The Chlorine Isotope Composition of Martian Meteorites 2. Implications for the Early Solar System and the Formation of Mars

Home , Nakhla meteorite, Sayh al Uhaymir 169

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 3.8 to +8.6&. The olivine- phyric shergottites are the isotopically lightest samples, with d37Cl mostly ranging from 4 to 2&. 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 1to0&, 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 4to3&. 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 4to3&. 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 Cl 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 0.2& (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 35Cl. 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 34SH (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 Cl. 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 Cl, which is condensed as a Cl- 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. 2014; Herd et al. 2014). However, the endmembers. The geochemical differences between crystallization age and derivation from a highly depleted groups require distinct depleted and enriched reservoirs source mantle source (Simon et al. 2014; Kayzar et al. existing since ca. 4.5 Ga (Borg et al. 1997; Brandon 2015) imply that it is distinct from either the et al. 2000, 2012; Debaille et al. 2008). The reservoirs shergottites or nakhlites. NWA 8159 has mineralogical may represent either two mantle sources or a depleted and geochemical evidence for the addition of a crustal mantle source and an ancient Martian crust (Herd et al. component (Shearer et al. 2015). 2002; Usui et al. 2012). Depleted shergottites crystallized under more reducing f(O2) conditions than Chassignites the enriched shergottites. Differences in f(O2) may be related to mantle sources (Borg et al. 1997; Brandon These are olivine-rich cumulates with two known et al. 2000, 2012; Debaille et al. 2008), incorporation of samples (Chassigny and NWA 2737). Crystallization an oxidized crustal component through assimilation ages, ejection ages, trace element, and isotopic (Herd et al. 2002), or oxidation during ascent and compositions (Brandon et al. 2000) are very similar to crystallization (Debaille et al. 2008; Peslier et al. 2010; nakhlites, suggesting a comagmatic origin (McCubbin Shearer et al. 2013). Dhofar 019 has been classified as et al. 2013). Chassignites contain biotite and kaersutite an olivine-phyric shergottite (Ennis et al. 2001) and a in addition to apatite as volatile-rich phases. The Cl basaltic shergottite (Taylor et al. 2002). It has more Fe- content of apatite included in olivine is lower than in rich silicates than many of the other olivine-phyric the groundmass (McCubbin and Nekvasil 2008; shergottites and may have experienced more fractional McCubbin et al. 2013). NWA 2734 has less secondary crystallization (Lentz and McSween 2001). It has a low- Cl-enrichment than Chassigny (McCubbin et al. 2013), temperature, terrestrial noble gas component and presumably experienced lesser contamination by a incorporated during weathering on Earth (Shukolyukov Cl-rich fluid. NWA 2737 is also most likely the deepest et al. 2002) and a slight positive D33S anomaly (Franz sourced sample from the Nahklite-Chassignite suite et al. 2014), suggestive of minor crustal contamination. (McCubbin et al. 2013). There is minor maskelynite in Chassigny, but not NWA 2737. Low temperature Nakhlites alteration minerals include carbonate, sulfates, and oxides (Wentworth and Gooding 1994; Bridges et al. Nakhlites are augite-rich igneous cumulate rocks 2001). formed in flows or as shallow-level intrusives with a ~1.3 Ga age (Treiman 2005). They are altered by near- Orthopyroxenite surface processes, possibly by infiltration of evaporitic waters (Gooding et al. 1991; Sawyer et al. 2000; Bridges The unique sample ALH 84001 is a coarse-grained et al. 2001; McCubbin et al. 2013), as evidenced by the orthopyroxenite with minor chromite, maskelynite, rare occurrence of halite, clays and oxy-hydroxides, and augite, apatite, pyrite, and carbonates (Mittlefehldt iddingsite (Treiman et al. 1993; Bridges and Grady 1994). It is of cumulate origin with a 4.091 Ga age, 1999). The presence of Cl-rich amphiboles suggests that and is extensively processed with a deep magma the magma was contaminated by Cl-rich surficial source similar to shergottites (Lapen et al. 2010). material (Sautter et al. 2006). McCubbin et al. (2013) Carbonates are indicative of near-surface interaction propose that the entire chassignite-nahklite body was (e.g., McSween and Harvey 1998; Eiler et al. 2002; exposed to Cl-rich fluid shortly after its shallow-level Halevy et al. 2011; Melwani Daswani et al. 2013). emplacement. Chlorapatite is the only phosphate There is evidence of shock, but delicate carbonate mineral and Cl content of nakhlites is higher than in features are preserved. 4 Z. Sharp et al.

Basaltic Breccia 3. Basaltic shergottites generally have d37Cl values in the range of 1to0&. NWA 7034 (Agee et al. 2013) and paired NWA 4. The cumulates (chassignites, nakhlites, and 7533 (Humayun et al. 2013) are an enriched breccia orthopyroxenite) all have positive d37Cl values, as with a significant crustal component. Distinct lithic does the breccia sample NWA 7034. The latter has clasts, many of which are apatite-bearing, have been an apatite grains with a d37Cl value over 8&. identified (Santos et al. 2013). Elevated D17O values 5. The augite basalt (NWA 8159) has a similar d37Cl suggest interaction with crust-atmosphere reservoir value to the related nakhlite NWA 5790, which is (Agee et al. 2013; Ziegler et al. 2013). NWA 7034 has thought to have formed at the highest levels of the very high bulk Cl and water content (Agee et al. 2013; nakhlite pile (Jambon et al. 2010). Humayun et al. 2013). An age of 4.43 Ga, disturbed at 6. There is no trend in the relationship between water- 1.7 Ga, is obtained from zircon (Humayun et al. 2013). soluble and structurally bound Cl. NWA 5790 and 37 This sample bears geochemical similarities to data Los Angeles have positive d Clsilicate – water soluble 37 obtained from spectroscopic surface measurements values and Zagami has a negative d Clsilicate – water (Agee et al. 2013). Shock level is low, as evidenced by soluble value. the lack of maskelynite (Agee et al. 2013). 7. There is no apparent correlation between d37Cl value and age. Mixed Lithology EETA79001 8. There is agreement between the d37Cl values measured in situ in apatite and bulk (Los Angeles This meteorite contains two distinct lithologies, A and NWA 7034). and B, related to two different magma sources (McSween and Jarosewich 1983), as well as a shock DISCUSSION glass, Lithology C, that famously contains trapped Martian atmospheric gases (Bogard and Johnson 1983; In order to understand the significance of the Becker and Pepin 1984). Lithology A is an olivine- Martian samples, it is necessary to set the framework normative feldspathic pyroxenite with anhedral for chlorine isotope variability as seen in other bodies orthopyroxene and olivine megacrysts, which may be (Fig. 2). Unaltered mantle-derived samples from Earth d37 & xenocrystic (Shearer et al. 2008). Lithology B is quartz- cover a narrow range of Cl values averaging 0.3 . normative, consisting of primarily pigeonite, augite, and C3 chondrites have a similar range, with somewhat d37 maskylinite-bearing basalt with glassy mesostasis. more scatter. Parnallee is anomalous with Cl values & Carbonates and sulfates are found in the lithology B of 4 (Sharp et al. 2013b). Lunar samples range from & d37 (Gooding et al. 1988). Oxidized chloride species and near zero to over 24 . The high Cl values are nitrate have been measured from EETA79001,652 rock thought to be due to anhydrous degassing of lunar d37 cuttings that include glass and carbonates, silica, and melts (Sharp et al. 2010). The lowest lunar Cl values sulfates (Kounaves et al. 2014). are similar to Earth and chondrites. High temperature equilibrium fractionation for Cl- d37 RESULTS bearing species is very small and cannot alter the Cl value of a material appreciably. Low temperature or All chlorine isotope data are reported in the near-surface processes that can cause significant standard delta notation, where d37Cl = Rsa 1 1000, fractionation include acid-degassing (Sharp et al. 2009), Rstd R = 37Cl/35Cl ratio of sample (sa) and standard pore water diffusion (Eggenkamp et al. 1994), biogenic (std). Samples are normalized to SMOC (standard mean and abiogenic perchlorate formation (Coleman et al. € ocean chloride) (Kaufmann et al. 1984) with a defined 2003; Sturchio et al. 2003; Bohlke et al. 2005), and value 0&. A number of general observations can be formation of organohalogen compounds (Reddy et al. made from the data shown in Fig. 1. 2002; Aeppli et al. 2013). These processes can account for all of the variation seen in terrestrial samples d37 1. The range of Cl values from this small data set is (Fig. 2). larger than for all terrestrial basalts and ultramafic samples (Barnes and Sharp 2006; Sharp et al. 2007, Two-Component Mixing 2013b; Barnes et al. 2008, 2009; Selverstone and Sharp 2011). The Martian data can be explained as a two- 2. The olivine-phyric shergottites and the basaltic component system. The olivine-phyric shergottites have shergottite Shergotty have d37Cl values ranging the lowest d37Cl values and are most representative of from 4to2&. the Martian mantle. The crustal contaminated samples The chlorine isotope composition of Martian meteorites 2 5

Ol-phyric shergottites NWA 7034 Basaltic shergottites 8.6 ‰ non-shergottites NWA 8159 SIMS bulk Chassigny NWA 5790

NWA 817 Mantle range ALH 84001 Los Angeles NWA 2737 Zagami NWA Shergotty 2975 DHO 019 EETA 79001B Tissint EETA 79001 A

Bulk LAR 06319 E RBT 04262

-4 -3 -2 -1 0 1 2 37Cl (‰ vs. SMOC)

Fig. 1. Chlorine isotope values of selected Martian meteorites. Triangles are ion probe analyses of single apatite grains, circles are for bulk rock structurally bound Cl. Duplicate analysis using both methods give overlapping results (Los Angeles and NWA 7034). Note: NWA 8159 and NWA 5790 may be related, an idea supported by the similar d37Cl values. Earth mantle value range shown by gray bar. Water-soluble data given in Table 1 are not shown. have the highest d37Cl values. Samples with intermediate All of the known inorganic fractionation processes d37Cl values, namely the majority of basaltic that could alter the chlorine isotope ratio of a mantle- shergottites, can be explained as a mixture of the mantle derived sample should lead to higher, not lower, d37Cl and crustal endmembers. In this scenario, the Martian values. Degassing should raise the d37Cl value of the mantle has a low d37Cl value of 3to4&, whereas residual melt. Contamination with the 37Cl-enriched the surﬁcial material has a variable, but high d37Cl crust would also raise the d37Cl value of the melt. value due to loss of light Cl to space (Fig. 1). Incorporation of typical chondritic material would drive the d37Cl value toward the typical 0& value The Mantle Component characteristic of chondrites (Sharp et al. 2013b). High- The olivine-phyric shergottites are thought to be the temperature igneous processes, such as differentiation most primitive basalts derived from the Martian mantle processes accompanying magma ocean and primordial (Peslier et al. 2010; Shafer et al. 2010; Gross et al. crust formation as well as partial melting and 2011), although contribution from a more evolved fractionation crystallization, should have no measurable crustal component has been suggested (Herd et al. 2002; effect on the d37Cl value of the melt or residue. Herd 2003). The olivine-phyric shergottites, along with On Earth, some marine sediments and volcanic Shergotty, have the lowest d37Cl values of any samples materials that have incorporated subducted sediments analyzed, and we assign a d37Cl value in the range of have negative d37Cl values (Fig. 2), but this is probably 4to3& to the mantle. We see no difference related to isotopically light organohalogen reactions between samples from depleted versus enriched (LREE/ occurring at the Earth’s surface. Even if some low- HREE) shergottite mantle sources (Williams et al. temperature process did produce light material on 2016), although small differences may emerge as Mars, there is no explanation as to why it would be additional samples are analyzed. incorporated only in the olivine-phyric shergottites (and 6 Z. Sharp et al.

volcanic ashes and lavas volcanic fumaroles marine sediments Liassic sediments (Switzerland) Crust altered oceanic crust ODP serpentinites (n >100) evaporites basement fluids Primitive Mantle-derived Unaltered (C3) Chondrites Other Chondrites

Mars

Moon

-4-20246810 12 1416 18 20 22 24 37Cl (‰ vs SMOC)

Fig. 2. Chlorine isotope composition of selected materials. The primitive mantle, evaporites, and ocean water account for the vast majority of Earth’s chlorine, and have d37Cl values close to 0&. Surﬁcial materials have a wider range of d37Cl values due to low-temperature interactions. The high d37Cl values of the Moon are related to anhydrous degassing. Sources of data: Bridges et al. (2004); Barnes and Sharp (2006); Bonifacie et al. (2007); Sharp et al. (2007, 2010, 2013b).

Shergotty). In lieu of a parent-body process to explain The occurrence of jarosite on Mars is evidence for the d37Cl values of the most primitive mantle-derived sulfuric acid reactions (Elwood Madden et al. 2004) samples, we conclude that the low d37Cl values of the that could produce HCl (g). olivine-phyric shergottites represent the unmodified High d37Cl values are found in all Martian samples Martian value that was inherited from the building that have evidence of crustal contamination. The blocks of Mars and probably reflect the solar nebula nakhlites are altered by near-surface processes, possibly value. by infiltration of evaporitic waters (Gooding et al. 1991; Sawyer et al. 2000; Bridges et al. 2001). Evidence The Crustal Component includes the occurrence of halite, clays, and oxy- Numerous isotopic systems demonstrate that the hydroxides (Bridges and Grady 1999), and iddingsite atmosphere of Mars has undergone mass-dependent (Treiman et al. 1993). The presence of Cl-rich fractionation during loss to space. This is most apparent amphiboles suggests that the magma was contaminated from the extremely high dD values measured in by evaporites (Sautter et al. 2006; Filiberto et al. 2014). numerous Martian meteorites (Zahnle et al. 1990; The augite basalt NWA 8159 is genetically related to Leshin 2000; Boctor et al. 2003; Greenwood et al. the nakhlites and has a similar d37Cl value (1.5 versus 2008). It is also seen in heavier isotope systems, such as 1.8&). Chassignites also have low-temperature Ar (Bogard 1997; Gillmann et al. 2011) and even Xe alteration minerals, including carbonate, sulfates, and (Pepin 1994). Early hydrodynamic escape during intense oxides (Wentworth and Gooding 1994; Bridges et al. extreme ultraviolet radiation (EUV) followed by 2001). ALH 84001 has carbonates indicative of near- prolonged atmospheric “erosion” explains the heavy surface interaction (e.g., McSween and Harvey 1998; isotope enrichments (Bogard 1997). Like other isotopic Eiler et al. 2002; Halevy et al. 2011; Melwani Daswani systems, Cl loss, presumably as HCl, would raise the et al. 2013) and very high 129Xe/132Xe ratios, consistent d37Cl value of residual material. HCl is a product of with atmospheric contamination (Swindle et al. 1995). volcanism and could also have formed by NaCl Finally, the impact melt breccia NWA 7034 (Agee et al. breakdown during reaction with sulfuric acid according 2013) has extensive evidence of crustal assimilation and to the reaction D17O anomalies (Ziegler et al. 2013). Not surprisingly, it has the highest d37Cl values measured to date. The conclusion that high d37Cl values are a þ ¼ þ : NaCl H2SO4(aq) NaHSO4(aq) HCl (g) (1) signature of surface alteration is supported by D33S data The chlorine isotope composition of Martian meteorites 2 7

(Williams et al. 2016). All samples with high d37Cl 2 values also have evidence for mass independent sulfur ) 2000ppm 33 C 1 isotope fractionation (D S anomalies), indicative of a O pm 1000p crust = 2‰

crustal/atmospheric contaminant (Franz et al. 2014). S 0 Cl isotope range d37 & s Three samples with Cl values less than 2 that v of basaltic shergottites also have published D33S data show no mass -1

‰

( independent sulfur anomaly. Of the olivine-phyric l -2 shergottites, only EETA79001A has evidence for mass C

37 independent fractionation, which may be explained by a -3 small amount of contamination by impact glass (see mantle (olivine-phyric shergottites) -4 Williams et al. [2016] for further details). EETA79001 0 0.02 0.04 0.06 0.08 0.1 also contains perchlorate (Kounaves et al. 2014), which X is almost certainly a near-surface contaminant. crust Perchlorates may in part contribute to the high Fig. 3. Calculated mixing curves between mantle and crust. d37Cl values of the surficial component, but this is Assumed endmembers are the following: mantle—20 ppm Cl, d37 = & — + & – difficult to assess. High perchlorate concentrations have Cl 3 ; crust 1000 ppm, 2 . Only 1 2% of crustal contamination will raise the d37Cl value of a mixed sample been detected on Mars (Hecht et al. 2009) and traces of (basaltic shergottites) from 3& to between 1 and 0&. perchlorate have been found in Martian meteorites (Kounaves et al. 2014). Thermodynamically, perchlorate should be enriched in 37Cl relative to chloride, but both crustal component (Longhi 1991; Norman 1999), but positive and negative fractionations have been measured retains mantle-like d37Cl and D33S values (Williams in natural terrestrial samples, suggesting strongly et al. 2016). Shergotty can also be modeled as a mixture nonequilibrium oxidation (Hoering and Parker 1961; between a depleted mantle source and <1% late trapped Coleman et al. 2003; Sturchio et al. 2003; Bohlke€ et al. liquid (Borg and Draper 2003). The Cl and S isotope 2005; Jackson et al. 2010). data are consistent with addition of a differentiated crustal melt that has not been contaminated by surficial Mixed Samples material. The basaltic shergottites Zagami, Los Angeles, An alternative to the two-component mixing NWA 2975, and EETA 79001B all have d37Cl values scenario outlined above is that the basaltic shergottites between 1 and +0.2& (Fig. 1). These samples are represent a third mantle source with d37Cl values similar easily explained in terms of mixing between the mantle to terrestrial basalts. The negative d37Cl value of and crustal source. This mixing of a crustal component Shergotty, however, argues against the three component may involve various types of processes such as Cl isotope model for Mars. The many geochemical assimilation, near-surface alteration, hydrothermal similarities of olivine-phyric shergottites and basaltic exchange, or addition of an impact component (impact shergottites (Barrat et al. 2002; Borg and Draper 2003; glass). As shown in Fig. 3, even a small addition of Shearer et al. 2008; Symes et al. 2008; Peslier et al. crustal material to a mantle-derived sample will raise 2010) further argues against a third reservoir and the d37Cl value appreciably. The mixing curve in Fig. 3 instead favors trace contamination of the basaltic is generated assuming that the mantle has a d37Cl value shergottites by a surficial component. of 3& and a Cl concentration of ~20 ppm, based on the lowest concentrations in Table 1 and from previous Comparison Between “Similar” Meteorites studies (Dreibus et al. 2006; Filiberto and Treiman 2009; Bogard et al. 2010). We conservatively estimate a The basaltic shergottites Shergotty and Zagami are minimum d37Cl value of 2& and a concentration of petrologically and geochemically similar (Shih et al. 1000 ppm for the surficial component, although 1982), as are the chassignites Chassigny and NWA concentrations from 0.5 to greater than 1 wt% have 2737. Nevertheless, Shergotty and NWA 2737 have been measured at the surface (Rieder et al. 2004; Keller mantle-like d37Cl values, whereas Zagami and et al. 2006; Litvak et al. 2014). With these boundary Chassigny have values consistent with crustal conditions, 1–2% crustal contamination will raise the contamination. We propose in this section that the d37Cl value from 3& to the 1to0& range found in different Cl isotope ratios of these similar meteorites is basaltic shergottites (Fig. 3). The large concentration due to the unique chemical behavior of Cl. Introduction difference between the mantle and crust makes Cl an of even minor amounts of low-temperature aqueous extremely sensitive indicator of crustal contamination. fluids along porosity or fractures can introduce Cl but Interestingly, Shergotty has geochemical evidence for a essentially no other elements, due to its unique 8 Z. Sharp et al.

Table 1. Chlorine isotope concentration and d37Cl values of selected Martian meteorites. Each SIMS analysis is the average of all analyses made on a single apatite grain. Reproducibility of d37Cl values is 0.26& for bulk analyses and approximately 0.5& for ion probe analyses. Blank columns indicate that the analysis was not made because the samples were too small or not measured. d37Cl apatite d37 d37 Enriched/ Cl content (ppm) Cl (structural) Cl (w.s.) (SIMS) Sample depleted Structural Water soluble & vs SMOC Fall/find Ol-phyric shergotites RBT 04262 Enriched 3.5 Find 2.5 2.8 LAR 06319 Enriched 2.8 Find 3.8 DHO 019 Depleted 0.6 Find 1.6 Tissint Depleted Fall Igneous fraction 69 136 2.9 0.0 Glass fraction 30 174 2.0 0.4 Bulk n.d. n.d. 0.6 n.d. EETA 79001A 180 n.d. 2.7 n.d. Find Basaltic shergotites Shergotty Enriched 3.3 Fall Los Angeles Enriched 115 10 0.3 0.5 0.4 Find 0.6 Zagami Enriched 49 28 0.9 0.4 Fall NWA 2975 Enriched 24 6 0.1 Below detect. Find EETA 79001B 42 n.d. 0.2 n.d. Find Other NWA 5790 Nakhlite 72 11 1.8 0.4 Find NWA 8159 Augite basalt n.d. n.d. 1.5 n.d. Find Chassigny Chassignite 0.3 Fall 0.4 NWA 2737 Chassignite n.d. n.d. 3.9 n.d. ALH 84001 Orthopyroxenite 1.4 Find NWA 7034 Impact melt 628 n.d. 1.0 n.d. 0.1 Find breccia 0.4 0.4 0.4 0.5 0.5 0.5 0.5 0.6 0.6 0.7 0.7 0.7 0.9 1.1 1.1 1.4 1.5 8.6 Avg. 1.1 The chlorine isotope composition of Martian meteorites 2 9 hydrophilic character (Sharp and Draper 2013). Cl 1994; Bridges et al. 2001), whereas NWA 2737 has isotope geochemistry is, therefore, a particularly little evidence for low-T alteration (Treiman et al. sensitive indicator of near-surface alteration. 2007). Shergotty and Zagami have similar mineralogy NWA 2737 underwent two shock events. The first (Stolper and McSween 1979; Treiman 1985), and REE was intense (~S5–S6 on the scale of Stoffler€ et al. 1992) patterns, Rb-Sr and Nd-Sm isochrons, and 39Ar/40Ar which resulted in brown shocked olivine (Treiman et al. ages (Shih et al. 1982). They have some of the highest 2007; Blaß€ et al. 2010). This event redistributed Cl d7Li values of any Martian meteorite (Magna et al. (Giesting et al. 2015), but did not necessarily result in 2015), have similar rare gas chemistry (Schwenzer et al. addition of surficial Cl. If an aqueous fluid had not 2007) and even crystal size distributions (Lentze and introduced surficial Cl to the chassignite precursor prior McSween 2000). There are several important chemical to shock, then there should be no change in the d37Cl difference between Zagami and Shergotty. Amphibole value of the rock. The second shock event affected both and apatite in these two meteorites have subtle to chassignites and is probably responsible for the ejection substantial differences in variations in Cl, F, and OH of these samples from Mars. These two shock events concentrations, and D/H ratios (e.g., Watson et al. may also contribute to the very high dD values of both 1994; Boctor et al. 2003; McCubbin et al. 2012). NWA 2737 and Chassigny (Giesting et al. 2015). Like Zagami has a D33S anomaly of 0.017&, whereas Zagami, the D33S value of Chassigny is nonzero at Shergotty has none (D33S = 0.003&) (Franz et al. 0.033& (sulfate fraction—acid-volatile sulfur was not 2014). The non-zero D33S value of Zagami is interpreted measured), indicating a crustal contribution. To our as evidence of surficial contamination (Franz et al. knowledge, the D33S value of NWA 2737 has not yet 2014). The same conclusion is obtained from the Cl been measured. isotope data, in which Shergotty has a mantle-like d37Cl The d37Cl value of Chassigny indicates that at some value and Zagami has a crustal signature. Therefore, point in its history, there was addition of crustally both sulfur and chlorine isotope data indicate the derived Cl. This may have accompanied the addition of addition of a low-temperature crustal component. carbonates and sulfates (the nonzero D33S value of Assuming that the incorporation of surficial Cl and S Chassigny sulfate supports a Martian origin). We would occurred at low temperatures, there is no reason to predict that apatite grains in melt inclusions hosted in expect that this event would necessarily be recorded by olivines should have a mantle signature and are in the other geochemical systems. Zagami and Shergotty both process of preparing for these analyses. have minor Xe isotope evidence of Martian atmosphere (Mathew et al. 1998), although this could be introduced Possible Explanations for the Light Cl Isotope along open fractures during shock related to ejection Composition of Primitive Martian Samples from Mars. NWA 2737 and Chassigny are olivine cumulates The d37Cl values of the Earth and of most that crystallized from primitive melts at depth carbonaceous CV3 chondrites are close to 0&.In (Nekvasil et al. 2007; Giesting et al. 2015). Chassigny contrast, the least processed Martian samples have is more evolved than NWA 2737. Chassigny has d37Cl values at least as low as 3&. We infer that these plagioclase while NWA2737 does not (Beck et al. 2006) low values represent the primitive Martian mantle and and NWA 2737 has more Mg-rich olivine than are not a result of later processing as there are no Chassigny (McCubbin et al. 2013). On the basis of the planet-body processes that we are aware of that could Cl content of apatites, McCubbin et al. (2013) lower the d37Cl value of a primitive mantle-derived concluded that NWA 2737 did not receive as much Cl- melt. Why then, does Mars have a negative bulk d37Cl enrichment as Chassigny. McCubbin and Nekvasil value when Earth, perhaps the Moon, and most (2008) suggested that the exogenous Cl was added after chondrites—materials which physically bracket the melt inclusions formed but before intercumulus melt position of Mars in the solar system—have near-zero crystallization, evidenced by the higher F contents in values? The answer may lie in the very different the melt inclusions hosted in olivines compared to the formation history of Mars compared to Earth. more Cl-rich apatites in the groundmass. NWA 2737 is Geochemical constraints derived from short-lived also thought to have formed stratigraphically below radioisotope systems suggest an age of Mars formation Chassigny (McCubbin et al. 2013) and may have as only a few million years after the first solids of the received less crustal contamination. Giesting et al. solar nebula (Dauphas and Pourmand 2011; Tang and (2015) suggested that the chassignites were emplaced at Dauphas 2014). Due to its small size, any magma shallow levels of <200 m depth. Chassigny has signs of ocean that existed on Mars would cool rapidly, within low-temperature alteration (Wentworth and Gooding 5 Ma (Elkins-Tanton 2008). In contrast to Earth, 10 Z. Sharp et al.

which formed as a protracted accretion of planetary 800 0.16 Cl/Na embryos, Mars’ evolution was arrested during the early 700 Cl (ppm) 0.14

stage of planetary formation (Brasser 2013). We ) 600 0.12

m 37 l

d N

suggest that the Cl value of the Martian mantle is p

( 500 0.1

(

c closer to that of the solar nebula, and that Earth, and m

o 400 0.08

s most chondrites, have been modiﬁed to higher values l

) by later processes. C 300 0.06 The d37Cl values of most chondrites range from 2 200 0.04 to +1& (Sharp et al. 2013b), although several, including 100 0.02 ordinary chondrites the least altered ordinary chondrite NWA 8276 (type 0 0 L3.00) and Parnallee (type LL3.6) have d37Cl values less CI CM CV CO CK H L LL R EH EL Chondrite type than 4& (Williams et al. 2016). We propose that these light values are, like Mars, representative of the solar Fig. 4. Cl content and Cl/Na ratios of various types of nebula. Chondrites with higher d37Cl values are chondrites. The low Cl contents of the ordinary chondrites explained by later incorporation of heavy Cl caused by suggests that they have not suffered late addition of HCl- hydrate. Data from compilation by Lodders and Fegley later fractionation in the solar nebula. (1998). In the early high-temperature phase of the inner solar nebula, Cl existed as HCl (g) and perhaps minor NaCl (g) below 1100 K. The first condensation of Cl fractionation associated with HCl hydrate formation occurs during the formation of sodalite (Na4Al3Si2O12Cl) would not have occurred so close to the Sun, so that between 900 and 950 K (Fegley and Lewis 1980). A the most pristine mantle samples, the olivine-phyric fraction of the nebular NaCl (g) is incorporated into shergottites, retain this early solar isotopic signature. solid sodalite and the remaining Cl condenses as NaCl (s) Martian samples with higher d37Cl values can be at 600 K (Sharp et al. 2013b). These two Cl fractions are completely explained by preferential volatile loss of incorporated into the precursors of Mars, chondrites, 35Cl. and Earth. The Cl isotope fractionation during these The low Cl content and d37Cl values of some high temperature condensation processes should be no ordinary chondrites suggest that they have escaped later more than 0.3& (Sharp et al. 2007), so that the d37Cl addition of HCl hydrate, in contrast to the other value of this early condensate reflects the solar value of chondrite groups. Alternatively, the low Cl content of ~5to3&. The low d37Cl values of early-formed ordinary chondrites could be related to Cl loss for this sodalite inclusions in CAIs (Sharp et al. 2007) may partly group only, but if this were the case then they should reflect the light solar value. have higher d37Cl values than the other groups. The Cl that does not form sodalite or NaCl (s) The Earth-Moon system formed inside the “ice . remains as HCl (g) until HCl hydrate (HCl 3H2O) line,” a region where formation of HCl-hydrate becomes stable at ~150 K (Zolotov and Mironenko probably did not occur. Addition of HCl-hydrate 2007). This reaction can only occur beyond the HCl cannot explain the high d37Cl values of the Earth. hydrate “ice line,” which is probably in the radial Instead, we suggest that the high d37Cl values are due to position of the asteroid belt. The Cl isotope protracted “impact erosion” that occurred during fractionation between HCl hydrate and HCl gas Earth’s formation and degassing of basaltic melts. The 37 (D ClHCl.3 H2O – HClg) at 150 K is 3–6& (Schauble and Cl content of Earth is drastically reduced relative to the Sharp 2011). If there had been partial condensation of expected value estimated from the planetary volatility HCl hydrate from HCl (g), then the newly formed HCl trend (Sharp and Draper 2013). The explanation for the hydrate would have had a heavy d37Cl value relative to depletion is Cl loss accompanying the multiple the light nebular value. Incorporation of this heavy Cl planetesimal/planetary embryo collisions during the into asteroids would raise their d37Cl value by several planet’s growth (Rubie et al. 2011). Volatile loss of Cl per mil. Zolotov and Mironenko (2007) propose during these energetic collisions could cause a addition of HCl hydrate to explain alteration and high preferential removal of light Cl to space. The Moon Cl contents of many chondrites. The ordinary preserves striking evidence of light Cl loss to space, with chondrites have lower Cl contents and Cl/Na values d37Cl values in excess of 20& (Sharp et al. 2010; than the other petrographic types (Fig. 4) suggesting Tartese et al. 2014; Treiman et al. 2014). Presumably that they have had minimal late addition of HCl the proto-Earth would have undergone a similar fate. It hydrate. They are also the only group that has low is therefore reasonable to conclude that the d37Cl of d37Cl values of 5to4&. Presumably Mars would Earth has been modified to higher values during have been inward of the HCl hydrate ice line. The large accretion, and that the 0& Earth value is not The chlorine isotope composition of Martian meteorites 2 11 representative of the solar nebula. Measurements of the supported by a grant from the NSF Instrumentation and Cl isotope composition of the solar wind, captured by Facilities Program. the Genesis mission, could settle this issue but the analyses of the miniscule amount of Cl retrieved from Editorial Handling—Dr. Justin Filiberto Genesis will require a precision in the permil range and solar wind fractionation effects will have to be REFERENCES quantified at this same level. We recognize that an alternative scenario is that the Aeppli C., Bastviken D., Andersson P., and Gustafsson O. 2013. 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