sign in sign up
<<
Home , Adirondack (Mars), Forsterite, Grant meteorite, Meteorite, Meteoritical Society, Olivine

& 48, Nr 5, 854–871 (2013) doi: 10.1111/maps.12092

Petrography, chemistry, and crystallization history of -phyric shergottite NWA 6234: A new melt composition

Juliane GROSS1*, Justin FILIBERTO2, Christopher D. K. HERD3, Mohit MELWANI DASWANI4, Susanne P. SCHWENZER4, and Allan H. TREIMAN5

1Department of and Planetary Sciences, American Museum of Natural History, Central Park West at 79th St, New York, New York 10024, USA 2Department of Geology, Southern Illinois University Carbondale, Carbondale, Illinois 62901, USA 3Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada 4CEPSAR, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK 5Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, Texas 77058, USA *Corresponding author. E-mail: [email protected] (Received 24 August 2012; revision accepted 24 January 2013)

Abstract–Knowledge of igneous and compositions is crucial for understanding ’ mantle evolution, including early differentiation, , and the chemical alteration at the surface. Primitive provide the most direct information about their mantle source regions, but most Martian either contain cumulate olivine or crystallized from fractionated melts. The new Martian Northwest Africa (NWA) 6234 is an olivine-phyric shergottite. Its most magnesian olivine cores (Fo78)arein Mg-Fe equilibrium with a of the bulk composition, suggesting that it represents a melt composition. Thermochemical calculations show that NWA 6234 not only represents a melt composition but is a primitive melt derived from an approximately Fo80 mantle. Thus, NWA 6234 is similar to NWA 5789 and Y 980459 in the sense that all three are olivine- phyric shergottites and represent primitive magma compositions. However, NWA 6234 is of special significance because it represents the first olivine-phyric shergottite from a primitive ferroan magma. On the basis of Al/Ti ratio of in NWA 6234, the minor components in olivine and , and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars’ upper . The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a last stage of crystallization, merrillite reacted with an OH-Cl-F-rich melt to form apatite. As this meteorite crystallized completely at depth and never erupted, it is likely that its apatite compositions represent snapshots of the volatile ratios of the source region without being affected by degassing processes, which contain high OH-F content.

INTRODUCTION Draper 2004; Musselwhite et al. 2006; Draper and Agee 2008; Usui et al. 2008). Olivine-phyric shergottites have The compositions of Martian basaltic magmas can been recognized as a significant and important subgroup provide crucial clues for understanding Mars’ mantle of the Martian shergottites (Goodrich 2002). Their evolution and its volatile budget. In particular, Martian relatively high bulk rock and olivine core basaltic shergottites have yielded many insights into the numbers (Mg# = molar Mg/[Mg+Fe]) suggests that they ’s bulk composition, its differentiation, the nature could represent primitive melts, i.e., unfractionated of distinct geochemical reservoirs, and the geologically liquids formed by direct of the mantle. recent ages of magmatic activities on Mars (e.g., Primitive melts can provide direct information about Dreibus and Waenke 1982, 1985; 1986; Waenke their mantle source regions, including compositions and 1991; Borg and Draper 2003; Treiman 2003; Agee and (e.g., Langmuir et al. 1992; Asimow and

© The , 2013. 854 Olivine-phyric shergottite NWA 6234 855

Longhi 2004). However, most olivine-phyric shergottites the pre-eruptive volatile history of NWA 6234 based on have a bulk rock Mg# that is too high to have been in merrillite/apatite interaction with an OH-Cl-F-rich melt. equilibrium with their most magnesian (e.g., Northwest Africa (NWA) 1068, (DaG) SAMPLE AND ANALYTICAL TECHNIQUE 476, Sayh al Uhaymir (SaU) 005, Elephant moraine (EET) A79001 -A, Dhofar 019). Therefore, Meteorite NWA 6234 was found in 2009 at an they may not represent magma compositions, but undisclosed location in Mali and purchased by an instead may contain cumulate olivine crystals and/or anonymous collector in February 2010. It was a 55.7 g have been affected by magmatic contamination (e.g., partly fusion-crusted stone, cross-cut on the interior by McSween and Jarosewich 1983; Wadhwa et al. 2001; several thin shock veins. A 3.31 g slice of the meteorite, Barrat et al. 2002; Goodrich 2002, 2003; Taylor et al. which included a possible shock , was purchased 2002; Shearer et al. 2008; Papike et al. 2009; Filiberto from Marmet Meteorites. The sample was split, and et al. 2010b, 2012; Filiberto and Dasgupta 2011). distributed to an international consortium team for Extensive work on olivine-phyric shergottites has investigations, including this study (Filiberto et al. already given clues to their crystallization history, 2011). Filiberto et al. (2012) reported on the bulk rock magma versus accumulation issues, volatile history, and of this meteorite. Analyses here are from fugacity of their source regions (e.g., Herd 2003, two polished thick sections that contain a melt vein that 2006; Filiberto and Treiman 2009; Filiberto et al. 2010b, cuts through the sections (Fig. 1). 2012; Gross et al. 2011; McCubbin et al. 2012). For Backscattered electron (BSE) images were taken example, experimental and mineralogical studies have with the Cameca SX100 electron microprobes (EMP) at shown that some olivine-phyric shergottites represent NASA Johnson Space Center (JSC) and at the magma compositions, while many others contain up to American Museum of Natural History (AMNH). These 30% cumulate (phenocrystic or xenolithic) material images were used to determine the textural (Treiman et al. 1994; Musselwhite et al. 2006; Usui et al. characteristics and the modal mineral abundance using 2008; Filiberto et al. 2010b; Filiberto and Dasgupta techniques described by Maloy and Treiman (2007). 2011; Gross et al. 2011). Based on the compositions of Mineral chemical compositions were obtained with apatite in olivine-phyric shergottites, the magmas in the Cameca SX100s at NASA JSC and at the AMNH. equilibrium with the apatite are thought to be enriched Operating conditions for other than apatite in chlorine compared with terrestrial , and to (see below) were: 15 kV accelerating voltage, 15–20 nA contain a range of water concentrations (Filiberto and beam current, focused electron beam (1 lm in size), and Treiman 2009; Patino~ Douce and Roden 2006; Patino~ peak and background counting times of 20–40 s per Douce et al. 2011; McCubbin et al. 2012; Gross et al., element. Analytical standards were well-characterized personal communication). Furthermore, studies of synthetic oxides and natural minerals including olivine-- equilibrium from olivine-phyric (Si, Ca, Mg,), olivine (Si, Mg, Fe), oligoclase, albite, shergottites provide evidence for low oxygen fugacities (Na), (Fe), rutile (Ti), corundum, (approximately 1–3 log units below the QFM buffer) in (Al), (Cr), Ni-diopside (Ni), rhodochrosite Martian basalts and their mantle source region; more (Mn), (K), and apatite (P). Data quality oxidized groundmass assemblages in these meteorites was ensured by analyzing standard materials as may reflect derivation and mixing of distinct magmas unknowns. from oxidized source regions, the effects of degassing or Apatite analyses were undertaken using the method internal fractionation of ferric , or as well as a of Goldoff et al. (2012) and Webster et al. (2009) to possible oxidizing agent within the Martian crust that minimize F, Cl, and Na loss during analysis. Goldoff has contaminated some magmas during eruption and/or et al. (2012) and Webster et al. (2009) showed that the emplacement (Herd et al. 2002; Herd 2003, 2006; Peslier best apatite analyses are obtained if Na, Cl, and F are et al. 2010). analyzed first with an acceleration voltage of 10 kV and Here, we describe the mineralogy, , and 4 nA beam current. Other elements (P, Si, Fe, Mg, Al, mineral chemistry of the new olivine-phyric shergottite Mn, Ti, Ca, K, S, and Ce) can be analyzed thereafter North West Africa 6234 (hereafter NWA 6234). From with an acceleration voltage of 15 kV and 20 nA beam petrographic observations, microprobe analyses, and current. All apatite analyses were obtained (and zonation patterns of minerals of a doubly polished thick calibrated) with a defocused electron beam (10 lmin section, we constrain various aspects of its petrologic diameter). Peak and background counting times were history, including conditions of crystallization (pressure, 30 s and 15 s per element. Analytical standards were temperature, and oxygen fugacity) as well as the well-characterized synthetic and natural oxides and crystallization sequence of this magma. We also constrain minerals including diopside (Si, Mg), K- (Al, 856 J. Gross et al.

Fig. 1. BSE images of NWA 6234. Larger image shows the overall fine-grained texture, which is composed of olivine (Ol) crystals (0.1–1.0 mm diameter), set in a finer grained groundmass (see detailed inset) of pyroxene (Pyx), (Mask), ferroan olivine (Ol), oxides (Ox), merrillite (Merr), and apatite (Ap). The arrow points to the melt vein in the rock that runs almost horizontally to the right.

K), (Ca), olivine (Fe), jade (Na), rutile (Ti), image analysis of a BSE image mosaic of a whole slab 2 MgF2 (F), rhodochrosite (Mn), boracite (Cl), face (135 mm ) using a technique similar to that of (S), CePO4 (Ce), and berlinite (P). Data quality was Maloy and Treiman (2007) resulting in 35% olivine; ensured by analyzing standard materials as unknowns. 38% pyroxene; 24% ; and 3% oxides, Hydroxyl in apatite cannot be measured directly by phosphates, and accessories. EPMA; however, the missing component in the X-site of the apatite was calculated from stoichiometry as OH- Olivine (assuming no vacancies or O2À substitutions), using the method of McCubbin et al. (2011). Olivine grains have euhedral to subhedral shapes. Ka X-ray intensity maps for P, Al, Ca, Cr, Fe, Mg, Their size distribution is seriate, i.e., grains range and Ti were obtained using the SX100 at the AMNH more or less continuously in size, from approximately with an accelerating voltage of 15 kV, beam current 10–15 lm in length up to approximately 1.0 mm between 40 and 100 nA, beam diameter of 1 lm, and (Fig. 1). The largest olivine grains contain inclusions of pixel spacing between 1 and 2 lm. Five images were chromite and of partially crystallized melts. The melt acquired simultaneously. With two cycles per image, 10 inclusions are commonly composed of glassy material elements could be mapped. Phosphorus was mapped on with tiny dendritic crystals of pyroxene and/or olivine three spectrometers simultaneously (a PET and TAP and/or plagioclase. The host olivine rarely shows Fe/Mg crystal and a large PET [LPET] crystal) to improve the zoning at the contact with the . counting statistics and the signal-to-noise ratio. Dwell Olivine grains larger than 200 lm in size are zoned. times ranging from 100ms to 260ms per point were The cores are as magnesian as Fo78 (Table 1; Figs. 2 and sufficient to detect phosphorus zoning. 3) and are homogeneous, with consistently higher Fo content than the mantle areas (Figs. 2 and 3). Outside the PETROLOGY AND MINERALOGY core zone, the Fo content slowly decreases (Fo76–62) with slowly increasing CaO content in the wide mantle area NWA 6234 is an olivine-phyric shergottite with an toward the rim. The rim has a Fo content from Fo75–68 unusual texture compared with other olivine-phyric (Figs. 2 and 3). Because olivine shows a seriate texture, shergottites. It is relatively unaltered, fine-grained, and the cores of smaller grains have a Fo content equal to composed of olivine crystals (0.1–1.0 mm diameter) set that of the mantle of slightly larger grains (Fig. 2). in a finer grained groundmass of pyroxene, maskelynite, Olivine grains smaller than 200 lm are not zoned and are ferroan olivine, spinel, ilmenite, merrillite, apatite, and iron-rich ranging from Fo55 to Fo43 in very small grains accessory Fe-sulfide (Fig. 1; inset). The modal (Fig. 2). Minor elements in olivine have almost no mineralogy, by volume, of the rock was determined by relationship with Fo content, as Fo decreases only MnO Olivine-phyric shergottite NWA 6234 857

Table 1. Representative microprobe analyses of olivine. Large olivine Medium olivine Core Mantle Rim Core Mantle Rim Matrix olivine Oxide wt% SiO2 39.17 38.34 37.83 37.39 35.94 36.15 37.83 37.39 36.30 36.28 36.26 35.22 34.10 34.05 TiO2 bd bd 0.02 0.04 0.05 0.02 0.02 0.04 0.02 bd 0.01 0.05 0.04 0.04 Al2O3 0.12 0.78 0.27 0.04 0.04 0.06 0.27 0.04 0.14 0.07 0.05 0.57 bd 0.25 Cr2O3 0.03 0.05 0.04 0.09 0.03 0.04 0.04 0.09 0.04 0.04 0.02 0.04 0.01 0.01 V2O3 na na 0.03 bd na na 0.03 bd bd 0.01 0.02 na na na FeO 20.36 19.81 26.94 26.79 34.82 34.18 26.94 26.79 31.40 31.19 36.95 38.29 45.53 44.54 MnO 0.43 0.42 0.50 0.54 0.63 0.67 0.50 0.54 0.61 0.56 0.72 0.77 0.88 0.87 MgO 40.06 40.11 33.99 35.20 28.30 29.43 33.99 35.20 30.78 31.38 26.14 25.10 20.02 19.54 CaO 0.15 0.12 0.23 0.09 0.16 0.14 0.23 0.09 0.13 0.11 0.13 0.18 0.13 0.22 NiO 0.08 0.10 0.05 0.07 0.07 0.04 0.05 0.07 0.02 0.10 bd 0.03 0.04 0.05 CoO bd 0.03 bd bd na na bd bd 0.04 0.01 0.03 0.06 na na ZnO 0.03 bd 0.04 0.04 na na 0.04 0.04 bd bd bd 0.05 na na P2O5 0.05 0.04 na na 0.03 0.04 na na na na na 0.04 0.05 0.12 Total 100.49 99.79 99.94 100.29 100.06 100.77 99.94 100.29 99.48 99.75 100.32 100.40 100.79 99.69 Normalized to 8 oxygen Si 1.00 0.99 1.01 0.99 1.00 0.99 1.00 1.01 1.00 0.99 1.01 0.99 0.99 1.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al 0.00 0.02 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.00 0.01 Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 V 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Fe 0.44 0.43 0.60 0.60 0.81 0.78 0.60 0.59 0.72 0.71 0.86 0.90 1.11 1.09 Mn 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 Mg 1.53 1.54 1.35 1.39 1.17 1.20 1.38 1.38 1.26 1.28 1.09 1.05 0.87 0.86 Ca 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 Ni 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Co 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Zn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 P 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total 2.99 3.00 2.99 3.00 3.00 3.01 3.00 2.99 3.00 3.01 2.99 3.00 3.00 2.99 Fo 77.81 78.30 69.22 70.08 59.16 60.55 69.86 70.21 63.60 64.20 55.78 53.88 43.94 43.89 na = not analyzed; bd = below detection limit. increases significantly, while CaO increases slightly Pyroxene (Fig. 3). NiO and Cr2O3 show no correlation. X-ray maps of phosphorus in the olivine (Fig. 4) Pyroxene occurs in the groundmass as subhedral to reveal oscillatory zoning patterns mostly parallel to the anhedral grains clustered together. Single grains are olivine edges, of thin (<10 lm) phosphorus-rich zones rare, the largest subhedral prismatic grain measures alternating with wider (50 to >150 lm) phosphorus-poor approximately 300 lm in length (Fig. 1). The cores are zones. The phosphorus zoning does not correlate with Mg-rich orthopyroxene (En70Fs26Wo4), which zone that of any other element. The larger grains typically outward by continuously increasing Ca and Fe contents have a thin phosphorus-rich core region followed by a to (En46-49Fs29–43Wo11–19) (Table 2; Fig. 5). A very wide (>150 lm) phosphorus-poor region, and strong second type of pyroxene found interstitially is a Ca-rich oscillatory zoning toward the rim. As the olivine grains augite (En37Fs26W37) (Table 2; Fig. 5). or melt show seriate texture, it is not surprising that some of the inclusions appear to be absent from pyroxene grains. smaller grains have strong oscillatory phosphorus zoning starting from the core toward the rim. The very small Oxides olivine grains with Fo55–43 appear to be homogeneous in terms of phosphorus and other elements. Areas The oxide minerals observed are ilmenite and immediately around melt inclusions and chromite spinel ranging in composition from chromite to inclusions show neither enrichment nor depletion in titanomagnetite. They are present as small grains and phosphorus. inclusions in other minerals throughout NWA 6234. 858 J. Gross et al.

Fig. 2. abundance in large olivine (>600 lm; circles), smaller sized olivine (<600 lm; squares), and the Fe-rich < l olivine in the groundmass ( 200 m; diamonds). Note the Fig. 4. Element map of phosphorous in olivine. The zoning overlap of the composition, suggesting that the olivines are has an oscillatory pattern mostly parallel to the olivine edges, cogenetic and seriate in nature. with thin (<10 lm) phosphorous-rich zones alternating with wider (50 to >150 lm) phosphorous-poor ones. The bright white spots in the groundmass are merrillite and apatite crystals.

Fig. 3. Chemical zoning profile across a large olivine crystal from core to rim. “Fo” is content (molar Mg/ (Mg+Fe) 9 100), left y-axis; other elements are in wt%, right y-axis. For details, see text.

Olivine crystals and their melt inclusions contain euhedral chromite grains (Fig. 1). Subhedral to anhedral Fig. 5. Pyroxene quadrilateral for NWA 6234. Pyroxene shows systematic chemical zoning, core to rim, from grains of chromite, ilmenite, and titanomagnetite occur in magnesian orthopyroxene () to pigeonite. A second the groundmass, typically as larger grains cored by type of pyroxene found interstitially is Ca-rich augite. The chromite with titanomagnetite rims or as ilmenite- range of olivine compositions is also shown at the bottom of titanomagnetite intergrowths interstitial to pyroxene and the pyroxene quadrilateral. The cores of large olivine grains plagioclase. Chemical analyses of oxide minerals are (brown circles) have slightly higher Mg# than the core pyroxene (orange circles); olivine mantles ( circles) have given in Table 3. Ferric iron (i.e., component) similar Mg# to intermediate pyroxene compositions (blue is calculated from charge balance, assuming that the circles). For detailed explanation, see text. oxides contain no cation vacancies. The chromite grains vary in composition according to (Usp = molar 2Ti/[2Ti+Cr+Al+Fe3+]) and 1–5% magnetite their setting. Chromite inclusions in olivine and cores component (Mt = molar Fe3+/[2Ti+ Cr+Al+Fe3+]). Several of groundmass grains are the most magnesian and larger grains in the groundmass are zoned, with rims as Cr-rich, with 79–83% chromite (Chr = molar Cr/ Ti-rich as Chr29Sp7Usp48Mt16. Fe-Ti oxide pairs in the [2Ti+Cr+Al+Fe3+]) and 11–12% spinel (Sp = molar Al/ groundmass consist of titanomagnetite of composition 3+ € [2Ti+Cr+Al+Fe ]) with 4–5% ulvospinel component Chr2–3Sp3–5Usp64–74Mt21–28 and ilmenite with 3–8% Olivine-phyric shergottite NWA 6234 859

Table 2. Representative microprobe analyses of pyroxene. Core Mantle-Rim High Ca Oxide wt% SiO2 53.16 55.10 52.89 50.74 49.95 49.26 TiO2 0.25 0.17 0.17 0.28 1.09 0.57 Al2O3 1.95 1.03 0.60 1.24 1.81 1.91 Cr2O3 0.27 0.86 0.37 0.48 0.13 0.34 FeO 18.39 16.73 20.66 19.86 15.79 17.72 MnO 0.54 0.62 0.79 0.82 0.58 0.61 MgO 24.75 24.89 20.53 16.85 12.67 12.16 CaO 1.20 2.06 4.91 9.22 17.73 15.31 NiO 0.02 0.02 na na na bd Na2O 0.11 0.03 0.06 0.11 0.25 0.24 K2O bd 0.01 bd bd 0.01 bd P O 0.03 0.06 0.01 0.07 0.51 0.04 2 5 Fig. 6. Magnetite versus ulvospinel€ content for in Total 100.66 101.59 100.99 99.65 100.52 98.16 NWA 6234. symbols are compositions of spinels Normalized to 12 oxygen enclosed in olivine; open symbols are from grains in the Si 1.94 1.98 1.97 1.94 1.91 1.93 groundmass. Ti 0.01 0.00 0.00 0.01 0.03 0.02 Al 0.08 0.04 0.03 0.06 0.08 0.09 in Table 4. The Mg# of merrillite ranges from 80.6 to Cr 0.01 0.02 0.01 0.01 0.00 0.01 69.3. Na2O content increases slightly with decreasing Fe 0.56 0.50 0.64 0.63 0.50 0.58 Mg# from 1 to 1.5 wt% (Fig. 8). Apatite is halogen- Mn 0.02 0.02 0.02 0.03 0.02 0.02 rich, with up to 1.9 wt% F and 3.4 wt% Cl. Mg 1.34 1.33 1.14 0.96 0.72 0.71 Water content of the apatite was determined Ca 0.05 0.08 0.20 0.38 0.72 0.64 indirectly from our EMP analyses of F and Cl, and the Ni 0.00 0.00 assumption of stoichiometry that F+Cl+OH = 1.00 Na 0.01 0.00 0.00 0.01 0.02 0.02 structural formula unit (sfu) (see McCubbin et al. 2011). K 0.00 0.00 0.00 0.00 0.00 0.00 P 0.00 0.00 0.00 0.00 0.01 0.00 This calculation implies that the apatite has up to 0.6sfu Cations 4.01 3.98 4.01 4.02 4.02 4.01 OH component (i.e., hydroxyl-apatite). This inferred total OH content represents a maximum value, in that it does 2À Mg# 70.58 72.62 63.91 60.19 58.85 55.01 not consider unmeasured X-site components like S , À Wo 2.40 4.15 9.90 19.14 37.19 33.25 O2 , carbonate, and vacancies. However, it could also En 68.89 69.60 57.58 48.66 36.96 36.72 represent a minimum value as F and Cl abundances are Fs 28.71 26.25 32.51 32.19 25.85 30.03 often overestimated in EMP analyses (Goldoff et al. na = not analyzed; bd = below detection limit. 2012). As outlined in the Methods section, we used a special routine to analyze apatite accurately, and we are therefore confident that F and Cl are not overstated in hematite (Fe2O3), 8–14% (MgTiO3), and our analyses. Within the Cl–F–OH ternary plot, apatite approximately 1.5% pyrophanite (MnTiO3) components. compositions plot toward the OH component (Fig. 9). The abundance of the magnetite component increases with increasing ulvospinel,€ as shown in Fig. 6, a plot of Other Phases Fe3+/(Fe3++2Ti+Cr+Al) [atomic%] versus 2Ti/(2Ti+ Cr+Al) [atom%]. Also notable is a gap in ulvospinel€ Plagioclase occurs in the groundmass and ranges content and a concomitant jump in magnetite content. in composition from An62–51 with up to 0.37 wt% K2O(Or2). It has been completely transformed by Phosphates shock to maskelynite (Irving et al. 2011). Iron sulfide is present in the groundmass; its S content ranges Two phosphates (merrillite and apatite) are present from 37.5 element-wt% to 38.8 element-wt%. in the groundmass of NWA 6234, with merrillite being more abundant than apatite. Both phosphates occur as Melt Vein intergrowths with each other. The grain size ranges from 10 to 100 lm in length (Fig. 7). They are in The melt vein in NWA 6234 cuts as a straight line textural equilibrium with pyroxene and plagioclase. through the rock and ranges from 10 lm to about Typical analyses of merrillite and apatite are presented 100 lm in thickness. The vein contains melt with 860 J. Gross et al.

Table 3. Representative microprobe analyses of oxides composition of the meteorite (Table 5) (Filiberto et al. in NWA 6234. 2012). The FeO content of the vein is slightly higher Chromite Titanomagnetite Ilmenite than in the bulk rock (23.8 wt% versus 21.3 wt%) and it is richer in TiO by approximately 0.3 wt%. A Oxide wt% 2 MgO 3.96 2.91 1.18 3.64 detailed investigation of the melt vein texture and

Al2O3 5.25 5.28 2.30 0.24 mineralogy will be presented elsewhere; however, we SiO2 0.38 0.33 0.16 0.74 note that it is similar to the vein reported by Walton CaO bd 0.02 0.26 0.55 et al. (2012) in NWA 4797. TiO2 0.95 0.81 19.76 49.51 Cr2O3 55.98 57.20 1.91 0.33 DOES NWA 6234 REPRESENT A MAGMA MnO 0.52 0.58 0.56 0.70 COMPOSITION? FeO 28.54 29.92 48.32 42.72 a Fe2O3 3.17 2.26 23.92 2.14 Total 98.75 99.31 98.37 100.57 For any , it is important to know Fe# 0.802 0.859 0.958 whether its bulk composition is that of a pure magma, or apfu whether it includes excess components such as cumulus Mg 0.21 0.16 0.07 0.13 crystals, xenocrysts, and/or assimilated material. This Al 0.22 0.22 0.10 0.00 question is particularly important for the Martian Si 0.01 0.01 0.00 0.02 basalts, as magma compositions are required to derive Ca 0.00 0.00 0.01 0.01 Ti 0.02 0.02 0.56 0.92 constraints on Martian mantle compositions and magma Cr 1.58 1.62 0.06 0.01 generation (e.g., Musselwhite et al. 2006; Filiberto et al. Mn 0.02 0.02 0.02 0.01 2010a; Filiberto and Dasgupta, 2011; Gross et al. 2011). Fe2+ 0.85 0.90 1.52 0.88 However, many Martian basalts are inferred to contain Fe3+ 0.08 0.06 0.68 0.04 cumulus or xenocrystic material (e.g., Stolper and Total cation 2.99 3.01 3.02 2.02 McSween 1979; McSween 1994, 2002; Papike et al. 2009; Molar compositionb Filiberto and Dasgupta 2011). Chr 0.80 0.81 0.02 Mt 0.04 0.03 0.22 To determine whether NWA 6234 represents a true Sp 0.11 0.11 0.03 magma composition, or contains cumulate grains or Usp 0.05 0.05 0.73 xenocrystic material, the chemical compositions of the fO2 estimates most magnesian crystals (in this case olivine) in the rock T(°C) 1103 922 674 843 were analyzed. These crystals were the first to crystallize D c IW 1.2 1.1 5.2 4.1 and have been in equilibrium with a melt of initial DQFMd À2.4 À2.7 1.0 0.27 magma composition. If that initial magma composition is Phases Ol, Pxe Ol, Pxe Ol, Pxf Oxidesg a equal to the measured bulk rock composition, this bulk Calculated from stoichiometry. rock composition would represent a magma composition bChr, chromite; Mt, magnetite; Sp, spinel; Usp, ulvospinel.€ cRelative to the Iron-Wustite€ (IW) buffer as defined by Herd (2008) (e.g., Shearer et al. 2008; Filiberto et al. 2009; Gross using the data of O’Neill and Pownceby (1993). et al. 2011). By comparing the chemistry of olivine with dRelative to the --Magnetite buffer as defined by the bulk rock composition, one can thus determine if the Wones and (1969). e olivine represents true . In Fig. 11, the Mg# Calculated according to the CT Server Ol-Px-Sp model (http:// of the olivine cores are compared with the bulk rock ctserver.ofm-research.org/Olv_Spn_Opx/index.php) using pyroxene Mg#. The blue dotted line (from Filiberto et al. Wo2.4En68.9Fs28.7 and olivine Fo67 at P = 1 bar. fCalculated according to the CT Server Ol-Px-Sp model (http:// 2012) represents the experimentally constrained Fe-Mg ctserver.ofm-research.org/Olv_Spn_Opx/index.php) using pyroxene equilibrium between olivine and basaltic magma, and = Fe/Mg Wo10.9En47.5Fs41.6 and olivine Fo49 at P 1 bar. Æ g reflects an equilibrium KD Olivine/Melt of 0.355 0.01 Calculated according to the Fe-Ti oxide oxythermobarometer of as determined from experiments on Martian basaltic Ghiorso and Evans (2008), using the oxide pair given in the table. compositions (both surface rocks and meteorites) by Filiberto and Dasgupta (2011). The NWA 6234 olivine schlieren, and fragments of crystals from the cores are within uncertainty of the calculated Mg# surrounding rock. Some of these crystals show rounded equilibrium of the melt’s bulk composition from which edges; others have clearly not reacted with the melt and they crystallized (Fig. 11). Hence, it is plausible that have sharp edges. In some areas, the melt vein cuts NWA 6234 represents a melt composition. through olivine and part of the crystal can be found in We can further test whether NWA 6234 represents the melt (Fig. 10). Although there are schlieren visible a primary mantle or fractionated melt using the inverse in the vein, the vein’s chemical composition is relatively experimental approach—i.e., determine if a melt of its homogeneous, and almost identical to the bulk bulk composition would be cosaturated with expected Olivine-phyric shergottite NWA 6234 861

Fig. 7. Schematic sketch of the intergrowth relationship between merrillite and apatite: merrillite is the primary magmatic phase that is replaced by apatite through the interaction with a Cl-F-OH-rich fluid derived from the crystallized NWA 6234 magma (after Shearer et al. 2011). ap = apatite; mer = merrillite; pyx = pyroxene; plag = plagioclase/maskelynite.

Fig. 8. Na (afu = atoms per formula unit) content versus Mg# of merrillite in NWA 6234 compared with other Martian meteorites (after Burger et al. 2012). a) The high Na content in merrillite in NWA 6234 suggests that plagioclase began crystallizing at approximately the same time or shortly after merrillite. b) The Na content in NWA 6234 slightly increases with decreasing Mg# in merrillite consistent with plagioclase cocrystallization. For further explanations, see text. mantle minerals (olivine, orthopyroxene, spinel, etc.) of 2004). Numerous experimental studies have used this likely chemical compositions at a reasonable mantle technique to constrain the pressures and temperatures pressure and temperature (e.g., Asimow and Longhi of formation, and mantle potential temperatures 862 J. Gross et al.

Table 4. Representative microprobe analyses of merrillite and apatite. Merrillite Apatite Oxide wt% Oxide wt% P2O5 45.93 45.35 45.12 P2O5 40.68 41.21 40.64 40.03 SiO2 0.20 0.25 0.22 SiO2 0.17 0.19 0.41 0.28 TiO2 bd 0.01 0.12 TiO2 bd 0.03 0.03 0.04 Al2O3 0.10 0.03 0.06 Ce2O3 na 0.04 0.05 0.03 Cr2O3 bd 0.02 bd Al2O3 0.04 0.01 0.04 0.02 FeO 1.77 2.36 1.46 FeO 0.86 0.73 1.06 1.07 MnO 0.10 0.09 0.10 MnO 0.11 0.09 0.17 0.13 MgO 3.19 3.07 3.42 MgO 0.13 0.10 0.16 0.11 CaO 46.38 47.96 47.96 CaO 54.88 55.09 53.53 52.59 Na2O 1.36 1.49 1.16 Na2O 0.09 0.06 0.07 0.23 K2O 0.06 na 0.03 SO3 na 0.01 bd 0.01 F bd 0.08 bd F 1.88 1.58 1.20 0.60 Cl bd 0.01 0.06 Cl 1.09 0.38 2.07 4.32 O=F,Cl 0.00 -0.03 -0.01 O=F,Cl -1.04 -0.75 -0.97 -1.23 Total 99.15 100.68 99.68 Total 98.88 98.78 98.44 98.24 Normalized to 56 Normalized to 8 cation Ca 17.931 18.442 18.548 Ca 4.971 4.964 4.899 4.889 Na 0.948 1.033 0.809 Na 0.015 0.009 0.011 0.039 K 0.028 0.000 0.016 Fe 0.061 0.051 0.076 0.078 sum 18.907 19.475 19.374 Mn 0.008 0.007 0.012 0.010 Mg 0.017 0.012 0.020 0.014 Fe 0.534 0.708 0.442 Al 0.004 0.005 0.004 0.002 Mn 0.029 0.028 0.030 Ce 0.001 0.001 0.001 Mg 1.718 1.640 1.839 Ti 0.000 0.002 0.002 0.002 Al 0.042 0.015 0.026 Cr 0.000 0.000 0.000 0.000 Cr 0.000 0.006 0.000 sum A 5.071 5.046 5.021 5.032 Ti 0.000 0.002 0.032 sum 2.339 2.399 2.368 P 2.911 2.934 2.939 2.940 Si 0.014 0.016 0.035 0.025 P 14.031 13.780 13.788 S 0.001 0.000 0.001 Si 0.072 0.090 0.078 sum B 2.926 2.951 2.974 2.965 Sum 14.103 13.870 13.866 Cation total 35.349 35.744 35.608 F 0.503 0.420 0.323 0.165 Mg# 76.303 69.848 80.621 Cl 0.156 0.054 0.299 0.635 OH# 0.341 0.525 0.378 0.201 sum X 1.000 1.000 1.000 1.000 Cation total 7.996 7.997 7.994 7.997 na = not analyzed; bd = below detection limit. for Mars (e.g., Musselwhite et al. 2006; Monders et al. uncertainty (Filiberto and Dasgupta 2011). We have 2007; Filiberto et al. 2008, 2010), the (e.g., Grove chosen a Fe-Mg Ol-Melt KD of 0.355 (Filiberto and and Vaniman 1978; Delano 1979, 1980; Elkins-Tanton Dasgupta 2011) and a mantle composition of Mg# = 80 et al. 2003; Draper et al. 2006), and the Earth (e.g., (Agee and Draper 2004). Using these values, the NWA Basaltic Volcanism Study Project, 1981). However, 6234 bulk composition is in equilibrium with a Martian experiments have not yet been conducted on the NWA mantle of Fo80, and thus most likely represent a 6234 composition. primitive, mantle-derived melt. The calculated average We calculate the temperature and pressure of equilibration pressure (with olivine + orthopyroxene formation of NWA 6234 from olivine-melt Mg-exchange bearing mantle) is 2.7 GPa with a temperature of thermometry (Putirka 2005; Lee et al. 2009) and melt approximately 1600 °C. This is significantly higher silica activity barometry (Albarede 1992; Lee et al. 2009). pressure than any other estimates for Martian basalt We chose this geothermobarometer (Putirka 2005; Lee formation (<2GPaand <1550 °C; e.g., Musselwhite et al. 2009) because it has been shown to reproduce et al. 2006; Monders et al. 2007; Filiberto et al. 2008; Lee experimental results for Martian basalts within et al. 2009; Filiberto et al. 2010; Filiberto and Dasgupta Olivine-phyric shergottite NWA 6234 863

Table 5. Composition of the bulk rock of NWA 6234 compared with the composition of the melt vein. NWA 6234 bulk rock composition* Melt vein (this study) Oxide wt% SiO2 44.6 44.54 TiO2 0.82 1.13 Al2O3 5.17 4.86 Cr2O3 0.64 1.04 FeOT 21.3 23.77 MnO 0.56 0.58 MgO 17.1 17.01 CaO 6.77 6.15 Na2O 1.04 0.90 K2O 0.08 0.07 Fig. 9. Ternary plots of apatite X-site occupancy (mol%) P2O5 0.81 0.82 from NWA 6234 using electron probe microanalysis. For Total 99.0 100.87 EPMA data, OH was calculated by stoichiometry based on 13 *Filiberto et al. (2012). anions and assuming 1ÀFÀCl = OH.

Fig. 11. Mg# (molar Mg/[Mg+Fe] 9 100) of bulk rock versus Fig. 10. BSE image of the melt vein cross cutting NWA 6234. Mg# in olivine cores, for selected Martian meteorites (after Note the schlieren and fragments of crystals in the vein. Papike et al. 2009). Blue dotted line represents the calculated Ol = olivine; Pyx = pyroxene; Mask = maskelynite. Mg# of melt in equilibrium with olivine cores (Filiberto et al. 2012); blue fine dashed lines represent the error of the calculated Mg# of melt. 2011; Gross et al. 2011). These results suggest that NWA 6234 was derived from a unique source region deeper within Mars than any other Martian basalt. Experiments mineral chemistry, petrography, and textural features and are needed on the NWA 6234 composition to fully relations. constrain the temperatures and pressure of formation of this basalt and the mantle potential temperature of its Crystallization Sequence source region. Fe/Mg Ratios CRYSTALLIZATION HISTORY The Fe/Mg ratios of olivines and pyroxenes in NWA 6234 can indicate which portions of these minerals The compositions and zoning patterns in minerals of crystallized simultaneously. The compositional NWA 6234 preserve a record of its igneous crystallization relationships between olivine and pyroxene are shown in history. In the next paragraphs, we place constraints on Fig. 5. The cores of orthopyroxene grains have a the formation conditions during crystallization and the composition of En70Fs26Wo4, and olivine cores are as Mg/Fe crystallization sequence of NWA 6234 based on the magnesian as Fo78. The observed KD Ol/low-Ca-px for 864 J. Gross et al. this pair (=0.73) is inconsistent with equilibrium values be interpreted to represent a chemical reaction between Mg/Fe from shergottite melts, where KD Ol/low-Ca-px = 1.2 magmatic merrillite and an OH-Cl-rich melt. This (Longhi and Pan 1989). Thus, the most magnesian olivine interpretation is not uncommon in Martian meteorites in NWA 6234 is too magnesian to have formed in (Greenwood et al. 2003; Shearer et al. 2011). A detailed equilibrium with the most magnesian orthopyroxene. investigation of the volatile fugacity ratios and pre- Mg/Fe Based on a KD Ol/low-Ca-px of 1.2, olivine of eruptive volatile history of NWA 6234 is presented composition Fo67 would be in equilibrium with the most elsewhere (Gross et al. 2012; Gross et al., personal magnesian orthopyroxene. Therefore, the cores of communication). pyroxene grains must have crystallized after the first olivine grains. The mantle compositions of the largest Phosphorus Zoning in Olivine olivines (and core composition of slightly smaller olivine grains) are consistent with Mg-Fe-equilibrium with the Olivine is the liquidus phase in NWA 6234, the first early pyroxene core composition and probably phase to crystallize from the magma. Therefore, its crystallized at approximately the same time. The same chemical composition can potentially record the relationship holds for the early pyroxene rims, the early evolution of mantle-derived magmas indicating magmatic Ca-rich , and the olivine rims. The smallest most conditions and timescales of crystallization. Recent Fe-rich olivine grains in the groundmass are, based on studies have shown that phosphorus zonation is common their KD, in equilibrium with the latest Ca-rich augites in igneous olivine in many types of igneous rocks and the latest low-Ca pyroxene rims. (; basalts; ; from Earth, Moon, Mars; meteorite suites; e.g., Boesenberg et al. 2004; Phosphates Milman-Barris et al. 2008; Stolper et al. 2009; Qian et al. Major and minor elements in merrillite, especially 2010; Spandler and O’Neill 2010; Peslier et al. 2010). Due Mg, Fe, and Na, can be used to place constraints on to its incompatible nature, the distribution of phosphorus the crystallization sequence of plagioclase and merrillite. in olivine can record the kinetic, dynamic, and High Na contents of merrillite in Martian meteorites are geochemical states of the magmatic system (Boesenberg thought to imply that it crystallized before plagioclase et al. 2004; Milman-Barris et al. 2008; Stolper et al. 2009; (Burger et al. 2012). In Fig. 8a, the Na content of Qian et al. 2010; Spandler and O’Neill 2010) and also merrillite in NWA 6234 is compared with those of other may preserve a record of the crystal growth rate Martian meteorites. In NWA 6234, merrillite has an variations because it diffuses more slowly through olivine Na2O content > approximately 1.1 wt%, which suggests than other elements. Dynamic crystallization experiments that plagioclase began crystallizing after merrillite. on basaltic compositions showed that the phosphorus However, plagioclase has a higher Ca/Na ratio than the variation in olivine is not caused by melt composition liquid from which it forms, and crystallization of variation resulting from magma mixing or transport of plagioclase should slightly enrich the residual magma in olivine as xenocrysts (Buseck and 1984; Peslier Na. Thus, the later-formed merrillite should also be et al. 2010). The most likely cause of the phosphorus slightly richer in Na, which is consistent with the slight oscillatory zoning is a variation in the crystal growth rate increase in Na content with decreasing Mg# in merrillite of olivine (Milman-Barris et al. 2008). High phosphorus (Fig. 8b). zones record rapid olivine growth, the olivine component The textural relationships, intergrowth between being actively depleted from the melt. If the phosphorus merrillite and apatite, and phosphorus content of the diffusion rate in the melt was equal to or slower than the melt yield insights into the igneous crystallization growth rate of the olivine, then phosphorus is sequence of the phosphates themselves (Jolliff et al. preferentially incorporated into the olivine over the melt 1993; McCubbin et al. 2011). The bulk composition of (Peslier et al. 2008). Thus, areas of the olivine with low NWA 6234 is rich in phosphorus (Filiberto et al. 2012) phosphorus are the result of slow growth as the boundary and fractional crystallization would have raised the layer between olivine and melt is temporarily depleted in activities of P2O5 until merrillite became stable. Further olivine component. This may reflect equilibrium fractional crystallization will increase the volatile (F2, crystallization conditions in which phosphorus behaves Cl2,H2O) activity in the melt while the P2O5 fugacity is as an incompatible element and goes preferentially into buffered by merrillite crystallization (Patino~ Douce and the melt. Roden 2006). At some point, the melt became saturated The high phosphorus zoning in the olivine cores in in apatite, at which time it cocrystallized with merrillite. NWA 6234 reflects initial undercooling followed by The textural relationship of apatite and merrillite is nucleation and a pulse of rapid crystal growth. The large consistent with this interpretation. Thus, the mantle area that shows low phosphorus concentration intergrowth of merrillite and apatite in NWA 6234 can (Fig. 4) reflects the slow equilibrium growth of the olivine Olivine-phyric shergottite NWA 6234 865

Fig. 12. Ti versus Al (afu = atoms per formula unit) for a) pyroxenes in NWA 6234 (core = orange circles; mantle-rim = blue circles, second-generation Ca-rich pyroxenes = green squares); and b) pyroxenes in NWA 5789 (core = dark blue circles; rim = light blue circles, mesostasis = green circles); in Y98 (experimental pyroxene; white circles); in NWA 1068 (experimental pyroxene, small red circles). Also, shown for comparison are simplified model depths of formation (modified from Nekvasil et al. 2004).

with the melt. During crystallization toward the end, molecules MgAl(AlSi)O6 and CaAl(AlSi)O6. This result alternating slow and fast growth rates must have has been confirmed for Martian magma compositions, occurred to explain the oscillatory zoning in phosphorus. after correction for the different Al/Ti ratio of the This change in growth rate seems to coincide with the parental magma (Filiberto et al. 2012), and is change in fO2 from iron-wustite€ (IW) + 1, at the time the consistent with experimental results on model chemical olivine were in equilibrium with the earliest pyroxene, systems (e.g., Gasparik 2000). to the time where the groundmass crystallized As this model is not calibrated for the exact bulk (approximately IW +4to+5); see the Temperature and rock composition of NWA 6234, it will only yield Oxidation State section below. High-temperature approximate pressures of crystallization. The pyroxenes annealing must have occurred for long enough to let in NWA 6234 have a constant Al/Ti ratio (Fig. 12a) other minor elements such as Ca, Mn, Al, Cr, and Ni from core to rim to Ca-rich augites, suggesting that they homogenize via diffusion (Clark et al. 1986; Milman- formed at constant pressure, consistent with upper Barris et al. 2008). crustal conditions (i.e., near the surface of Mars). Thus, one can infer that the parent magma of NWA 6234 Pressure crystallized completely at shallow depth inside Mars and never erupted. This history is unlike those of NWA Minor elements in pyroxenes, especially Al and Ti, 5789 and Yamato-980459 (Y98 hereafter), which both can be used to place constraints on the pressure crystallized deeper within the crust and then erupted conditions at which the mineral crystallized. The Al/Ti onto the (Fig. 12b). These differing ratios for pyroxenes from the groundmass and those histories are consistent with the petrographies of the from the mesostasis are compared in Fig. 12 with the meteorites: NWA 6234 has a fine, seriate grain size model of Nekvasil et al. (2004, 2007) modified for distribution, whereas NWA 5789 and Y98 show a Martian compositions (Filiberto et al. 2012). This strong bimodal grain size distribution (Usui et al. 2008; model illustrates the pressure dependence of the Al/Ti Gross et al. 2011). ratio for pyroxenes in equilibrium with a basaltic magma and is calibrated from experiments on a suite Temperature and Oxidation State of terrestrial alkalic basalts. Nekvasil et al. (2004) showed that pyroxenes crystallized from basaltic melts The temperature and redox conditions of had higher Al/Ti ratios when grown at high pressures. crystallization of NWA 6234 can be calculated via In crystal-chemical terms, this means that high- mineral thermometers and oxybarometers, particularly pressure pyroxenes have greater proportions of from appropriate mineral assemblages of olivine- octahedral Al in the Tschermak’s component pyroxene-spinel (Ol-Px-Sp), and from the ferric iron 866 J. Gross et al.

Table 6. Summary of oxygen fugacity estimates for NWA 6234 early phases using the Ol-Px-Sp oxybarometer. a b c P(kbar) T(°C) Log10fO2 Log10fO2 (IW ) Log10fO2 (QFM ) Fe# 0.001 837 À17.3 0.7 À3.1 0.875 0.001 922 À15.2 1.1 À2.7 0.852 0.001 900 À15.9 0.8 À3.0 0.859 0.001 1103 À12.1 1.2 À2.4 0.802 Average 0.001 940 Æ 114 0.9 Æ 0.2 À2.8 Æ 0.3 0.847 Æ 0.032 10 905 À15.6 1.0 À2.8 0.875 10 994 À13.7 1.3 À2.4 0.852 10 971 À14.3 1.1 À2.6 0.859 10 1185 À10.7 1.4 À2.1 0.802 Average 10 1014 Æ 120 1.2 Æ 0.2 À2.5 Æ 0.3 0.847 Æ 0.032

All calculations involved Fo67 olivine, and Wo2.4En68.9Fs28.7 orthopyroxene. Uncertainties on the average are 2 sigma standard deviation of the mean. aTemperature calculated from the CT Server Ol-Px-Sp oxybarometer, which uses the olivine-spinel geothermometer of Ghiorso and Sack (1991). bRelative to the Iron-Wustite€ (IW) buffer as defined by Herd (2008) using the data of O’Neill and Pownceby (1993). cRelative to the Quartz-Fayalite-Magnetite buffer as defined by Wones and Gilbert (1969). contents of early- and late-formed oxides. The most on the lower Ti, Fe3+ or higher Ti, Fe3+ side of the magnesian olivine in NWA 6234 crystallized prior to gap were used, the oxygen fugacity results are the same, the most magnesian orthopyroxene, based on Mg-Fe approximately IW +5. Temperatures obtained from 3+ exchange KD; therefore, we can only study the redox spinels on the low-Ti, Fe side of the gap yield higher conditions once orthopyroxene started to crystallize, temperatures (approximately 870 versus approximately assuming no subsolidus equilibration. We used the 720 °C), suggesting that these spinels are closer to composition of the most magnesian orthopyroxene we equilibrium with the olivine and low-Ca pyroxene than 3+ analyzed, Wo2.4En68.9Fs28.7, olivine in Fe/Mg the higher Ti, Fe spinels. The choice of pyroxene– equilibrium with orthopyroxene (Fo67), and chromite olivine pair does not significantly affect estimates. Our compositions. We selected four compositions from the best estimate for this stage of crystallization is fO2 = IW chromite grains enclosed in olivine, favoring those +4.8 Æ 0.1, T = 873 Æ 64 °C(n = 7, 2r deviation of the with high Cr# (molar Cr/[Cr+Al]) and low Fe# (molar mean). Fe/[Fe+Mg]), following the criteria of Goodrich et al. The most Ti, Fe3+-rich titanomagnetite and ilmenite (2003). Oxygen fugacity and temperature were are present in the groundmass. The application of the calculated from the Ol-Opx-Sp oxybarometer, Fe-Ti oxide oxybarometer of Ghiorso and Evans (2008) implemented in the online calculator on the CT Server to these pairs yields variable results, from as low as IW (http://ctserver.ofm-research.org/Olv_Spn_Opx/index.php); +2.3 to IW +4.1. This variability may be attributed to this calculator is based on the thermodynamic models disequilibrium between oxides, likely as a result of of Sack and Ghiorso (1989, 1991, 1994). Calculations subsolidus re-equilibration. Selection of the most Fe3+- were made for both 1 bar and 10 kbar pressure, and rich ilmenite with a Fe3+-rich titanomagnetite (from results are shown in Table 3. Temperatures from above the gap in Fig. 6) yields IW +4.1 and T = 1040 °C. olivine-chromite thermometry (Sack and Ghiorso 1991a) Application of the Ca-QUIlF model (Andersen et al. range from 900 to 1185 °C. The highest temperature 1993) to the same pair yields IW +4.1 and T = 843 °C comes from the lowest Fe# chromite (Table 3). The (Table 3), in agreement with the results from the lowest temperatures are given by the highest-Fe# Ghiorso-Evans model; a good indication that these oxide , which suggest subsolidus Fe-Mg exchange compositions are in equilibrium. Inclusion of this same with olivine hosts. For both sets of chromites, olivine- oxide pair with Fe-rich olivine and low-Ca pyroxene (as spinel oxybarometry gives oxygen fugacities above) yields IW +4.9 and T = 781 °C using Ca-QUIlF, approximately 1 log unit above the iron-wustite€ (IW) and although the temperature is lower, it is within buffer. Pressure has no effect within uncertainties uncertainty of the two thermodynamic models. This (Table 6), and subsolidus equilibration has apparently suggests that all of these and oxide compositions had little effect on the oxygen fugacity recorded by the were in equilibrium during the final stages of chromites. crystallization. Ol-Opx-Sp thermombarometry on the groundmass The results from late-stage oxides and oxide+silicate minerals yields significantly lower temperatures and phases suggest that oxygen fugacity increased higher oxygen fugacities. Regardless of whether spinels approximately 4 log units between crystallization of the Olivine-phyric shergottite NWA 6234 867 earliest olivine (IW +1) and the groundmass (IW +5). A possibility of secondary magmatic processes that could similar increase is observed in NWA 1068/1110, from IW disturb the parental magmatic volatile abundances prior +1toIW+4.5 (Herd 2006). Although the most Fe3+-rich to apatite crystallization (i.e., precrystallization secondary titanomagnetites in NWA 6234 are not as enriched in processes). The precrystallization secondary processes can magnetite (Fe3+/(Fe3++2Ti+Cr+Al) to 42%; Fig. 6) as include (but are not limited to) degassing/fluid loss from those in NWA 1068/1110 (Fe3+/[Fe3++2Ti+Cr+Al] up to the melt and/or assimilation/mixing with other lithologic 66%), the latest oxides in NWA 1068/1110 and NWA components/fluids (McCubbin et al. 2011). The effect of 6234 (i.e., with 2Ti/[2Ti+Cr+Al] = 80–90%) do show shock on volatile gain or loss has not been studied for some overlap, and agree in oxygen fugacity within Martian apatites. However, Ostertag et al. (1985) studied uncertainties (IW +4.8 Æ 0.3 for NWA 1068/1110 using terrestrial apatites from Haughton and Ca-QUIlF; Herd [2006] compared with IW +4.8 Æ 0.1 showed that their chemistry is not affected by the impact for NWA 6234 using the same method). NWA 6234 and shock process. The solubility of H species in silicate crystallized in a closed system with respect to volatile melts is strongly pressure-dependent and any melt abundance (see the Volatile History of NWA 6234 subjected to low pressures (including those at the During Late Stage Crystallization section) and the Martian surface or in shallow magma chambers) will increase in oxygen fugacity between early and late phases have undergone magmatic degassing even at very low in this closed system may be explained by buildup of hydrogen concentrations (as summarized by Burnham ferric iron as crystallization proceeds, similar to that 1994; McMillan 1994). observed in the LAR 06319 olivine-phyric shergottite NWA 6234 crystallized under upper crustal pressure (Peslier et al. 2010). conditions and never erupted onto the surface, thus a degassing/fluid loss event seems unlikely under these Volatile History of NWA 6234 during Late conditions. Furthermore, degassing should cause the Crystallization melt to lose significant proportions of its fluid-soluble elements (notably H, Cl, and Li), but apatite in NWA Despite the importance of water to Martian geology 6234 is rich in H (up to 60% hydroxyl component), and its potential in affecting the course of igneous which is inconsistent with significant degassing. petrogenesis, Martian meteorites are rather dry: Similarly, the variation and abundance of Li- in approximately 50–150 ppm H2O (Leshin et al. 1996; NWA 6234 suggest that the rock was never affected by Leshin 2000; Leshin and Vicenzi 2006). However, they degassing or interaction with a fluid (Filiberto et al. could have degassed upon eruption and thus no longer 2012). In addition, NWA 6234 shows no evidence of record the original volatile concentrations in their late aqueous fluids that might have affected the parental, pre-eruption magmas. There is significant composition of apatite; the meteorite contains no disagreement on the pre-eruption volatile contents of hydrous silicate phases (e.g., serpentine, smectite) such Martian basaltic magmas, with estimates ranging from as might form during aqueous alteration. Thus, it seems nearly anhydrous (based on the bulk concentrations, reasonable to infer that NWA 6234 crystallized in a experimental petrology, and mineral chemistry), through closed system with respect to volatile abundance, and nearly 2 wt% H2O (based on experimental petrology, the apatite composition (rich in OH) reflects that of a crystallization temperatures, and mineral chemistry); see late-stage melt, which in turn reflects the proportions McCoy et al. (2011) and references therein. and composition of volatile species in the parental Apatite records the volatile contents (OH, F, Cl) of mantle-generated melt. With the OH-F contents of the its parental magma and thus has been used in the past to apatites in NWA 6234 being among the highest values evaluate the pre-eruptive volatile concentrations of measured for any , it is reasonable to terrestrial, Martian, and lunar magmas (Stormer and infer that the actual content of fluorine, chlorine, and Carmichael 1971; Westrich 1982; Mathez and Webster water of the Martian mantle, parental to NWA 6234, 2005; Patino~ Douce and Roden 2006; Filiberto and may be higher than previously thought estimates based Treiman 2009; Boyce et al. 2010; McCubbin et al. 2010, on other Martian basalts. A detailed investigation of the 2011, 2012; Patino~ Douce et al. 2011). In the absence of volatile fugacity ratios and pre-eruptive volatile history postcrystallization secondary processes that can affect of NWA 6234 is presented elsewhere (Gross et al. 2012; apatite volatile contents, apatite compositions can be Gross et al., personal communication). used to determine the magmatic volatile abundances at the time of apatite crystallization. However, the volatile CONCLUSIONS abundances of the parental liquid (and from these, volatile abundances in the magmatic source region) are The Martian meteorite NWA 6234 is an olivine- difficult to constrain from apatite analyses due to the phyric shergottite with an unusual texture compared 868 J. Gross et al. with other olivine-phyric shergottites. It is relatively Andersen D. J., Lindsley D. H., and Davidson P. M. 1993. unaltered, fine-grained, and composed of a seriate olivine QUIlF: A Pascal program to assess equilibria among texture set in an even finer grained groundmass of Fe-Mg-Mn-Ti oxides, pyroxenes, olivine, and quartz. Computers and Geosciences 19:1333–1350. pyroxene, maskelynite, ferroan olivine, spinel, ilmenite, Asimow P. D. and Longhi J. 2004. The significance of merrillite, apatite, and accessory Fe-sulfide. The olivine multiple saturation points in the context of polybaric – core compositions with Fo78 are in equilibrium with the near-fractional melting. Journal of Petrology 45:2349 bulk rock Mg# of 59. Olivine-melt Mg-exchange 2367. thermometry, and silica activity in the melt barometry Barrat J. A., Gillet P., Sautter V., Jambon A., Javoy M., Goepel C., Lesourd M., Keller F., and Petit E. 2002a. Petrology and calculations show that NWA 6234 not only represents a chemistry of the basaltic shergottite Northwest Africa 480. melt composition but is a primitive melt derived from an Meteoritics & Planetary Science 37:487–499. approximately Fo80 mantle.Inthissense,itissimilarto Barrat J. A., Jambon A., Bohn M., Gillet P., Sautter V., the other olivine-phyric shergottites NWA 5789 and Y98, Goepel C., Lesourd M., and Keller F. 2002b. Petrology which also represent magma compositions. However, the and chemistry of the picritic shergottite Northwest Africa 1068 (NWA 1068). Geochimica et Cosmochimica Acta calculated pressures of mantle melt formation for NWA 66:3505–3518. 6234 may be higher than previous estimates for Y98, Boesenberg J. S., Ebel D. S., and Hewins R. H. 2004. An NWA 5789, and surface basalts. experimental study of phosphoran olivine and its Based on textural relationships, minor element significance in group (abstract #1366). 35th mineral chemistry, and zoning patterns NWA 6234 Lunar and Planetary Science Conference. CD-ROM. Borg L. E. and Draper D. S. 2003. A petrogenetic model for crystallized fully within the lower crust at shallow the origin and compositional variation of the Martian depth, and never erupted onto the surface. Thus, the basaltic meteorites. Meteoritics & Planetary Science volatile content of the apatite composition most likely 38:1713–1731. represents a true snapshot of the primitive original Boyce J. W., Liu Y., Rossman G. R., Guan Y., Eiler J. volatile content of the melt from which the meteorite M., Stolper E. M., and Taylor L. A. 2010. Lunar apatite with terrestrial volatile abundances. Nature crystallized. The Cl/F/OH ratios in apatite suggest that 466:466–469. the H2O content of the melt in the upper crust is higher Burger P. V., Shearer C. K., Papike J. J., and McCubbin F. M. than previously thought. These results, the higher 2012. Crystal chemistry of merrillite in Martian basalts and pressure of the mantle melt formation, and the ferroan its significance to interpreting basalt petrogenesis (abstract nature of its composition, suggest that NWA 6234 may #1178). 43rd Lunar and Planetary Science Conference. CD-ROM. represent a magma from a unique, previously untapped Burnham C. W. 1994. Development of the Burnham model source region (at least in terms of olivine-phyric for prediction of H2O solubility in magmas. In in shergottites) with unaltered volatile composition. magmas, edited by Carroll M. R. and Holloway J. R. Reviews in Mineralogy, vol. 30. Washington, D.C.: – Acknowledgments—We are grateful to A. Peslier for Mineralogical Society of America. pp. 123 129. Buseck P. R. and Clark J. 1984. —a assistance with the EMP analyses at NASA JSC, and containing pyroxene and phosphoran olivine. C. Bryden for help with oxygen fugacity calculations. Mineralogical Magazine 48:229–235. We thank Drs. L. Hallis, S. Symes, and M. McCanta Clark A. H., Pearce T. H., Roeder P. L., and Wolfson I. 1986. for helpful reviews and comments on this manuscript, Oscillatory zoning and other microstructures in magmatic as well as editor C. Floss for handling this manuscript olivine and augite; Nomarski interference contrast observations on etched polished surfaces. American and helpful thoughts and comments. This study was Mineralogist 71:734–741. supported by NASA MFR grant NNX09AL25G, Delano J. W. 1979. green glass—Chemistry and Natural Sciences and Engineering Research Council of possible origin. Proceedings, 10th Lunar and Planetary Canada Discovery Grant 261740-08, and an SIU-C Science Conference, pp. 275–300. startup package. Delano J. W. 1980. Chemistry and liquidus phase relations of Apollo 15 red glass: Implications for the deep lunar interior. Proceedings, 11th Lunar and Planetary Science Editorial Handling—Dr. Christine Floss Conference. pp. 251–288. Draper D. S. and Agee C. B. 2008. Fundamental importance REFERENCES of returned samples to understanding the Martian interior. Ground truth from Mars 2008. http://www.lpi.usra.edu/ Agee C. B. and Draper D. S. 2004. Experimental constraints captem/msr2008/presentations/Draper.pdf on the origin of Martian meteorites and the composition Draper D. S., duFrane S. A., Shearer J. C. K., Dwarzski R. of the Martian mantle. Earth and Planetary Science Letters E., and Agee C. B. 2006. High-pressure phase equilibria 224:415–429. and element partitioning experiments on Apollo 15 green Albarede F. 1992. How deep do common basaltic magmas C picritic glass: Implications for the role of garnet in the form and differentiate? Journal of Geophysical Research deep lunar interior. Geochimica et Cosmochimica Acta 97:10997–11009. 70:2400–2416. Olivine-phyric shergottite NWA 6234 869

Dreibus G. and Waenke H. 1982. of the SNC Goodrich C. A. 2002. Olivine-phyric Martian basalts: A new meteorites: Chemistry, size and formation. Meteoritics type of shergottite. Meteoritics & Planetary Science 37:31– 17:207–208. 34. Dreibus G. and Waenke H. 1985. Mars, a volatile-rich planet. Goodrich C. A. 2003. Petrogenesis of olivine-phyric Meteoritics 20:367–381. shergottites Sayh al Uhaymir 005 and Elephant Moraine Elkins-Tanton L. T., Chatterjee N., and Grove T. L. 2003. A79001 lithology A. Geochimica et Cosmochimica Acta Experimental and petrological constraints on lunar 67:3735–3772. differentiation from the Apollo 15 green picritic . Goodrich C. A., Herd C. D. K., and Taylor L. A. 2003. Meteoritics & Planetary Science 38:515–527. Spinels and oxygen fugacity in olivine-phyric and Filiberto J. and Dasgupta R. 2011. Fe2+-Mg partitioning lherzolitic shergottites. Meteoritics and Planetary Science between olivine and basaltic melts: Applications to genesis 38:1773–1792. of olivine-phyric shergottites and conditions of melting in Greenwood J. P., Blake R. E., and Coath C. D. 2003. the Martian interior. Earth and Planetary Science Letters microprobe measurements of O(18)/O(16) ratios of 304:527–537. phosphate minerals in Martian meteorites ALH 84001 and Filiberto J. and Treiman A. H. 2009. Martian magmas Los Angeles. Geochimica et Cosmochimica Acta 67:2289– contained abundant chlorine, but little water. Geology 2298. 37:1087–1090. Gross J., Treiman A. H., Filiberto J., and Herd C. D. K. Filiberto J., Treiman A. H., and Le L. 2008. Crystallization 2011. Primitive olivine-phyric shergottite NWA 5789: experiments on a basalt composition. Petrography, mineral chemistry, and cooling history imply Meteoritics & Planetary Science 43:1137–1146. a magma similar to Yamato-980459. Meteoritics & Filiberto J., Jackson C., Le L., and Treiman A. H. 2009. Planetary Science 46:116–133. Partitioning of Ni between olivine and an iron-rich basalt: Gross J., Filiberto J., Treiman A., Herd C. D. K., Melwani Experiments, partition models, and planetary implications. Daswani M., and Schwenzer S. P. 2012. Petrography, American Mineralogist 94:256–261. mineral chemistry, and crystallization history of olivine- Filiberto J., Dasgupta R., Kiefer W. S., and Treiman A. H. phyric shergottite NWA 6234: A new intermediate melt 2010a. High pressure, near-liquidus phase equilibria of the composition (abstract #2693). 43rd Lunar and Planetary basalt Fastball and melting in the Martian Science Conference. CD-ROM. mantle. Geophysical Research Letters 37:L13201, doi: Grove T. L. and Vaniman D. T. 1978. Experimental petrology 13210.11029⁄12010GL043999. of very low Ti (VLT) basalts. In Mare Crisium: The view Filiberto J., Musselwhite D. S., Gross J., Burgess K., Le L., from Luna 24, edited by Merrill R. B. and Papike J. J. and Treiman A. H. 2010b. Experimental petrology, New York: Pergamon Press. pp. 445–471. crystallization history, and parental magma characteristics Herd C. D. K. 2003. The oxygen fugacity of olivine-phyric of olivine-phyric shergottite NWA 1068: Implications for Martian basalts and the components within the mantle and the petrogenesis of ‘‘enriched’’ olivine-phyric shergottites. crust of Mars. Meteoritics & Planetary Science 38:1793–1805. Meteoritics & Planetary Science 45:1258–1270. Herd C. D. K. 2006. Insights into the redox history of the Filiberto J., Abernethy F., Butler I. B., Cartwright J., Chin E. NWA 1068/1110 Martian basalt from mineral equilibria J., Day J. M. D., Goodrich C., Grady M., Gross J., and oxybarometry. American Mineralogist Franchi I. A., Herd C. D. K., Kelley S. P., Ott U., 91:1616–1627. Penniston-Dorland S., Schwenzer S. P., and Treiman A. Herd C. D. K. 2008. Basalts as probes of planetary interior H. 2011. Maximizing the science return from 3.3 g of redox state. Reviews in Mineralogy and Geochemistry Martian meteorite: A consortium study of olivine-phyric 68:527–553. shergottite Northwest Africa 6234. EOS. Meteoritics & Herd C. D. K., Borg L. E., Jones J. H., and Papike J. J. 2002. Planetary Science 46(Suppl.):A108. Oxygen fugacity and geochemical variations in the Filiberto J., Chin E. J., Day J. M. D., Franchi I. A., Martian basalts: Implications for Martian basalt Greenwood R. C., Gross J., Penniston-Dorland S., petrogenesis and the oxidation state of the Schwenzer S. P., and Treiman A. H. 2012. Geochemistry of Mars. Geochimica et Cosmochimica Acta 66:2025–2036. of intermediate olivine-phyric shergottite Northwest Africa Irving A., Herd C., Gellissen M., Kuehner S., and Bunch T. 6234, with similarities to basaltic shergottite Northwest 2011. Paired fine grained, permafic olivine-phyric Africa 480 and olivine-phyric shergottite Northwest Africa shergottites northwest Africa 2990⁄5960⁄6234⁄6710: Trace 2990. Meteoritics & Planetary Science 47:1256–1273, doi: element evidence for a new type of Martian mantle source 10.1111/j.1945-5100.2012.01382.x. or complex lithospheric assimilation processes (abstract Gasparik T. 2000. An internally consistent thermodynamic #5232). Meteoritics 46:A108. model for the system CaO-MgO-Al2O3-SiO2 derived Jolliff B. L., Haskin L. A., Colson R. O., and Wadhwa M. primarily from phase equilibrium data. The Journal of 1993. Partitioning in REE-saturating minerals: , Geology 108:103–119. experiment, and modeling of whitlockite, apatite, and Ghiorso M. S. and Evans B. W. 2008. Thermodynamics of evolution of lunar residual magmas. Geochimica et rhombohedral oxide solid solutions and revisions of the Cosmochimica Acta 57:4069–4094. Fe-Ti two-oxide geothermometer and oxygen-barometer. Jones J. H. 1986. A discussion of isotopic systematics and American Journal of Science 308:957–1039. mineral zoning in the shergottites: Evidence for a 180 m.y. Goldoff B., Webster J. W., and Harlov D. E. 2012. igneous crystallization age. Geochimica et Cosmochimica Characterization of fluor-chlorapatites by electron probe Acta 50:969–977. microanalysis with a focus on time—dependent intensity Langmuir C., Klein E., and Plank T. 1992. Petrological variation of halogens. American Mineralogist 97:1103–1115. systematics of mid-ocean ridge basalts: Constraints on 870 J. Gross et al.

melt generation beneath ocean ridges. Geophysical from the Gusev plains, , Mars. Meteoritics & Monograph-American Geophysical Union 71:183–183. Planetary Science 42:131–148. Lee C.-T. A., Luffi P., Plank T., Dalton H., and Leeman W. P. Musselwhite D. S., Dalton H. A., Kiefer W. S., and Treiman 2009. Constraints on the depths and temperatures of A. H. 2006. Experimental petrology of the basaltic basaltic magma generation on Earth and other terrestrial shergottite Yamato-980459: Implications for the thermal using new thermobarometers for mafic magmas. structure of the Martian mantle. Meteoritics & Planetary Earth and Planetary Science Letters 279:20–33. Science 41:1271–1290. Leshin L. A. 2000. Insights into Martian water reservoirs from Nekvasil H., Dondolini A., Horn J., Filiberto J., Long H., analyses of Martian meteorite QUE 94201. Geophysical and Lindsley D. H. 2004. The origin and evolution of Research Letters 27:2017–2020. silica-saturated alkalic suites: An experimental study. Leshin L. A. and Vicenzi E. 2006. Aqueous processes recorded Journal of Petrology 45:693–721. by Martian meteorites: Analyzing Martian water on Earth. Nekvasil H., Filiberto J., McCubbin F. M., and Lindsley D. Elements 2:157–162. H. 2007. Alkalic parental magmas for chassignites? Leshin L. A., Epstein S., and Stolper E. M. 1996. Hydrogen Meteoritics & Planetary Science 42:979–992. geochemistry of SNC meteorites. Geochimica et O’Neill H. S. and Pownceby M. I. 1993. Thermodynamic data Cosmochimica Acta 60:2635–2650. from redox reactions at high-temperatures 1. An Longhi J. and Pan V. 1989. The parent magmas of the SNC experimental and theoretical assessment of the meteorites. Proceedings, 19th Lunar and Planetary Science electrochemical method using stabilized zirconia Conference. pp. 451–464. electrolytes, with revised values for the Fe–FeO, Co–CoO, Maloy A. K. and Treiman A. H. 2007. Evaluation of image Ni–NiO and Cu–Cu2O oxygen buffers, and new data for classification routines for determining modal mineralogy of the W–WO2 buffer. Contributions to Mineralogy and rocks from X-ray maps. American Mineralogist 92:1781– Petrology 114:296–314. 1788. Ostertag R., Robertson P. B., Stoffler D., and Wohrmeyer C. Mathez E. A. and Webster J. D. 2005. Partitioning behavior 1985. First results of a multidisciplinary analysis of the of chlorine and fluorine in the system apatite-silicate melt- , Devon Island, Canada. III. fluid. Geochimica et Cosmochimica Acta 69:1275–1286. Petrography and shock . Proceedings, 16th McCoy T. J., Corrigan C. M., and Herd C. D. K. 2011. Lunar and Planetary Science Conference. pp. 633–634. Combining meteorites and missions to explore Mars. Papike J. J., Karner J. M., Shearer C. K., and Burger P. V. Proceedings of the National Academy of Sciences 108: 2009. Silicate mineralogy of Martian meteorites. 19159–19164. Geochimica et Cosmochimica Acta 73:7443–7485. McCubbin F. M., Smirnov A., Nekvasil H., Wang J., Hauri Patino~ Douce A. E. and Roden M. 2006. Apatite as a probe E., and Lindsley D. H. 2010. Hydrous magmatism on of halogen and water fugacities in the terrestrial planets. Mars: A source of water for the surface and subsurface Geochimica et Cosmochimica Acta 70:3173–3196. during the . Earth and Planetary Science Patino~ Douce A. E., Roden M. F., Chaumba J., Fleisher C., Letters 292:132–138. and Yogodzinski G. 2011. Compositional variability of McCubbin F. M., Jolliff B. L., Nekvasil H., Carpender P. K., terrestrial mantle apatites, thermodynamic modeling of Zeigler R. A., Steele A., and Lindsley D. H. 2011. apatite volatile contents, and the halogen and water and chlorine abundances in lunar apatite: budgets of planetary mantles. Chemical Geology 288:14–31. Implications for heterogeneous distributions of magmatic Peslier A. H., Woodland A. B., and Wolff J. A. 2008. Fast volatiles in the lunar interior. Geochimica et Cosmochimica ascent rates estimated from hydrogen diffusion Act 75:5073–5093. profiles in xenolithic olivines from Southern Africa. McCubbin F. M., Hauri E. H., Elardo S. M., Vander Kaaden Geochimica et Cosmochimica Acta 72:2711–2722. K. E., Wang J., and Shearer C. K., Jr. 2012. Hydrous Peslier A. H., Hnatyshin D., Herd C. D. K., Walton E. L., melting of the Martian mantle produced both depleted and Brandon A. D., Lapen T. J., and Shafer J. T. 2010. enriched shergottites. Geology 40:683–686. Crystallization, melt inclusion, and redox history of a McMillan P. F. 1994. Water solubility and speciation models. In Martian meteorite: Olivine-phyric shergottite Larkman Volatiles in magmas, edited by Carroll M. R. and Holloway Nunatak 06319. Geochimica et Cosmochimica Acta 74: J. R. Reviews in Mineralogy, vol. 30. Washington, D.C.: 4543–4576. Mineralogical Society of America. pp. 131–156. Putirka K. D. 2005. Mantle potential temperatures at , McSween H. Y. 1994. What we have learned about Mars Iceland, and the mid-ocean ridge system, as inferred from from SNC meteorites. Meteoritics & Planetary Science olivine phenocrysts: Evidence for thermally driven mantle 29:757–779. plumes. Geochemistry Geophysics Geosystems 6:1–4, doi: McSween H. Y. 2002. The rocks of Mars, from far and near. 10.1029/2005gc000915. Meteoritics & Planetary Science 37:7–25. Qian Q., O’Neill H. St. C., and Hermann J. 2010. McSween H. Y. and Jarosewich E. 1983. Petrogenesis of the Comparative diffusion coefficients of major and trace Elephant Moraine A79001 meteorite—Multiple magma elements in olivine at approximately 950 °C from a pulses on the Shergottite parent body. Geochimica et xenocryst included in dioritic magma. Geology 38:331– Cosmochimica Acta 47:1501–1513. 334. Milman-Barris M. S., Beckett J. R., Baker M. B., Hofmann Sack R. O. and Ghiorso M. S. 1989. Importance of A. E., Morgan Z., Crowley M. R., Vielzeuf D., and considerations of mixing properties in establishing an Stolper E. 2008. Zoning of phosphorus in igneous olivine. internally consistent Thermodynamic database— Contributions to Mineralogy and Petrology 115:739–765. Thermochemistry of minerals in the system Mg2SiO4- Monders A. G., Médard E., and Grove T. L. 2007. Phase Fe2SiO4-SiO2. Contributions to Mineralogy and Petrology equilibrium investigations of the Adirondack class basalts 102:41–68. Olivine-phyric shergottite NWA 6234 871

Sack R. O. and Ghiorso M. S. 1991a. Chromian spinels as Stormer J. C. and Carmichael I. S. E. 1971. Fluorine- petrogenetic indicators: Thermodynamic and petrologic Hydroxyl exchange in apatite and biotite: A potential applications. American Mineralogist 76:827–847. igneous geothermometer. Contributions to Mineralogy and Sack R. O. and Ghiorso M. S. 1991b. An internally consistent Petrology 31:121–131. model for the thermodynamic properties of Fe-Mg- Taylor L. A., Nazarov M. A., Shearer C. K., Jr., McSween H. titanomagnetite-aluminate spinels. Contributions to Y., Jr., Cahill J., Neal C. R., Ivanova M. A., Barsukova Mineralogy and Petrology 106:474–505. L. D., Lentz R. C., Clayton R. N., and Mayeda T. K. Sack R. O. and Ghiorso M. S. 1994a. Thermodynamics of 2002. Martian meteorite Dhofar 019: A new shergottite. multicomponent pyroxenes. 1. Formulation of a general- Meteoritics & Planetary Science 37:1107–1128. model. Contributions to Mineralogy and Petrology 116: Treiman A. H. 2003. Chemical compositions of Martian 277–286. basalts (shergottites): Some inferences on basalt formation, Sack R. O. and Ghiorso M. S. 1994b. Thermodynamics of mantle metasomatism, and differentiation in Mars. multicomponent pyroxenes. 2. Phase-relations in the Meteoritics & Planetary Science 38:1849–1864. quadrilateral. Contributions to Mineralogy and Petrology Treiman A. H., Lindstrom D. J., and Martinez R. R. 1994. 116:287–300. The parent magma of in shergottite EETA79001: Sack R. O. and Ghiorso M. S. 1994c. Thermodynamics of Bulk and trace element composition inferred from multicomponent pyroxenes. 3. Calibration of Fe2+(Mg)(-1), magmatic inclusions. 25th Lunar and Planetary Science TiAl2(MgSi2)(-1), TiFe2(3+)(MgSi2)(-1), AlFe3+(MgSi)(-1), Conference. p. 1709. NaAl(CaMg)(-1), Al(-2)(MgSi)(-1) and Ca(Mg)(-1) Usui T., McSween H. Y., Jr., and Floss C. 2008. exchange-reactions between pyroxenes and silicate melts. Petrogenesis of olivine-phyric shergottite Yamato Contributions to Mineralogy and Petrology 118:271–296. 980459, revisited. Geochimica et Cosmochimica Acta Shearer C. K., Burger P. V., Papike J. J., Borg L. E., Irving 72:1711–1730. A. J., and Herd C. D. K. 2008. Petrogenetic linkages Wadhwa M., Lentz R C. F., McSween H. Y., Jr., and Crozaz among Martian basalts: Implications based on trace G. 2001. A petrologic and trace element study of Dar al element chemistry of olivine. Meteoritics & Planetary Gani 476 and Dar al Gani 489: Twin meteorites with Science 43:1241–1258. affinities to basaltic and lherzolitic shergottites. Meteoritics Shearer C. K., Burger P. V., Papike J. J., Sharp Z. D., and & Planetary Science 36:195–208. McKeegan K. D. 2011. Fluids on differentiated : Waenke H. 1991. Chemistry, , and evolution of mars. Evidence from phosphates in differentiated meteorites Space Science Reviews 56:1–8. GRA 06128 and GRA 06129. Meteoritics & Planetary Walton E. L., Irving A. J., Bunch T. E., and Herd C. D. K. Science 46:1345–1362. 2012. Northwest Africa 4797: A strongly shocked Spandler C. and O’Neill H. St. C. 2010. Diffusion and ultramafic shergottite related to compositionally partition coefficients of minor and trace elements in San intermediate Martian meteorites. Meteoritics & Planetary Carlos olivine at 1,300C with some geochemical Science 47:1449–1474. implications. Contributions to Mineralogy and Petrology Webster J. D., Tappen C. M., and Mandeville C. W. 2009. 159:791–818. Partitioning behavior of chlorine and fluorine in the Stolper E. M. and McSween H. Y. 1979. Petrology and origin system apatite-melt-fluid II: silicate systems at of the shergottite meteorites. Geochimica et Cosmochimica 200 MPa. Geochimica et Cosmochimica Acta 73:559–581. Acta 43:1475–1498. Westrich H. R. 1982. F-OH exchange equilibria between - Stolper E., Baker M. B., Beckett J., McCanta M., and Saal A. mineral pairs. Contributions to Mineralogy and 2009. Phosphorus zoning in olivine: A new source of Petrology 78:318–323. information on the early magmatic histories of igneous rocks Wones D. R. and Gilbert M. C. 1969. Fayalite-magnetite- (abstract #V74B-01). American Geophysical Union, quartz assemblage between 600° and 800 °C. American Spring Meeting 2009. Journal of Science 267A:480–488.