Beatrice Pelorosso

Costanza Bonadiman
email: costanza.bonadiman@unife.it

Theodoros Ntaflos

Michel Gregoire

Silvia Gentili

Alberto Zanetti

Massimo Coltorti

An insight into the first stages of the Ferrar magmatism: ultramafic cumulates from Harrow Peaks, northern Victoria Land, Antarctica

Keywords: ultramafic xenoliths, high-Mg magmatic olivines, orthopyroxenite, Karoo-

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

Introduction

The West Antarctic Rift system (WARS) is one of the largest continental rift areas in the world and is comparable to the East African Rift in scale [START_REF] Martin | Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre[END_REF]Le Masurier and Thomson 1990).

Since the Cenozoic it was affected by alkaline magmatism represented by the products of the Mc.

Murdo volcanic and Meander intrusive groups [START_REF] Kyle | Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica[END_REF]. The most primitive volcanic products from Victoria Land contain mafic and ultramafic xenoliths in a large spectrum of lithologies that testify for a complex mantle/crust evolution of the Subcontinental Lithospheric Mantle (SCLM).

Evidence of mantle partial melting and enrichment events (due to both alkaline metasomatism and refertilisation by tholeiitic melts) as well as crystallization of alkaline melts at the Moho depth have been largely documented [START_REF] Gamble | Metasomatized xenoliths from Foster Crater, Antarctica: implications for lithosphere structure and processes beneath the Transantarctic Mountains[END_REF][START_REF] Beccaluva | Nature and evolution of subcontinental lithospheric mantle of Antarctica: evidence from ultramafic xenoliths of the Melbourne volcanic province (northern Victoria Land, Antarctica)[END_REF][START_REF] Cooper | Geology, geochronology and geochemistry of a basanitic volcano, White Island, Ross Sea, Antarctica[END_REF][START_REF] Martin | Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF]. Geochemical and isotopic data trace back the refertilisation process of SCLM in the northern Victoria Land prior to the Cenozoic [START_REF] Melchiorre | The role of eclogite in the rift-related metasomatism and Cenozoic magmatism of Northern Victoria Land, Antarctica[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF], suggesting a possible connection with the most important magmatic activity in Antarctica, which occurred before the Cenozoic: the Jurassic magmatism, part of the Karoo-Ferrar Large Igneous Province. The Jurassic magmatism produced outcrops that stretch over 3500 km from the Theron Mountains of Antarctica to southeast Australia (Elliot & Fleming 2004), including several volcanic bodies such as mafic sills, flood basalts (i.e. Kirkpatrick basalt, Vestfjella ferropicrites), phreatomagmatic volcanic rocks, layered mafic intrusions (i.e. Ferrar dolerite sill) and mafic dykes (Kyle et al., 1989;[START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF][START_REF] Elliot | Paleovolcanological setting of the middle Jurassic Mawson Formation: evidence from the Prince Albert Mountains, Victoria Land[END_REF][START_REF] Storey | The links between large igneous provinces, continental break-up and environmental change: evidence reviewed from Antarctica[END_REF][START_REF] Heinonen | Mineral chemical evidence for extremely magnesian subalkaline melts from the Antarctic extension of the Karoo large igneous province[END_REF]Bedard et al., 2007). In the Harrow Peaks (74.0 2785°S 164.47466°E, Fig. 1), where the alkaline Cenozoic magmatism is dominant as well as in many other northern Victoria Land magmatic localities, lavas brought to the surface ultramafic xenoliths. The samples available for this study were collected during the XX Italian Expedition organised by PNRA (Programma Nazionale Ricerche in Antartide) in the 2004/05 Austral summer. They are mainly (amphibole bearing) spinel peridotites, but composite xenoliths are also abundant. The Harrow Peaks samples include dunites and one orthopyroxenite, which are rarely observed or absent in the xenolith populations sampled in the region and carried by the same magmatic system.

A few of these xenoliths were studied by Gentili et al. (2005) in order to evaluate the role of amphibole formation in the modification of the original redox condition of this mantle domain. The quoted authors revealed extremely high redox conditions (Δlog QFM fO2 = ~ +5, +7) for the amphibole formation, in clear disequilibrium with the peridotite matrix.

Here we present new petrological and geochemical data of this unique group of ultramafic xenoliths, which provide evidence for a magmatic origin and show to what extent these rocks have been affected by metasomatic event(s), and their relationships with Victoria Land SCLM.

Sample description and Petrography

The ultramafic xenoliths found in the Harrow Peaks lavas are rather small (2 to 5 cm) and subrounded in shape; samples showing textural and geochemical evidence of host-basalt contamination [START_REF] Bonadiman | Metasomatism vs host magma infiltration: A case study of Sal mantle xenoliths, Cape Verde Archipelago[END_REF] were included in the study for a comparative purpose.

Three out of the eight samples selected for this study are composite xenoliths. Hydrous phases (mainly amphibole) occur in all samples, showing significantly different grain size and modal contents from sample to sample.

The mineral modal proportion was determined by point counting, averaging two runs with more than 2,000 points for each thin section (2.5 x 4.0 cm): the results were confirmed by thin section colour code analyses (ImageJ© software) and reported in Table 1. Based on the classification diagram for mafic and ultramafic rocks (Fig. 2; Streickeisen, 1974), the Harrow Peaks xenoliths consist of spinel-bearing lherzolites (HP151, HP164 and HP166), one harzburgite (HP144) and one orthopyroxenite (HP163).

The composite xenoliths combine various lithologies that make difficult to apply the standard nomenclature for ultramafic rocks. HP124 and HP143 are made up of a dunite matrix crossed by clinopyroxene (HP143, Fig. 3a) and clinopyroxene + amphibole + rare phlogopite (HP124, Fig. 3b) veins; HP121 is also composite with a harzburgite matrix (for containing up to 6% and 4% of modal orthopyroxenes and clinopyroxenes, respectively), crosscut by a large amphibole vein (Fig. 3c). The textural similarities and the high olivine modal contents (Fig. 2; Table 1) led us to equate HP121 sample with the rest of the composite dunites.

Regardless of the lithology, most of the samples exhibit tabular to equigranular textural types [START_REF] Mercier | Textures and Fabrics of the Upper-Mantle Peridotites as illustrated by Xenoliths from Basalts[END_REF] with equidimensional, polygonal in shape olivines (1 mm) delimited by boundaries often converging at 120° (i.e. Figs. 4a,b). The modal content of the orthopyroxene is higher in lherzolites (18 -26 volume. %) with respect to the harzburgite (13 vol. %). The clinopyroxene in lherzolites/harzburgite is small and interstitial (Fig. 4e); small irregular spinel is scattered within the olivine grains. Amphibole is present both as disseminated crystals (i.e. HP124; Fig. 4b) or in veins (i.e. HP121; Fig. 4d), and it is associated with rare phlogopite (HP124). The "layered" textural effect evidenced in some samples (i.e. HP143 or HP151; Figs. 3a,e) is marked by a trail of small irregular spinels within the olivine grains, and/or by the amphibole/clinopyroxene veins in clear textural disequilibrium with the dunite matrix (HP121; HP124 and HP143).

The predominant equigranular texture exhibited by Harrow Peaks xenoliths is completely absent in most of the xenolith suits from the same volcanic district (i.e. Baker Rocks and Greene Point), where the protogranular texture is commonly observed. In addition, secondary textures (i.e. glassy patches and sieved pyroxenes, spongy rims) are rarer with respect to other Antarctic xenolith occurrences [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF]. They consist of glassy thin menisc between minerals (including disseminated amphiboles) or small pools (<100 microns) containing microlites of olivine, spinel and clinopyroxene (HP124, HP143, HP144, and HP151).

Analytical Methods

Bulk major and trace element analyses were precluded due to the small size of the xenoliths.

Instead, thin and thick sections were prepared for in situ analytical protocols.

Major element compositions of minerals and glass were determined by a CAMECA SX100 electron microprobe equipped with four wavelengths dispersive (WD) and one Energy Dispersive (ED) spectrometer, at the Department of Lithospheric Research, University of Wien (Austria). The operating conditions were 15 kV accelerating voltage, 20 nA beam current and 20 s counting time on peak position.

In order to minimise Na and K loss, a 5μm defocused beam and 10 s counting time on peak position were applied for glass analyses. Natural and synthetic standards were used for calibration and PAP corrections were applied to the intensity data [START_REF] Pouchou | Quantitative Analysis of Homogeneous or Stratified Microvolumes Applying the Model "PAP[END_REF]. The concentrations of trace elements in pyroxene and glass were obtained by a Laser Ablation Microprobe-Inductively Coupled Plasma Mass Spectrometry (LAM-ICP-MS) at Geosciences Montpellier Université de Montpellier and at igg CNR, Pavia. Both laboratories applied the same analytical protocol and processing data system. Each analysis took 120s: 60s background acquisition (gas blank) and 60s acquisition for the sample.

The analyses were corrected with internal standards using CaO for clinopyroxene and glass, and SiO2 for orthopyroxene. The detection limit is a function of the ablation volume and counting time, and is therefore calculated for each analysis. The ablation volume, in fact, greatly depends on the instrument configuration, consequently, the detection limit decreases if spot size, beam power and cell gas flow are reduced. A beam diameter of 40-100 µm and a scanning rate of 20 µm / s were used.

The theoretical limit of detection ranges between 10 and 20 ppb for REE, Ba, Th, U, and Zr and 2 ppm for Ti. Data were processed using the Glitter ® software (van Achterbergh et al., 2001), and element concentrations were calibrated against the NIST612 certified reference material, using the values of [START_REF] Pearce | A compilation of new and published major and trace data for NIST SRM 610 and NIST SRM 612 glass reference materials[END_REF].

Mineral chemistry

Major element data collected by electron microprobe and reported as wt % oxides are provided for olivine, orthopyroxene, clinopyroxene and spinel in Tables S1, S2 and S3, as well as for amphibole in Table S4 of Supplementary Material.

Mineral major element compositions

Olivine is the ubiquitous phase of the Harrow Peaks peridotites; however, despite the variety of the lithological types and the presence of composite xenoliths, it has a relatively narrow range of composition. The forsterite (Fo) content, calculated as [Mg/(Mg+Fetot) x100 atomic formula units], varies from 86.60 to 90.45, and the NiO content from 0.26 to 0.47 wt %, reported as sample average (Fig. 5a; Table S1 of Supplementary Material); Fo values of olivine (~3% in volume) in orthopyroxenite HP163 and lherzolite HP164 are not included in this compositional range; they are in the range of 80.45-85.88, reflecting the common chemical zoning of cumulus olivines (Fig. 5a; Table S1 of Supplementary Material).

Orthopyroxene varies in the range of En66.62-83.89Fs0.19-0.21-Wo0.038-0.17 with Mg# [Mg# = molar Mg/(Mg+Fetot) x100] = 83.27-89.60 showing an apparent equilibrium with the coexisting olivine in all the investigated samples, and remarkable differences from those of the residual peridotites.

Orthopyroxene presents Al2O3 contents mostly in the range of 1.24 to 3.32 wt %, and Cr2O3 and TiO2 (as sample compositional averages) always <0.50 and <0.20 wt %, respectively (Fig. 5b; Table S2 of Supplementary Material). S3 of Supplementary Material).

Spinel is chemically heterogeneous, varying in composition

Spinel from harzburgite HP144 and lherzolite HP151 present the highest and the lowest Cr# values, respectively, at similar low Fe2O3 (2.54-3.71 wt%) and (lowest) TiO2 (<0.22 wt %) (Fig. 5c). The spinel composition in terms of Cr# is also compared to the Fo content in the Arai (1994, Fig. 6) in which Harrow Peaks samples plot outside the mantle array theoretical field.

Clinopyroxene occurs in this group of ultramafic xenoliths in evident textural disequilibrium with the olivine + spinel + (orthopyroxene) assemblage. It is Cr-rich augite following the Morimoto (1989) classification scheme, and heterogeneous in composition from one grain to another.

Irrespective of lithology, Mg# values span from 85.63 to 93.89 with TiO2< 0.55 wt % and Al2O3 < 4.13 wt % as sample averages (Figs.,7a,b;Table S4 of Supplementary Material). Only a few grains of clinopyroxene concentrated in veins of both HP124 and HP143 dunites have TiO2 and Al2O3 contents that reflect a host basalt low-pressure perturbation (Figs. 7 a,b). Among the possible exchanging cations in M1 and M2 clinopyroxene sites, the only correlation observed is the progressive Na enrichment at lower Mg#.

Amphibole is texturally related to the clinopyroxene and generally occurs as vein in dunites, while it is small and disseminated in lherzolites and the orthopyroxenite. It is chemically classified as kaersutite (HP121, HP143 and HP163), magnesio-hastingsite (HP124) and ferri-kaersutite (HP164), following the [START_REF] Leake | Nomenclature of amphiboles: Report of the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names[END_REF] scheme, with Mg# ranging from 82.86 to 90.39 and TiO2 and Al2O3 contents varying from 1.05 to 4.19 wt% and 11.43 to 14.86 %, respectively (Gentili et al., 2015, Figs. 7c, d;Table S5 of Supplementary Material). The Harrow Peaks amphibole group had already been investigated for a crystallochemical study that led to the discovery of a new amphibole type: the ferri-kaersutite [START_REF] Gentili | Oxoamphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)[END_REF]. Based on the crystallochemical model of [START_REF] Oberti | A crystal-chemical re-evaluation of amphibole/melt and amphibole/ clinopyroxene D Ti in petrogenetic studies[END_REF], Harrow Peaks amphibole have lattice parameters primary ascribable to the magmatic-type [START_REF] Gentili | Oxoamphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)[END_REF], whereas amphiboles from the rest of Antarctic xenoliths have chemical and structural characteristics that fall within the "mantle-type" [START_REF] Oberti | A crystal-chemical re-evaluation of amphibole/melt and amphibole/ clinopyroxene D Ti in petrogenetic studies[END_REF][START_REF] Bonadiman | Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica[END_REF].

Phlogopite grains occurring in HP124 appear chemically zoned (Mg# ~ 91-92), coherent with the observed variability of the other phases forming the vein. They have TiO2 and Al2O3 contents between 1.41-2.19 wt% and 13.88-15.86 wt%, respectively; alkalis are instead constant (Na2O ~0.60 wt%, K2O ~ 9.60 wt%, Table S5 of Supplementary Material). S6 of Supplementary Material) with SiO2 contents ranging from 56.21 to 68.35 wt %, in relation to the textural position and adjacent mineral type, as well as the xenolith's lithology.

Glass is extremely variable in composition (Table

Pyroxene trace elements

Trace element concentrations of pyroxenes averaged per sample are reported in Table 2, whereas all the LA-ICP-MS data are in Tables S7 -S10 of Supplementary Material.

In the chondrite-normalised REE+Sr+Zr+Hf+Ti+Y diagram, orthopyroxene displays a H-MREE downward trend, with the slope increasing in the Yb and Lu region and with an ubiquitous positive Ti anomaly (Fig. 8). HP121 and HP143 are composite xenoliths with orthopyroxene-bearing dunite matrix, but only HP143 contains a few orthopyroxenes large enough for in situ trace element analyses. In detail, the chondrite-normalised trace element distribution of HP143 orthopyroxene mimics that of the HP164 lherzolite (clinopyroxene modal contents ∼11 in volume %) and, to a lesser extent, that of the HP151 lherzolite (clinopyroxene ∼9 in volume %), with YbN/GdN =4.16 and remarkable Ti (Ti/Ti*=5.6) positive and Sr (Sr/Sr*=0.84) negative anomalies. HP166 lherzolite (clinopyroxene ∼7 in volume %) displays orthopyroxene with high Zr, Hf (and Ti) and M-HREE contents, depicting a M-HREE chondrite normalised profile ∼7 times higher, but LREE and LILE contents comparable to the orthopyroxene in lherzolites (and dunite). HP166 orthopyroxene shows coupled strong Ti and Zr (Hf) positive anomalies. In turn, orthopyroxene in HP144 harzburgite (clinopyroxene ∼3 and orthopyroxene ∼13 in volume %) contains the lowest M-HREE (plus Ti), but the highest LREE and LILE concentrations of the entire xenolith population (Table 2), exhibiting almost flat or slightly L-REE enriched patterns (CeN/NdN ~ 2), (Fig. 8, Table 2). Disseminated clinopyroxene is scarce in Harrow Peaks xenoliths. HP164 is the clinopyroxenerichest sample with 11 volume % of modal content (Table 1). Clinopyroxene is extremely variable in terms of trace element contents and, coherently with major elements behaviour, without any correlation with the "fertile" character of the rock (Fig. 9a; Table 2). In the chondrite normalised incompatible multi-element variation diagram, it shows negative Sr and Ti anomalies. The large compositional variability is even more evident in chondrite normalised REE diagrams (Fig. 9b) with overall enriched (LaN 9.02-89.9 and YbN 3.30-20.0 times chondrite) patterns, ranging from almost flat / slightly (i.e. HP143 LaN/YbN= 2.36) to highly (i.e HP124: LaN/SmN = 6.89; LaN/Yb N= 12.53) LREE enriched.

Strongly MREE enriched upward convex patterns (HP166:

GdN/YbN=2.36;GdN/LaN=2.17) can also be observed. The clinopyroxene of HP151 and HP166 lherzolites displays significant negative Eu anomalies (Fig. 9b).

Clinopyroxene (∼7 volume%) in orthopyroxenite HP163 is compositionally variable; most of the grains reproduce the convex upward pattern shown in HP166 lherzolite (at a lower degree of enrichment, Fig. 9b), and a few small crystals evidence a progressive LREE enrichment (LaN = 15.4-179, Fig. 9b).

Clinopyroxene concentrated in veins in composite xenoliths (HP143, HP124) has generally low HREE contents (YbN=3.03 for HP143 and 6.85 for HP124), with less pronounced or absent negative Sr and Ti anomalies with respect to the disseminated clinopyroxene of lherzolites and harzburgite.

It shares (almost) flat chondrite-normalised REE distribution patterns, but different degrees of LREE enrichments (LaN/YbN=2.80-8.65). As observed in lherzolites HP151 and HP166, a Eu anomaly also characterises the small clinopyroxene in the amphibole-bearing vein of HP121 composite xenolith.

Amphibole trace elements

Amphibole is characterised by incompatible trace element chondrite-normalised patterns of similar shape but at various degrees of trace element enrichment (i.e. BaN=13.1-95.6; NdN=11.1-46.1, Fig. 11a) and with systematic Nb and Ti positive anomalies (Fig. 10a). The trace element poorest contents are recorded by the rare amphibole in the thick clinopyroxene vein of the HP143 composite xenolith (Figs. 3a, 4c and Fig. 10b). They perfectly mimic the coexisting clinopyroxene REE pattern (Fig. 10b). Conversely, the highest trace element concentrations are measured in veined amphibole in HP121 composite xenolith that reflect a pure alkaline-like REE pattern (i.e. ~ LaN 43.1, NdN 46.1, YbN 11.3). Veined amphibole in HP124 composite xenolith is also characterised by a gently convex REE pattern (Fig. 10b), but depleted in LREEs with respect to the associated clinopyroxene. In HP164 lherzolite, the disseminated amphibole roughly displays a pattern similar to the coexisting clinopyroxene (Fig. 9b), but with higher LREE contents (i.e. amphibole: LaN= 32.2; clinopyroxene LaN=11.0, Fig. 9b). Overall, the Harrow Peaks amphibole appears more heterogeneous and trace element depleted with respect to that of the nearby Baker Rocks mantle xenolith population (Figs.

10a, b; [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF].

Discussion

Geochemical and textural features of Harrow Peaks ultramafic xenoliths highlighted remarkable differences between this locality and others in Antarctica. Primarily, their modal compositions do not depict any trend attributable to a common mantle residual path (Fig. 2; [START_REF] Bodinier | Orogenic, ophiolitic, and abyssal peridotites[END_REF]Workman and Hart, 2005) and secondly, the Fo content of olivine, in relation to the modal content, is difficult to ascribe to i) residual (melting) trend defined by the mantle peridotites from other localities in Antarctica (Fig. 5a), or ii) alkaline cumulate rocks such as those sampled from Mt.

Overlord (~ 50 Km from Harrow Peaks, [START_REF] Perinelli | Cumulate xenoliths from Mt. Overlord, northern Victoria Land, Antarctica: A window into high pressure storage and differentiation of mantle-derived basalts[END_REF] or in thoeleiitic magmatic products as those of the Basement Sills (Bedard et al., 2007, Fig. 5a). In addition, there is not an observed correlation between spinel and olivine, as expected in a potential residual trend, with the sole exception of the composite xenolith HP21, where spinel Fo and Cr# values of the dunitic matrix intercept the mantle array curve in a region that reflect a moderate fertile lherzolite (melting degree < 20%), inconsistent with the rock lithotype (Fig. 6).

Moreover, clinopyroxene, which is extremely low in modal content, tends to have Mg# values comparable to those of the clinopyroxene from Antarctica mantle peridotites (Fig. 7a,b), but without any relationship with potential residual (i.e. decreasing of Al2O3 with the increasing of Mg#) or metasomatic (i.e. enrichment in TiO2 or Al2O3 at comparable Mg#) trends [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Perinelli | Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)[END_REF][START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF]2011;[START_REF] Armienti | Cenozoic thermal evolution of lithospheric mantle in northern Victoria Land (Antarctica): evidences from mantle xenoliths[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF][START_REF] Pelorosso | Role of percolating melts in Antarctic subcontinental lithospheric mantle: New insights from Handler Ridge mantle xenoliths (northern Victoria Land, Antarctica)[END_REF].

Thus, all the mentioned characteristics suggest that xenoliths from Harrow Peaks do not represent a simple peridotite residuum after melt extraction. Consequently, we discuss the origin of these xenoliths as potentially representing cumulates that crystallised from various silicate melts migrating through the mantle and separated at mantle-crust boundary P-T conditions.

Formation of Harrow Peaks xenoliths

T-P-fO2 equilibration conditions of the Harrow Peaks xenolith suite have been previously explored by [START_REF] Gentili | Oxoamphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)[END_REF], using a crystallochemical approach based on the amphibole dehydrogenation. The study highlighted a strong discrepancy between the redox conditions recorded by the amphibole dehydration equilibrium (∆QFM = +5 -+ 6.8) and those of the coexisting peridotite mineral assemblage (ΔQFM = -2.78 --0.2) finally equilibrated at ~ 854 -940°C

(calculated at P= 1.5 GPa). Using the same approach, this decoupling was not detected for the amphibole-bearing lherzolites and harzburgites of the nearby area of Baker Rocks where amphiboles and peridotite matrix converge to fO2 values between QFM and ΔQFM -1.78 [START_REF] Bonadiman | Crystal chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica[END_REF].

We re-examined the Harrow Peaks samples in view of the suggested possible magmatic origin and integrated previously determined thermobarometric results with new chemical data and new samples.

Temperature values calculated using the combination of olivine-spinel Fe-Mg exchange thermometers of Wood and Virgo (1989) and [START_REF] Ballhaus | High pressure experiment calibration of the olivineorthopyroxene-spinel oxygen barometer: implication for the oxidation state of the mantle[END_REF] in the lherzolites, harzburgite and dunitic portions of composite xenoliths, suggest that these xenoliths finally equilibrated between 800 (orthopyroxenite HP163) and 1200°C (lherzolite HP151).

Presuming that the initial crystallisation of olivine is the main forming process of this group of xenoliths, the potential temperature of crystallisation was also calculated based on olivine/melt equilibrium, assuming that the Fe/Mg ratio of olivine (on sample average) was still close to the initial crystallization conditions. This is reasonable since they were extracted from the forming melts, and cumulated as monomineralic or bimineralic systems. Experimental determinations of Fe-Mg diffusivity in such a system reveal that for an olivine composition of Fo83-90 the closure temperature is reached fast, and the estimated length scales for diffusion in a given time strongly decreased [START_REF] Chakraborty | Rates and mechanisms of Fe-Mg interdiffusion in olivine at 980-1300 °C[END_REF][START_REF] Costa | Short time scales of magmatic assimilation from diffusion modeling of multiple elements in olivine[END_REF].

The FeO-MgO (and SiO2) contents of the theoretical melt/s in equilibrium with dunitic portions of composite xenoliths were inferred comparing various Fe/Mg Kd olivine/melt [START_REF] Norman | Olivine-hosted melt inclusions in Hawaiian picrites: equilibration, melting, and plume source characteristics[END_REF][START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF][START_REF] Oeser | Processes and time scales of magmatic evolution as revealed by Fe-Mg chemical and isotopic zoning in natural olivines[END_REF] models. Using the equation [21] of [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF], and assigning a maximum H2O content of 1wt %, the best fit obtained for the high Mg# HP121 is a melt composition having Mg# 71.5. For HP124, HP143, the majority of lherzolites (Mg# 88) and the harzburgite (Mg# 89), the melt in equilibrium (melt 2) is obtained by fractionation of ca.4 % olivine and 0.8 % of spinel from melt 1 (Tables 3,4).

Furthermore, the olivine in the orthopyroxenite sample (Mg# 80.46) results to be in equilibrium with a slightly evolved melt (melt 3) obtained by additional fractionation of ca. 5% of olivine, 4 % of orthopyroxene, 1 % of clinopyroxene and 0.5 of plagioclase from melt 2 (Table 4).

Overall, the calculated melts show compositions that span between high-Mg tholeiites (picrites?)

and typical tholeiitic basalts, in a co-genetic evolution of the liquid line of descent. In this respect, the HP121 dunitic portion represents the primitive crystallised product of the Kirkpatrick basalt precursor [START_REF] Kyle | Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica[END_REF][START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF]. The olivine potential crystallisation temperature, obtained by iterative calculations using simultaneously melt compositions and Fo-contents [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF], is in the narrow range of 1314-1202± 27 °C, with, coherently, the highest values recorded for the olivine of the HP121.

The barometric conditions under which the Harrow Peak xenoliths were finally equilibrated are firstly deduced by the presence of spinel as the sole aluminium phase (0.9 < P < 2.5 GPa, Wood, 1974). However, an attempt to evaluate the region where the crystallised phases olivine, olivine+ (orthopyroxene) and orthopyroxene + (olivine) separated from the hypothetical (calculated melt/s) was carried out applying the geobarometers of [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF] and [START_REF] Nimis | Clinopyroxene geobarometry of magmatic rocks: part 1. An expanded structural geobarometer for anhydrous and hydrous, basic and ultrabasic systems[END_REF], based on orthopyroxene/melt and clinopyroxene/melt equilibrium, respectively. The obtained values (Table 3) limit the stationing of Harrow Peaks cumulates in the P field of 1.3 (HP143) -0.5 (HP163)

GPa. Considering that the estimated error (for the method) is ± 0.2 GPa, the obtained values are coherent with those reported by [START_REF] Perinelli | Thermal evolution of the lithosphere in a rift environment as inferred from the geochemistry of mantle cumulates; Northern Victoria Land, Antarctica[END_REF][START_REF] Perinelli | Cumulate xenoliths from Mt. Overlord, northern Victoria Land, Antarctica: A window into high pressure storage and differentiation of mantle-derived basalts[END_REF], which constrain the crust/mantle boundary (Moho).

Parental Melt modelling

The predominant mineral assemblages (olivine+spinel) of the composite xenoliths, the occurrence of orthopyroxenite and the calculated olivine (and orthopyroxene)/melt equilibrium, reveal that the parental liquid had high MgO contents (Mg# 72) and was close to silica saturation. Such melts are associated with an anomalous high temperature (excess Tex) melting of the Phanerozoic mantle (picrites) or the Archean cratons (komatiites). Picrites or komatiites either present cumulitic olivine or are olivine-saturated even at high pressure, so that they can reach such high MgO and Mg# [START_REF] Lee | The development and refinement of continental arcs by primary basaltic magmatism, garnet pyroxenite accumulation, basaltic recharge and delamination: insights from the Sierra Nevada, California[END_REF]Class, 2008).

All the studied Harrow Peaks xenoliths (including orthopyroxenite HP163), have olivine as cumulus phase, whose composition differs from most of the Antarctic ultramafic olivine cumulates, commonly ascribed to the Cenozoic alkaline magmatism of the West Antarctic Ridge System (Fig. 5a; [START_REF] Gamble | The origins of glass and amphibole in spinel-wehrlite xenoliths from Foster Crater, McMurdo Volcanic Group, Antarctica[END_REF][START_REF] Gamble | Metasomatized xenoliths from Foster Crater, Antarctica: implications for lithosphere structure and processes beneath the Transantarctic Mountains[END_REF][START_REF] Perinelli | Cumulate xenoliths from Mt. Overlord, northern Victoria Land, Antarctica: A window into high pressure storage and differentiation of mantle-derived basalts[END_REF]. Harrow Peaks' rocks instead represent products crystallised at mantle depth, close to the Moho discontinuity, from the precursor melt/s of the Ferrar Group tholeiitic magmatism (Kyle 1990[START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF], or from the high Mg-tholeiites (low Ti -picrites), similar to those of Vestfalja [START_REF] Heinonen | Jurassic dikes of Vestfjella, western Dronning Maud Land, Antarctica: Geochemical tracing of ferropicrite sources[END_REF]2010).

The calculated primitive melt forming the dunite matrix of HP121 has Mg# (up to 72) higher than any documented melt (Mg# 59-68), directly or indirectly (i.e. mantle metasomatic melt) related to the Cenozoic magmatic system, which is responsible for xenoliths rising to the surface [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Perinelli | Metasomatism induced by alkaline magma on upper mantle of the Northern Victoria Land (Antarctica): an experimental approach[END_REF][START_REF] Nardini | Sr-Nd-Pb-He-O isotopeand geochemical constraints to the genesis of Cenozoic magmas from the West Antarctic rift[END_REF]. In this respect, the HP121 dunite represents the primitive crystallised product of the Kirkpatrick basalt (Ferrar Group) precursor (low Ti picrites), whereas dunite portions of HP124 and HP143 composite xenoliths and harzburgite /lherzolite samples are fractionated and accumulated by melt/s close in composition to the exposed Ferrar thoeleiitic magmas (i.e. Kirkpatrick basalt; dolerites; [START_REF] Kyle | Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica[END_REF][START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF] in a cogenetic liquid line of descent (Table 4). The scarceness of olivine and the low Mg# of orthopyroxene in HP163 orthopyroxenite, point towards a cumulate of the same evolved melt (Table 4).

In such a frame, it is not easy to understand the role of clinopyroxene (and amphibole). Fe/Mg Kd values (0.18-0.32) that cannot be equilibrated with most of the calculated melts (Table 3). This is clear for the HP121 and HP124 composite xenoliths ( Fe/Mg Kd clinopyroxene/melt = 0.18-0.15), whereas the HP143 composite xenolith, where the clinopyroxene almost entirely forms the monomineralic large vein (Figs. 2,3), is the only sample that records a potential equilibrium of clinopyroxene with its matrix olivine, and with a calculated melt ( Fe/Mg Kd clinopyroxene/melt = 0.27) having Mg#=68 (Table 3). HP143 clinopyroxene is characterised by notably lower REE contents with a slight LREE-enrichment and flat M-REE patterns (LaN/CeN=1.25). Moreover, it does not present the high Eu negative anomaly, which instead characterises most of the Harrow Peaks clinopyroxenes. Overall, the HP143 REE profile is coherent with the equilibrium with a tholeiitelike forming melt and is clearly distinct from the alkaline-type clinopyroxene of HP121 and HP124 dunites (Fig. 9b).

Furthermore, we use the REE of the HP143 clinopyroxene to evaluate the HP143 parental melt REE contents. The REE Kd clinopyroxene/melt dataset used for the calculations is reported in Table S11 of Supplementary Material. The resulting profile mimics those of the Ferrar tholeiites and, coherently with the major element results, it suggests a potential equilibrium between the HP143 clinopyroxene and a parental thoeleiitic melt (Fig. 11).

To sum up, HP121 and HP163 are cumulates from hypothetical parental melts that represent two extremes: the most primitive and the most evolved terms of a hypothetical liquid line of descent, respectively, suggesting that the parental melt of the Kirkpatrick basalt was a (low alkaline) picrite, which stationed in the shallow mantle. The melt system, modified in time and space, produced the rest of the Harrow Peaks ultramafic rocks, including the orthopyroxenite, at a shallower level. For all, but HP143, clinopyroxene and amphibole were formed by a different magmatic or metasomatic episode/s associated with more evolved melts that reasonably also crystallised plagioclase; this phase, even if not present in these cumulus fragments, may have imprinted the scattered Eu negative anomaly (Fig. 9).

Geodynamic implications

The study of the Harrow Peaks xenoliths is clearly relevant as they represent the initial stages of the Ferrar magmatism. The importance of the Ferrar magmatism in West Antarctica is widely attested [START_REF] Kyle | Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica[END_REF][START_REF] Fleming | Chemical and isotopic variations in an iron-rich lava flow from the Kirkpatrick Basalt, north Victoria Land, Antarctica: implications for low-temperature alteration[END_REF][START_REF] Elliot | Paleovolcanological setting of the middle Jurassic Mawson Formation: evidence from the Prince Albert Mountains, Victoria Land[END_REF]; the large volume of magmas generated by this event permeated part of the lithospheric domains beneath northern Victoria Land, refertilising the residual peridotitic upper mantle [START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF].

The Ferrar large igneous system includes various magmatic bodies; among them, the doleritic Basement Sill from the McMurdo Dry Valleys represents the plumbing system of the Ferrar flood basalt eruptions (Bedard et al., 2007). The rock forming the sill displays a cumulate texture similar to that observed in layered intrusions and is characterised by an "orthopyroxene enriched base" (Bedard et al., 2007); in that view, the unique investigated orthopyroxenite HP163 could be representative of the same basement.

Orthopyroxenites are quite rare among the Antarctica xenoliths suites. To the best of our knowledge the only other one documented is associated with clinopyroxenites and peridotite xenoliths hosted in basalts of the Mount Morning eruptive centre [START_REF] Martin | Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre[END_REF].

In northern Victoria Land, orthopyroxenites were also found as part of the layered sequence from Niagara Icefall (northern Victoria Land, Tribuzio et al., 2008); they were interpreted as cumulates [START_REF] Hergt | The geochemistry of Jurassic dolerites from Portal Peak, Antarctica[END_REF][START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF][START_REF] Elliot | Paleovolcanological setting of the middle Jurassic Mawson Formation: evidence from the Prince Albert Mountains, Victoria Land[END_REF]. Thus the hypothesis that the investigated suite of samples could represent cumulus rocks from the most primitive Ferrar melt seems the most plausible.

Tracing the origin of Harrow Peaks cumulates back to the Jurassic, it is reasonable that they remained located within the shallow lithospheric mantle (P ~1 GPa) up to the Cenozoic (~ 50 Ma), when the alkaline magmatism associated with the West Antarctic Rift brought them to the surface.

The Cenozoic (late stage) alkaline metasomatism pervaded large portions of mantle domains and cumulates suites in Victoria Land [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Perinelli | Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)[END_REF][START_REF] Perinelli | Metasomatism induced by alkaline magma on upper mantle of the Northern Victoria Land (Antarctica): an experimental approach[END_REF][START_REF] Perinelli | Thermal evolution of the lithosphere in a rift environment as inferred from the geochemistry of mantle cumulates; Northern Victoria Land, Antarctica[END_REF][START_REF] Martin | Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF][START_REF] Pelorosso | Role of percolating melts in Antarctic subcontinental lithospheric mantle: New insights from Handler Ridge mantle xenoliths (northern Victoria Land, Antarctica)[END_REF], and affected the Harrow Peaks lithospheric portion, as testified by the presence of alkaline-like amphiboles (phlogopite) and the LREE-enrichment in clinopyroxene (HP121 and HP124).

Conclusions

1. The textural and geochemical characteristics of Harrow Peaks xenoliths evidence that these ultramafic rocks represent deep crustal/mantle cumulates crystallised from a primitive picritic melt with a potential temperature ~ 1300 °C;

2. The presence of orthopyroxenite in the system demonstrates that the inferred melt was saturated in SiO2;

3. The mineral/melt equilibrium allowed to identify the parental melt as corresponding to the first stages of the Ferrar magmatism; Greene Point [START_REF] Melchiorre | The role of eclogite in the rift-related metasomatism and Cenozoic magmatism of Northern Victoria Land, Antarctica[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF], Baker Rocks [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Perinelli | Geochemical and O-isotope constraints on the evolution of lithospheric mantle in the Ross Sea rift area (Antarctica)[END_REF][START_REF] Perinelli | Metasomatism induced by alkaline magma on upper mantle of the Northern Victoria Land (Antarctica): an experimental approach[END_REF] and Handler Ridge [START_REF] Pelorosso | Role of percolating melts in Antarctic subcontinental lithospheric mantle: New insights from Handler Ridge mantle xenoliths (northern Victoria Land, Antarctica)[END_REF] are also reported (green stars). The map of northern Victoria Land is modified from [START_REF] Estrada | Continuous Neoproterozoic to Ordovician sedimentation at the East Gondwana margin -implications from detrital zircons of the Ross Orogen in northern Victoria Land, Antarctica[END_REF], the Antarctica view is from Google Earth. from PM (Primordial Mantle, McDonough and Sun, 1995) to clinopyroxene consumption is also calculated (dashed line) using theoretical and experimental results of Jensing and Holland (2015) and [START_REF] Herzberg | Geodynamic information in peridotite petrology[END_REF]. trails. Important to note that the real modal content of spinel do not correspond to the black areas as in the thin section. b) HP124 is characterised by dunitic matrix and the thinnest vein containing also phlogopite. c) HP121 with the largest monomineralic vein that has been partly removed for crystallochemical investigations [START_REF] Gentili | Oxoamphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)[END_REF]. Harzburgite HP144 (d) and high clinopyroxene lherzolite (e). High orthopyroxene lherzolites (f, g) and orthopyroxenite (h). Victoria Land peridotites [START_REF] Coltorti | Amphibole genesis via metasomatic reaction with clinopyroxene in mantle xenoliths from Victoria Land, Antarctica[END_REF][START_REF] Martin | Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre[END_REF][START_REF] Pelorosso | Pervasive, tholeiitic refertilisation and heterogeneous metasomatism in northern Victoria Land lithospheric mantle (Antarctica)[END_REF][START_REF] Pelorosso | Role of percolating melts in Antarctic subcontinental lithospheric mantle: New insights from Handler Ridge mantle xenoliths (northern Victoria Land, Antarctica)[END_REF], Mt

Overlord cumulates [START_REF] Perinelli | Cumulate xenoliths from Mt. Overlord, northern Victoria Land, Antarctica: A window into high pressure storage and differentiation of mantle-derived basalts[END_REF] and Basement sill leucotroctolites (Bedard et al., 2007) are reported for comparison. 3. The reconstructed melt 2, assumed in equilibrium with HP143 clinopyroxene, is obtained using REE Kd cpx/melt data from GERM. The resulting profile converges on a tholeiitic melt that resembles the Ferrar basalt [START_REF] Kyle | Development of heterogeneities in the subcontinental mantle: evidence from the Ferrar Group, Antarctica[END_REF][START_REF] Fleming | Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica[END_REF]

  from sample to sample without any clear relationship with the lithological types. In composite xenoliths HP124 and HP143 it shows high variable Cr# [Cr# = molar Cr/(Cr + Al )x100] (24.25-61.69) and Mg# (60.81-52.26) values, with TiO2 (0.5-1.04 wt%) contents approaching the spinel composition of HP163. HP121 contains relatively low-Ti spinel (TiO2=0.26-0.36 wt %) with the lowest Fe2O3 (0.71 wt % as sample average) recorded by the Harrow Peaks xenolith population (Fig.5c; Table

  The few clinopyroxene grains of HP121 and those concentrated in amphibole-bearing veins in HP124 and HP143, as well as the clinopyroxene occurring in harzburgite and lherzolites, generally show high Mg# (90-91) and Fe/Mg clinopyroxene/olivine distribution coefficients ( Fe/Mg Kd 0.59-0.97) out of the range of equilibrium values ( Fe/Mg Kd clinopyroxene/olivine = 0.83-0.86, Putirka 2008). Applying the Fe/Mg Kd clinopyroxene/melt formulation of Putirka et al. (2008) and Mollo et al. (2017) to the previous calculated Harrow Peaks parental melts, the ideal liquidus clinopyroxene has a Fe/Mg Kd clinopyroxene/melt of 0.24-0.27. The Harrow Peaks clinopyroxene instead, is characterised by

  of a boninitic melt related to the development of a back arc basin in an active continental margin. Similarities between the Harrow Peaks ultramafic cumulates and the Niagara Icefall layered rocks include: i) the occurrence of high Mg# dunites (Mg# values = 91-94) coexisting with orthopyroxenites (Mg# values=84-88); ii) the matching of REE abundances and profiles of clinopyroxenes from the Niagara Icefall gabbronorite with that of the HP143 clinopyroxene (Fig. 11). Therefore, considering the mentioned examples from different geodynamic settings, we cannot exclude a priori that the Harrow Peaks cumulates represent fragments of cumulus "layers" crystallised and separated from tholeiitic or boninitic melts. However, Harrow Peaks cumulates are hosted in lavas sampled in the Wilson Terrane, where the Ferrar outcrops are represented by the Ferrar Dolerite Dufek Intrusions and the Kirkpatrick Basalts

4.Figure 1 .

 1 Figure Captions

Figure 2 .

 2 Figure 2. Harrow Peaks xenoliths in the olivine-orthopyroxene-clinopyroxene IUGS classification diagram. (*) Dunitic matrix of composite xenoliths. The residual path (annotated by melting %)

Figure 3 .

 3 Figure 3. Thin sections of Harrow Peaks mantle xenoliths, which are mainly characterised by equigranular textural type. HP143, HP124, HP121composite xenoliths (a, b, c) consist of large clinopyroxene and amphibole veins cross-cutting a dunitic matrix. a) HP143 with evident spinel

Figure 4 .Figure 5 .

 45 Figure 4. Photomicrographs of representative microstructures from Harrow Peaks xenoliths. Polygonal large olivine with boundaries converging at 120° (a, b), detail of the large clinopyroxene vein with embryonic amphibole (c). Tabular olivine zone and amphibole vein in sample HP121 (d).Lherzolite HP166 characterised by equigranular olivine, orthopyroxene and small clinopyroxene (e). Large orthopyroxene in the orthopyroxenite HP163 (f)

Figure 6 .

 6 Figure 6. Plot of spinel Cr# vs Fo content of coexisting olivine of Harrow Peaks ultramafic xenoliths.(*) Dunitic matrix of composite xenoliths. The olivine-spinel mantle array is from[START_REF] Arai | Characterization of spinel peridotites by olivine-spinel compositional relationships: review and interpretation[END_REF].Melting trend (annotated by melting %) is from[START_REF] Park | Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr-Nd-Hf isotopic studies on peridotite xenoliths in trachybasalt[END_REF]; peridotite fields are modified from[START_REF] Park | Evolution of the lithospheric mantle beneath Mt. Baekdu (Changbaishan): Constraints from geochemical and Sr-Nd-Hf isotopic studies on peridotite xenoliths in trachybasalt[END_REF].

Figure 7 .

 7 Figure 7. Harrow Peaks clinopyroxene (a, b) and amphibole (c, d) compositions in term of Mg# vs. TiO2 and vs. Al2O3. Compositional fields reported for comparison in (a, b) are as in figure 5.

Figure 8 .

 8 Figure 8. Chondrite-normalized trace element (REE+Sr+Zr+Hf+Ti+Y) distribution for Harrow Peaks orthopyroxene (represented as sample average). Chondrite values from McDonough and Sun (2005) Figure 9. Chondrite-normalized trace-element patterns a) and Rare Earth Elements (REE) patterns b) for Harrow Peaks clinopyroxene represented ad sample average, except for HP163 (orthopyroxenite). Due to the large trace element variability, all the analyses of the few HP163 clinopyroxenes are reported. Chondrite values from McDonough and Sun (2005)

  . In turn, HP143 clinopyroxene profile mimics those occurring in the ultramafic sequence (cumulates) from Niagara Ice Falls (Tribuzio et al., 759 2008). Chondrite-normalized values from McDonough and Sun (2005).

Acknowledgements

XXXX.

M.C. is acknowledged for his fieldwork during the XX PNRA expedition in Northern Victoria Land (Austral summer 2004(Austral summer -2005)).

The authors would like to thank Barbara Galassi (Brighton, UK) for checking the English language in this paper. This work was funded by PNRA (National Programme Antarctic Research) project: 2013-2015 "Hydrous phases stability in the lithospheric mantle of the large continental rift systems: a petrological/experimental study of the mantle xenoliths and lavas of the Northern Victoria Land (principal investigator; C.B) B.P was supported by MIUR-2015 20158A9CBM Grant ((principal investigator; C.B).