Water content in the Martian mantle: A Nakhla perspective

Accepted Manuscript Water content in the Martian mantle: a Nakhla perspective Franz A. Weis, Jeremy J. Bellucci, Henrik Skogby, Roland Stalder, Alexander A. Nemchin, Martin J. Whitehouse PII: DOI: Reference:

S0016-7037(17)30337-X http://dx.doi.org/10.1016/j.gca.2017.05.041 GCA 10311

To appear in:

Geochimica et Cosmochimica Acta

Received Date: Accepted Date:

13 February 2017 27 May 2017

Please cite this article as: Weis, F.A., Bellucci, J.J., Skogby, H., Stalder, R., Nemchin, A.A., Whitehouse, M.J., Water content in the Martian mantle: a Nakhla perspective, Geochimica et Cosmochimica Acta (2017), doi: http:// dx.doi.org/10.1016/j.gca.2017.05.041

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Water content in the Martian mantle: a Nakhla perspective Franz A. Weisa,b*, Jeremy J. Bellucci a, Henrik Skogbya, Roland Stalderc, Alexander A. Nemchina,d, Martin J. Whitehousea a

Swedish Museum of Natural History, Dept. of Geosciences, Box 50007, SE-104 05

Stockholm, Sweden, [email protected], [email protected], [email protected], [email protected] b

Uppsala University, Dept. of Earth Sciences, Center of Experimental Mineralogy, Petrology

and Geochemistry (CEMPEG), SE-752 36 Uppsala, Sweden, [email protected] c

Innsbruck University, Inst. for Mineralogy and Petrography, A-6020 Innsbruck, Austria,

[email protected] d

Curtin

University,

Dept.

of

Applied

Geology,

Perth,

WA

6845,

Australia,

[email protected]

*corresponding author Word count: Abstract: 289 Main text: 6869 Acknowledgements: 54 Figure captions: 542

Keywords: Nakhla, hydrogen in NAMs, Mars, Martian water, magmatic water content

Abstract Water contents of the Martian mantle have previously been investigated using Martian meteorites, with several comprehensive studies estimating the water content in the parental melts and mantle source regions of the shergottites and Chassigny. However, no detailed 1

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studies have been performed on the Nakhla meteorite. One possible way to determine the water content of a crystallizing melt is to use the water content in nominally anhydrous minerals (NAMs) such as clinopyroxene and olivine. During or after eruption on the surface of a planetary body and during residence in a degassing magma, these minerals may dehydrate. By reversing this process experimentally, original (pre-dehydration) water concentrations can be quantified. In this study, hydrothermal rehydration experiments were performed at 2 kbar and 700 °C on potentially dehydrated Nakhla clinopyroxene crystals. Rehydrated clinopyroxene crystals exhibit water contents of 130 ± 26 (2σ) ppm and are thus similar to values observed in similar phenocrysts from terrestrial basalts. Utilizing clinopyroxene/melt partition coefficients, both the water content of the Nakhla parent melt and mantle source region were estimated. Despite previous assumptions of a relatively dry melt, the basaltic magma crystallizing Nakhla may have had up to 1.42 ±0.28 (2σ) wt. % H2O. Based on an assumed low degree of partial melting, this estimate can be used to calculate a minimum estimate of the water content for Nakhla’s mantle source region of 72 ±16 ppm. Combining this value with values determined for other SNC mantle sources, by alternative methods, gives an average mantle value of 102 ± 9 (2σ) ppm H2O for the Martian upper mantle throughout geologic time. This value is lower than the bulk water content of Earth’s upper mantle (~250 ppm H2O) but similar to Earth’s MORB source (54-330 ppm, average ~130 ppm H2O).

1. Introduction The presence of liquid water on Earth has been an important factor for developing features that clearly distinguish it from other planets in the inner Solar System, including a clear distinction between the continental and oceanic crust and formation of plates in the lithosphere, which have remained in sustained motion for billions of years (Martin et al.,

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2008) and the presence of life. Therefore, any differences in tectonic styles, magmatic differentiation, and the ability to support life are intertwined with the hydrological cycle on a planetary body. For example, Mars has inferred surface water in the form of brines (MartinTorres et al., 2015) and frozen water at the poles. However, the quantity of water in the Martian mantle and how it compares to Earth remains an outstanding question. This question can be investigated using Martian meteorites, which are the only physically accessible samples of Mars at present. Martian meteorites are divided into three main groups: shergottites (basalts), nakhlites (clinopyroxene cumulates) and chassignites (dunites) known collectively as the SNCs. In addition, two samples fall outside these main groups; orthopyroxenite ALH84001 and a regolith breccia represented by several paired stones, including the most studied NWA 7034 and NWA 7533. All these meteorites cover almost the entire history of Mars ranging in age from 4.4 Ga to 160 Ma (e.g., Nyquist et al., 2001; Agee et al., 2013; Humayun et al., 2013). A simple approach to quantify the water content of the Martian mantle is to estimate the magmatic water contents of parental melts of available Martian rocks (e.g., McCubbin et al., 2010a, 2012). Results from previous studies for water contents in the SNC parental melts range from 0.1 to 2.83 wt. % (Karlsson et al., 1992; Leshin et al. 1996; Dann et al., 2001, Boctor et al., 2003; McCubbin et al., 2010a, 2013; Usui et al., 2012), similar to magmatic water contents on Earth (0.1 to >6.0 wt. %; e.g., Danyushevsky, 2001; Wallace, 2005; Plank et al., 2013). Several methods have been used to determine pre-eruptive magmatic water contents in the parental melts for shergottites and Chassigny, such as the analysis of phase assemblages and melt compositions (e.g., Dann et al., 2001), water contents of apatite crystals (e.g., McCubbin et al., 2012), or the measurement of volatile contents directly from quenched volcanic glass or melt inclusions and minerals within them (e.g., Boctor et al., 2003; McCubbin et al., 2010a; Usui et al., 2012). To date, however, no detailed study has

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focused on the quantification of magmatic water in the Nakhla parental melt. Previously, hydrogen extraction studies and bulk rock analysis for Nakhla revealed whole-rock water contents averaging around 0.1 wt. % (e.g., Karlsson et al., 1992; Leshin et al., 1996; Bridges and Warren, 2006). However, these studies do not account for possible effects of magma degassing or dehydration after solidification. Additionally, some of the approaches previously used for other SNC meteorites cannot be universally employed. Melt inclusions, for example, may not always be present at a size suitable for analysis or they may have undergone varying degrees of water loss or gain (e.g., Baker, 2008; Esposito et al., 2014; Le Voyer et al., 2014). Similarly, the water content in apatite is easily affected by changes in the melt’s water content (Ustunisik et al., 2011; Boyce et al., 2014) and estimated water partition coefficients for apatite and a basaltic melt vary (Nakamura et al. 1982; McCubbin et al., 2010b, 2014; Vander Kaaden et al., 2012). Lastly, it is critical to know the degree of crystallization before apatite formation, which is difficult to estimate in lavas such as Nakhla that underwent fractional crystallization (Stockstill et al., 2005; Bridges and Warren, 2006). An alternative method to determine the water content of a magma is to use nominally anhydrous minerals (NAMs) such as clinopyroxene and olivine, which are more ubiquitously present in igneous rocks (e.g., Aubaud et al., 2004; Wade et al., 2008; Nazzareni et al., 2011; Okumura, 2011; Weis et al., 2015). Olivine and clinoyroxene were also among the first minerals to crystallize in the Nakhla melt (e.g., Stockstill et al. 2005). NAMs incorporate small amounts of protons as charge balance associated with cation vacancies or coupled substitutions (forming OH-groups with oxygen of the crystal lattice) during growth from a hydrous magma. A significant advantage of using NAMs such as clinopyroxene is the ability to experimentally rehydrate them after potential dehydration during emplacement on or ejection from the surface of a planetary body, yielding more accurate, primary OH- concentrations (Weis et al., 2015; 2016). Employing mineral/melt

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partition coefficients based on crystal chemistry (e.g., O’Leary et al., 2010) subsequently allows determining magmatic water contents. Here the water contents in clinopyroxene and olivine phenocrysts from the Nakhla meteorite have been measured by Fourier Transform Infrared Spectroscopy (FTIR). Additionally, rehydration experiments were performed on Nakhla clinopyroxene to quantify any potential degassing effects and the rehydrated crystals were used to determine a value for magmatic water of the Nakhla parent melt. That value was then used to calculate the water content of the Martian mantle from where Nakhla was derived. Lastly, this value has been compared to all available literature data for the SNC meteorites to quantify the range and present the most comprehensive value for the water content of the Martian mantle, to date.

2. Sample Nakhla is a clinopyroxenite that fell in Egypt on June 28th, 1911 and was immediately recovered, limiting the possibility of significant terrestrial contamination. The meteorite contains olivine, clinopyroxene, un-shocked plagioclase, mesostasis K-feldspar, oxides, FeS, and chalcopyrite (e.g., Harvey and McSween, 1992). The clinopyroxene is zoned augite with Fe enrichment at the rims and the olivine is fayalite (Harvey and McSween, 1992; Treiman 1990). Nakhla is a cumulate rock similar to “Theo’s flow” which is an extrusive Archean cumulate package on Earth (Friedman et al., 1995). The portion of the cumulate pile from which Nakhla is thought to have been derived has estimated cooling rates of 0.0005-0.04 °C/hr (Mikouchi & Miyamoto, 2002), and also could have had interaction with metasomatic fluids. The crystallization age of Nakhla is 1.38±0.08 Ga (Shih et al., 2010) and it was ejected from the vicinity of the Martian surface at 10.8 ±0.8 Ma with shock pressures estimated to be less than ~15 GPa (Eugster et al., 2002; Stöffler et al., 1991; Greshake et al., 2004).

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3. Background and experimental approach Hydrogen in NAMs is incorporated by structural defects such as cation vacancies (e.g. 2H+vs. Mg2+) and charge deficiencies (e.g. Al3+ + H+ vs.Si4+) where it is bonded to oxygen (OH-) and, regarded as an oxide component, which can be expressed as water concentration (i.e., wt. ppm H2O). Upon ascent from the mantle or during volcanic eruptions NAMs in mantle derived magmas (e.g., SNCs) can dehydrate by hydrogen diffusion out of the crystals via the relatively fast and reversible redox reaction OH- + Fe2+ ↔ O2- + Fe3+ + ½ H2 (Eq.1) (e.g., Skogby, 1994; Ingrin et al., 1995, Stalder and Skogby, 2007; Sundvall et al., 2009; Weis et al., 2015, 2016; Liu et al. 2016). Importantly, the Fe in this reaction is not a medium that creates or resets hydrogen-associated defects, but serves solely as a charge-balancing medium upon dehydration. This redox reaction is the main mechanism for hydrogen exchange as well as dehydration of the crystals as the charge balance can easily be retained. The reduction or oxidation of Fe occurs in almost a 1:1 relationship with the hydrogen incorporation or loss (Weis et al. 2015, 2016). Sometimes it is difficult to observe this 1:1 mechanism with exact values since the concentration of hydrogen is always very small compared to Fe in the crystals. It is noteworthy that a close relationship between the content of tetrahedral trivalent cations in clinopyroxene crystals and hydrogen incorporation has also been observed. Hydration experiments on synthetic, dry crystals grown under anhydrous conditions have shown a near 1:1 correlation for H incorporation and the amount of available tetrahedral trivalent cations indicating explicitly charge compensation (Skogby 1994). Hydrogen incorporation still was governed by reduction of Fe3+ and in some cases more Fe3+ was reduced than hydrogen incorporated, thereby limiting the hydrogen intake to the available defects rather than available Fe3+. The experiments on dry synthetic crystals showed that charge imbalances, and specifically tetrahedral ones, are preferred for hydrogen incorporation as soon as small amounts of hydrogen are available. Considering a natural volatile-bearing

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melt it can thus be assumed that likely unbalanced tetrahedral and octahedral defects will be associated with hydrogen. Various studies have shown that hydrogen diffusion out of Fe-rich clinopyroxene crystals following the redox reaction occurs within days to minutes at temperatures between 600 and 1000 °C (see reviews by Ingrin and Blanchard, 2006 and Farver, 2010). The hydrogen-associated defects in the crystal structure, however, will remain after the redoxdehydration since reaction kinetics for vacancy and cation diffusion are many orders of magnitude slower than the redox-processes (e.g., Ingrin et al., 1995; Cherniak and Dimanov, 2010). NAMs, especially clinopyroxene phenocrysts in volcanic rocks, are therefore, expected to keep a “memory” of the hydrogen content during original crystallization from a magma. Thus, potentially dehydrated clinopyroxene crystals can be rehydrated by thermal annealing in hydrogen gas or under hydrothermal pressure, since the redox-reaction is to large extents reversible (e.g., Skogby & Rossman 1989; Sundvall et al. 2009; Weis et al., 2015, 2016). Specifically, rehydration experiments under hydrothermal pressures of 2 to 3 kbar seem to be able to restore water contents in clinopyroxene crystals that then provide a reasonable estimate for magmatic water contents at the depth of clinopyroxene crystallization (Weis et al., 2016). While temperature is important regarding the kinetics of redox-reaction (Eq.1), pressure is an important parameter to reconstruct the potential initial water content of crystals at depth as with lower pressure in volcanic systems clinopyroxene crystals begin to lose their volatile hydrous component. The pressure chosen for experiments in this study is 2 kbar which represents conditions within the upper to middle crust for much of Mars (Turcotte and Schubert, 1982). Furthermore, this pressure was chosen on the basis of studies that propose 1 to 2 kbar for crystallization conditions of the Nakhla melt (Bridges and Warren 2006, Filiberto et al. 2014, Harvey and McSween 1992). Various studies provide an estimate for temperatures between 1100 and 700 °C under 7

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which Nakhla crystallized (Mikouchi and Miyamoto 2002, Treiman 2005, Filiberto et al. 2014). However, when performing rehydration experiments it is explicitly not intended to fully reconstruct conditions in the magma chamber, but only to restore the “memory” of the dehydrated crystal. The experimental temperature of 700 °C was chosen since the hydrogen diffusion coupled to redox reaction Eq.1 has been shown to be active with convenient kinetics for laboratory experiments (Skogby and Rossman, 1989; Skogby, 1994; Bromiley et al., 2004; Koch-Müller et al., 2007; Sundvall and Skogby, 2011). Clinopyroxene crystals in this study show XFe/(Fe+Mg) > 0.18 (Table 1) and thus the kinetics for redox reaction Eq.1 are similar to the kinetics for hydrogen self-diffusion (Ingrin and Blanchard, 2006). Under the set temperature conditions the reaction kinetics for hydrogen self-diffusion are logD = -11.5 and hence at least 4.5 orders of magnitude higher than for vacancy and cation diffusion (logD ≤ -16). Cation and vacancy diffusion involve the resetting of structural defects associated with hydrogen incorporation (Ingrin and Skogby, 2000; Ingrin and Blanchard, 2006; Cherniak and Dimanov, 2010), which has to be avoided in a rehydration experiment. Yet, for the given experimental temperature and time intervals, resetting of structural defects is therefore not expected. An unknown, potentially critical factor in the approach taken here is the effect of shock on any structural defects in clinopyroxene. However, Nakhla was ejected with limited shock pressures <~15 GPa (Stöffler et al., 1991; Greshake et al., 2004) and typical features of intense shock such as granulated rims, clinopyroxene twinning, and maskelynite are absent. Therefore, chemical consequences of shock in Nakhla are assumed to be minor (Treiman, 2005), which thus also implies a negligible effect on the hydrogen-associated defects such as cation vacancies. Furthermore, new experiments support that there is no effect of shock on hydrogen associated defects in clinopyroxene (Peslier et al., 2016). Consequently rehydrated

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crystals analyzed here may be used as a proxy for the magmatic water content of the Nakhla parent melt and the Martian mantle.

4. Methods 4.1. Electron probe micro analysis Analyses of major elements (Al, Ti, Fe, Mg, Na, K, Si, Ca, Mn, Cr) in clinopyroxene and olivine crystals were carried out at the Department of Earth Sciences, Uppsala University using a Field Emission-EPMA JXA-8530F JEOL hyperprobe. For clinopyroxene, the analysis was done after annealing experiments on crystal Nakhla cpx1 (Table 1) while for olivine a separate untreated crystal was taken. Five to six spots were analyzed on the crystals (Fig.1) using a beam current of 10 nA with an acceleration voltage of 15 kV with 10 seconds on peak and 5 seconds on lower and upper background. Standards used were fayalite for Fe, magnesium oxide (MgO) for Mg, pyrophanite (MnTiO 3) for Mn and Ti, aluminum oxide (Al2O3) for Al, wollastonite (CaSiO3) for Ca and Si, chromium oxide (Cr2O3) for Cr, nickel oxide (NiO) for Ni as well as albite NaAlSi3O8, orthoclase (KAlSi3O8) and apatite Ca5(PO4)3(OH,F,Cl) for Na, K and P, respectively. For each crystal an average composition was calculated from all analyzed spots. A detailed description of the EPMA analysis and the analytical uncertainties is presented in Barker et al. (2015). From the obtained weight percentages, the number of atoms per formula unit in each crystal was calculated on the basis of a four cation-normalization. In order to distinguish between and quantify Fe2+ and Fe3+ in clinopyroxenes, results from Mössbauer analysis were applied (see section 4.3).

4.2. Re-hydration under pressure

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For pressure experiments, selected crystals were welded into Au-capsules with an outer (inner) diameter of 5.0 (4.6) mm together with 12 µl of water. To prevent any dissolution of the clinopyroxene during the experiments, about 2 mg of diopside powder (CaMgSi2O6) were added to the capsule. Au-capsules were sealed using a Lampert PUK U3 welding device (equipped with tungsten electrode, flushed with argon gas). Possible leaks causing water loss were identified by weighing the capsules before and after heating in an oven at 120 °C for 15 min. Pressure treatment was performed in Rene 41 steel-bombs in cold seal pressure vessels at Innsbruck University using water as pressure medium. All experiments were performed at a temperature of 700 °C, a pressure of 2 kbar and time intervals of 66 and 96 hours (Table 2). The temperature was measured by Ni-CrNi thermocouples and pressures were measured with a Heise gauge and kept constant within 0.05 kbar during the whole run duration. The rehydration experiments were not buffered in order to control the oxygen fugacity. However, due to the hedenbergite component in the clinopyroxene crystals (see Table 2) we assume that the oxygen fugacity is controlled by the hedenbergite-magnetitequartz buffer, which imposes redox conditions similar to QFM (cf. Gustafson, 1974; Xirouchakis and Lindsley, 1998). Water fugacities for pressure experiments behave in a linear relation to and do not deviate strongly from the nominal pressure (cf. Pitzer and Sterner, 1994).

4.3. FTIR spectroscopy Individual clinopyroxene and olivine crystals of a size suitable for analysis (≥ 300 μm) were hand-picked under a binocular microscope. The crystals were then mounted in thermoplastic resin for further processing. With the help of crystal morphology and optical 10

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microscopy (extinction angles), the selected clinopyroxene crystals (n = 3) were oriented along their crystallographic c-axis and their (100) and (010) crystal faces, on which the directions of the main refractive indices (α, β and γ) occur. A detailed procedure of the crystal alignment is described in Stalder and Ludwig (2007). An orientation of the selected olivine crystal was not successful due to the lack of morphology and the overall straight extinction. Various particle size-grades of Al2O3-grinding paper (final grade 2 μm) were used to thin and polish the crystals to thicknesses between 150 and 350 μm. Before and after annealing experiments, polarized FTIR spectra in the range 2000-5000 cm-1 were acquired on the oriented clinopyroxene crystals along the directions of the main refractive indices (α, β and γ) to obtain the total absorbance: Atotal=Aα+Aβ+Aγ. Since it was not possible to orientate the olivine crystal properly, only unpolarized spectra were acquired. The polished untreated crystals were analyzed with a Bruker Vertex 70 spectrometer equipped with a NIR source (halogen lamp), a CaF2 beamsplitter, a wiregrid polarizer (KRS5) and an InSb detector. After annealing experiments, crystals were polished again to remove any remaining diopside powder. Subsequently, the spectra were measured using a Bruker Hyperion 3000 microscope equipped with a Globar source, a KBr beamsplitter and a MCT detector. Crystal thickness varied between 150 and 350 μm for both the (100) and (010) orientations. Cracks and inclusions in the crystals were avoided by applying small apertures (100 to 200 μm) for masking during analysis. For each individual spectrum, 128 scans with a spectral resolution of 4 cm-1 were performed and averaged. The obtained spectra were baseline corrected by a polynomial function and the individual OH bands were fitted with the software PeakFit. The corresponding water contents were then calculated using both the wavenumber-dependent calibration function established by Libowitzky and Rossman (1997) and the mineral-specific (augite) calibration of Bell et al. (1995).

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4.4. Mössbauer spectroscopy The oxidation states of Fe in pristine Nakhla clinopyroxene and olivine were obtained by Mössbauer spectroscopy. A powdered single crystal for each mineral was analyzed using a

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Co point source (active diameter 0.5 mm). The powdered single crystal

was mixed and ground with thermoplastic resin and formed into a ~1 mm3 cylinder that was mounted on a strip of tape for analysis with the point source. The Mössbauer measurements were performed at incident angles of 90° to the γ-rays. The obtained spectrum was calibrated against an α-Fe foil, folded and reduced from 1024 to 512 channels. The spectral fitting was done with the Mössbauer spectral analysis software MossA (Prescher et al., 2012). During the fitting process, two doublets were assigned to Fe2+ and one doublet to Fe3+ in the octahedral positions. From the area of the doublets, the percentage of each oxidation state relative to the total Fe content of the sample was obtained, assuming similar recoil-free fractions for Fe2+ and Fe3+. Due to high baseline counts (> 6 x106) the estimated analytical error for the obtained Fe m+/Fetot ratio is ± 0.4 %. No crystal specific compositional analysis was carried out on the Mössbauer sample due to the destructive nature of sample preparation. Due to a very weak Fe3+ peak in olivine, the parameters for centroid shift and quadrupole splitting were constrained to values previously determined by Dyar (2003).

4.5. Magmatic water content calculation Parental magmatic water contents of an equilibrium magma from which the studied clinopyroxene crystals formed were calculated using the obtained clinopyroxene water contents after rehydration and utilizing appropriate partition coefficients for H2O. To calculate partition coefficients for water between clinopyroxene and basaltic melt, the chemical data from the EPMA and the equation of O’Leary et al. (2010) was employed 12

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(lnD=-4.2(±0.2)+6.5(±0.5)VI[Al3+]-1.0(±0.2)[Ca2+]). This equation is specifically designed for Carich clinopyroxene and is based on the amount of tetrahedral aluminum that is strongly interlinked with hydrogen incorporation into clinopyroxene due to charge balancing processes. Parental magmatic water contents were then calculated for each individual clinopyroxene crystal (n = 3) and subsequently, an average was produced for the whole rock sample. Since during the sample preparation the rims of our Nakhla clinopyroxenes were lost during polishing, the calculated magmatic water contents are based on the core composition of the crystals, thus representing magmatic H2O at an early stage of crystallization.

5. Results 5.1. Electron probe micro analysis The clinopyroxene and olivine compositional data and structural formulas obtained by EPMA are shown in Table 1. Both analyzed crystals show no zonation and structural formulas were calculated based on an average composition from all analyzed spots on each crystal. The analyzed clinopyroxene crystal from the Nakhla meteorite is an augite (Wo39.5En37.0Fs23.5; Mg# = 62) while the analyzed olivine is a fayalite (Fa60.0Fo38.7Tp1.3); Mg# = 39).

5.2. Pressure experiments, FTIR and magmatic water contents Prior to rehydration experiments all three Nakhla clinopyroxene crystals did not show water contents above the detection limit of ~10 ppm. The same was observed for the analyzed olivine crystal (Fig.2). After thermal annealing, water contents in the clinopyroxene crystals rose to values between 126 and 134 ppm (Table 2). The absorption bands increased significantly in height in all three directions (α, β, and γ) after thermal annealing. The

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strongest increase in absorbance was notably always observed for the band at 3630 cm-1. Values determined with the mineral-specific calibration by Bell et al. (1995) were about 25 % higher, but the authors note that their mineral-specific (augite) calibration is primarily valid for samples with similar OH-spectra. The augite spectra do not show a major absorption band at ~3460 cm-1 for Eǀǀα and Eǀǀβ and are different from the spectra presented in Bell et al. (1995). As such, the values in this study are derived through the calibration by Libowitzky and Rossman (1997), which has previously been used successfully for synthetic as well as natural clinopyroxene samples (e.g., Stalder, 2004; Stalder and Ludwig, 2007; Sundvall and Stalder, 2011; Mosenfelder and Rossman, 2013). Utilizing these values, the calculated magmatic water contents for the Nakhla parental melt average around 1.44 ±0.28 wt. % (Table 2). Potential uncertainties for calculated water contents can arise from baseline correction and measurements of the crystal thickness. However, due to the quality of the spectra and the relatively large thickness of the crystals a maximum error of ±10 % is assumed for the precision of the calculated clinopyroxene water contents. In addition, the uncertainty regarding the accuracy of the values due to the calibration for absorption coefficients is another ±10 % (cf. Libowitzky and Rossman, 1997) resulting in an overall uncertainty of ±20 % for the calculated clinopyroxene. No hydrogen diffusion profiles were observed in the clinopyroxene crystals and hence, the magmatic water contents. A propagation of error for calculated magmatic water contents showed that the total maximum error is similar to the uncertainty of the FTIR analysis and thus ±20 %.

5.3. Mössbauer spectroscopy The results for the Mössbauer spectroscopy of Nakhla clinopyroxene and olivine are shown in Table 3. Mössbauer spectra with doublets for Fe2+ and Fe3+ are shown in Figure 3.

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Clinopyroxene shows Fe3+/Fetot = 3 % while the analyzed olivine crystal has Fe3+/Fetot = 0.4 %. Neither metallic Fe, which would indicate excessive reduction, nor oxidation products such as magnetite or hematite were observed.

6. Discussion 6.1. Water in Martian clinopyroxenes and olivines The absence of water in the Martian clinopyroxene and olivine crystals may be due to dehydration within a volcanic system, during eruption, or during residence on the Martian surface and impact (e.g., Dyar, 2003; Dyar et al., 2004; Weis et al., 2015). For Nakhla, the water content in a clinopyroxene crystal has previously been measured by secondary ion mass spectrometry (SIMS) and gave an estimate of 97 ppm (Hallis et al., 2012). The 2σ-error for this estimate is very large and equal to ±408 ppm, however, the water contents obtained for San Carlos olivine standards in the study correspond well with values obtained in other SIMS and FTIR analyses (e.g., Miller et al., 1987; Kurosawa et al., 1997). In contrast to the values obtained in this present study, the crystal studied by Hallis et al. (2012) might not have completely dehydrated. Large variation in dehydration of clinopyroxene crystals can occur within individual lava samples (e.g., Weis et al., 2015). The water contents of rehydrated Nakhla clinopyroxene crystals, however, are similar to the value determined by Hallis et al. (2012) and water contents in phenocrysts from basalt lavas on Earth (see review by Peslier, 2010).

Instead of rehydration experiments, it was previously attempted to estimate the primary water contents in potentially dehydrated clinopyroxene and olivine from Martian meteorites on the basis of their Fe3+contents, assuming that all Fe3+ formed by the dehydration reaction Eq.1 (e.g., Dyar, 2003; Dyar et al., 2005).The Mössbauer analysis conducted here

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revealed that 3 % of the total Fe in the clinopyroxene is Fe3+, which is similar to results obtained by Dyar (2003). Expressing the Fe3+ content in atoms per formula units (apfu) gives a value of 0.013 (Table 1). The average water intake into the crystals of 131 ppm corresponds to 0.003 apfu hydrogen and thus about 23 % of the Fe3+ content assuming a 1:1 relation for the redox-reaction (1) upon hydrogen incorporation. If all the Fe3+ in the crystal was due to dehydration via Eq.1 during the ejection and shock event the crystals should have had a water content of 528 ppm. This value is higher than most water contents of terrestrial clinopyroxenes (see review by Peslier, 2010). However, it seems unlikely that all the Fe3+ was generated by dehydrogenation because of other substitutions involving monovalent cations. The Nakhla clinopyroxene contains Na+ and thus a portion of the Fe3+ might simply be associated with for example an aegirine component (Na+Fe3+) instead of hydrogen-associated defects (Skogby and Rossman, 1989; Skogby, 1994; Purwin et al., 2009). Since the oxygen fugacity for the Nakhla and nakhlite parent melt is estimated to be near QFM, and thus similar to basaltic melts on Earth, the occurrence of Fe3+ not related to dehydration processes is expected (Righter et al., 2008; Szymanski et al., 2010). Similar to clinopyroxene, the Fe3+ in the Nakhla olivine can be tested as a proxy for the water content in the crystal prior to dehydration. Considering that Fe3+/Fetot = 0.4 %, the amount of Fe3+ is 0.005 apfu (Fetot = 1.199). If the Fe3+ was generated solely by dehydration via redox reaction Eq.1, this would represent an initial water content of ~250 ppm in the crystal. This value is much higher than the average water contents in olivine crystals on Earth, especially considering pheno- and xenocrysts in basaltic melts (see review by Peslier, 2010). Assuming a minimum partition coefficient for hydrogen of 0.0007 (Le Voyer et al., 2014) between basaltic melt and olivine, this would result in an unreasonably high water content for the melt. Olivine from Nakhla includes magnetite and augite bearing symplectic exsolutions which likely formed from oxidation of olivine under magmatic

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conditions (Mikouchi et al., 2000). While some of the Fe3+ present in the olivine may have resulted from dehydration via Eq.1, it seems, however, more likely that most of it was generated by oxidation in a magmatic system. Thus the Fe3+ content in NAMs cannot simply be applied as a proxy for primary water contents and rehydration of the hydrogen associated defects seems to produce more original values.

6.2. Water contents of the Nakhla parental melt From this study of magmatic water content of Nakhla, its melt has an average of 1.44 ±0.28 wt. %. The clinopyroxene water content reported by Hallis et al. (2012) would correspond to 1.07 ±4.50 wt. % H2O in the Nakhla melt, slightly lower but yet similar to values in the current study (Table 2). Partial dehydration of the crystal in Hallis et al. (2012) can’t be excluded, which could account for the somewhat lower value. To further test the derived magmatic water content measured here and to obtain a less precise but independent estimate of H2O in Nakhla’s parent melt, compositional data of augite and a clinopyroxene hygrometer can be applied (Perinelli et al. 2016). The hygrometer was specifically calibrated and applied to trachybasalts yet it may be used for magma compositions ranging from basalt to andesite (Perinelli et al. 2016). A limiting factor of this method is that precise information about temperature and pressure conditions is needed to obtain more precise estimates for water contents. Choosing a medium crystallization pressure of 1.5 kbar for Nakhla (Harvey and McSween 1992, Treiman 1993, Bridges and Warren 2006, Filiberto et al. 2014,) and a temperature of 1100 °C reveals a magmatic water content of 1.5 ±0.45 wt. % which is consistent with the value derived through our rehydration experiments. The magmatic water contents obtained through clinopyroxene crystals in this study are higher than previously published values of ~0.1 wt. % from hydrogen extraction and whole rock studies (Fig.4, Karlsson et al., 1992; Leshin et al., 1996; Bridges and Warren, 2006). These literature

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values, however, do not account for possible degassing of the crystals and the melt upon ascent, eruption and ejection shock. Although not quantified, a low water content for the Nakhla parent melt was also supported based on the comparison of Li and B concentration patterns in Nakhla and Shergotty clinopyroxenes (Lentz et al., 2001). The identified Li and B patterns which supported a hydrous Shergotty, but a dry Nakhla parental melt, were only partly observed in other studies and could, in addition to magma degassing, also be explained by possible element re-distribution due to impact shock (Herd et al., 2005). Rehydrated clinopyroxenes may thus provide a better estimate of the magmatic water content of Nakhla’s parent melt. There are several reasonable concerns in the analytical approach performed here. These concerns are the unknown influence of other volatiles (e.g., CO2) on the hydrogen content in the crystals or excessive hydration due to reduction of Fe3+ (see Weis et al. 2016). Yet, the reduction of Fe3+ seems to have little to no influence on the actual generation of hydrogen-associated defects (see also section 3). However, the method applied here reproduced data obtained by independent methods and thus, is assumed to be accurate and the aforementioned uncertainties may not have a great influence on the final numbers. Additionally, there exist other uncertainties, specifically for Nakhla and its crystallization history. For example, the Nakhla magma body may have cooled slowly and may have been influenced by crustal Cl-rich fluids. There are two contrasting models for the formation of the Nakhlites. The first treats Nakhla and other nakhlites as a separate standalone magma body (e.g., Stockstill et al. 2005, Bridges and Warren 2006, Filiberto et al. 2014). The second hypothesis proposed that the nakhlites and Chassigny formed as a single magma body with Nakhla crystallizing at an intermediate stage during an evolving crystallization sequence (e.g., McCubbin et al. 2013). The contrasting hypotheses both include some interaction of the Nakhla melt with hydrothermal, Cl-rich fluids. This brings two questions regarding the 18

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applied approach in this study: 1) Do the reconstructed magmatic water contents represent the primary melt from which Nakhla crystallized and/or 2) do the values measured here represent a more fractionated melt at an intermediate stage of crystallization of a large igneous body? An additional consideration should be raised as to how the interaction of fluids potentially influenced hydrogen-associated defects in the crystals and subsequently what the rehydrated water contents represent (fluid vs. magma). Cl-rich fluids interacting with Nakhla have been proposed as a result of ongoing crystallization and Cl-saturation (e.g., McCubbin et al. 2013) or to originate from the initial primary Nakhla melt (Filiberto et al. 2014). If the fluids originated from Nakhla itself and were part of the primary magma then reconstructed water contents from defects here would account for this and thus represent the initial parental melt in equilibrium with the Cl-rich fluid phase. Subsequently, fluid interaction was proposed to have occurred at temperatures ~700 °C or lower (McCubbin et al. 2013, Filiberto et al. 2014). At these temperatures, hydrogen-associated defects can be expected to remain stable (see section 3). If the Nakhla melt, regardless whether it was an individual magma body or part of a crystallization sequence, expelled Cl-rich fluids, the net effect on the melt would be a shift to more oxidizing conditions (Bell and Simon 2011) as well as a loss in fluid pressure and both processes could lead to a dehydration of clinopyroxene crystals. Both models considering the crystallization and fluid activity of the Nakhla melt have in common that clinopyroxene crystallized first, before fluid interaction. Due to temperature and the difference in kinetics interaction with Clrich fluids can be assumed to have had little effect on hydrogen-associated defects other than leading to potential dehydration governed by redox-reaction Eq.1. Thus rehydrated defects would represent a minimum estimate of initial magmatic water contents. The question of when exactly Nakhla crystallized, however, is a more substantial issue. If the nakhlites are seen as an individual magma body, where the primitive basaltic

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Nakhla melt crystallized relatively early with little to no impact of magma fractionation (Stockstill et al. 2005) the rehydrated clinopyroxene and determined magmatic water contents are representative of the Nakhla parental melt and can provide a close approximation of the mantle water content. Analyses in this study are done on augite cores which in Nakhla have been interpreted to be more representatives of the parental melt (Wadhwa and Crozaz 1995). This also includes the assumption that no substantial magma degassing occurred prior to Nakhla crystallization. This assumption is justified, as Nakhla is seen to have crystallized within a closed system (Wadhwa and Crozaz 1995, Treiman 2005, Day et al. 2006, Udry et al. 2012). Yet, if the model of a common magma body for Chassigny and Nakhla is considered, the rehydrated defects would not represent the initial mantle melt but a more fractionated melt at an intermediate stage in the crystallization sequence and which in addition had been previously enriched by fluids (Nakamura et al. 1982, McCubbin et al. 2013). If this were the case, the values estimated for the mantle water content from magmatic H 2O content would be overestimated and also difficult to evaluate more precisely since the degree of fractionation and volatile enrichment is difficult to estimate. It remains under discussion which model is correct. This study considers the nakhlites to originate from an independent magmatic body with Nakhla representing a primitive unfractionated melt. Yet, the values presented in this study may be valid for either interpretation. One more critical issue is the cooling rate of the Nakhla melt, which was estimated to be relatively slow (0.0005–0.04 °C/h) and a cooling range from ~1100–700 °C (Mikouchi and Miyamoto 2002). The higher temperature and the slow cooling rate bring cation and vacancy diffusion to faster kinetics and thus, potentially resetting of the hydrogen associated defects. Further, if magma degassing occurred, the solubility, i.e. the creation of new hydrogen-associated defects in equilibrium with the new and lower magmatic volatile content, could have been affected. In this case, the measured rehydrated defects in this study provide a

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minimum estimate for the water content of the melt, if initial hydrogen associated defects were reset due to cation and defect diffusion.

6.3. Water content of the Nakhla source mantle Using the calculated parental magmatic H2O contents an estimate for the water in the Martian mantle can be deduced accounting for the degree of partial melting of the Nakhla mantle source region. Models of the rare-earth element (REE) distribution in Nakhla indicate that the REE composition of the parental melt could be achieved by low degrees of partial melting of a garnet-bearing Martian mantle (~0.5 % to ≤ 0.1 %; Nakamura et al. 1982; Wadhwa, 1994; Shih et al. 1999, Borg et al. 2003; Treiman 2005; Shih et al., 2010; Jones, 2015). Assuming a maximum of 0.5 % for partial melting of the Martian mantle and subsequent equilibrium crystallization of clinopyroxene (Stockstill et al. 2005) provides a maximum estimate for the volatile content for the Nakhla mantle source. Using the calculated water content in the Nakhla parental melt indicates thus maximum mantle water contents of 70 ±14 to 74 ±16 ppm H2O for Nakhla’s mantle source (Table 4, Fig.5). These values overlap with the combined value of 68 ±113 ppm H2O determined for the Chassigny and nakhlite source (Filiberto et al., 2016). This model assumes equilibrium crystallization and no fractionation of the Nakhla parent melt. Yet, 50 % were proposed as a minimum for fractional crystallization for Nakhla (Nakamura et al. 1982). Thus, the calculated water content in this study for the parental melt and a garnet-bearing mantle source would potentially be overestimated. However, assuming a model to obtain REE concentrations in Nakhla clinopyroxene proposing fractional crystallization of at least 50 % for the parent melt, requires also ~5 % of partial melting of a garnet-bearing mantle source (Nakamura et al. 1982). This would shift the water content of the mantle source towards higher values. Assuming that the water content of the fractionated

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melt thus was 1.44 wt. %, as suggested in this study, the mantle source region would have had at least 350 ±70 ppm H2O. This value is outside of analytical uncertainty of the 250 ppm H2O that have been suggested for the Chassigny mantle source (McCubbin et al. 2010). The slightly higher value determined for Nakhla in this case could be a consequence of the addition of Cl-rich fluids to the system prior to clinopyroxene crystallization (McCubbin et al. 2013). Fractional crystallization cannot be entirely excluded for nakhlites based on major element abundances (Borg et al. 2003), yet melt inclusions in Nakhla seem to represent a primitive unfractionated basaltic melt undergoing equilibrium crystallization of clinopyroxene and subsequent fractionation (Stockstill et al. 2005). Most studies modelling the REE patterns in Nakhla (Nakamura et al. 1982, Shih et al. 1999, Borg et al. 2003) assume low degrees of partial melting which is why a maximum degree of 0.5 % is thus also considered in this study and hence a mantle water content of ~72 ±16 ppm.

6.4. Water in the Martian Mantle Using the available data from the literature (Fig.5; Boctor et al., 2003; McCubbin et al., 2010a, 2012; Usui et al., 2012) and the values obtained in this study the average concentration of water in the source regions for the shergottites, nakhlites, and chassignites are 125 ±8 ppm, 72 ±16 ppm, 109 ±3 ppm, respectively (Fig.6, Table 4). A weighted mean of these values is 102 ±9 ppm (2). This value is significantly lower than most of Earth’s upper mantle which ranges from 54 to >1000 ppm and has an estimated bulk value of ~250 ppm (Table 4), but is, however, similar to Earth’s MORB source (54 to 330 ppm H2O, average ~130 ppm) (Michael, 1988; Sobolev and Chaussidon, 1996; Wallace, 1998; Saal et al., 2002; Simons et al., 2002; Salters and Stracke, 2004; Workman and Hart, 2005; Keppler and Bolfan-Casanova, 2006; Marty and Yokochi, 2006; Albarede, 2009; Hirschmann, 2010; Bizimis and Peslier 2015). This difference indicates that the Martian mantle is

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significantly drier than that of Earth. Additionally, the range of crystallization ages (1.35 Ga 0.16 Ga) and similarities in water content indicate a relatively homogenous Martian mantle over geologic time. A possible explanation for the apparent lower volatile content in Mars’ mantle relative to Earth is a profound volatile loss in the early history of the planet most likely enabled by an early outgassing of Martian mantle either as a result of magma ocean crystallization or extensive magmatic activity during the first few hundreds of millions of years, which is also indicated by a number of observations such as the high surface concentrations of halogens (e.g., Taylor et al., 2010; Taylor, 2013; Bellucci et al., 2017). Widespread volatile loss from the mantle should have formed an early Martian atmosphere, which had probably been lost given that Mars does not appear to show consistent evidence for the processes similar to Earth’s plate tectonics (Martin et al., 2008) capable of reintroducing volatiles back to the mantle. This loss of atmosphere is also suggested based on a significant deficit of light isotopes of noble gases in the Martian atmosphere (Pepin, 1991) as well as the observed substantial enrichment of Martian water in D relative to H and enrichment of Martian regolith in 17O (Bjoraker et al., 1989; Krasnopolsky et al., 2007; Nemchin et al., 2014). Extreme ultraviolet (EUV) radiation flux from the young Sun combined with the relatively low gravity of Mars and early disappearance of magnetic field (Lammer et al., 2013) should have a significant impact on the early loss of Martian atmosphere, reducing the overall content of volatiles on the planet.

7. Conclusions This study provides new measurements of the water and Fe redox states in nominally anhydrous minerals from Mars. The results show that olivine as well as clinopyroxene crystals from Nakhla may have completely dehydrated upon eruption or shock.

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Previous proposals that the content of Fe3+ in theses minerals could be used as a proxy for water contents prior to dehydration do not seem to be applicable. Rehydration experiments on clinopyroxene crystal cores reveal that crystals may have held 130 ±13 ppm H2O which would correspond to a water content of 1.44 ±0.28 wt. % H2O in the Nakhla parent melt. Determining the water contents in rehydrated clinopyroxenes may provide the current best alternative to melt inclusions or apatite for the estimation of magmatic water in planetary mafic rocks. The results from this study indicate that mantle source regions for all three SNC meteorites seem to have had similar water contents that stayed constant throughout geologic time. The weighted mean value of 102 ±6 ppm for the Martian mantle is lower than for the bulk upper mantle on Earth. The drier mantle of Mars may be the reason for the absence of Earth style plate tectonic activity.

Acknowledgements This work was partially funded by grants from the Knut and Alice Wallenberg Foundation (2012.0097) and the Swedish Research Council to HS (VR 2011-5430), MJW and AAN (VR 621-2012-4370), and JJB (VR 2016-03371). The data for this paper are available in the text, tables and references therein and from the corresponding author on request.

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Figure captions Figure 1: BSE images of a) clinopyroxene and b) olivine crystals and analysis spots for EPMA. The crystals were homogenous and showed no zonation.

Figure 2: Representative IR-spectra of a) olivine and b) clinopyroxene before (dashed line) and after (solid line) annealing at 2 kbar and 700 °C. Polarized measurements with Eǀǀα and Eǀǀγ were done on the (010) crystal face while Eǀǀβ was measured on (100). Absorbances have been normalized to 1 mm thickness. The clinopyroxene spectra show the three main vibrational bands of water at 3630 cm-1, 3530 cm-1, 3460 cm-1, which are expected for diopside (Skogby, 2006) and relate to different OH-dipole orientations (see text for details). The increase in peak intensity and thus water content after pressure annealing is apparent. Some small peaks around 2900 cm-1 can be seen and are most likely from epoxy residue. The epoxy, however, had no effect on the water range of the spectrum. For olivine no OH-bands are observed.

Figure 3: Representative Mössbauer spectra for a) clinopyroxene (untreated) and b) olivine in the Nakhla meteorite. The spectra show the typical Fe2+ and Fe3+ doublets which were used to determine the concentration of both iron oxidation states in the crystal.

Figure 4: Magmatic water contents determined for Martian meteorite parental melts. Different colors indicate the analytical method used to determine the melt water content. Error bars show the 2σ error. Where no error bars are shown, the error is smaller than the symbol. Reference data was taken from available literature. Nakhla: this study, Karlsson et al., 1996; Leshin et al., 1996; Bridges and Warren, 2006; Hallis et al., 2012; Shergotty: Dann et al.,

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2001; Lentz et al., 2001; McSween et al., 2001; McCubbin et al., 2012; QUE942012, Zagami, Los Angeles, GRV990274: McCubbin et al., 2012; Chassigny: Boctor et al., 2003; McCubbin et al., 2010a.

Figure 5: Mantle water concentrations determined for Martian meteorites in comparison to Earth and the Moon. Nakhla: this study; Shergotty, Zagami, Los Angeles, GRV99027, QUE94201: McCubbin et al., 2012; Chassigny: Boctor et al., 2003; McCubbin et al., 2010a; Y-980459: Usui et al., 2012, Earth: Michael, 1988; Sobolev and Chaussidon, 1996; Wallace, 1998; Saal et al., 2002; Simons et al., 2002; Salters and Stracke, 2004; Workman and Hart, 2005; Keppler and Bolfan-Casanova, 2006; Marty and Yokochi, 2006; Albarede, 2009; Hirschmann, 2010; Bizimis and Peslier 2015; Moon: McCubbin et al., 2010b; Hauri et al., 2011.

Figure 6: Martian mantle water concentrations vs. crystallization ages of meteorites (e.g., Nyquist et al., 2001). The grey symbol represents an average for the Shergottite meteorite group. Boxes on the right represent the range of Earth’s upper mantle and MORB water contents (see Table 4; for MORB one outlier is not shown for simpler presentation). Black marks in the boxes represent Earth’s bulk and a MORB average value. A bulk value for the Martian upper mantle derived on the basis of averages for the SNC meteorites is much lower than the water content for the terrestrial upper mantle but yet similar to Earth’s MORB mantle source (Michael, 1988; Sobolev and Chaussidon, 1996; Wallace, 1998; Saal et al., 2002; Simons et al., 2002; Salters and Stracke, 2004; Workman and Hart, 2005; Keppler and Bolfan-Casanova, 2006; Marty and Yokochi, 2006; Albarede, 2009; Hirschmann, 2010; Bizimis and Peslier 2015).

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Weis et al. Nakhla_MS_13.02.2017

Figures Figure 1

Figure 2

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Weis et al. Nakhla_MS_13.02.2017

Figure 3

Figure 4

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Weis et al. Nakhla_MS_13.02.2017

Figure 5

Figure 6

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Weis et al. Nakhla_MS_13.02.2017

Tables

Table 1. Nakhla compositional data Major elements (wt. %) Cpx 51.27

SiO2

0.91

Al2O3

Atoms per formula unit

Ol 33.53 0.03

Si

Cpx

Ol

4+

1.958

1.000

3+

0.041

0.001

0.726

0.773

0.017

-

Al

2+

MgO

12.76

17.40

Mg

Na2O

0.23

-

Na+

0.45

MnO

0.24

TiO2

2+

1.00

Mn

0.015

0.025

0.03

4+

0.007

0.001

Ti

+

K2O

-

-

-

-

CaO

18.99

0.45

Ca2+

0.777

0.014

FeO

14.04

48.10

Fe2+ a

0.436

1.193

3+ a

0.013

0.006

3+

0.011

0.000

4.000

3.014

-

Fe2O3

-

Cr2O3

0.35

0.01

Total

99.26

100.56

K

Fe

Cr

Total

a

Values for Fe2+ and Fe3+ are based on Mössbauer results from Table 3. Composition in wt.% refers to an average of all analyzed spots on the crystal.

Table 2. Water contents of the Nakhla clinopyroxene and their parental melt [H2O]cpx after annealing (ppm)

Si4+

Nakhla cpx1a

134 ±26

1.958

0.042

0.777

-4.704

b

132 ±26

1.958

0.042

0.777

126 ±26

1.958

0.042

97 ±408

1.958

0.042

Sample

Nakhla cpx2

b

Nakhla cpx3

Nakhla augite

c

[H2O]Melt (wt. %)

Average

0.009

1.48 ±0.30

1.44 ±0.28

-4.704

0.009

1.46 ±0.30

0.777

-4.704

0.009

1.39 ±0.28

0.777

-4.704

0.009

1.07 ±4.50

(IV)

Al3+ Ca2+

lnD(cpx-melt) D(cpx-melt)

a

Crystal annealed for 66 hours, 700 °C, 2 kbar Crystal annealed for 96 hours, 700 °C, 2 kbar c Data from Hallis et al., 2012 b

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Weis et al. Nakhla_MS_13.02.2017

Table 3. Mössbauer spectroscopy results inta (%)

fwhm (mm/s)

cs (mm/s)

dq (mm/s)

Fe2+ (1)

24.0

0.37

1.18

2.54

2+

Fe (2)

73.0

0.38

1.16

1.92

3+

3.0

0.24

0.46

0.72

Fe2+ (1)

68.5

0.31

1.15

2.79

2+

Fe (2)

31.1

0.22

1.18

3.01

Fe3+ (octahedral site)

0.4

0.19

0.48b

0.65b

Sample Nakhla cpx

Fe (octahedral site) Nakhla ol

int - intensity in percentage of total absorption area a= Fem+/Fetotal fwhm - full width at half maximum (including source width) cs - centroid shift dq - quadrupole splitting Estimated uncertainty for Fem+/Fetot intensities is ± 0.4 %. Fe2+ (1) and Fe2+ (2) are related to the M1 and M2 sites respectively in both minerals, but the intensity distribution between the two sites bears greater uncertainty due to strong overlap of the two doublets. b Values taken from Dyar 2003.

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Weis et al. Nakhla_MS_13.02.2017

Table 4. Water contents of mantle sources of the SNC meteorites Sample

Melt H2O (wt. %)

Initial Melt H2O (ppm)

Nakhla

Based on 0.5 % partial melting of mantle source

Nakhla cpx1

1.48 ±0.30

14800 ±3000

74 ±16

Nakhla cpx2

1.46 ±0.30

14600 ±3000

73 ±16

Nakhla cpx3

1.39 ±0.28

13900 ±2800

70 ±14

Average

Mantle water content (ppm)

Based on 10 % partial melting of mantle source

Shergotty Apatite 1

2.17 ±0.07

2167 ±67

217 ±7

Shergotty Apatite 2

1.77 ±0.07

1767 ±67

177 ±7

Shergotty Apatite 3

1.83 ±0.07

1833 ±67

183 ±7

Shergotty Apatite 4

1.97 ±0.07

1967 ±67

197 ±7

Shergotty Apatite 5

2.60 ±0.10

2600 ±100

260 ±10

Shergotty Apatite 6

1.73 ±0.07

1733 ±67

173 ±7

Shergotty Apatite 7

1.53 ±0.07

1533 ±67

153 ±7

Shergotty Apatite 8

2.87 ±0.10

2867 ±100

287 ±10

Shergotty Apatite 9

1.00 ±0.07

1000 ±67

100 ±7

Shergotty Apatite 10

1.50 ±0.07

1500 ±67

150 ±7

Average

McCubbin et al., 2012

190 ±7

QUE94201 Apatite 1

1.77 ±0.17

1767 ±167

177 ±17

QUE94201 Apatite 2

2.13 ±0.20

2133 ±200

213 ±20

QUE94201 Apatite 3

1.83 ±0.20

1833 ±200

183 ±20

QUE94201 Apatite 4

1.40 ±0.13

1400 ±133

140 ±13

QUE94201 Apatite 5

1.40 ±0.13

1400 ±133

140 ±13

QUE94201 Apatite 6

1.27±0.13

1267 ±133

127 ±13

QUE94201 Apatite 7

0.90 ±0.10

900 ±100

90 ±10

QUE94201 Apatite 8

0.73 ±0.07

733 ±67

Average

McCubbin et al., 2012

73 ±7 143 ±14

Los Angeles Apatite 1

0.87 ±0.03

867 ±33

87 ±3

Los Angeles Apatite 2

0.80 ±0.03

800 ±33

80 ±3

Los Angeles Apatite 3

0.60 ±0.03

600 ±33

60 ±3

Los Angeles Apatite 4

1.90 ±0.10

1900 ±100

190 ±10

Los Angeles Apatite 5

1.53 ±0.07

1533 ±67

153 ±7

Los Angeles Apatite 6

2.07 ±0.10

2067 ±100

207 ±10

Average

McCubbin et al., 2012

129 ±6

Y980459 MI 1

0.15

146 ±4

15

Y980459 MI 2

0.25

251 ±12

25 ±1

Y980459 MI 3

0.18

183 ±6

18 ±1

Y980459 MI 4

0.47

466 ±26

47 ±3

46

This study

72 ±16

Shergottites

Average

References

Usui et al., 2012

26 ±1

Weis et al. Nakhla_MS_13.02.2017

Table 4. (continued) Water contents of mantle sources of the SNC meteorites Sample

Melt H2O (wt. %)

Initial Melt H2O (ppm)

Mantle water content (ppm)

References

Zagami Apatite 1

1.17 ±0.17

1167 ±167

117 ±17

McCubbin et al., 2012

GRV990274 Apatite 1

1.43 ±0.00

1433

143

McCubbin et al., 2012

Average for shergottites Chassigny

125 ±8 Based on 3 % partial melting of mantle source

Chassigny Amphibole 1

0.43

4300

129

Chassigny Amphibole 2

0.84

8400

252

CHAS-MI-1

0.27 ±0.01

2740 ±120

82 ±4

CHAS-MI-2

0.13 ±0.01

1292 ±82

39 ±2

CHAS-MI-3

0.19 ±0.01

1940 ±80

58 ±2

CHAS-MI-4

0.32 ±0.02

3170 ±170

95 ±5

Average

McCubbin et al., 2010a Boctor et al., 2003

109 ±3

Earth Mantle Sources OIB source

750 ±210

Simons et al., 2002

OIB source

120 ±27

OIB source

332 ±65

OIB source

450 ±190

Subduction zone

1190-1900

Simons et al., 2002 Bizimis and Peslier, 2015 Wallace 1998 Sobolev and Chaussidon 1996 Simons et al., 2002

MORB

54 ±12

MORB

142 ±82

MORB

80-330

MORB

100

MORB

116

MORB

70-135

Average MORB

130

Bulk Upper Mantle

250

47

Saal et al., 2002 Sobolev and Chaussidon 1996 Hirschmann, 2010 Salters and Stracke, 2004 Workman and Hart, 2005 Keppler and BolfanCasanova, 2006

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