domain: sdu sde

text:
Anastassia Y Borisova
email: anastassia.borisova@get.omp.eu

Klaus Peter Jochum
email: k.jochum@mpic.de

Sophie Gouy
email: sophie.gouy@get.omp.eu

Mineralogical and geochemical features of the Allan Hills tephra, South Victoria Land: Implications for mid-Pleistocene volcanic activity in Antarctica

Keywords: volcanic tephra, Antarctica, Allan Hills, mantle plume, Pleistocene

More than 100 volcanoes have been discovered in Antarctica, however, the sources of many Antarctic tephras are not yet constrained. In this study, four tephra samples recovered from the environment of Site 27 on an area of blue ice at Allan Hills, South Victoria Land, provide information on magma and crustal sources for the tephra erupted in Antarctica. We report data on the chemistry of Antarctic tephra glasses and minerals analysed by electron microscopy, electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry. The tephra amphibole chemistry indicates magma residence at ~1 GPa and ~1100 °C. Abundant Ca-Al-Si-rich minerals imply that the magma ascended from a reservoir at 29.4 ± 2.7 km depth was affected by the assimilation of calc-silicate crust. The high Fe and Ti contents in the magmatic minerals and the trace element patterns of the glasses typical of a hydrothermally altered oceanic crust suggest that the Allan Hills tephra originated from a mantle plume-derived magma. The fragmented morphology of the glasses implies that the magmatic activity was associated with explosive eruptions during the mid-Pleistocene. The chemistry of the glasses and the presence of rhönite and Al-rich spinel minerals suggest that the magma might be genetically related to the currently active McMurdo volcanic group.

Introduction

Antarctica is a volcanically active region [START_REF] Kyle | Erebus Volcanic Province[END_REF][START_REF] Wörner | Lithospheric dynamics and mantle sources of alkaline magmatism of the Cenozoic West Antarctic Rift System[END_REF][START_REF] Van Wyk De Vries | A new volcanic province: an inventory of subglacial volcanoes in West Antarctica[END_REF][START_REF] Iverson | The first physical evidence of subglacial volcanism under the West Antarctic Ice Sheet[END_REF]. A number of "dust" bands visible on the surface of blue ice at Allan Hills, South Victoria Land are reported to be volcanic ashes (tephra) [START_REF] Nishio | Volcanic ash layers in base ice areas near the Yamato Mountains, Dronning Maud Land and the Allan Hills, Victoria Land, Antarctica[END_REF][START_REF] Dunbar | Geochemical composition and stratigraphy of tephra layers in Antarctic blue ice: Insight into glacial tephrochronology[END_REF][START_REF] Spaulding | Ice motion and mass balance at the Allan Hills blue ice area, Antarctica, with implications for paleoclimate reconstructions[END_REF]2013;[START_REF] Iverson | Eruptive history and magmatic stability of Erebus volcano, Antarctica: Insights from englacial tephra[END_REF][START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF]. Tephra layers in blue ice result from explosive eruptions, thus providing excellent time-lines for the assessment of climate change, thermal anomalies and volcanic activity in polar regions [START_REF] Keys | Tephra and debris layers in the Skelton Neve and Kempe glacier, South Victoria Land, Antarctica[END_REF][START_REF] Ponomareva | Identification of a widespread Kamchatkan tephra: A middle Pleistocene tie-point between Arctic and Pacific paleoclimatic records[END_REF][START_REF] Dunbar | New Zealand supereruption provides time marker for the Last Glacial Maximum in Antarctica[END_REF][START_REF] Narcisi | A 16,000-yr tephra framework for the Antarctic ice sheet: a contribution from the Talos Dome core[END_REF]2016;[START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF]. However, the sources and ages of many Antarctic tephras are not yet constrained. This fact hampers the use of tephra for establishing correlations with paleo-climatic archives. This study aims to (i) identify the mineralogical and chemical composition of the dated tephra particles from the environment of Site 27 in the Allan Hills blue ice area (BIA), South Victoria Land, (ii) interpret this information in terms of the physical-chemical conditions of magmatic crystallization and (iii) obtain better constraints on the origin of tephra. Moreover, the results presented here may be used to improve our interpretation of paleo-environmental records [START_REF] Keeling | Atmospheric CO2 records from sites in the SIO air sampling network[END_REF] in Antarctica. In this study, we investigate four tephra samples recovered from the environment of Site 27, allowing us to elucidate the presence of multiple magma sources in the Southern Victoria Land.

Materials and Methods

Four samples (labelled BIT10, BIT12, SURF and CORE) were collected from englacial tephras at Site 27 in the Allan Hills BIA (Fig. 1, Table 1). Glaciological studies suggest steady-state conditions allowing the long-term continuity of the ice areas as well as that old ice is located at the surface [START_REF] Spaulding | Ice motion and mass balance at the Allan Hills blue ice area, Antarctica, with implications for paleoclimate reconstructions[END_REF]2013). Each tephra sample is derived from a cubic meter of ice. The ice is first melted, followed by decantation of the water and storage of the complete (unfiltered) sample. The insoluble residue of the tephra is extracted from melted ice samples using 0.2-μm Nuclepore polycarbonate filters. The melt water volume for each of these samples is from ~150 mL to ~250 mL. Map coordinates for each sample are given in Table 1. SURF was collected from the same ash layer as BIT10, while CORE was sampled 24.25 m up flow from SURF, extracted from an ice core at a depth of ~3.5 m. Stratigraphically, BIT10, SURF and CORE all represent the same tephra layer, which is dated at ~114.7 ka, according to a time scale developed for Site 27 or ice core S27 [START_REF] Spaulding | Ice motion and mass balance at the Allan Hills blue ice area, Antarctica, with implications for paleoclimate reconstructions[END_REF]2013;[START_REF] Higgins | Atmospheric composition 1 million year ago from blue ice in the Allan Hills, Antarctica[END_REF], whereas the glass-rich BIT12 sample (Fig. 2) is dated by 40 Ar/ 39 Ar method at 202 ± 7 ka [START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF].

The tephra samples were carefully embedded into epoxy resin, polished with SiC paper and diamond pastes and then investigated at the Géosciences Environnement Toulouse (GET) laboratory in France using a JEOL JSM-6360 LV scanning electron microscope, with energy dispersive spectroscopy (EDS) coupled with an automatic particle analyzer controlled by the "Esprit" program working in the "Feature" module. The backscattered electron (BSE) imaging was performed coupled with EDS mapping (Table 1, Supplementary Dataset) following the analytical protocol previously developed [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013) and described below. The solid particles (glasses and minerals) in each sample were initially distinguished based on their grey tones on the BSE images, being classified using the EDS technique (Table 1, Supplementary Dataset), and then analysed by electron probe microanalysis (EPMA) (Figs. 2, S1-S3). The most representative particles include glass, plagioclase, quartz, alkali feldspar, albite, spinels, iron silicate, clinopyroxene, orthopyroxene, kaersutite and Ca-Al-rich amphibole, ilmenite, rhönite and carbonates. These latter particles, together with other unclassified particles, are then used to interpret the tephra provenances. The minimum size of the analysed particles is 12 to 80 μm, with a maximum size ranging from 100 to 200 µm, depending on the sample (see Figs 2, S1-S3). The number of compositionally homogeneous particles analyzed by BSE-EDS method ranges from 190 to 270 per sample. Table 1 and the Supplementary Dataset summarize the mineral and glass contents in the four tephra samples obtained by back-scattered-electron-energy-dispersive Xray spectroscopy (BSE-EDS) mapping.

Major-and minor-element compositions of glasses and minerals were determined on embedded samples using a CAMECA SX-50 instrument (GET, Toulouse, France), following the analytical protocol previously developed [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013). Additionally, the BIT12 glasses were analysed using a CAMECA SX-Five instrument (Centre Castaing, Toulouse, France). Tables 2,S1-S9 and the Supplementary Dataset present the complete dataset and analytical conditions applied during EPMA and femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP-MS). For the spot analyses and mineral profiling, the electron beam was set at an accelerating voltage of 15 kV, with a probe current of 10-20 nA for minerals, 10 nA for glasses and a spot diameter on the sample of 4 μm (CAMECA SX-50) to defocused 10 µm (CAMECA SX-Five). Sodium was analysed first of all to assure minimal loss during EPMA; the applied analytical conditions allowed an optimal signal for the major and minor elements and a better internal precision. Such conditions are in accordance with recommendations given by [START_REF] Kuehn | INTAV Intercomparison Participants The INTAV intercomparison of electron-beam microanalysis of glass by tephrochronology laboratories: Results and recommendations[END_REF] for use in tephrochronology to ensure minimal sodium loss during EPMA. The following synthetic or natural reference materials were used: albite (Na), periclase (Mg), corundum (Al), sanidine (K), wollastonite (Si and Ca), pyrophanite (Ti), hematite (Fe), chromium oxide (Cr), and topaz (F). An acquisition time of 10 s was used per element on the peak (by CAMECA SX-50 and CAMECA SX-Five) in glasses and minerals and an acquisition time of 5 s was used per peak of Na (by CAMECA SX-Five) in the BIT12 glasses and 5 s per element on the background (by CAMECA SX-50 and CAMECA SX-Five). The analytical uncertainties (1-3 wt.% relative for major elements) are evaluated using MPI-DING reference glasses (T1-G, ATHO-G and StHs6/80-G, Table S3) as described by [START_REF] Jochum | MPI-DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios[END_REF]. The detection limits for major and minor elements in the main minerals, as well as glasses, are similar to those previously described by [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]. The detection limit for F under the CAMECA SX-50 conditions is 700 ppm.

The major and trace element composition of the BIT12 glasses and minerals, as well as the glass-hosted inclusions of micrometric size, were measured and identified by fs-LA-ICP-MS at the Max Planck Institute for Chemistry, Mainz, Germany [START_REF] Jochum | Non-matrix-matched calibration for the multi-element analysis of geological and environmental samples using 200 nm femtosecond LA-ICP-MS: A comparison with nanosecond lasers[END_REF] (see Table 2, Tables S1 andS2). Nine line scan analyses of 500 µm length each were performed on the BIT12 sample. Each of nine scan fs-LA-ICP-MS spectra has been thoughtfully analysed, and the spectra of the predominant "matrix" have been identified and separated from the "inclusion" spectra based on the characteristic peaks of Si and Na in those of "inclusions" (Fig. 2). The both analyzed "matrix" glasses and "inclusions" are essential components of the tephra analyzed by fs LA-ICP-MS (Fig. 2). The "matrix" composition was obtained based on 333 single measurements by fs LA-ICP-MS, whereas "inclusions" have been quantified based on the well-known method of inclusion analysis by laser ablation (e.g., [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013). The analyzed "inclusions" are not always visible on the images obtained by electron microscopy (Fig. 2), since their identification and quantification is based solely on the fs LA-ICP-MS spectra. For the evaluation of the "matrix" and "inclusions" data, we have rejected the first 1 to 4 measurements per a scan line corresponding to a range of unstable conditions (unstable laser; relative higher proportion of (contaminated) surface material).

The LA-ICP-MS system was equipped with a femtosecond laser NWRFemto200 (Electro Scientific Industries, New Wave Research Division, Portland, OR, USA) [START_REF] Jochum | Non-matrix-matched calibration for the multi-element analysis of geological and environmental samples using 200 nm femtosecond LA-ICP-MS: A comparison with nanosecond lasers[END_REF] and coupled with a Thermo Finnigan sector-field ICP-MS Element 2. Table S2 lists the main operating parameters, in addition to a detailed description of the Thermo Element 2 mass spectrometer at MPIC already given by [START_REF] Jochum | Validation of LA-ICP-MS trace element analysis of geological glasses using a new solid-state 193 nm Nd:YAG laser and matrix-matched calibration[END_REF]. This system is fitted with a Ti-sapphire-based ultra-fast amplifier delivering laser pulses shorter than 130 fs.

The amplifier produces a laser output wavelength of 200 nm via highly efficient harmonic generation of the fundamental wavelength. The laser beam is introduced into a high-performance New Wave Large Format cell, which is a two-volume ablation cell capable of generating wash-out times <1 s and accommodating multiple samples and reference materials. We used NIST 610 as the calibration material and new reference values [START_REF] Jochum | Determination of reference values for NIST SRM 610-617 glasses following ISO guidelines[END_REF]. Spot size and repetition rate (used for the investigation of the tephra "matrix" and the reference glasses) were 40 µm and 50 Hz, respectively, at a fluence of 0.7 J cm -2 .

We used line scan analysis, with a scan speed of 5 µm s -1 . The micro-size "inclusions" were investigated with a spot size of 25 µm at a fluence of 0.6 J cm -2 , with a scan speed of 1 µm s -1 .

The ICP-MS measurements were performed in low mass resolution mode (M/∆M = 300), using a combination of magnetic and electrical scan modes. The analysis time for a 1pass measurement is about 1.5 s, while the whole ablation time is 100 s for the "matrix" and 50 s for "inclusions" numbered 1 to 10. The following isotopes were used in the multielement analyses: 7 [START_REF] Jochum | Validation of LA-ICP-MS trace element analysis of geological glasses using a new solid-state 193 nm Nd:YAG laser and matrix-matched calibration[END_REF]. In addition, the major element concentrations (including CaO wt.%) were adjusted (independently from trace elements) to a total oxide content of 97 wt.%, which is minimal content of the total oxide sum obtained by EPMA (Table S3, the Supplementary Dataset).

The reproducibility (% RSD, relative standard deviation) of the fs-LA-ICP-MS analyses is 1-3% as demonstrated by the analyses of the most homogeneous glasses, such as, basaltic glass KL2-G (MPI-DING series) [START_REF] Borisova | In situ multi-elemental analysis of ATHO-G rhyolitic glass by femtosecond LA-ICP-MS: Evidence for significant heterogeneity at mm-scale in respect to Zn, Mo, Cs and Pb[END_REF][START_REF] Jochum | MPI-DING reference glasses for in situ microanalysis: New reference values for element concentrations and isotope ratios[END_REF]. For example, the concentration data for KL2-G agree with the reference values within 1-5% [START_REF] Jochum | Non-matrix-matched calibration for the multi-element analysis of geological and environmental samples using 200 nm femtosecond LA-ICP-MS: A comparison with nanosecond lasers[END_REF]. Because of sample heterogeneities, the reproducibility of the "matrix" measurements of BIT12 tephra is lower (about 10-25%). The uncertainty of the data of the small (µm-size) "inclusions" is strongly dependent on the concentration and generally varies between 10% and 30%. The proposed contents of K2O in the BIT12 tephra glasses given in Figure 3 are calculated according to the measured Na2O contents and an average Na2O/K2O ratio characteristic of that measured in the BIT12 glasses by EPMA (see Table S3, Supplementary Dataset).

Results

The common feature of the investigated tephra samples is the presence of magmatic Ca amphiboles, titanomagnetites (up to 20 rel.%, Table 1), Ti-augites (up to 19 rel.%), plagioclases (up to 18 rel.%), and mildly peralkaline to alkaline glasses with quartznormative compositions (up to 84 rel.%, see Fig. 2, Table S3, Supplementary Dataset).

BIT10 is a fine grained tephra (≤100 µm) with fragments and agglomerations of the volcanic particles composed of ferrokaersutite, kaersutite, ferro-edenite and plagioclase, as well as Ti-augite, apatite, anorthoclase, olivine, ilmenite, rhönite grains, and glasses of trachyandesitic, trachytic to phonolitic composition (Figs. 3,S1). The amphiboles are identified according to the classification of [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]. Table 1 andTables S4-S9 show that the BIT10 sample contains magmatic Ca amphibole (Mg# 37-47, TiO2 = 3.8-5.8 wt.%), Ti-augite (TiO2 = 4.7-5.7 wt.%), olivine (Fo77-80, CaO = 0.4-1.0 wt.%), plagioclase (An40-66, FeO = 1.2 wt.%), titanomagnetite (Usp89), ilmenite (Ilm92-94) and F-apatite. Two Ca-Mg-Fe carbonate and two quartz particles are detected by the BSE-EDS method (Table 1, Supplementary Dataset). Moreover, abundant metamorphic Ca-Al-rich tschermakite amphiboles are identified in this sample. Three ilmenite particles (P216, P217 and P218) were detected and analysed by BSE-EDS mapping of the sample, being used here to estimate the magmatic temperature and oxygen fugacity (fO2) based on the Fe-Ti oxide geothermobarometer [START_REF] Ghiorso | Thermodynamics of rhombohedral oxide solid solutions and a revision of the Fe-Ti two-oxide geothermometer and oxygenbarometer[END_REF] (Table S8). Pairs of ilmenitetitanomagnetite reflect the magmatic mineral equilibrium at a temperature of 1105 ± 15 °C with redox conditions (expressed in logarithmic units, log(fO2), relative to the redox buffer of nickel-nickel oxide, NNO) varying from NNO-1.3 to NNO-0.8 (Table S8).

BIT12 is a fragmented glass-rich tephra sample composed of 50 -200 μm-sized glass fragments of trachytic composition with typical bubble-wall appearance and fluidal texture (SiO2 = 58 -68 wt.%; Na2O + K2O = 7.6 -9.4 wt.%), plagioclase (An33-57, FeO = 0.14 -0.34 wt.%), titanomagnetite (Usp 83-86) analysed by EPMA (Fig. 3, Tables S3,S5, S8), as well as SiO2 grains and glass-hosted "inclusions" identified and analysed by fs-LA-ICP-MS (Figs. 2, 4, Table 2). Additionally, we have found particles of anorthoclase, Ca-Fe-rich pyroxene and rhönite (see Table S9, Supplementary Dataset). Compared to the "particles" identified by BSE-EDS and analysed by EDS and EPMA method, the analyzed "inclusions" are not always visible on the images obtained by electron microscopy (Fig. 2), since their identification and quantification is based solely on the fs LA-ICP-MS spectra.

The glasses analysed by EPMA and fs-LA-ICP-MS, using several line scans on a micrometric scale, are characterized by a nearly homogeneous trachytic "matrix" composition (Table 2, Figs. 2,4) with high concentrations of Cs, Rb, Pb, REE, Nb, Th, U, As and Zr relative to primitive mantle composition [START_REF] Lyubetskaya | Chemical composition of Earth's primitive mantle and its variance: 1. Method and results[END_REF]. The "matrix" glass has a quartz-normative composition (Table S3). The primitive mantlenormalized pattern of the average composition is similar to the patterns of the McMurdo trachyte rocks [START_REF] Kyle | Mineralogy and geochemistry of a basanite to phonolite sequence at Hut Point Peninsula, Antarctica, based on core from Dry Valley Drilling Project Drillholes 1, 2 and 3[END_REF][START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica[END_REF][START_REF] Kelly | Geochemistry and mineralogy of the phonolite lava lake, Erebus volcano, Antarctica: 1972 -2004 and comparison with other lavas[END_REF], marine sediments [START_REF] Ben Othman | The geochemistry of marine-sediments, island-arc magma genesis, and crust mantle recycling[END_REF] and global subducting sediment (GLOSS) [START_REF] Plank | The chemical composition of subducting sediment and its consequences for the crust and mantle[END_REF].

The BIT12 glasses also show an inhomogeneous distribution of large-ion lithophile elements (LILE) (Li, B, As, Ba, Rb, Sr and LREE), which can be distinguished on the laser ablation patterns as sub-micrometric "inclusions" in the host "matrix" glass. An inclusion of the SiO2 mineral (number 4, Fig. 2, Table 2) has very low Na2O and Al2O3 contents (Na2O SiO2 and Al2O3 SiO2 contents are below detection limits by fs-LA-ICP-MS). An inclusion of Si-Al-rich glass with relatively low Na2O and high Rb (~270 ppm), Ba (~9000 ppm), Sr (~550 ppm), and Eu (3.4 ppm) contents (number 6, Table 2), is also detected by fs-LA-ICP-MS. The glass and mineral "inclusions" numbered 1 to 3 and 7 to 10 have elevated SiO2 (66-68 wt.%), Al2O3 (15-22 wt.%), Na2O (6.5-8.0 wt.%) contents comparable to alkali feldspar (Na-rich plagioclase or anorthoclase). The most abundant "matrix" glasses of trachyte composition are characterized by a Xnf/X ratio of 0.74 ± 0.04 (the Xnf/X value necessary for the estimation of amphibole-melt partitioning is the sum of the molar fractions of network-forming cations (Xnf) normalized to the molar fraction of the total number of cations (X) in the melt) according to [START_REF] Tiepolo | Trace-element partitioning between amphibole and silicate melt[END_REF]. A single Ca-rich carbonate particle in this sample was also detected using BSE-EDS (Table 1, Supplementary Dataset).

SURF is a tephra sample with fragmented volcanic particles 50-200 μm in size. Table 1, Tables S4-S6, S8 and S9 report the mineralogy in the analysed zone of this sample, which contains plagioclase (An37-57, FeO = 0.25-0.85 wt.%), magmatic Ca amphibole (Mg# 40-62, TiO2 = 3.2-5.3 wt.%), titanomagnetite (Usp 54-82), rhönite, olivine (Fo68-85, CaO = 0.3-1.0 wt.%), Ti-augites (TiO2 = 2.7-3.8 wt.%), glasses of trachyandesite, trachyte to phonolite composition (Figs. 3,S3) and dolomite (SURF-20). Moreover, abundant metamorphic Ca-Al-rich tschermakite amphiboles are identified (see Supplementary Dataset). Four Ca-Mg-Fe carbonate particles are also detected using the BSE-EDS method (Table 1, Supplementary Dataset).

CORE is a finely grained tephra sample with particles of volcanic material finer than 50 µm. Table 1 andTables S4-S8 show that the sample contains plagioclase (An33-68, FeO = 0.6-1.0 wt.%), Ti-augite (TiO2 = 2.3-5.5 wt.%), magmatic Ca amphibole (Mg# 38-76, TiO2 = 3.9-5.6 wt.%), olivine (Fo84-87, CaO = 0.2-0.4 wt.%), titanomagnetite (Usp49-70), ilmenite (Ilm0.91-1.0), glasses of trachyandesite to trachyte compositions (Figs. 3,S2), and rhönite particles. One Ca-Mg-Fe carbonate particle is detected using the BSE-EDS method (Table 1, Supplementary Dataset). Moreover, abundant metamorphic Ca-Al-rich tschermakite amphibole and four particles of Al-rich oxide of spinel group are identified (see Table S8 and Supplementary Dataset). The composition of the magmatic ilmenite particle (P171) detected and analysed by BSE-EDS method, coupled with that of titanomagnetite, yields a temperature of 1092 ± 100 °C and redox conditions slightly below the NNO buffer (from NNO-0.58 to NNO-0.19, Table S8).

Thus, in addition to typically magmatic minerals (ferrokaersutite, kaersutite, ferroedenite, and plagioclase, along with Ti-augite, apatite, anorthoclase, olivine, ilmenite), and rhönitea mineral belonging to the aenigmatite group [START_REF] Kyle | Occurrences of rhönite in alkali lavas of the McMurdo Volcanic Group, Antarctica, and Dunedin Volcano, New Zealand[END_REF] as well as quartz-normative glasses of mildly peralkaline to alkaline composition (Table 2), all tephra samples contain metamorphic phases, such as carbonates, quartz, metamorphic Ca-Al-rich amphiboles and Al-rich spinels.

Discussion

Derivation of Allan Hills tephra from a mantle plume

A magmatic system that evolves as an open system (e.g., through assimilation-fractional crystallization, magma mixing) may nevertheless provide evidence of local equilibrium. For example, the mineral and glasses found in volcanic ashes formed due to crustal assimilation and magma mixing frequently coexist in local equilibrium at the µm scale (e.g., [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013). In the investigated system, we applied partition coefficients between magmatic minerals and mafic melts, principally those of alkaline affinity, to determine whether the mineral phases could be in equilibrium with the co-existing glasses.

In contrast to mid-ocean ridge magmas, mantle plume-derived or intraplate magmas typically represent high-temperature melts that are regularly enriched in Fe and Ti (e.g., Hawaii and Iceland) [START_REF] Giehl | Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study[END_REF]. The investigated samples contain abundant differentiated glasses and associated Fe-Ti-rich minerals (e.g., Ti-augite, titanomagnetite, ilmenite and rhönite). The average TiO2 content of 0.32 ± 0.23 wt.% in the BIT12 glasses (Table 2 andTable S3) corresponds to the value obtained by assuming a theoretical

D TiO2

Amph/melt partition coefficient of 6 ± 2 according to the Xnf/X ratio described by [START_REF] Tiepolo | Trace-element partitioning between amphibole and silicate melt[END_REF]. This Ti partitioning between amphibole and melt means that the BIT12 tephra glasses are close to equilibrium with the magmatic amphiboles in BIT10, SURF and CORE, with an average TiO2 Amph content of 4.9 ± 0.6 wt.% (Table S4). The predominant "matrix" glass is also enriched in iron (average FeO glass 5.2 ± 0.6 wt.%, Table 2 and Table S3), similar to the alkaline melts produced under water-saturated magmatic conditions [START_REF] Giehl | Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study[END_REF]. According to experimental Fe partitioning between plagioclase and melt, D FeO Plag/melt = 0.14-0.22 for An40-60 [START_REF] Bindeman | Ion microprobe study of plagioclasebasalt partition experiments at natural concentration level of trace elements[END_REF], the BIT12 trachytic glasses are in equilibrium with the plagioclases An55 ± 12, with average FeO Plag content of 0.84 ± 0.25 wt.% (BIT10, SURF, CORE, Table S5). A common volcanic origin is implied by the mid-Pleistocene age of the Allan Hills tephra samples (BIT12 is estimated to have been formed

~202 ka ago [START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF], while samples BIT10, SURF, and CORE represent a tephra layer dated at ~115 ka) and the similar major element compositions of the mineral and glass particles in all tephra samples. The typical chemical features of the Allan Hills tephra glasses, mostly of trachytic to phonolitic composition, suggest that they originated from a common magma generated by mantle plume activity. The fragmented morphology and micrometric size of the analysed particles and glass fragments, as well as the typical bubble-wall appearance and fluidal texture of the glasses (BIT12, Fig. 2), suggest a Plinian-or Subpliniantype explosive eruption (e.g., [START_REF] Ponomareva | Identification of a widespread Kamchatkan tephra: A middle Pleistocene tie-point between Arctic and Pacific paleoclimatic records[END_REF][START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013).

The occurrence of olivine Fo68-87 micro-crystals in samples BIT10, CORE and SURF, which are likely from the same eruption, implies their common magmatic source. According to experimental data on equilibrium Ca partitioning between olivine and aluminosilicate melt (D CaO Ol/melt = 0.14-0.28 at Fo68-87) [START_REF] Libourel | Systematics of calcium partitioning between olivine and silicate melt: implications for melt structure and calcium content of magmatic olivines[END_REF], the Allan Hills olivines with CaO Ol = 0.3-1.0 wt.% (Table S6) were crystallized from silicate melt with CaO melt ranging from 1.1 to 5.2 wt.%. The fact that the BIT12 glasses have comparable CaO glass of 1.0 ± 0.6 wt.% (Tables 2,S3) indicates that the Allan Hills olivines (BIT10, CORE and SURF) are mostly represented by magmatic crystals close to equilibrium with the analysed BIT12 trachytic glasses. By contrast, the most CaO-rich (~ 1.0 wt.% CaO) olivine may have also crystallized from Ca-rich melt formed due to the assimilation of calc-silicate crust [START_REF] Deegan | Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: Insights from experimental petrology[END_REF][START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF].

Furthermore, the BIT12 Allan Hills trachytic glasses are also strongly enriched in trace elements such as Cs, Rb, Ba, Pb, B, La, Ce, Nb, Th, U, As and Zr compared to the primitive mantle composition (Fig. 4). A hydrothermally altered oceanic lithosphere, which is similar in composition to oceanic sediments, is enriched in volatiles (H2O, B, As), LILE (such as Rb, Ba, Cs, Pb, LREE), and high field strength elements (HFSEs; such as Al, Zr, Nb, Th, and U) relative to heavy rare earth elements (HREE). High concentrations of volatiles (B, As), LILE, and HFSE elements relative to HREE in the BIT12 glasses (e.g., "matrix" Table 2 and Table S3) are signatures of either highly altered silicate rocks or pelagic sediments, such as GLOSS.

Similar to the Icelandic hot spot [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF], an altered oceanic lithosphere is an appropriate candidate for a mantle plume source for the Allan Hills magma. This similarity suggests that the Allan Hills magma draws its origin from a mantle plume derived from a hydrothermally altered oceanic lithosphere. Indeed, the West Antarctic Rift System may be related to a hot spot activity [START_REF] Behrendt | Crustal and lithospheric structure of the West Antarctic Rift System from geophysical investigationsa review[END_REF]; the magmatic activity of Mount Erebus, Ross Island, is related to a mantle plume known as the Erebus plume [START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica[END_REF]. The Allan Hills glass compositions are different from the Cenozoic rift magmas, which are characterized by negative Pb anomalies in the primitive mantle-normalized patterns [START_REF] Rocchi | Intraplate strike-slip tectonics as an alternative to mantle plume activity for the Cenozoic rift magmatism in the Ross Sea region, Antarctica[END_REF]. Therefore, the Allan Hills tephra glasses are chemically distinct from magmatic products related to intraplate strike-slip tectonic activity. The alkalinity of the Allan Hills glasses is similar to that of the McMurdo volcanic rocks and glasses (Fig. 3). Moreover, the Allan Hills "matrix" trachytic glass displays mantle-normalized trace element patterns which are similar to those of the McMurdo trachytes (Fig. 4); in addition, the Allan Hills tephra contain a rare mineral called rhönite (Tables 1,S9, Supplementary Dataset). Similarly, the Erebus and Melbourne volcanic products in the currently active McMurdo volcanic group include rhönite-bearing-and Al-rich spinel-bearing trachytic lavas as well as pyroclastic fall deposits related to explosive volcanism, (e.g., Kyle and Prie, 1975;[START_REF] Kyle | Mineralogy and geochemistry of a basanite to phonolite sequence at Hut Point Peninsula, Antarctica, based on core from Dry Valley Drilling Project Drillholes 1, 2 and 3[END_REF][START_REF] Kyle | Erebus Volcanic Province[END_REF][START_REF] Iverson | Eruptive history and magmatic stability of Erebus volcano, Antarctica: Insights from englacial tephra[END_REF][START_REF] Kelly | Geochemistry and mineralogy of the phonolite lava lake, Erebus volcano, Antarctica: 1972 -2004 and comparison with other lavas[END_REF]. These data strongly support a hypothesis that the studied Allan Hills tephra may be genetically related to the currently active McMurdo volcanic group. Thus, the Allan Hills tephra represent mid-Pleistocene ashes produced due to intraplate magmatism, likely derived from mantle plume activity [START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica[END_REF], and likely associated with the magmatic activity of the McMurdo volcanic group, in particular, Erebus volcanic province.

Physical-chemical conditions of magma crystallization

In the Allan Hills tephra, the abundant mildly peralkaline to peralkaline glasses, together with the presence of Ca amphiboles in the volcanic tephra, and the fragmented morphology of the tephra particles (Figs. 2, S1-S3), imply an explosive eruption of the volcanic material due to saturation of the magma with an aqueous fluid, [START_REF] Martel | Magma storage conditions and control of eruption regime in silicic volcanoes: Experimental evidence from Mt[END_REF]. Application of the amphibole geobarometer equation [START_REF] Schmidt | Amphibole composition in tonalite as function of pressure: an experimental calibration of the Al-in-hornblende barometer[END_REF] to the Allan Hills amphiboles (Table S4) yields an average pressure of 0.98 ± 0.09 GPa for magma residence in a deep reservoir, likely in the presence of an H2O-CO2 fluid. Similarly, application of the amphibole geobarometer of [START_REF] Ridolfi | Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes[END_REF] implies a pressure of 1.1 ± 0.1 GPa. These high-pressure conditions suggest that the magma may have resided in a chamber under mantle lithospheric conditions (29.3 ± 2.7 km depth, Fig. 5), below the continental crust [START_REF] Behrendt | Crustal and lithospheric structure of the West Antarctic Rift System from geophysical investigationsa review[END_REF][START_REF] Cooke | An amphibole perspective into magmatic processes at Sangeang Api volcano, Indonesia[END_REF], rather than below a thick oceanic crust (e.g., Central Kerguelen Plateau) [START_REF] Borisova | Lead isotope signatures of Kerguelen plume-derived olivine-hosted melt inclusions: Constraints on the ocean island basalt petrogenesis[END_REF][START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF].

The high Ti contents in the BIT10, CORE, and SURF amphiboles (Ti C ≥ 0.5 apfu, atoms per formula unit, Table S4) and Ti-augites (X cpx CaTi up to 0.20, Table S7) reflect an enrichment in the Ca-Ti-Tschermak's molecule (CaTiAl2O6). This signature suggests a high temperature during the magmatic crystallization. Given that TiO2 contents in the amphibole vary from 3.2 to 6.8 wt.%, the graphical method of Ernst and Liu (1998) implies magmatic temperatures above 900 °C at a pressure of ~1 GPa. Calculations according to Equation (4)

from [START_REF] Féménias | Calcic amphibole growth and composition in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)[END_REF] and geothermometer of [START_REF] Ridolfi | Stability and chemical equilibrium of amphibole in calc-alkaline magmas: an overview, new thermobarometric formulations and application to subduction-related volcanoes[END_REF] suggest crystallization temperatures of 1135 ± 80 °C and 1057 ± 50 °C, respectively (Table S4). The clinopyroxene-only geothermometer indicates 1196 ± 90 °C at 0.9 GPa [START_REF] Putirka | Thermometers and barometers for volcanic systems[END_REF] (Table S7). These pressure-temperature conditions suggest deeper and hotter magmatic reservoir(s) compared to the shallow magmatic system feeding the Erebus lava lake over the last 20 kyr (Moussalam et al., 2013).

Moreover, the composition of rare ilmenite-titanomagnetite pairs from samples CORE and BIT10 (Table S8) indicates reducing conditions below the NNO buffer (NNO-1.0 to NNO-0.19, Table S8). These reducing conditions are also corroborated by the presence of rhönite containing Ti 3+ [START_REF] Kyle | Occurrences of rhönite in alkali lavas of the McMurdo Volcanic Group, Antarctica, and Dunedin Volcano, New Zealand[END_REF]. These redox conditions are comparable to those (between QFM and QFM-1.0) established by Moussallam et al. (2017) for the shallow magmatic system feeding the Erebus lava lake over the last 20 kyr. The presence of abundant Ca-Al-rich tschermakite (Table 1, Supplementary Dataset) in the tephra suggests high temperature-pressure conditions of magma residence or transport in deep crustal rocks (e.g., [START_REF] Cooke | An amphibole perspective into magmatic processes at Sangeang Api volcano, Indonesia[END_REF].

Evidence for mineral dissolution and calc-silicate crust assimilation

Calc-silicate crust xenoliths observed in different volcanic products are systematically enriched in Ca-Al-rich minerals (e.g., amphibole, anorthite), carbonates, Ca-rich glasses and SiO2-rich phases such as metamorphic quartz [START_REF] Deegan | Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: Insights from experimental petrology[END_REF][START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF]2016). Several carbonates, SiO2 particles and an inclusion of SiO2 with low Na2O contents are also detected in the BIT10 and BIT12 tephra samples (Tables 1 and2). Low Na2O and Al2O3 contents in the SiO2 "inclusion" (number 4, Fig. 2, Table 2) are detected by fs-LA-ICP-MS in the host "matrix" glass. This implies that the mineral phase is a quartz microxenocryst rather than magmatic cristobalite [START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF] crystallized from the associated melt with high Na2O and Al2O3 contents (Table 2).

The occurrence of dolomite and mylonitized limestones is documented in the Ross orogenic structures [START_REF] Kleinschmidt | The "Ross orogenic" structures in the Shackleton Range and their meaning for Antarctica[END_REF], and metamorphic calc-silicate rocks are present elsewhere in Antarctica [START_REF] Yamakawa | The crystal structure of scapolite in the Lutzow-Holm Bay region, East Antarctica[END_REF]. The presence of dolomite, other carbonates and quartz particles, as well as the "matrix" glass-hosted inclusion of quartz (number 4, Table 2 and Table S9) formed as micro-xenocrysts strongly supports crustal assimilation by the magma. Indeed, available experimental data suggest a dolomite-quartz stability limit at 500 °C at low pressures [START_REF] Müller | Metastable prograde mineral reaction in contact aureoles[END_REF]. Moreover, the thermal stability of limestone is below 820-870 °C at 0.1 MPa pressure [START_REF] Ar | Calcination kinetics of high purity limestones[END_REF]. Given that the established temperature conditions of the peralkaline magma crystallization are ~1100 °C, the crustal dolomite was also likely thermodynamically unstable with the magma at high pressure. Therefore, the crustal quartz-carbonate association may have briefly survived in the magma as crustal micro-xenoliths, (e.g., [START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF]2016), and could have been incorporated into the volcanic tephra due to very rapid ascent of the Allan Hills magma from its reservoir during an explosive (Plinian-or Subplinian-type) eruption.

The calc-silicate assimilation process is supported by the elevated concentration of Sr glass in one glass (Sr ≈ 550 ppm content in glass "inclusion" number 6) and the calculated Sr melt content up to 620 ppm (based on the Sr content in plagioclase "inclusion" 5, Table 2) in the host BIT12 "matrix" glass [START_REF] Deegan | Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: Insights from experimental petrology[END_REF][START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF]2016). Moreover, the observed Al-rich spinels are either representative of carbonate-rich metamorphic rocks (e.g., galaxite) [START_REF] Essene | Crystal chemistry and petrology of coexisting galaxite and jacobsite and other spinel solutions and solvi[END_REF] or crystallized in the melt due to incongruent dissolution of calc-silicate minerals such as anorthite [START_REF] Morgan | An experimental study of anorthosite dissolution in lunar picritic magmas: Implications for crustal assimilation process[END_REF]. The high abundance of the main Ca-rich minerals (plagioclase, augites with elevated molar fractions of Ca-Tschermak

X cpx

CaTs up to 0.1, and Ca-Al-rich tschermakite amphiboles, accounting for up to 58 rel.%, Tables 1,S7), as well as accessory dolomite and other carbonates, quartz and the Al-rich spinel particles in the tephra samples (Tables S8 andS9; Supplementary Dataset), are signatures of calc-silicate crust assimilation (e.g., [START_REF] Mollo | Carbonate assimilation in magmas: A reappraisal based on experimental petrology[END_REF][START_REF] Deegan | Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: Insights from experimental petrology[END_REF]. In addition, the observed dispersion in alkalinity of the glasses of trachyandesitic, trachytic and phonolitic to tephri-phonolitic composition (Fig. 3) may be also directly related to the carbonate-bearing material assimilation by magmas [START_REF] Mollo | Carbonate assimilation in magmas: A reappraisal based on experimental petrology[END_REF].

The compositions of the BIT12 glass "inclusion" imply inhomogeneity of the melt with respect to most major, volatile and LILE trace elements (Li, B, As, Ba, Rb, Sr, REE, Fig. 4) on a sub-micron scale, suggesting magma mixing. For example, the "inclusions" numbered 1 to 3 and 7 to 10 analysed by fs-LA-ICP-MS have elevated SiO2, Al2O3, and Na2O contents, and are variably enriched in REE, Sr, Rb and Ba (Table 2). These inclusions may be formed as glasses or as minerals due to progressive dissolution/crystallization of alkali feldspars in the melt. Indeed, the presence of anorthoclase particles in the BIT12 sample is recorded in our work (see Supplementary Dataset) and by [START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF].

Additionally, formation of Si-Al-rich glass of dacite/rhyolite composition with high Sr, Rb, Ba and Eu contents (glass "inclusion" number 6, Table 2) is likely due to the dissolution of quartz ± plagioclase, and also requires thermodynamic instability triggered, for example, by crustal assimilation. Indeed, similar rhyolite series TD 1387B-iii is recently recorded by [START_REF] Narcisi | A new Eemian record of Antactic tephra layers retrieved from Talos Dome ice core (Northern Victorial Land)[END_REF] in the Talos Dome ice core (Northern Victoria Land); and it is dated at ~116 ± 2 ka. Although such felsic glasses are regularly considered to be genetically related to an extra-Antarctic source [START_REF] Narcisi | A 16,000-yr tephra framework for the Antarctic ice sheet: a contribution from the Talos Dome core[END_REF]2016), the presence of the described inclusions within the Allan Hills trachytic "matrix" glass requires an in situ origin due to mineral (quartz ± plagioclase) dissolution during magma differentiation in the lithosphere of Antarctica. The volcanic particles might be accumulated during a single volcanic eruption and represent a result of syn-eruptive processes of mineral dissolution e.g., [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]2013). Mineral dissolution and mixing of melts by chemical diffusion can be directly related to magmatic events, in particular magma recharge and crustal assimilation (e.g., [START_REF] Deegan | Magma-carbonate interaction processes and associated CO2 release at Merapi volcano, Indonesia: Insights from experimental petrology[END_REF][START_REF] Borisova | Highly explosive 2010 Merapi eruption: Evidence for shallow-level crustal assimilation and hybrid fluid[END_REF]2016;[START_REF] Iverson | Cherecterization and correlation of englacial tephra from blue ice areas and ice cores, Antarctica[END_REF]. In the case of the Allan Hills tephras, which are chemically similar to the McMurdo volcanic group rocks and associated glasses (Fig. 3), the hypothesis of calc-silicate crust assimilation is strengthened by the presence of crustal xenoliths and xenocrysts in the Erebus and Melbourne Volcanic Province trachytes and more highly differentiated volcanics. These latter are characterized by variably radiogenic Sr isotope composition [START_REF] Wörner | The Mt. Melbourne Volcanic Field (Victoria Land, Antarctica) II Geochemistry and magma genesis[END_REF][START_REF] Kyle | Erebus Volcanic Province[END_REF][START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica[END_REF][START_REF] Wörner | Lithospheric dynamics and mantle sources of alkaline magmatism of the Cenozoic West Antarctic Rift System[END_REF], providing additional arguments for either magma mixing or crustal assimilation associated with fractional crystallization or a combination of both.

Thus, the 202 -115 ka Allan Hills magma might be derived from a mantle plume, likely currently located beneath the McMurdo volcanic group, and then accumulated in a deep magmatic reservoir at 29.4 ± 2.7 km depth. The magma was affected by calc-silicate crust assimilation in the crustal reservoir(s) under pre-eruptive and/or syn-eruptive conditions.

Alternatively, lower crust of McMurdo volcanic region composed of altered oceanic crust (which may partially include calc-silicate materials) might be partially melted by underlying hot plume to produce the trachytic melts. This hypothesis may better explain differentiated and hydrous compositions of the Allan Hills tephra, and incorporation of partially dissolved crustal materials into the magma. The magma was explosively erupted due to an excess of H2O-CO2 fluid phase in the pre-eruptive reservoir(s) (Fig. 5). The existence of such reservoir(s) may be interpreted based on the total oxide contents of the well-polished glasses measured by EPMA (Table S3, Supplementary Dataset), which indicate up to 3 wt.% volatiles (H2O-CO2) dissolved in the melts upon quenching. Such dissolved volatile contents imply the melt saturation with H2O-CO2 fluid at pressure below 0.7 GPa corresponding to crustal depths (<20 km depth) [START_REF] Iacovino | H2O-CO2 solubility in mfic alkaline magma: applications to volatile sources and degassing behavior at Erebus volcano, Antarctica[END_REF], assuming the fraction of XH2O fluid ranges from 0.1 to 0.2.

Implications and conclusions

This study provides evidence for the origin of the mid-Pleistocene Allan Hills tephra in Antarctica, which are considered to represent volcanic products due to the explosive activity of a mantle plume-derived magma. Our data indicate that the mantle plume responsible for the origin of the S27 tephra is currently located beneath the McMurdo volcanic group. The chemistry of the magmatic Ca amphiboles implies that the magma resided at a depth of 29.3 ± 2.7 km. The presence of abundant magmatic Ca-rich minerals (e.g., plagioclase and augite), metamorphic Ca-Al-rich amphiboles, Ca-Fe pyroxenes as well as dolomite and other Hills tephra sample from site S27, Table S1: Operating parameters of the laser ablation system used for the fs-LA-ICP-MS analyses, Table S2: Typical operating parameters of the ICP mass spectrometer used for the fs-LA-ICP-MS analyses, Table S3: Composition of glasses in the BIT12 sample (EPMA and CIPW norms in wt.%) and the ATHO-G reference material, Table S4: Compositions of calcic amphiboles in the tephra samples (EPMA in wt.%), formula units, Table S5: Compositions of anorthoclases and plagioclases in the tephra samples (EPMA in wt.%) The "matrix" (red diamonds) and "inclusions" (rose field) are normalized to composition from [START_REF] Lyubetskaya | Chemical composition of Earth's primitive mantle and its variance: 1. Method and results[END_REF]. The "matrix" and "inclusions" patterns are compared to the patterns of the bulk-rock composition of the McMurdo volcanic group trachyte lavas and pyroclastic rocks (labelled as McMurdo trachyte lavas) [START_REF] Kyle | Mineralogy and geochemistry of a basanite to phonolite sequence at Hut Point Peninsula, Antarctica, based on core from Dry Valley Drilling Project Drillholes 1, 2 and 3[END_REF][START_REF] Kyle | Erebus Volcanic Province[END_REF][START_REF] Kyle | Petrologic evolution of anorthoclase phonolite lavas at Mount Erebus, Ross Island, Antarctica[END_REF][START_REF] Kelly | Geochemistry and mineralogy of the phonolite lava lake, Erebus volcano, Antarctica: 1972 -2004 and comparison with other lavas[END_REF], marine sediments [START_REF] Ben Othman | The geochemistry of marine-sediments, island-arc magma genesis, and crust mantle recycling[END_REF], and global subducted sediments (GLOSS) [START_REF] Plank | The chemical composition of subducting sediment and its consequences for the crust and mantle[END_REF]. The order of elements on the plot corresponds to the increasing ratio of charge to ionic radius [START_REF] Borisova | Processes controlling the 2010 Eyfjallajökull explosive eruption[END_REF]. The arrow indicates an increase in the Sr concentration in the glass "inclusion" (number 6) compared to the average Sr content in the "matrix" glass (Table 2). S4). d

Ilmenite particles from samples BIT10 and CORE (major element compositions obtained by BSE-EDS) are used for temperature and fO2 estimations using a Fe-Ti oxide geothermobarometer as in [START_REF] Ghiorso | Thermodynamics of rhombohedral oxide solid solutions and a revision of the Fe-Ti two-oxide geothermometer and oxygenbarometer[END_REF] (see Table S8). e Metamorphic Ca-Mg-Fe or Ca-Mg carbonates are identified among the unclassified particles (P 44, P 47, P 155, P 157 in the SURF; P 149 in the CORE; P 50 and P 22 in the BIT10, and P 84 in the BIT12, see Supplementary Dataset). A dolomite particle (analysis SURF-20, Table S9) is detected by BSE-EDS and analysed by EPMA in the SURF sample (see Table S9). Four particles of Al-rich spinel are identified (P166,167,168, in the CORE sample (see Table S8; Supplementary Dataset).

Figure 1 .

 1 FIGURE CAPTIONS:

Figure 2 .

 2 Figure 2. (A) Back-scattered electron image of the BIT12 Allan Hills tephra sample

Figure 3 .

 3 Figure 3. Total alkalis versus silica (TAS) diagram (Na2O+K2O versus SiO2, wt.%)

Figure 4 .

 4 Figure 4. Pattern of the trace element compositions normalized to primitive mantle

Figure 5 .

 5 Figure 5. Scheme of the proposed model for residence and transport of the Allan

  Figure 2.

  Figure 3.

  Figure 5.

gas blank corrected ion intensities of the trace element isotopes of interest, relative to 43 Ca. Routine calculations programmed in Excel rejected obvious outliers by

  Li, 11 B, 23 Na,25 Mg,27 Al,29 Si,43 Ca, 47 Ti, 51 V, 53 Cr, 55 Mn, 57 Fe, 59 Co, 62 Ni, 65 Cu, 66 Zn, 75 As, 85 Rb, 88 Sr, 89 Y, 90 Zr, 93 Nb, 133 Cs, 137 Ba, 139 La, 140 Ce, 141 Pr, 146 Nd, 147 Sm, 151 Eu, 157 Gd, 159 Tb, 163 Dy, 165 Ho, 167 Er, 169 Tm, 173 Yb, 175 Lu, 178 Hf, 181 Ta, 208 Pb, 232 Th, and 238 U. Offline data reduction was performed by the same mode by calculating the

Table S6 :

 S6 Compositions of olivines in the tephra samples (EPMA in wt.%), TableS7: Compositions of Ti-augites in the tephra samples (EPMA in wt.%), formula units, TableS8: Compositions of titanomagnetites and Al-rich spinel in the tephra samples (EPMA in wt.%), TableS9: Compositions of apatite, dolomite, and rhönite in the

tephra samples (EPMA in wt.%), Supplementary Dataset: BSE-EDS mapping and EPMA data for the BIT10, BIT12, SURF and CORE samples.

Table 1 .

 1 Coordinates and contents of minerals and glass in the Allan Hills BIA tephra samples (Antarctica). PN = Number of particles investigated by BSE-EDS mapping. Relative% = Relative percentage by area (normalized to 100%) calculated based on the EDS analysis using the "Feature" program. N.A.-not calculated, because the particles were identified by EPMA, not by BSE-EDS . "-"-no data on the mineral or glass particles. b SiO2 particle detected by BSE-EDS mapping in the BIT10 and BIT12 samples and SiO2 inclusion analysed by fs-LA-ICP-MS in the BIT12 glassy sample. c Abundant metamorphic Ca-Al rich amphiboles are tschermakites (see Supplementary Dataset), whereas magmatic Ca amphiboles, such as ferrokaersutite, kaersutite and ferro-edenite were identified by EPMA in the BIT10, SURF, and CORE samples (see Table

	Samples

a

Acknowledgments

: The authors thank Editor Takashi Yamanouchi and several anonymous reviewers for important suggestions on the initial versions of the manuscript. The authors are grateful to Brigitte Stoll and Ulrike Weis for technical assistance with the fs-LA-ICP-MS technique and Christine Cavaré-Hester and Anne-Marie Cousin for drafting the figures. The authors thank Andrei Kurbatov, Nelia Dunbar, Nicole Elizabeth Spaulding and William C. McIntosh for providing the tephra samples. Prof. Nelia Dunbar (2MBA project) provided information on four tephra samples. The authors thank Jean-Luc Bouchez, Saskia Erdmann, Azam Soltanmohammadi, and Anne Nedelec for important suggestions on earlier versions of the manuscript. This paper incorporates data from the Masters study of Elena Zakervashevitch carried out during 2011-2012 at Toulouse University III, Toulouse, France. Dr M.S.N. Carpenter post-edited the English style and grammar.

Funding: Funding for this work was provided by INSU "Aleas" and "Cessur" (2014 -2017) in France.

carbonate minerals, Al-rich spinel, quartz xenocrysts and felsic glass "inclusions", suggests that the magma assimilated Ca-Si-Al-rich calc-silicate crustal material. The formation of Si-Al-rich glass of dacite/rhyolite composition (glass "inclusion" number 6, similar to rhyolite series TD 1387B-iii recorded by [START_REF] Narcisi | A new Eemian record of Antactic tephra layers retrieved from Talos Dome ice core (Northern Victorial Land)[END_REF] in the Talos Dome ice core, Northern Victoria Land, and dated at ~116 ka) requires an in situ origin due to mineral (quartz ± plagioclase) dissolution during magma differentiation in the lithosphere of Antarctica. The obtained data suggest that an attention should be made to interpret correctly isotopic composition of the contaminated lavas and glasses in terms of mantle plume composition.

We assume that the elevated atmospheric CO2 concentrations recorded in the Antarctica region during the mid-Pleistocene [START_REF] Higgins | Atmospheric composition 1 million year ago from blue ice in the Allan Hills, Antarctica[END_REF] might be due to at least two processes: (1) mantle plume activity and magma degassing due to saturation with a H2O-CO2 fluid phase [START_REF] Ernst | Experimental phase-equilibrium study of Al-and Ti-contents of calcic amphibole in MORB -A semiquantitative thermobarometer[END_REF][START_REF] Kelly | Geochemistry and mineralogy of the phonolite lava lake, Erebus volcano, Antarctica: 1972 -2004 and comparison with other lavas[END_REF], and (2) contact metamorphic decarbonation reactions due to interactions of the magma with calc-silicate crust. Since volcanic activity is frequently related to an elevated CO2 flux (e.g., [START_REF] Ernst | Experimental phase-equilibrium study of Al-and Ti-contents of calcic amphibole in MORB -A semiquantitative thermobarometer[END_REF][START_REF] Toutain | Structure and CO2 budget of Merapi volcano during inter-eruptive periods[END_REF], the mantle plume activity may also be responsible for the high CO2 anomalies recorded in Antarctica ice cores. Such coupling of the volcanic tephra hosted in Antarctic ice cores and the host ice core composition is a highly promising approach for developing robust models of the terrestrial climate. This represents a high priority objective for Antarctic Science according to the current scientific programs (e.g., Antarctic Roadmap Challenges) [START_REF] Kennicutt | Synthesis delivering 21st century Antarctic and Southern Ocean science[END_REF].

Supplementary Materials:

Conflicts of Interest:

The authors declare no conflict of interest. on the partition coefficients between plagioclase and melt according to [START_REF] Bindeman | Ion microprobe study of plagioclasebasalt partition experiments at natural concentration level of trace elements[END_REF] and analysis № 5 for the plagioclase. Blanks in the columns represent values below the detection limits of fs-LA-ICP-MS. 1-10 are the compositions of minerals and glass "inclusions" in the glassy matrix ("matrix"). $ Bracketed number indicates number of points per analysis.


Zhuan Zhang

Marie Cordier

Pierre H Dixneuf

Jean-François Soulé

We report herein P(III)-directed C-H bond alkenylation of (dialkyl)-and (diaryl)-biarylphosphines using internal alkynes. Chloride-free [Rh(COD)OAc)]2 acts as a better catalyst than commercially available [RhCl(COD)]2. Conditions were developed to control the mono-and difunctionalization depending on the alkyne stoichiometry. One of these novel bis-alkenylated (dialkyl)biarylphosphine was employed for the preparation of a palladium(II) complex and some of these functionalized ligands have outperformed their corresponding unfunctionalized phosphines in Pdcatalyzed amidation with sterically hindered aryl chlorides.

).

Figure 1. Relevant & Structural features of Biarylphosphines

However, the traditional synthetic approaches of biarylphosphines involve multistep synthesis with polar organometallic reagents making challenging the production of banks of multifunctional ligands to search for optimal catalytic activities. Recently, late-stage diversification-via C-H bond functionalization-was comfortably ensconced to optimize the design of pharmaceuticals, [START_REF] Yamaguchi | Emerging Synthetic Tools for Natural Products and Pharmaceuticals[END_REF] and organic materials; [START_REF] Wencel-Delord | C-H Bond Activation Enables the Rapid Construction and Late -Stage Diversification of Functional Molecules[END_REF] but it was barely employed for generation of ligand's bank. [START_REF] Zhang | Late Stage Modifications of P-containing Ligands using Transition-Metal-Catalyzed C-H Bond Functionalisation[END_REF] Since the discovery by Hartwig that regioselective C-H bond functionalization can be directed by P(III) atom, [START_REF] Shelby | Unusual in Situ Ligand Modification to Generate a Catalyst for Room Temperature Aromatic C-O Bond Formation[END_REF] this strategy has been applied to quickly modify biaryl phosphines by C-H bond borylation; [START_REF] Crawford | Phosphine-Directed C-H Borylation Reactions: Facile and Selective Access to Ambiphilic Phosphine Boronate Esters[END_REF] and by C-H bond arylation, [START_REF] Qiu | Rhodium(I) -Catalyzed Tertiary Phosphine Directed C-H Arylation: Rapid Construction of Ligand Libraries[END_REF] and C-H bond silylation. [START_REF] Wang | Rhodium(II)-Catalyzed Dehydrogenat ive Silylation of Biaryl-Type Monophosphines with Hydrosilanes[END_REF] In 2019, our group reported ortho'-C-H bond alkylation biarylphosphines allowing the preparation of 40 multifunctional phosphines with unique efficiency for catalytic carboxylation reaction. (Figure 1b). [START_REF] Zhang | Rh(I)-Catalyzed P(III) -Directed C-H Bond Alkylation: Design of Multifunctional Phosphines for Carboxylation of Aryl Bromides with Carbon Dioxide[END_REF] At the same time, Shi's group reported similar reactions, albeit using [RhCl(COD)]2. [START_REF] Wang | Rhodium-Catalyzed Direct Hydroarylation of Alkenes and Alkynes with Phosphines Through Phosphorous-Assisted C-H Activation[END_REF] They have also shown that terminal alkynes can be employed leading ortho'-monoalkenylated biarylphosphines (Figure 1c). [START_REF] Wang | Rhodium-Catalyzed Direct Hydroarylation of Alkenes and Alkynes with Phosphines Through Phosphorous-Assisted C-H Activation[END_REF] In contrast to alkenes, with which bis-ortho'dialkylation easily occurs; with terminal alkynes, only the mono-alkenylation happened. As there was no example of mono-and di-alkenylation with internal alkynes, we investigate Rh(III) and Rh(I) catalytic system to reach this target (Figure 1d). We selected Cy-JohnPhos (1a) and diphenylacetylene (2a) as model substrates to investigate the C-H bond alkenylation directed by P(III) atom (Table 1). As the first attempt, we evaluated the conditions described by Fagnou for the hydroarylation of internal alkynes (namely, 2 mol% Cp*Rh(CH3CN)3(SbF6)2 associated with PivOH in toluene) [START_REF] Schipper | Rhodium(III)-Catalyzed Intermolecular Hydroarylation of Alkynes[END_REF] but no reaction occurred (entry 1). The use of [Cp*RhCl2]2 as catalyst also failed to deliver the functionalized phosphine 3aa or 4aa, likely due to strong P(III)-Rh(III) bonds (entry 2) . We then evaluated Rh(I) catalyst, as we and others previously demonstrated their efficiency in P(III)-directed C-H bond activation/functionalization. 7d, 8, 10-11 Moreover, Rh(I) catalysts have been also employed in alkenylation of arene C(sp 2 )-H bond with alkynes using a bidental directing group [START_REF] Shibata | Rhodium -Catalyzed Alkenylation of C-H Bonds in Aromatic Amides with Alkynes[END_REF] or nitrogen group. [START_REF] Lim | Rhodium(I)-Cat alyzed ortho-Alkenylation of 2-Phenylpyridines with Alkynes[END_REF] The Shi's conditions for alkenylation with terminal alkyne afforded 3aa in only 23% yield (entry 3). The use of the Wilkinson catalyst [RhCl(PPh3)3] gave a lower yield (entry 4). However, higher activity was observed using 2 mol% chloride-free [Rh(OAc)(COD)]2 catalyst affording the mono-alkenylated phosphine 3aa in 71% among with 21% of bis-alkenylated phosphine 4aa (entry 5). Interestingly, in the presence of a base, the formation of the bis-ortho'-alkenylated phosphine 4aa can be decreased, and the best result was obtained using 0.25 equivalent of KOAc affording 3aa in 86% isolated yield (entries 6-10). To get the bis-ortho'alkenylated phosphine 4aa selectively, we increased the amount of diphenylacetylene (2a) (entries 11 and 12). In the presence of 3 equivalents of 2a and 4 mol% of Rh catalyst, 4aa was isolated in 75% yield (entry 13). Using 0.5 equiv. of KOAc, the isolated yield can rise up to 78% (entry 14). [Rh(OAc)](COD)]2 KOAc (0.25) 55 43 12 [b] [Rh(OAc)](COD)]2 KOAc (0.25) 30 62 13 [b,c] [Rh(OAc)](COD)]2 KOAc (0.25) 15 75 14 [b,c] Having determined the best reaction conditions for the mono-and di-alkenylnation of biarylphosphine through P(III)-assisted C-H bond activation, we turned our attention to the scope of the reaction. Firstly, we investigated the mono-functionalization using 2 mol% of [Rh(OAc)(COD)]2 associated with 25 mol% KOAc in toluene at 120 ºC, and biarylphophine/alkyne 1:1 ratio (Scheme 1).

Table 1. Optimization of the Reaction Conditions

The reaction of biphenyl bearing (diphenylphosphino), or (di-iso-propylphosphino) with diphenylacetylene have afforded the corresponding phosphine-alkene ligands 3ba, 3ca in 76% and 81% yield, respectively. In contrast to the previous report on Rh(I)catalyzed hydroarylation of terminal alkynes, [START_REF] Wang | Rhodium-Catalyzed Direct Hydroarylation of Alkenes and Alkynes with Phosphines Through Phosphorous-Assisted C-H Activation[END_REF] in which a sterically hindered t-Bu-JohnPhos (1d) is not reactive, the reaction with diphenylacetylene lead to the formation of mono-alkenylated phosphine 3da in 54% yield. Ortho'substituted biarylphosphines such as MePhos (1e) and DavePhos (1f) also underwent C-H bond alkenylation at ortho'-position affording the phosphines 3ea and 3fa in good yields.

Rh(I)-catalyzed ortho'-C-H bond alkenylation of 2-dicyclohexylphosphino-4-(N,Ndimethylamino)-1,1'-biphenyl (1g) afford 3ga in 79% yield. These reaction conditions are tolerant to heteroaromatic moieties. Indeed (cataCXium®P) with N-phenylpyrrole scaffold -developed by Beller's group for cross-couplings of aryl chlorides- [START_REF] Rataboul | New Ligands for a General Palladium-Cataly z ed Amination of Aryl and Heteroaryl Chlorides[END_REF] is alkenylated on the phenyl unit to provide the phosphino-alkene 3ha in 77% yield. CM-Phos developed by Kwong for cross-coupling reactions with aryl mesylates, [START_REF] So | Palladium-Catalyzed Amination of Aryl Mesylates[END_REF] undergoes indolinyl C3-H bond alkenylation to afford the 3ia in moderate yields. Less reactive hex-3-yne (2f) and dec-5-yne (2g) were also suitable substrates in C-H bond alkenylation affording the novel phosphino-alkene ligands 3bf, 3hf, 3ag, and 3bg in good yields. However, the use of non-symmetrical alkynes (e.g., ethyl 3-phenylpropiolate or 1-phenyl-1propyne) led to non-harvestable mixtures of isomers.

We then explored the difunctionalization of biarylphosphine ortho' C-H bonds using an excess amount of internal alkynes and 4 mol% of [Rh(OAc)(COD)]2 associated with 50 mol% KOAc in toluene at 120 ºC (Scheme 2).

The steric factor of phosphorus substituent appears critical. The (diphenylphosphino), or (di-iso-propylphosphino) were efficient directing groups in the bis-ortho'-alkenylation affording 4ba and 4ca in 60% and 74% yield respectively. While the reaction with tBu-JohnPhos (1d) failed to deliver the difunctionalized phosphine 4da and we only observed the formation of monofunctionalized phosphine 3da in 54% yield. The introduction of a strong electron-donating group, such as NMe2, did not affect the reactivity as the dialkenylated product 4ga was isolated in 70% yield. The two-fold-C-H bond activation/functionalization of cataCXium®P (1h) afford 4ha in 75% yield. Bis-C-H bond alkenylation of 1a were also carried out with other symmetrical para-substituted diarylacetylene (i.e., R = Br, Me, OMe) affording 4ab-4ad in 73-75% yields. (CataCXium®P) with N-N-phenylindole scaffold 1j was bis-alkenylated on the phenyl unit to provide the phosphine-bisalkene 4ja in 78% yield.

Scheme 2. Scope of Rhodium(I)-Catalyzed Two-Fold ortho', ortho' C-H Bonds Alkenylation of Biarylphosphines with Alkynes

To estimate the coordination ability of phosphine 4aa, we prepared the palladium(II) complex 5a (Scheme 3). Mixing ligand 4a and PdCl2(PhCN)2 in CH2Cl2 at room temperature led to the complete formation of complex 5a as indicated by 31 P{1H} NMR with a downfield shift of the phosphorus atom from δ = -7.7 ppm in the free phosphine to δ = 67.0 ppm in the complex. In the X-ray crystallographic structure, complex 5a was found to be a dimeric structure through a chloro-bridge unit with a distorted square planar coordination geometry. The Pd-Pd distance of 3.50 Å was somewhat longer than those of the corresponding SPhos compound (2.24 Å) and the Pd-P distance of 2.26 Å was similar to the SPhos-Palladium compound (2.25 Å). In contrast to JohnPhos and SPhos, which adopted a closed conformation, 17 5a presents an open conformation, i.e., the orientation of the secondary aryl ring is pointing away from the metal center, avoiding a metal-arene interactions. This conformation switch might be due to the introduction of the very bulky 1,2diphenylvinyl substituents. Finally, we would to demonstrate the positive effect of the introduction of alkenyl group on biarylphosphines by comparing the catalytic activity of a couple of these novels (bis)-alkenylated (dialkyl)biarylphosphines with their parent phosphine often used in Pd-catalyzed crosscoupling reaction. Aryl chlorides are much less reactive substrates in Pd-catalyzed cross-coupling reactions, albeit they are more abundant and less expensive than bromide or iodide equivalents. Their cross-coupling with amides remains challenging as specific phosphine ligands have to be designed to promote the oxidative addition to aryl chlorides and reductive elimination of an amidate ligand from Pd(II) centers which can generate κ2amidate palladium intermediate complexes (A, Scheme 4) resulting in the inhibition of reductive elimination and by the way catalytic efficiencies. [START_REF] Ikawa | Pd-Catalyzed Amidations of Aryl Chlorides Using Monodentate Biaryl Phosphine Ligands: A Kinetic, Computational, and Synthetic Investigation[END_REF] We selected electron-rich and sterically hindered 2-chlorotoluene and acetamide as model substrates to evaluate the potential of our bifunctional biarylphosphines in Pd-catalyzed amidation (Scheme 4).

In the presence of 2.5 mol% Pd(dbda)2/2phosphine and 2 equivalents of Cs2CO3 in t-BuOH, the commercially available (dicyclohexyl)biarylphosphines [JohnPhos (1a), MePhos (1e), DavePhos (1f), DavePhos analogue (1g) cataCXium®Pin (1i) and XPhos] were almost inactive. Interestingly, the use of phosphinoalkene hybrid ligands 3ea, 3fa, or 3ia gave the desired N-arylamide 6a in 15-37% yields. Surprisingly higher yields were observed using the phosphines bearing two alkenyl units 4aa and 4ga leading to the formation of 6a in 95% and 82% yield, respectively. This positive rise of activity is expected to arise from the presence of the very bulky alkenyl group at both ortho'-positions which might stabilized Pd intermediates by noncovalent interactions [START_REF] Mahmudov | Noncovalent Interactions in Metal Complex Catalysis[END_REF] or/and may avoid the formation of κ2-amidate palladium intermediate complexes (A).

Using the most active bifunctional phosphine 4aa, we confirmed that cross C-N bondcoupling nicely occurred affording the aryl amide 6b-6f in high yields (Scheme 4 , bottom).

Scheme

4.

Application of Bis-Alkenylated (Dialkyl)biarylphosphine in the Pd-Catalyzed Amidation of Aryl Chlorides.

In summary, we report the first example of C-H bond alkenylation of aromatic C-H bonds of biarylphosphines with internal alkynes through P(III)-directed the C-H bond cleavage with Rh(I)(OAc) catalysts. The use of KOAc associated with chloride-free Rh(I) catalyst allowed the mono-functionalization or the difunctionalization depending on the stoichiometry of reaction. A new class of phosphino-alkene(s) ligands with different steric and electronic properties are obtained in good yields. The introduction of these very bulky vinyl substituents induces a conformation switch of palladium complex from closed to open form. A significant consequence for the catalytic performance is that monophosphines that have typically been ineffective in the amidation of aryl chlorides were morphed into ligands that create highly active palladium catalysts in amidation of ortho-substituted aryl chlorides. Applications of the developed ligand libraries and more detailed coordination studies are currently ongoing in our laboratory.

The Supporting Information is available free of charge on the ACS Publications website. Crystallographic information file (CIF) for CCDC 968419 (3aa) Crystallographic information file (CIF) for CCDC 1968421 (4aa) Crystallographic information file (CIF) for CCDC 1982983 (5a) Additional procedures and data including characterizations, crystal structures, and NMR results (PDF)

Figure 2 .

 2 Figure 2. Late-stage Diversification of Biarylphosphines

  [Rh(OAc)](COD)]2 KOAc (0.5) 10 78 [a] 2 equiv. of 2a; [b] 3 equiv. of 2a , [c] 4 mol% of Rh catalyst

Scheme 1 .

 1 Scheme 1. Scope of Rhodium(I)-Catalyzed Ortho' C-H Bond Alkenylation of Biarylphosphines with Alkynes

Scheme 3 .

 3 Scheme 3. Stabilization of Pd(II) with the Bulky Phosphine ligand 4aa

  s c r i p t

We thank the CNRS, UR1 for providing financial support and ZZ acknowledge the China Scholarship Council (CSC) for his PhD grant (201706780007).