The Chlorine Isotope Composition of Martian Meteorites 2. Implications for the Early Solar System and the Formation of Mars
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Meteoritics & Planetary Science 1–16 (2016) doi: 10.1111/maps.12591 The chlorine isotope composition of Martian meteorites 2. Implications for the early solar system and the formation of Mars Zachary SHARP1,2,*, Jeffrey WILLIAMS1, Charles SHEARER3, Carl AGEE3, and Kevin McKEEGAN4 1Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131–0001, USA 2Center for Stable Isotopes, University of New Mexico, Albuquerque, New Mexico 87131–0001, USA 3Institute of Meteoritics, University of New Mexico, Albuquerque, New Mexico 87131–0001, USA 4Earth and Space Sciences, University of California, Los Angeles, California 90095–1567, USA *Corresponding author. E-mail: [email protected] (Received 15 July 2015; revision accepted 29 October 2015) Abstract–We determined the chlorine isotope composition of 16 Martian meteorites using gas source mass spectrometry on bulk samples and in situ secondary ion microprobe analysis on apatite grains. Measured d37Cl values range from À3.8 to +8.6&. The olivine- phyric shergottites are the isotopically lightest samples, with d37Cl mostly ranging from À4 to À2&. Samples with evidence for a crustal component have positive d37Cl values, with an extreme value of 8.6&. Most of the basaltic shergottites have intermediate d37Cl values of À1to0&, except for Shergotty, which is similar to the olivine-phyric shergottites. We interpret these data as due to mixing of a two-component system. The first component is the mantle value of À4toÀ3&. This most likely represents the original bulk Martian Cl isotope value. The other endmember is a 37Cl-enriched crustal component. We speculate that preferential loss of 35Cl to space has resulted in a high d37Cl value for the Martian surface, similar to what is seen in other volatile systems. The basaltic shergottites are a mixture of the other two endmembers. The low d37Cl value of primitive Mars is different from Earth and most chondrites, both of which are close to 0&. We are not aware of any parent-body process that could lower the d37Cl value of the Martian mantle to À4toÀ3&. Instead, we propose that this low d37Cl value represents the primordial bulk composition of Mars inherited during accretion. The higher d37Cl values seen in many chondrites are explained by later incorporation of 37Cl-enriched HCl-hydrate. INTRODUCTION formation (e.g., Pepin 1997; Genda and Abe 2005; Sharp et al. 2010). Mars has not suffered from The concentrations of nonrefractory elements on much of the later processing that affected both the differentiated planetary bodies decrease in relation to Earth and Moon. It formed at the outer periphery of their volatility, or condensation temperature. The the inner protoplanetary disk, incorporating primitive sources and terrestrial abundance of highly volatile planetesimals scattered beyond the edge of the truncated elements, including the critical life-forming elements C, disk (Hansen 2009; Walsh et al. 2011; Izidoro et al. N, and H, are particularly difficult to model due to 2014; O’Brien et al. 2014). It is far smaller than Earth uncertainties in how they were incorporated into solids and Venus, and not much larger than a planetary in the early solar system and lost during later embryo (Kokubo and Ida 1998; Brandon 2011; Brasser volatilization. Stable isotope ratios have been used to 2013). Finally, it is also thought to be significantly older constrain the sources and abundance of these elements than the Earth and Moon, with a calculated core on Earth (Alexander et al. 2012; Marty 2012), but these formation age of only several million years after estimates are complicated by potential isotopic collapse of the solar nebula (Dauphas and Pourmand fractionation that may occur by volatile loss (e.g., 2011; Tang and Dauphas 2014). Martian meteorites hydrodynamic escape) both during and after planetary may therefore provide geochemical and isotopic 1 © The Meteoritical Society, 2016. 2 Z. Sharp et al. information on volatile element concentrations and Leachates and extracted ClÀ from silicates are converted isotopic compositions that were erased on the Earth by to AgCl following the procedure of Eggenkamp (1994). later energetic processing. AgCl is reacted with excess CH3I in evacuated glass Volatile loss has certainly occurred on Mars. Mars tubes at 80 °C for 48 h to quantitatively convert AgCl was a wet planet early in its history (Carr and Head to CH3Cl. Excess CH3I is separated from CH3Cl using 2003), but probably lost most of its water within the gas chromatography (Barnes and Sharp 2006) and the first 10 Myr (Erkaev et al. 2014) due to interaction with purified CH3Cl is measured in a Finnigan MAT Delta solar wind (Chassefiere and Leblanc 2004), impact XL Plus mass spectrometer in continuous flow mode. erosion, and/or hydrodynamic escape (Hunten 1993). Minimum sample size is ~10 lg Cl. Multiyear analyses The isotopic composition of the Martian atmosphere of an inhouse serpentinite standard EL05-14 reflects mass-dependent loss to space (Bogard et al. (d37Cl = 0.9& versus SMOC; Barnes and Sharp 2006) 2001), with elevated 38Ar/36Ar ratio relative to have a reproducibility of Æ0.2& (Selverstone and Sharp chondrites and Earth (Swindle et al. 1986). The average 2015). 15N/14N ratio of Mars is 1.7 times than the Earth value In situ ion microprobe analyses were made on the (Owen et al. 1977), and CO2 and H2O are also enriched large radius Cameca 1270 ion microprobe at UCLA in the heavy isotope due to volatile loss (Mahaffy et al. using a Cs+ primary beam focused to ~10–20 lm, 2013; Webster et al. 2013). following procedures in our previous work (Sharp et al. The water content of the Martian mantle is poorly 2010). Ion beams for mass 35 and 37 are measured known, with some researchers arguing that mantle simultaneously on Faraday cup detectors with water contents could be equal to those of Earth equivalent count rates of 2 to 5 9 107cps for 35ClÀ. The (Treiman 1985; Johnson et al. 1991; McSween et al. measurements are made at high mass resolution, 2001; McCubbin et al. 2012; Gross et al. 2013) and sufficient to eliminate all isobaric interferences, others suggesting a far less-hydrous mantle (Dreibus including 34SHÀ (Layne et al. 2004). We have developed and Wanke€ 1987; Wanke€ and Dreibus 1988; Filiberto two apatite standards for calibration: Durango apatite and Treiman 2009). Recently, it was suggested that with a concentration of 0.37 wt% Cl and a d37Cl value Martian magmas were rich in Cl and poor in H2O, of 0.33& and a synthetic Cl-apatite from the University making Cl a critical component in the melting and of Heidelberg with a Cl concentration of 5.5% and a crystallization properties of Martian melts (Filiberto d37Cl value of 2.8&. The d37Cl values of these and Treiman 2009; Jones 2015). Chlorine also exists in standards were determined using the gas source mass high concentrations at the surface (Rieder et al. 1997, spectrometry method described above. Precision of 2004; Bridges et al. 2001; Gellert et al. 2004; Keller individual spot analyses ranged from 0.2& to 1.2& et al. 2006; Boynton et al. 2007; Bogard et al. 2010), depending on concentration. External reproducibility is with evidence for significant concentrations of in the range of 0.5& for samples with percent-level Cl perchlorate (Hecht et al. 2009; Franz et al. 2013). In concentrations. this contribution, the chlorine isotope compositions of different types of Martian meteorites were measured in SAMPLES order to constrain the mantle value and that of the more evolved crust. A companion paper examines the For the purposes of this study, we divide Martian interaction between these reservoirs (Williams et al. meteorites into olivine-phyric shergottites and basaltic 2016). Here, we place the Cl-isotopic composition of shergottites; nakhlites (clinopyroxene-cumulate); Mars into the context of origin of early solar system chassignites (olivine cumulate); and ungrouped samples Cl-isotopic reservoirs and the accretion of Mars. ALH84001, an orthopyroxenite; NWA 8159, a unique augite basalt; basaltic breccia NWA 7034; and mixed ANALYTICAL METHODS lithology EETA 79001. Relevant to this study is the following description of each class. Stable chlorine isotope ratios were measured on bulk samples by gas source mass spectrometry at the Shergottites University of New Mexico. Samples are crushed to a fine powder and leached in 18MΩ deionized water to Shergottites are mafic to ultramafic igneous rocks extract soluble ClÀ. The leached samples are heated derived from the Martian mantle, and make up about ¾ to melting in a H2O vapor stream (pyrohydrolysis) to of the known Martian meteorites. There have been a extract silicate-bound ClÀ, which is condensed as a ClÀ- number of classification schemes that have been bearing aqueous solution. The solutions are reacted proposed based on petrography and geochemistry with concentrated nitric acid to remove excess S. (McSween and Treiman 1998; Goodrich 2002; Meyer The chlorine isotope composition of Martian meteorites 2 3 2006; Irving 2012; Treiman and Filiberto 2015). For this shergottites (Bogard et al. 2010). Shock is minimal for discussion, we divide samples into basaltic the nakhlites, with fully crystalline, normal birefringent (clinopyroxene, plagioclase rich) and olivine-bearing plagioclase present (Treiman 2005). (and olivine- orthopyroxene-phyric and lherzolitic) shergottites, the latter group representing crystallization Augite Basalt products of more primitive melt compositions (e.g., Gross et al. 2011). Meteorite NWA 8159 is an augite-bearing basalt Geochemically, the shergottites can be classified as with 50% high-Ca pyroxene and ~40% plagioclase + depleted, enriched, and intermediate based on their maskelynite and nearly pure endmember magnetite. It REE patterns and their isotopic systematics (initial had been proposed that this sample may be the 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf). The intermediate crystallized differentiate of the nakhlite suite (Agee shergottites are likely a mixture of the other two et al.