dick | Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas[END_REF][START_REF] González-Jiménez | Metamorphism on chromite ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)[END_REF][START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil[END_REF][START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF][START_REF] Ishii | Petrological studies of peridotites from diapiric serpentinite seamounts in the Izu-Ogasawara-Mariana Forearc, Leg 125[END_REF][START_REF] Marques | Sulfide mineralization in an ultramafic-rock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF][START_REF] Mellini | Hydrothermally reset magmatic spinels in retrograde serpentinites: Formation of "ferritchromit" rims and chlorite aureoles[END_REF][START_REF] Pearce | Geochemistry and tectonic significance of peridotites from the South Sandwich arc-basin system, South Atlantic[END_REF][START_REF] Saumur | Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic[END_REF].

In ultramafic rocks, the typical metamorphic alteration pattern of spinels consists in variously developed Al-depleted rims, relatively enriched in Fe and Cr, belonging to the Cr-Fe-spinel system (e.g., ferritchromite, [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF][START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] González-Jiménez | Metamorphism on chromite ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)[END_REF][START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Mellini | Hydrothermally reset magmatic spinels in retrograde serpentinites: Formation of "ferritchromit" rims and chlorite aureoles[END_REF][START_REF] Saumur | Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic[END_REF]. In extreme stages, iron oxide rims depleted in Al, Mg, and Cr are produced, corresponding to Cr-rich magnetite and magnetite (e.g., [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF]. These exsolved elements participate in the formation of clinochlore, which precipitates in the form of blades surrounding spinel grains (e.g., [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF]. Large Fe-rich alteration rims are generally interpreted as an evidence of high metamorphic grades, making spinel alteration patterns a widely used metamorphic indicator (e.g., amphibolite facies with T < 550°C, [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF][START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] González-Jiménez | Metamorphism on chromite ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)[END_REF][START_REF] Mellini | Hydrothermally reset magmatic spinels in retrograde serpentinites: Formation of "ferritchromit" rims and chlorite aureoles[END_REF]. However, some studies show that spinel chemistry can be strongly affected by mantle metasomatism [START_REF] El Dien | Cr-spinel records metasomatism not petrogenesis of mantle rocks[END_REF] or after emplacement at low temperatures and low metamorphic grades (e.g., [START_REF] Barra | Alteration patterns of chromian spinels from La Cabaña peridotite, south-central Chile[END_REF][START_REF] Saumur | Zoned Cr-spinel and ferritchromite alteration in forearc mantle serpentinites of the Rio San Juan Complex, Dominican Republic[END_REF], particularly in hydrothermal environments (e.g., [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Marques | Sulfide mineralization in an ultramafic-rock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF]. For instance, [START_REF] Marques | Sulfide mineralization in an ultramafic-rock hosted seafloor hydrothermal system: From serpentinization to the formation of Cu-Zn-(Co)-rich massive sulfides[END_REF] showed evidence of significant spinel zonation in serpentinites from the active Rainbow hydrothermal vent field. [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF] identified similar patterns in serpentinites that constitute the relics of a Neoproterozoic (ca. 760 Ma) fossil black-smoker hydrothermal system. These studies suggest that major mineralogical changes in spinels can be strongly dependent on the nature of the metamorphic/alteration event. Hence, the use of the spinel alteration state as an indicator of the metamorphic grade should be carefully considered.

Minerals from the Cr-Fe spinel suite have specific magnetic properties depending on Fe-Cr ion substitutions during alteration processes from which they originate (e.g., [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Horen | Magnetic characterization of ferrichromite in soils developed on serpentinites under temperate climate[END_REF]Kadziałko-Hofmokl et al., 2008;[START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF]. [START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF] and references therein discussed the magnetism of the Fe 3 O 4 -FeCr 2 O 4 system from a theoretical and experimental point of view. However, very few studies discussed the magnetic properties of Cr-spinel alteration products, notably of the intermediate phases between Chromite and Magnetite endmembers [START_REF] Horen | Magnetic characterization of ferrichromite in soils developed on serpentinites under temperate climate[END_REF][START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Kądziałko ; Hofmokl | Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to paleomagnetic results of the dunites[END_REF][START_REF] Kądziałko ; Hofmokl | Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of Sudetic paleozoic ophiolite[END_REF]. In their mineralogical and paleomagnetic studies, [START_REF] Kądziałko ; Hofmokl | Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to paleomagnetic results of the dunites[END_REF][START_REF] Kądziałko ; Hofmokl | Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of Sudetic paleozoic ophiolite[END_REF] detected "highly magnetic Cr-magnetite with Curie temperature (T c ) of 530°C" as well as "less magnetic phases with T c ranging from about 100 to 450°C". These results highlight the potential of magnetic methods in the study of these alteration products.

Our study aims to identify magnetic signatures of spinel alteration products, but also to explore whether these signatures can help to identify the processes from which they originate. We apply different approaches for the determination of magnetic mineralogy that we couple with Mössbauer spectrometry and mineral chemistry. We present and compare magnetic, Mössbauer and geochemical data from four Neoproterozoic ultramafic units from Brazil and Morocco representing different geological settings and different degrees of spinel alteration. This integrated approach allows us to associate particular magnetic signatures commonly observed in mafic and ultramafic rock in order to chemically constrain spinel alteration products. Our results testify that magnetic measurements efficiently identify poorly recognized Cr/Fe minerals and assess their chemical variability.

The Chromium-Iron Spinel System

The Cr-Fe spinel system is expressed with the formula Fe 2+ (Fe 1-n Cr n ) 2 O 4 , where 0 < n < 1 represents the molar fraction of chromite (Figure 1). The two end-members of the system are pure chromite (FeCr 2 O 4 ) for n = 1 and magnetite (Fe 3 O 4 ) for n = 0 (e.g., [START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF].

The phases in the Fe-Cr-spinel system share an almost ideal cubic structure with an arrangement of oxygen atoms gathered either in tetrahedral or octahedral sites (Figure 1b). Tetrahedral sites generally host divalent cations like Mg 2+ , Fe 2+ and accessory Ni 2+ , Zn 2+ , Mn 2+ , etc… while octahedral sites host trivalent cations such as Fe 3+ , Cr 3+ , or Al 3+ . Thermodynamic models of the Fe-Cr-O system (e.g., [START_REF] Decterov | Thermodynamic modelling of the Al2O3-CaO-CoO-CrO-Cr2O3-FeO-Fe2O3-MgO-MnO-NiO-SiO2-S system and applications in ferrous process metallurgy[END_REF][START_REF] Taylor | A thermodynamic assessment of the Cr-Fe-O system[END_REF] calculate the cation distribution in the spinel structure, depending on temperature and composition. The cation distribution calculated with the FactSage software and the FToxid database [START_REF] Bale | FactSage thermochemical software and databases, 2010-2016[END_REF] at 300°C is provided in Figure 1c.

The chromite end-member (FeCr 2 O 4 ) presents a normal spinel structure where each type of cation is situated in their dedicated sites as described above. Chromite is ferromagnetic with a T c of -202.15°C; therefore it does not carry a remanent magnetization at ambient temperature (Ziemniak & Castelli, 2003, Figure 1a). Conversely, the pure magnetite end-member (FeFe 2 O 4 ) carries a strong remanent magnetization with a T c of 580°C [START_REF] Petersen | Curie temperature[END_REF] (Figure 1a). It displays an inverse spinel structure where the octahedral sites host only trivalent cations of Fe 3+ , which are also present in the tetrahedral sites together with divalent Fe 2+ [START_REF] Lenaz | Structural changes and valence states in the MgCr2O4-FeCr2O4 solid solution series[END_REF]) (Figures 1b and1c).

Between the two end-members, the physical and structural properties of the Cr-Fe spinel system [Fe 2+ (Fe 1- n Cr n ) 2 O 4 ] depend on the different contributions of normal and inverse structures. Thus, properties such as [START_REF] Decterov | Thermodynamic modelling of the Al2O3-CaO-CoO-CrO-Cr2O3-FeO-Fe2O3-MgO-MnO-NiO-SiO2-S system and applications in ferrous process metallurgy[END_REF][START_REF] Bale | FactSage thermochemical software and databases, 2010-2016[END_REF].

the T c or saturation magnetization vary as a function of n (e.g., [START_REF] Robbins | Magnetic properties and site distributions in the system Fe-Cr 2 O 4 -Fe 3 O 4 (Fe 2 +Cr 2-x Fe x 3+ O 4 )[END_REF][START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF]. Both parameters decrease with the increase of Cr content for n varying between 0 and ca. 0.7 corresponding to the inverse spinel structure (Figure 1). For n between ca. 0.7 and 1, the normal spinel structure exhibits a continued decrease of the T c while saturation magnetization slightly increases with n (Figure 1a).

Ferritchromite and chromium magnetite (Cr-magnetite) terms are used to describe intermediate phases of the spinel system but the n value corresponding to the transitions between these two minerals are not well constrained yet. At room temperature, Cr-Fe spinels become ferrimagnetic when the solid solution is characterized by n ≤ 0.6. [START_REF] Robbins | Magnetic properties and site distributions in the system Fe-Cr 2 O 4 -Fe 3 O 4 (Fe 2 +Cr 2-x Fe x 3+ O 4 )[END_REF][START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF]. This limit (n = 0.6) has therefore been applied to define the superior compositional boundary of ferritchromite in terms of Cr content (Figure 1a). [START_REF] Kądziałko ; Hofmokl | Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to paleomagnetic results of the dunites[END_REF] defined the Cr-magnetite according to its magnetic properties, that is, a ferritchromite with a magnetic behavior close to that of magnetite in terms of saturation magnetization and T c . Yet, Cr-magnetite is still not defined in terms of its chemical composition, and a better constraint on this transition and the associated n value is needed.

In natural Fe-Cr spinels, other cations, such as Al, Mg, Mn, Ni, and Ti, are also present in varying amounts and may influence the spinel physical properties. Indeed, incorporation of impurities in pure magnetite produces a widening and a shift toward lower temperatures of the magnetite Verwey transition (initially at 121 K) (e.g., [START_REF] Barlow | Concentration dependence of the Verwey transition in germanium-and fluorine-substituted magnetite[END_REF][START_REF] Brabers | Impurity effects upon the Verwey transition in magnetite[END_REF][START_REF] Honig | Analysis of the Verwey transition in magnetite[END_REF][START_REF] Kakol | Magnetic and transport properties of magnetite in the vicinity of the Verwey transition[END_REF][START_REF] Özdemir | The effect of oxidation on the Verwey transition in magnetite[END_REF]. Nevertheless, experiments of [START_REF] Chhaya | Magnetic properties of the mixed spinel NiAl 2x Cr x Fe 2-3x O 4[END_REF] in synthetic spinel systems demonstrated that minor amounts of Al or Ni in chromite (FeCr 2 O 4 ) do not significantly modify the relationship between n and T c . Thus, the determination of T c should be a powerful way to identify different Cr-Fe spinel phases, providing a new way to monitor the spinel alteration state in mafic and ultramafic rocks.

Geological Setting, Host Rocks, and Cr-Spinel Alteration State

This study is based on extensive sampling of ultramafic units from Neoproterozoic ophiolites in Brazil (the Araguaia Belt, [START_REF] Hodel | A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil[END_REF] and Morocco (Bou Azzer and Khzama, [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF][START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF]. All samples investigated are fully serpentinized peridotite (hydrated mantle) that have been differently affected by obduction-related metamorphism and, for some of them, by intense hydrothermal alteration [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF]. Due to these different metamorphic/hydrothermal histories, Cr-spinels from these serpentinites exhibit various alteration patterns and intensities (Figure 2). Hence, they constitute prime targets to study Cr-spinel alteration processes as well as the resulting alteration products and their magnetic properties. The main serpentinites petrographic features from the four studied units are listed below.

Serpentinites From Araguaia Belt Ultramafic Units

Serpentinites from Araguaia Belt ultramafic units are located between the Amazonian Craton and the Paranaïba basin in central Brazil (see the detailed petrographic and petrological study in [START_REF] Hodel | A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil[END_REF]. They display a classical pseudomorphic mesh texture mainly constituted of lizardite/chrysotile assemblages (low-temperature serpentine varieties), attesting to a static oceanic-like serpentinization and a regional metamorphism that involved temperatures below 350°C (e.g., [START_REF] Evans | The serpentinite multisystem revisited: Chrysotile is metastable[END_REF][START_REF] Hodel | A Neoproterozoic hyper-extended margin associated with Rodinia's demise and Gondwana's build-up: The Araguaia Belt, central Brazil[END_REF][START_REF] Mével | Serpentinization of abyssal peridotites at mid-ocean ridges[END_REF][START_REF] Schwartz | Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites[END_REF]. Spinels of these serpentinites are generally well-preserved and weakly altered (Figures 2a and2b).

Serpentinites From the Khzama Ophiolite

Serpentinites from the Khzama ophiolite are located in the Sirwa Inlier, in the Moroccan Anti-Atlas (see the detailed petrographic and petrological study in [START_REF] Hodel | Neoproterozoic serpentinites: A window on the oceanic lithosphere associated with the Rodinia break-up[END_REF][START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF]. These rocks are essentially antigorite and generally present a nonpseudomorphic texture characteristic of high-temperature serpentinization (T > 350°C, [START_REF] Evans | The serpentinite multisystem revisited: Chrysotile is metastable[END_REF][START_REF] Schwartz | Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites[END_REF]. Lizardite/chrysotile assemblages, sometimes occurring in the form of nodules within the antigoritic matrix, seem to indicate a first episode of serpentinization at a lower temperature (<350°C, [START_REF] Hodel | Neoproterozoic serpentinites: A window on the oceanic lithosphere associated with the Rodinia break-up[END_REF], prior to antigoritization which likely occurred during the obduction of the ophiolitic sequences [START_REF] Hodel | Neoproterozoic serpentinites: A window on the oceanic lithosphere associated with the Rodinia break-up[END_REF][START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF]. Despite this important metamorphism episode involving temperatures above 350°C, spinels of these serpentinites are only weakly altered (Figures 2c and2d). It is worth to note that large oxide grains in BSE images (i.e., tens to hundreds µm large) are not serpentinization related pure magnetite, which are much smaller, but small Cr-spinel grains or Cr-spinel alteration rims fragments. (j-l) SEM multi-elemental maps displaying Cr, Fe, and Al repartition in the same altered grains (g, h, i, respectively) and in the surrounding clinochlore blades (Al-rich phases, in green around spinels grains). Cr-Mgt, Crmagnetite; Fch, ferritchromite; Mgt, magnetite; SEM, scanning electron microscope; Spl, preserved spinel cores; Stch, stichtite. (g-l) were modified after [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF].

Serpentinites From the Southern Ultramafic Unit of the Aït Ahmane Ophiolitic Sequence

Serpentinites from the southern ultramafic unit of the Aït Ahmane ophiolitic sequence (Bou Azzer ophiolite, Moroccan Anti-Atlas, see the detailed petrographic and petrological study in [START_REF] Hodel | Neoproterozoic serpentinites: A window on the oceanic lithosphere associated with the Rodinia break-up[END_REF][START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF]. These serpentinites are very similar to those of the Khzama ophiolite. They also present nodules made of lizardite/chrysotile assemblages within an antigoritic matrix. Thus, a two-step serpentinization scenario, similar to the one proposed for Khzama serpentinites, is likely for these serpentinites. As in the case of the Khzama serpentinites, the spinels of these serpentinites are also weakly altered (Figures 2e and2f).

Serpentinites of the Northern Ultramafic Unit of the Aït Ahmane Ophiolitic Sequence

Serpentinites of the northern ultramafic unit of the Aït Ahmane ophiolitic sequence [START_REF] Hodel | Neoproterozoic serpentinites: A window on the oceanic lithosphere associated with the Rodinia break-up[END_REF][START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF]. They generally display pseudomorphic texture essentially made of lizardite/chrysotile assemblages attesting a static oceanic-like serpentinization and a weak obduction related metamorphism that involved temperatures below 350°C (e.g., [START_REF] Evans | The serpentinite multisystem revisited: Chrysotile is metastable[END_REF][START_REF] Mével | Serpentinization of abyssal peridotites at mid-ocean ridges[END_REF][START_REF] Schwartz | Pressure-temperature estimates of the lizardite/antigorite transition in high pressure serpentinites[END_REF]. [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF] showed that these serpentinites are often strongly altered due to an intense black-smoker type hydrothermal activity which affected them before their obduction. The spinels of these serpentinites are highly altered, being strongly affected by this hydrothermal episode [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF] (Figures 2g-2l).

Methods

Petrography and Mineral Chemistry

Back-scattered electron (BSE) images and elemental mappings were performed using a JEOL JSM-6360LV Scanning Electron Microscope (SEM) at the Geoscience Environnement Toulouse laboratory (France) operating at 20 kV with an Electron Dispersive System (EDS), which allows for in-situ qualitative characterization of the chemical composition. The quantitative chemical composition of spinels and their different alteration phases were determined using a Cameca SXFive electron microprobe at the Raimond Castaing Microcharacterization Center (University of Toulouse III, Paul Sabatier) in France and a JEOL JXA-8230 electron microprobe at the Microanalysis Laboratory of the Federal University of Ouro Preto in Brazil. On the Cameca SXFive, the measurements were done under a voltage of 15 kV and a current of 20 nA. The size of the analysis spot was about 4 μm. The standards used were: albite (Na), corundum (Al), periclase (Mg), sanidine (K), wollastonite (Ca, Si), pyrophanite (Mn, Ti), hematite (Fe), chromite (Cr), NiO (Ni), sphalerite (Zn), tugtupite (Cl), barite (Ba), and topaz (F). The detection limits are 0.01 wt. %. On the JEOL JXA-8230, the analytical conditions were a voltage of 15 kV and a current of 20 nA. The size of the analysis spot was about 5 μm. The standards used were: anorthoclase (Na), olivine (Mg), corundum (Al), microcline (K), quartz (Si), microcline (K) and Cu-augite (Ca), ilmenite (Mn, Ti), almandine (Fe), chromite (Cr), Glass-Rhyolitic IR-X standard (Ni), gahnite (Zn), scapolite (Cl), barite (Ba), and CaF2 (F). The detection limits are 0.01 wt. %. Fe 2 O 3 concentrations were calculated according to stoichiometric compositions of the spinel system [START_REF] Droop | A general equation for estimating Fe 3+ concentrations in ferromagnesian silicates and oxides from microprobe analyses, using stoichiometric criteria[END_REF].

Magnetic Measurements

The magnetic susceptibility variations as a function of temperature were measured using an AGICO KLY-3 kappabridge coupled with an AGICO CS-3 heating unit at the Geosciences Environnement Toulouse (GET) laboratory in France and at the Laboratorio de Paleomagnetismo da Universidade de São Paulo (USPMag) in Brazil. Each measurement cycle consisted of continuous measurements of magnetic susceptibility over a heating/cooling cycle from room temperature to a peak temperature of 700°C with a heating/cooling rate of 14°C per minute, under an inert Argon atmosphere. In a previous study, [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF] used the tangent method to determine T c s. Given the important Cr content of the investigated magnetic phases, we here used the inverse susceptibility method in order to avoid any bias. These two methods are described in [START_REF] Petrovský | On determination of the Curie point from thermomagnetic curves[END_REF]. The data were then processed using AGICO Cureval 8.0.1 software. In order to quantify the relative contribution of each magnetic species to the magnetic susceptibility, we used the normalized gradient (or its absolute values) of the magnetic susceptibility curve in heating [i.e., the first derivative: (KT t + 1 -KT t )/(T t + 1 -T t ), where KT is the magnetic susceptibility at a given temperature and T the corresponding temperature] as described in [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF].

Measurements of magnetic susceptibility at low temperatures were carried out at the Laboratoire de Chimie de Coordination (LCC) in Toulouse, France. They were acquired with an MPMS XL from Quantum design with frequencies of 1,000 Hz and 50 Hz.

Hysteresis parameters determinations, 15 first-order reversals curves (FORC) and 56 acquisitions of isothermal remanent magnetization (IRM) were performed on vibrating sample magnetometers (VSM, Princeton Measurements Corporation) at the Institut de Physique du Globe de Paris (IPGP) in France and at the US-PMag in Brazil. The determinations carried out for the Ait Ahmane samples are already reported in [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF].

IRM curve acquisitions were performed with an increasing induced field (100 steps) up to 1 T. The statistical analysis of the IRM acquisition curves, following the approach proposed by [START_REF] Kruiver | Quantification of magnetic coercivity components by the analysis of acquisition curves of isothermal remanent magnetisation[END_REF], [START_REF] Egli | Analysis of the field dependence of remanent magnetization curves[END_REF], 2004[START_REF] Heslop | Using time-and temperature-dependent Preisach models to investigate the limitations of modelling isothermal remanent magnetization acquisition curves with cumulative log Gaussian functions[END_REF], were processed using the Maxunmix software of [START_REF] Maxbauer | MAX UnMix: A web application for unmixing magnetic coercivity distributions[END_REF]. This software has the advantage of being able to analyze magnetic mineral components with coercivities that are not log-normal, common in natural samples, by incorporating skewness [START_REF] Egli | Analysis of the field dependence of remanent magnetization curves[END_REF][START_REF] Heslop | Using time-and temperature-dependent Preisach models to investigate the limitations of modelling isothermal remanent magnetization acquisition curves with cumulative log Gaussian functions[END_REF][START_REF] Maxbauer | MAX UnMix: A web application for unmixing magnetic coercivity distributions[END_REF]. The magnetic components were chosen to obtain the best fit with the total IRM acquisition using the fewest possible numbers of components. For each sample, 100 FORCs (First order Reversal Curve) were measured with an average time on each point of 200 ms and a one-second pause between each measurement. FORC diagrams were generated using FORCinel v3.0 software [START_REF] Harrison | FORCinel: An improved algorithm for calculating first-order reversal curve distributions using locally weighted regression smoothing[END_REF] with a smoothing factor [START_REF] Roberts | First-order reversal curve diagrams: A new tool for characterizing the magnetic properties of natural samples[END_REF] of 5 (for all samples) and paramagnetic correction.

Mössbauer Spectroscopy

Mössbauer spectroscopy analyses were performed at the LCC Laboratory in France. Spectra were collected on a constant-acceleration conventional spectrometer with a 0.30 GBq source of 57 Co (Rh matrix) at 80 K, using a JANIS cryostat. The thermal scanning was monitored by a temperature controller LAKESHORE 335. The absorber was a sample of about 100 mg of powder that was enclosed in a 20 mm diameter cylindrical plastic sample holder. Mössbauer's parameters were fitted to a Lorentzian model using the Recoil software. The isomer shift values are given with respect to metallic iron at room temperature. A table of parameters is provided in Data Set S2.

Magnetic Scanning Microscopy

The distribution of magnetization in the thin section was obtained using a custom-made magnetic scanning microscope in the Instrumentation and Magnetic Measurement Laboratory of the Physics Department of Pontificia Universidade Catolica do Rio de Janeiro (PUC-Rio) in Brazil. The equipment was developed by [START_REF] Pereira | Novel scanning dc-susceptometer for characterization of heat-resistant steels with different states of aging[END_REF], and its potential use in rock thin sections was outlined by [START_REF] Araujo | Scanning magnetic microscope using a gradiometric configuration for characterization of rock samples[END_REF]. The microscope operates with a pair of commercial Hall effect sensors. Both sensors were mounted with their sensitivity axes in opposite directions forming an axial gradiometer to measure the magnetic properties of minerals in thin sections (30 μm thick). A magnetic field of 140 mT was applied perpendicular to the surface of the sample. Maps were performed with steps of 250 μm for low resolution maps and 50 μm for higher resolution maps. The sensitivity to magnetic moment is 8.8 × 10 -11 Am 2 . Punctual hysteresis curves were obtained after magnetic scanning microscopy mappings. Acquisition consists in applying a magnetic field DC(T) to the sample while the reading system measures the induced magnetic field J (mT) in the z direction of the sample. Although these curves do not have a theoretical model necessary to obtain the exact magnetic moment of the rock, they nonetheless allow for a qualitative assessment of the local mineralogy.

Results

Spinels Alteration State: Petrography and Mineral Chemistry

Spinels of the different serpentinite units display various degrees of alteration, characterized by more or less wide alteration rims, with a wide range of chemical compositions. Major element compositions of spinel alteration rims in serpentinites from the four studied ophiolitic units are given in Data Set S3. Spinels in the analyzed samples show contrasting behavior. Spinels from Araguaia Belt, Khzama, and South Aït Ahmane ultramafic units are generally weakly altered (Figures 2a-2f). They display large preserved cores, surrounded by thin, sometimes discontinuous rims composed of two types of Cr-Fe-spinels. The first one presents Cr 2 O 3 contents between 0 and 5 wt.%, which correspond to 0 < n < 0.08 in the Cr-Fe-spinel system formula (Fe 2+ (Fe 1-n Cr n ) 2 O 4 ). The second one is richer in Cr 2 O 3 that is rarely below 13 wt.% of Cr 2 O 3 (n = 0.21) (Figure 3). Spinel alteration aureoles of Araguaia Belt serpentinites are generally thicker compared to the Khzama and Aït Ahmane ones. They, sometimes, obliterate the entirety of the primary spinel. Rims from the Araguaia Belt serpentinites are sometimes very rich in SiO 2 (up to 18.50 wt.%) and MgO (21.24 wt.%), which probably reflects a mixture with serpentine or other magnesium-silicate phases due to the porous nature of the rims (e.g., [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF].

In contrast to the other studied units, the spinels of the North Aït Ahmane serpentinites are much more altered (Figures 2 and3a) [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF]. They often display large Cr-Fe-spinel rims, sometimes replacing the entirety of the initial spinel grains [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF] (Figures 2i and2l). These rims show a wide range of compositions (Figures 2 and3a) but the Cr 2 O 3 content of these alteration phases is never lower than 9.70 wt.% (n = 0.15).

Thermomagnetic Curves

Thermomagnetic curves of serpentinites with both weakly altered (Araguaia, Khzama, and South Aït Ahmane) and strongly altered (North Aït Ahmane) spinels show several mineralogical transformations and Curie points (Figures 4 and5).

Thermomagnetic curves display a sudden increase in magnetic susceptibility at around 130°C during the first heating. The magnetic susceptibility continues to increase until 300-350°C where it reaches a maximum. These increases are not reversible, while the rest of the curves are almost reversible. The first decrease in magnetic susceptibility continues until between 400 and 450°C. Then, the curves are generally stable up to 500°C (Figure 4a). These two transitions are tentatively associated with ferritchromite destabilization and its partial conversion into maghemite which is then thermally destabilized from ca. 320°C (e.g., [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Horen | Magnetic characterization of ferrichromite in soils developed on serpentinites under temperate climate[END_REF]. To investigate if these features that appear during heating could represent fingerprints of alteration processes, we estimated the gradient of these two steps, the increase and the decrease in magnetic susceptibility centered around 300-350°C. We plotted the peak values of the gradients associated with ferritchromite and maghemite against each other. Interestingly, there is a strong linear correlation between these two magnetic signals (Figure 4b). At higher temperatures, curves present either one or two marked transitions at 515 ± 11°C (ranging from 504 to 526°C for our sampling) and ca. 580°C, corresponding respectively to the Curie points of Cr-magnetite and pure magnetite (Figures 456). The concomitant presence of these two Curie points is only noticeable in the serpentinites with strongly altered spinels of the North Aït Ahmane unit. The 515 ± 11°C transition is never found in samples from the South Aït Ahmane and Khzama units, and rarely in the Araguaia Belt except for sample QT73. All samples from these serpentinites with weakly altered spinels display the Curie The destabilization of ferritchromite in maghemite from 130°C is also visible. The dotted curve corresponds to the normalized gradient of the magnetic susceptibility heating curve, making it possible to quantify the relative contribution of each magnetic species to the susceptibility (red dotted arrows). (b) Maghemite contribution to the magnetic susceptibility against that of the ferritchromite (normalized gradient of ferritchromite is expressed in absolute value for a better readability). Linear correlation attests to the link between these two magnetic species. Interestingly, slopes vary for the different units. It might attest to a different alteration pattern. "Norm. gradient" of the magnetic susceptibility curve corresponds to peak values reached by the normalized prime derivative of the heating curve in heating [(KT t + 1 -KT t )/(T t + 1 -T t )] for the temperature range of ferritchromite and maghemite destabilization. point of near-pure magnetite ca. 580°C (Figures 5 and6). We occasionally notice a higher T c , concerning a minor part of the total magnetic susceptibility, which likely corresponds to small amounts of hematite (e.g., in samples QT73 and AA0101F, E, see Figure 6) or a remaining maghemite phase [START_REF] Gehring | High temperature stability of natural maghemite: A magnetic and spectroscopic study[END_REF].

Low-temperature susceptibility curves (Figure 7) highlight the magnetic transitions below ambient temperature. Two samples of serpentinites from the North Aït Ahmane unit have been selected for these analyses: AH14 and AA0101E, representing respectively weakly altered (Figures 2g and2j) and strongly altered Figure 6. Thermomagnetic curves of bulk serpentinites samples and backscattered electrons microphotographs of typical spinels of the corresponding samples from strongly altered spinels from the North Aït Ahmane (Morocco), compared to weakly altered spinels from the Araguaia Belt ultramafic units (Brazil). The dashed line corresponds to the normalized gradient of the thermomagnetic curves (during heating) that is used to quantify the relative contribution of each magnetic species to the total magnetic susceptibility. spinels (Figures 2i and2l). In both samples, a magnetic transition is observed around 120 K and corresponds to the Verwey transition of magnetite (Figure 7). This transition is wider and shifted toward lower temperatures for the more altered sample AA0101E (Figure 7b). This behavior is classically attributed to a higher amount of Fe substitution in magnetite or oxidation [START_REF] Barlow | Concentration dependence of the Verwey transition in germanium-and fluorine-substituted magnetite[END_REF][START_REF] Özdemir | The effect of oxidation on the Verwey transition in magnetite[END_REF]. Another transition is clearly visible toward 40-50 K for the two samples. It appears at slightly higher temperature and is wider in sample AH14 (Figure 7a) than in sample AA0101E (Figure 7b). A similar transition with temperatures varying from 38 to 100 K has been identified in synthetic Cr-Fe-spinels nanoparticles with different amounts of Cr and was interpreted as a result of the compositional and grain-size dependence of the blocking temperature in these very fine magnetic particles (e.g., [START_REF] Sandu | Magnetic properties of glass-ceramics obtained by crystallization of ironrich borosilicate glasses[END_REF].

Mössbauer Characterization

Mössbauer spectra were measured for four samples from the North Aït Ahmane unit at 80 K (Figure 8). Variations between samples seem related to the degree of alteration. The two spectra of samples containing little Cr-magnetite, hence showing weak alteration state, display characteristic magnetite fine structures. The spectra can be fitted with the five characteristic components of natural magnetite below the Verwey transition [START_REF] Doriguetto | Characterization of a natural magnetite[END_REF]. Upon alteration, the magnetite fraction decreases, while the paramagnetic Fe 3+ content grows up. Also, the magnetite spectra progressively round up, and only four of the five components are useful for the fitting. For the central paramagnetic components, the main absorption peaks stay in the same position. The spectra were then analyzed with a set of five paramagnetic contributions. These signatures are in agreement with those identified in lizardite materials (O'Hanley and Dyar 1993) and other ferritchromite materials [START_REF] Lenaz | Structural changes and valence states in the MgCr2O4-FeCr2O4 solid solution series[END_REF].

Hysteresis and Magnetic Granulometry

Results of magnetic hysteresis analyses carried out on serpentinites from the four studied ophiolitic units are given in Data Set S4.

Hysteresis parameters provide an initial assessment of the grain size of the magnetic grain populations. These parameters are conventionally represented in a Day diagram [START_REF] Day | Hysteresis properties of titanomagnetites: Grain-size and compositional dependence[END_REF]Dunlop, 2002a[START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF] in which different areas represent different domain states of magnetite and Ti-magnetite (Dunlop, 2002a[START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF] including single-domain (SD), pseudo-single domain (PSD) and multi-domain (MD) grains, from the smallest to the coarser (Figure 9). All samples plot near the theoretical mixing line of the mixture of SD and MD magnetites [START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF]) (Figure 9). However, it is important to mention that this diagram does not distinguish mixtures of grains with different sizes (i.e., domain states).

All samples with weakly altered spinels show magnetic hysteresis properties comparable to those of other serpentinites previously reported in the literature (e.g., [START_REF] Bonnemains | Magnetic signatures of serpentinization at ophiolite complexes[END_REF][START_REF] Fujii | Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization[END_REF][START_REF] Maffione | Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes[END_REF][START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF] (gray shadow in Figure 9). Serpentinites from the Brazilian ultramafic units of the Araguaia Belt exhibit a wide variation of magnetic grain sizes with M rs /M s and H cr /H c ratios between 0.03 and 0.43 and 1.62 and 4.3, respectively. The only exception is again the sample QT40 with a H cr /H c ratio [START_REF] Day | Hysteresis properties of titanomagnetites: Grain-size and compositional dependence[END_REF] for serpentinites from Araguaia Belt, Khzama, and South Aït Ahmane ultramafic units. The different areas bounded by the black lines represent areas of different grain sizes, from the smallest to the coarsest: SD for a single domain, PSD for pseudo-single domains, and MD for multi-domains. The black curve is a theoretical mixing curve between SD and MD magnetite grains, the percentages corresponding to the MD fraction [START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF]. The red field is that of hydrothermalized serpentinites from the North Aït Ahmane unit, the most altered falling always into the MD domain (from [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF]. The gray field on the diagram corresponds to literature data for serpentinites and serpentinized peridotites from [START_REF] Bonnemains | Magnetic signatures of serpentinization at ophiolite complexes[END_REF], [START_REF] Fujii | Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization[END_REF], [START_REF] Maffione | Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes[END_REF][START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF], FORC diagrams (in the middle) and IRM curves deconvolutions (right) of the Araguaia Belt serpentinites also depict how the grain size evolves through the diagram for the less altered serpentinites. FORC, first-order reversals curves; IRM, isothermal remanent magnetization. of 4.92, which corresponds to a spinel rich serpentinized dunite (Figure 9). Similarly, the South Aït Ahmane and Khzama units of the Moroccan serpentinites display hysteresis parameters mostly into the PSD domain (Figure 9), the serpentinites of South Aït Ahmane showing a wider magnetic grains size range than those of the Khzama ophiolite, with M rs /M s ratios varying between 0.04 and 0.27 and 0.04 and 0.08, respectively, and H cr /H c ratios between 1.53 and 3.97 and 2.16 and 4.21, respectively. In contrast, serpentinites from the North Aït Ahmane unit (indicated by the red shadow in Figure 9) exhibit much coarser grain sizes occupying entirely the MD field in the Day diagram.

To better constrain the magnetic grain size distributions (Figure 9), we performed IRM and FORC acquisitions on the Brazilian serpentinites of the Araguaia Belt. IRM curves and FORCs denote the presence of SD, PSD, and MD grains in these samples (Figure 9). Statistical analysis of IRM acquisition curves [START_REF] Maxbauer | MAX UnMix: A web application for unmixing magnetic coercivity distributions[END_REF] displays two to four distinctive magnetic components with variable contributions to the total IRM. When necessary, we add skewness to obtain the best fit. The main populations are characterized by moderate to high coercivities (mean coercivity from 1.5 to 2) that probably reflect a mixture of MD and SD/PSD magnetites. Sample QT57 with the highest M rs /M s ratio (0.43) is composed of four populations of magnetic oxides (Figure 9). It exhibits a predominant population of magnetite with high coercivities. This corroborates the predominance of SD/PSD magnetites in weakly altered samples. The contribution of the moderate coercivities population increases with the alteration. Hematite is sometimes detected in samples with larger magnetite grain sizes (e.g., in samples QT73 and AA0101F, E). However, this contribution to the total IRM remains minor.

In FORC diagrams, samples with intermediate M rs /M s and H cr /H c ratio (e.g., sample QT55) show a typical single-domain behavior with a ridge along with the origin and relatively high H c values whereas sample QT40 displays well-expressed low-moderate coercivity components on the IRM curves and FORC diagram characteristic of MD magnetic grains, with a wide vertical distribution close to the origin and relatively low H c values.

Magnetic Scanning Microscopy

We performed magnetic microscopic scans on Cr-spinel grains displaying alteration rims in order to investigate the magnetization of the Cr-spinel's alteration products. Figure 10 shows the perpendicular component of the magnetic field response to 35 × 35 mm and 15 × 15 mm scans of a thin section of the North Aït Ahmane sample AA0101F. The 35 × 35 mm map highlights the scattered distribution of magnetic minerals in the serpentine matrix. A higher magnification image (Figure 10c) emphasizes magnetic zonation in highly altered spinel grains, with an increasing magnetization from partially preserved spinel cores toward the strongly magnetic ferritchromite Cr-magnetite rims. Comparison between SEM observations (BSE image and elemental maps) and magnetic maps clearly shows that the preserved pristine spinel core (Cr and Al rich zone in Figures 10h and10i) shows much weaker magnetization compared to the altered Fe-rich rims. Thus, magnetic scanning makes it possible to reveal the scale of the magnetization intensity corresponding to ferritchromite and Cr-magnetite at the grain level. Moreover, a punctual hysteresis curve performed on the spinel grain's alteration rim using the magnetic scanning microscope (Figure 10d) exhibits low coercivity typical of magnetite. This attests that Cr-magnetite's magnetic behavior is very similar to that of pure magnetite. Another punctual hysteresis curve for which saturation is not reached testifies to the presence of hematite (Figure 10e).

Discussion

All the measurements and observations performed in this study tend to dissociate the North Aït Ahmane serpentinites from the three other units (Araguaia Belt, Khzama, and South Aït Ahmane) in terms of petrography, chemistry of Cr-spinel alteration phases and magnetic mineralogy.

Petrographical observations indicate that the spinels of the North Aït Ahmane serpentinites are mostly highly altered compared to their counterparts from the other three units (Figure 2). Spinels of the North Aït Ahmane serpentinites are rimmed by a wide range of Cr-Fe minerals that, in extreme cases, entirely replace the pristine spinel cores (Figures 2g-2l). On the contrary, other serpentinites host spinels only partially replaced by sharp and weakly developed Cr-Fe mineral rims (Figures 2a-2e). These alteration products also differ in terms of chemistry. Sharp alteration rims of the Araguaia Belt, Khzama and South Aït Ahmane serpentinite mostly consist either of ferritchromite which rarely show Cr 2 O 3 content below 13 wt.% or magnetite containing less than 5 wt.% of Cr 2 O 3 . In contrast, North Aït Ahmane serpentinites display a complete range of Cr-Fe minerals composition, which nevertheless never show Cr 2 O 3 content below 9.70 wt.% (Figure 3a).

Magnetic measurements also point out the major differences between the North Aït Ahmane serpentinites and those from the three other units. For all units, thermomagnetic curves evidence several magnetic minerals related to Cr-spinel alteration in addition to the magnetite produced by the serpentinization reaction (Figure 5). Among these magnetic minerals, one showing a T c at 515 ± 11°C (Figure 5b) as well as a widening of the Verwey transition near 121 K (Figure 7), is exclusive to the North Aït Ahmane serpentinites. While serpentinites from the Araguaia Belt, Khzama and South Aït Ahmane contain a mixture of PSD and MD grains characterized by H cr /H c ratios below four, as commonly observed in serpentinites worldwide, those from the North Aït Ahmane host coarser MD magnetic grains with H cr /H c ratios reaching six (Figure 9). Below, we discuss the particular magnetic features of the different Cr-spinel alteration products in the studied serpentinites. We compare them with mineral chemistry and Mossbauer spectroscopy results to better constrain their nature as well as the processes involved in their formation.

Magnetic Identification of Cr-Magnetite

The North Aït Ahmane serpentinites differ from others by displaying two T c s in the thermomagnetic curves (Figures 456). In addition to the pure magnetite T c ca. 580°C, they present a T c at 515 ± 11°C (Figure 5b). They also show a widening of the Verwey transition in low-temperature magnetometry (Figure 7). These two features correspond to a typical signal of magnetite containing impurities, Cr in this case. Indeed, the presence of Cr in the crystalline lattice of magnetite results in a decrease of the Verwey transition (initially at 121 K for pure magnetite) or to the suppression of the latter passing from an inverse spinel to monoclinic structure during cooling [START_REF] Gattacceca | Low temperature magnetic transition of chromite in ordinary chondrites[END_REF]. This Cr-rich magnetite, detected only in the serpentinites of the North Aït Ahmane unit hosting highly altered Cr-spinels, is thus likely to be a spinel alteration product (Figures 5 and6).

Hysteresis parameters denote unusually large magnetic grains in North Aït Ahmane serpentinites (Figure 9, [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF]. [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF] proposed that these coarser magnetite grains likely correspond to Cr-spinel alteration products, notably Cr-magnetite. This is confirmed by our magnetic scanning microscope observations showing that altered spinel grains carry strong magnetization, with a magnitude similar to that of pure magnetite produced by sea-floor serpentinization (Figure 10). Electron microprobe analyses reveal that these large magnetic grains correspond to a wide range of Fe-Cr oxides replacing Cr-spinel grains (Figures 2g-2l and6). These Fe-Cr oxides contain up to 86.10 wt.% of FeO T , which corresponds to a n of 0.15 in the structural formula of the series, Fe 2+ (Fe 1-n Cr n ) 2 O 4 ) (Figure 1). A value of n = 0.15 would theoretically correspond to a T c of 478°C (Figure 1) in the model of [START_REF] Ziemniak | Immiscibility in the Fe 3 O 4 -FeCr 2 O 4 spinel binary[END_REF], which is lower than the one we measured in our samples from North Aït Ahmane. Indeed, the T c at 515 ± 11°C that we observe is supposed to correspond to a n between 0.085 and 0.114 in the cited model. Interestingly, [START_REF] Kądziałko ; Hofmokl | Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to paleomagnetic results of the dunites[END_REF][START_REF] Kądziałko ; Hofmokl | Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of Sudetic paleozoic ophiolite[END_REF]) also observed a "double magnetite T c " in the range 500-580°C in the Devonian serpentinites of the Grochowiec unit (Braszowice-Brzeźnica ophiolite, Poland), with a T c of ca. 530°C, similar to those observed in the serpentinites of the North Aït Ahmane unit. These authors also interpreted this T c ca. 530°C as reflecting the presence of Cr-magnetite. As in the case of North Aït Ahmane spinels, the n measured for spinel's alteration products in the Grochowiec serpentinites is always greater than 0.1 (Figure 3b). It is also important to note that Kądziałko-Hofmokl et al (2010) describe another unit (the Mnich unit) which contains phases with n values between 0 and 0.1 (i.e., greater amount of iron). This one only displays the Curie point of pure magnetite ca. 580°C, as it is the case for the serpentinite units hosting weakly altered spinels in the present study (i.e., Araguaia Belt, Khzama, South Aït Ahmane) (Figures 2a-2e, 3a, and5a).

The compositions that we measured in alteration rims of spinels of the North Aït Ahmane serpentinites are very similar to those measured in the Grochowiec serpentinites by [START_REF] Kądziałko ; Hofmokl | Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of Sudetic paleozoic ophiolite[END_REF] (Figure 3). According to these analyses, the magnetic phase having a Curie point at 515 ± 11°C in North Aït Ahmane serpentinites, and ca. 530 for Grochowiec serpentinites [START_REF] Kądziałko ; Hofmokl | Magnetic mineralogy and paleomagnetism of serpentinized ultramafic rocks from the Braszowice-Brzeźnica fragment of Sudetic paleozoic ophiolite[END_REF] corresponds to a Cr-Fe-spinel having n between 0.1 and 0.2 (i.e., a Cr 2 O 3 content between about 6 and 13 wt.%) (Figures 3 and 11). The less chromiferous Fe-rich spinel phases (n < 0.1, i.e., Cr 2 O 3 < 5 wt. %) observed in the North Aït Ahmane and also in the other serpentinites from Araguaia, Khzama and South Aït Ahmane do not seem to have a Curie point distinguishable from that of pure magnetite or, alternatively, their signal is masked by the high susceptibility of the pure magnetite produced by the serpentinization reactions (Figures 3 and 11). Providing a more precise interval of n values than 0.1 < n < 0.2 for the Cr-magnetite is challenging and could be achieved only through an experimental approach. Indeed, in natural samples the measured T c corresponds to that of a mixing of different Cr-magnetites. Cr-magnetite becomes richer in iron from the cores toward the rim of altered spinel grains. This is reflected by the change in magnetization intensity in the scanning magnetic maps. The proportion of Fe-richer Cr-magnetite versus Fe-poorer Cr-magnetite varies for each spinel grain to another according to its morphology, fracturation, exposition to more or less acid and oxidizing fluid, making it impossible to better constrain the Cr-magnetite composition within each sample.

The different magnetic methods that we have used here highlight the similarity of the magnetic properties of Cr-magnetite (as defined here) and pure magnetite, with the exception of the T c s. At microscopic scale, the innovative scanning magnetic maps, allowing both the estimation of the induced magnetization changes at grain scale and their hysteresis curves, permit to directly compare these parameters to chemistry data at the same scale. This makes this innovative technique promising for further studies.

Taken together, our observations provide better constraints on the Cr-magnetite composition domain generally used in studies treating the chemistry of spinels and their alteration products, initially based on the work of [START_REF] Barnes | The range of spinel compositions in terrestrial mafic and ultramafic rocks[END_REF]. Fe-rich Cr spinels with a n greater than 0.2 are non (easily) detectable by their magnetic signature. Those with a n lower than 0.1 appear indistinguishable from pure magnetite. We thus propose a new definition for the Cr-magnetite: a Fe-rich Fe-Cr spinel with a n between 0.1 and 0.2 (Figure 11) that could carry a strong magnetization and a T c at ca. 500-530°C, distinguishable from that of pure magnetite.

The Ferritchromite Signal in Thermomagnetic Curves

In most of the studied samples, ferritchromite is evidenced by a specific thermomagnetic signature that consists of a sudden increase of magnetic susceptibility between 130 and 150°C during the first heating (Figures 456). This likely corresponds to a chemical transformation due to the destabilization of ferritchromite phases [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Horen | Magnetic characterization of ferrichromite in soils developed on serpentinites under temperate climate[END_REF]. This sudden increase continues with a milder increase up to 300-400°C forming a bulge that does not remain during cooling. This indicates the formation of a new phase which is then transformed again at higher temperatures (Figures 456). Except for this bump, curves are generally almost reversible, that is, they regain almost the same magnetic susceptibility value when they return to room temperature (Figures 5 and6). This clearly indicates a transformation of a paramagnetic phase into a magnetic phase that destabilizes itself into a non-magnetic or weakly magnetic phase over 350-400°C. [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF] proposed that this particular trend should be due to the transformation of paramagnetic ferritchromite into a transitional phase, possibly maghemite. Indeed, the shape of the bulge and the destabilization temperature are characteristic of maghemite (e.g., [START_REF] Gehring | High temperature stability of natural maghemite: A magnetic and spectroscopic study[END_REF]. Such a transformation could be linked to oxidation leading to cation mobility in the different sites by dynamic segregation (e.g., [START_REF] Domenichini | Dynamic segregation during ferrite oxidation revealed by XPS[END_REF][START_REF] Horen | Magnetic characterization of ferrichromite in soils developed on serpentinites under temperate climate[END_REF]. Otherwise, the transition visible at 40-50 K on the low-temperature magnetic susceptibility curves (Figure 7) could correspond to chromite as described by [START_REF] Gattacceca | Low temperature magnetic transition of chromite in ordinary chondrites[END_REF] in ordinary chondrite.

When plotted together, values of the normalized gradient of heating curves of the ferritchromite destabilization versus these of maghemite destabilization (Figure 4b) display linear correlations. Interestingly, trends Cr 3+ , and Fe 3+ illustrating the domains of the composition of the different Cr-Fe-spinels produced by mantle spinels alteration after [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF] and [START_REF] Barnes | The range of spinel compositions in terrestrial mafic and ultramafic rocks[END_REF]. We specify the composition field for Cr-magnetite.

are different for the different serpentinite units implying the maghemite resulting from the destabilization of ferritchromite is specific to each unit of serpentinites. For instance, more maghemite seems to be produced following the destabilization of a given amount of ferritchromite in the case of the North Aït Ahmane serpentinites compared to the Araguaia belt serpentinites (Figure 4b). This is probably linked with the composition of the ferritchromite itself. Hence, these differences in covariation of ferritchromite versus maghemite thermomagnetic signals could attest different alteration patterns and hence different alteration processes.

A Way to Decipher Cr-Spinel Alteration Processes

Here we show that magnetic methods can help to characterize the alteration processes that a given spinel-bearing rock has endured. Chemical composition and magnetic properties of spinel alteration products, namely ferrichromite and Cr-magnetite, which show wide ranges of geochemical variability governed by Fe-Cr substitutions, are closely controlled by the nature of the involved metamorphism (e.g., P, T, fluid, etc., e.g., [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF][START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] González-Jiménez | Metamorphism on chromite ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)[END_REF]. Hence, the magnetic detection of the different spinel alteration products, the quantification of their relative abundance and the assessment of their chemical variability allow to partially reconstruct the alteration history of a given rock sample.

Advanced spinel alteration, is generally associated with high metamorphic grade, involving temperatures of 500-600°C or higher (e.g., [START_REF] Ahmed | Fluid-related modifications of Cr-spinel and olivine from ophiolitic peridotites by contact metamorphism of granitic intrusions in the Ablah area, Saudi Arabia[END_REF][START_REF] Barnes | Chromite in Komatiites, II. Modification during greenschist to mid-amphibolite facies metamorphism[END_REF][START_REF] González-Jiménez | Metamorphism on chromite ores from the Dobromirtsi Ultramafic Massif, Rhodope Mountains (SE Bulgaria)[END_REF][START_REF] Mellini | Hydrothermally reset magmatic spinels in retrograde serpentinites: Formation of "ferritchromit" rims and chlorite aureoles[END_REF]. However, the North Aït Ahmane serpentinites, which endured much lower temperatures (below 300°C, [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF], host highly altered spinels showing a wide range of ferritchromite and Cr-magnetite compositions (Figures 2g-2l and3a). As presented in detail in [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF], serpentinites from the North Aït Ahmane unit were variously impacted by abyssal hydrothermalism leading to important Cr-spinel alteration. Interestingly, T c of Cr-magnetite measured in these serpentinites is highly correlated to the intensity of their hydrothermal signature as quantified by the (Eu/Eu*) N ratio (see [START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF] (Figure 12). Such correlation is likely to be due to enhanced Fe-Cr substitutions, promoted by warm (ca. 280°C), acidic and oxidizing fluids [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF]. This produces greater amounts and Fe-richer Cr-magnetite rims around spinel grains compared to less affected serpentinite. This brings new evidence concerning the link between the hydrothermal episode endured by the North Aït Ahmane serpentinites and intense spinel alteration despite the relatively low temperatures of these processes [START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF][START_REF] Hodel | Fossil black smoker yields oxygen isotopic composition of Neoproterozoic seawater[END_REF].

In addition to the magnetic evidence, cation exchanges under oxidizing conditions induced by abyssal hydrothermalism (T < 300°C) are also evidenced by Mossbauer spectroscopy, which clearly attests changes in the iron oxidation state with increasing weathering (Figure 8).

Conversely to North Aït Ahmane, serpentinites of the South Aït Ahmane and Khzama units, which experienced higher temperature (>350°C) and do not show major hydrothermal imprint, host only very weakly altered spinels (Figures 2c-2f).

Therefore, in addition to temperature, spinel alteration also appears to be controlled by the type of metamorphic event endured by their host rocks, notably when important amounts of fluids are involved (e.g., [START_REF] Barra | Alteration patterns of chromian spinels from La Cabaña peridotite, south-central Chile[END_REF]. In this case, the fluid-rock ratio, as well as the physico-chemical properties of the fluid, could also play a key role in the spinel alteration. Particularly, the fluid redox state (oxidizing conditions favoring trivalent cations mobility), its pH (acidic pH promoting transition metals mobility) but also anionic ligands availability (the latter allowing transition metals mobility as well) have to be considered [START_REF] Holser | Hydrothermal magnetite[END_REF][START_REF] Kalczynski | Hydrothermal alteration, mass transfer and magnetite mineralization in dextral shear zones, western Hudson Highlands, New York, United States[END_REF]Purtov et al., 1989). Significant spinel alteration (major to complete obliteration of the primary spinel by Cr-magnetite and ferritchromite), associated to evidence of low metamorphic conditions (e.g., T < 350°C), likely constitutes a good indicator of significant hydrothermal activity.

Hence, this study shows that phases produced by spinel alteration processes can be easily tracked with magnetic methods. As seen before, each of the four units studied here display its proper magnetic fingerprint (Figures 4-6 and9). We show that thermomagnetic curves constitute a prime method to (1) identify the products of Cr-spinel alteration (ferritchromite, Cr-magnetite), ( 2) assess the different chemical compositions of these phases (e.g., Figures 4b and12), and (3) quantify their relative contribution to the magnetization, and so their relative abundance in a given sample. Hence, this study opens the door to the use of magnetic methods to investigate the Cr-spinel alteration in mafic and ultramafic rocks. Rock magnetism methods are cost effective, weakly destructive and do not necessitate important amounts of material. They could also be appropriate to investigate extraterrestrial spinel-bearing rock samples, allowing to track fluid/ rock interaction in primitive meteorites and thus possibly providing important clues about the origin of water on Earth (e.g., [START_REF] Piani | The role of chlorine in the formation of magnetite skarns[END_REF].

Conclusion

• By combining microprobe, microscopy, Mössbauer, and magnetic data, this study reveals the potential of magnetic measurements to detect and identify the different Cr-Fe-spinels produced by the alteration of chromiferous spinels ubiquitous in mafic and ultramafic rocks. • Magnetic scanning microscopy confirms that coarser magnetic grain size in the North Aït Ahmane serpentinites compared to other serpentinites worldwide, as identified by hysteresis parameters (e.g., H cr / H c > 4), is related to Cr-spinel alteration phases showing a strong magnetization, similar in magnitude to serpentinization related pure magnetite for external rims. • Electron microprobe analyses reveal that these phases consist in a continuum of Fe-Cr spinel minerals, namely from ferritchromite to Cr-rich magnetite, characterized by FeO Total and Cr 2 0 3 contents ranging from 45.96 to 86.10 and 9.70 to 43.05 wt.%, respectively. • Thermomagnetic curves show that Cr-magnetite is characterized by a Curie point of 515 ± 11°C in our samples, which occurs together with the typical Curie point for pure magnetite at 580°C leading to a "double magnetite T c " feature. • We propose a new definition for the Cr-magnetite: a Cr-Fe-spinel with a Curie point of ca. 500-530°C defined by the structural formula Fe 2+ (Fe 1-n Cr n ) 2 O 4 ) with an n between 0.2 and 0.1, corresponding to Cr 2 O 3 concentrations in the range of 6-13 wt.%. For Cr-rich magnetite with Cr 2 O 3 contents <6 wt.%, the associated T c is indistinguishable from that of pure magnetite. • Thermomagnetic curves also allow us to characterize ferritchromite, the latter being highlighted by its destabilization into maghemite around 130°C. This produces an increase in magnetic susceptibility from this temperature, until maghemite destabilization starting at ca. 300°C. • We also show that a standardized variation curve of the magnetic susceptibility upon heating allows relative quantification of the contributions of the different Cr-spinel alteration phases to the bulk magnetic susceptibility. This allows us to establish a link between the destabilization of ferritchromite around 130°C into maghemite and the destabilization of this maghemite from 300°C. Interestingly, specific covariation trends between these two magnetic species for each studied unit likely result from different alteration processes. • In addition to petrographic observations, Mossbauer spectrometry and low temperature measurements, a clear positive correlation between the T c measured for Cr-magnetite and the intensity of the hydrothermal signature recorded by Eu/Eu* ratios in the North Aït Ahmane serpentinites confirms that intense spinel alteration is a consequence of the hydrothermalism endured by these serpentinites despite low temperatures (<300°C). This suggests that fluid-rock interactions can also strongly control spinel alteration even at low temperature, implying that significant spinel alteration is not necessarily related to high metamorphic grade. • Cr-spinel alteration phases (ferritchromite and Cr-magnetite) are easily identifiable via the variation of magnetic susceptibility as a function of temperature and tracked by magnetic scanning microscopy. This opens the doors to magnetic investigations of the Cr-spinel alteration state in terrestrial and extraterrestrial mafic and ultramafic rocks.

Figure 1 .

 1 Figure 1. (a) Curie temperature and magnetic moment evolution of the spinel system [Fe 2+ (Fe 1-n Cr n ) 2 O 4 ] showing the transition from normal spinel (i.e., chromite pole, FeCr 2 O 4 , n = 1) to inverse spinel (Mag. = magnetite pole, Fe 3 O 4 , n = 0). Ferritchromite and Cr-magnetite (Cr-Mag.) are inverse spinel intermediate phases between these two end-members. After Dunlop and Ozdemir (1997), Horen et al. (2014), Kadzialko-Hofmokl et al., (2008), Robbins et al. (1971), and Ziemniak and Castelli (2003). (b) Crystallographic structure of the spinel system for n = 0.15 [Fe 2+ (Fe 0.85 Cr 0.15 ) 2 O 4 ]. (c) Cation distribution of the spinel system [Fe 2+ (Fe 1-n Cr n ) 2 O 4 ] calculated with the FactSage software (see Data Set S1,[START_REF] Decterov | Thermodynamic modelling of the Al2O3-CaO-CoO-CrO-Cr2O3-FeO-Fe2O3-MgO-MnO-NiO-SiO2-S system and applications in ferrous process metallurgy[END_REF][START_REF] Bale | FactSage thermochemical software and databases, 2010-2016[END_REF].

Figure 2 .

 2 Figure 2. (a-f) Examples of backscattered electron microphotographs (using SEM) of very weakly altered spinels in serpentinites from the Araguaia Belt (a and b), Khzama (c and d) and South Aït Ahmane (e and f). (g-i) Examples of BSE microphotographs of variously altered spinels from the North Aït Ahmane unit. Alteration state increasing from left to right.It is worth to note that large oxide grains in BSE images (i.e., tens to hundreds µm large) are not serpentinization related pure magnetite, which are much smaller, but small Cr-spinel grains or Cr-spinel alteration rims fragments. (j-l) SEM multi-elemental maps displaying Cr, Fe, and Al repartition in the same altered grains (g, h, i, respectively) and in the surrounding clinochlore blades (Al-rich phases, in green around spinels grains). Cr-Mgt, Crmagnetite; Fch, ferritchromite; Mgt, magnetite; SEM, scanning electron microscope; Spl, preserved spinel cores; Stch, stichtite. (g-l) were modified after[START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF].

Figure 3 .

 3 Figure 3. Ternary plot of trivalent cation concentrations (Al 3+ , Cr 3+ , and Fe 3+ ) in spinel alteration rims from (a) the four studied serpentinite units (data for North Aït Ahmane serpentinites are from Hodel et al., 2017; core compositions are from Hodel et al., 2017, 2019) and (b) from the Grochowiec serpentinites (Poland) in which Kądziałko-Hofmokl et al. (2010) detected a spinel alteration phase with a T c ca. 530°C.

Figure 4 .

 4 Figure 4. (a) Thermomagnetic curves (sample AA0101F from the North Aït Ahmane unit) with a Curie point at 526°C characteristic of Cr-magnetite.The destabilization of ferritchromite in maghemite from 130°C is also visible. The dotted curve corresponds to the normalized gradient of the magnetic susceptibility heating curve, making it possible to quantify the relative contribution of each magnetic species to the susceptibility (red dotted arrows). (b) Maghemite contribution to the magnetic susceptibility against that of the ferritchromite (normalized gradient of ferritchromite is expressed in absolute value for a better readability). Linear correlation attests to the link between these two magnetic species. Interestingly, slopes vary for the different units. It might attest to a different alteration pattern. "Norm. gradient" of the magnetic susceptibility curve corresponds to peak values reached by the normalized prime derivative of the heating curve in heating [(KT t + 1 -KT t )/(T t + 1 -T t )] for the temperature range of ferritchromite and maghemite destabilization.

Figure 5 .

 5 Figure 5. Thermomagnetic curves (heating only) of bulk serpentinite samples from the four studied ultramafic units. (a) Serpentinites hosting weakly altered Cr-spinels: Araguaia Belt, Khzama, and South Aït Ahmane units, (b) Serpentinites hosting highly altered Cr-spinels: North Aït Ahmane units. Most of the curves display several mineralogical transformations (highlighted by blue and red shaded rectangles), around 130, 300-450, 515 ± 11, and 580°C. Note that only North Aït Ahmane serpentinites show a Curie point around 515°C.

Figure 7 .

 7 Figure 7. In-phase (m″) and out-of-phase (″) magnetic susceptibility measurements at low temperature for two serpentinites of the North Aït Ahmane ultramafic unit: (a) sample AH14 whose spinels are weakly altered and (b) sample AA0101E whose spinels are most of the time completely replaced by ferritchromite and Cr-magnetite.

Figure 8 .

 8 Figure 8. Mössbauer spectra collected at 80 K. Dots: experimental data; green line and area: global computed curve; colored areas: elemental components. The gray arrow illustrates the increasing hydrothermal alteration of these serpentinites and of their constitutive spinels.

Figure 9 .

 9 Figure 9. Representation of hysteresis parameter ratios in a Day diagram[START_REF] Day | Hysteresis properties of titanomagnetites: Grain-size and compositional dependence[END_REF] for serpentinites from Araguaia Belt, Khzama, and South Aït Ahmane ultramafic units. The different areas bounded by the black lines represent areas of different grain sizes, from the smallest to the coarsest: SD for a single domain, PSD for pseudo-single domains, and MD for multi-domains. The black curve is a theoretical mixing curve between SD and MD magnetite grains, the percentages corresponding to the MD fraction[START_REF] Dunlop | Theory and application of the Day plot (M rs /M s versus H cr /H c ) 1. Theoretical curves and tests using titanomagnetite data[END_REF]. The red field is that of hydrothermalized serpentinites from the North Aït Ahmane unit, the most altered falling always into the MD domain (from[START_REF] Hodel | Unusual massive magnetite veins and highly altered Cr-spinels as relics of a Cl-rich acidic hydrothermal event in Neoproterozoic serpentinites (Bou Azzer ophiolite, Anti-Atlas, Morocco)[END_REF]. The gray field on the diagram corresponds to literature data for serpentinites and serpentinized peridotites from[START_REF] Bonnemains | Magnetic signatures of serpentinization at ophiolite complexes[END_REF][START_REF] Fujii | Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization[END_REF],[START_REF] Maffione | Magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes[END_REF][START_REF] Oufi | Magnetic properties of variably serpentinized abyssal peridotites[END_REF], FORC diagrams (in the middle) and IRM curves deconvolutions (right) of the Araguaia Belt serpentinites also depict how the grain size evolves through the diagram for the less altered serpentinites. FORC, first-order reversals curves; IRM, isothermal remanent magnetization.

Figure 10 .

 10 Figure 10. (a-c) Magnetic maps using magnetic scanning microscopy showing the induced magnetization into a highly hydrothermalized sample from the North Aït Ahmane unit (AA0101F). As described by Hodel et al. (2017) serpentinization related magnetite is less present in these samples. Magnetization is partly carried by the Cr-magnetite resulting from Cr-spinel grains alteration. (d and e) Punctual hysteresis measurements for points 1 and 2 in c and b, respectively. (f) Corresponding microphotograph of a highly altered spinel grain. (g) BSE image of the same grain. (h and i) SEM elemental mapping of the same grain (Fe, Cr in h and Fe, Cr, Al in i). Cr-and Al-rich parts correspond to preserved and partially preserved spinel cores while Fe-richer phases (in red) are ferritchromite and Cr-magnetite.

Figure 11 .

 11 Figure 11. Triangular diagram of concentrations of trivalent cations Al 3+ , Cr 3+ , and Fe 3+ illustrating the domains of the composition of the different Cr-Fe-spinels produced by mantle spinels alteration after Ahmed and Surour (2016) and[START_REF] Barnes | The range of spinel compositions in terrestrial mafic and ultramafic rocks[END_REF]. We specify the composition field for Cr-magnetite.

Figure 12 .

 12 Figure 12. Graphical representation of the measured T c of Cr-magnetite versus (Eu/Eu*) N ratio. The (Eu/Eu*) N corresponds to Eu/[(Gd + Sm)/2] normalized to the chondrite (Barrat et al., 2012) and quantifies the Eu positive anomaly produced by abyssal hydrothermalism in the serpentinites. (Eu/Eu*) N ratios values are from Hodel et al. (2018).

  Authors are grateful to Fabienne De Parseval for thin sections preparation, to Philippe De Parseval and to Marco Paulo de Castro for their assistance on electron microprobes, to Thierry Aigouy for it assistance on the SEM and to Plinio Jaqueto for his help on the VSM. Authors are also grateful to the Microscopy and Microanalysis Laboratory (LMic) of the Universidade Federal de Ouro Preto, a member of the Microscopy and Microanalysis Network of Minas Gerais State/Brazil/FAPEMIG. This work has been funded by Research Grant 2016/06114-6 of the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Brazil), TelluS-SYSTER program of Institut national des sciences de l'Univers (INSU, CNRS, France), French Ministère de l'Éducation nationale, de l'Enseignement supérieur et de la Recherche (MENESR) and the PICS-CNRS (NanoPol) program. F. Hodel thank IODP France for his postdoc fellowship. The authors are also grateful to GW. ter Maat and an anonymous reviewer for their careful reading and suggestions, which significantly improved this contribution.

Acknowledgments

Data Availability Statement

All data used in this manuscript (Supplementary data 1, 2, 3, 4) are available in [START_REF] Hodel | The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment[END_REF] (http:// doi.org/10.5281/zenodo.3877929). Further queries and information requests should be directed to the lead author F.H. (florent.hodel.geo@gmail.com).