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A New Melt Composition Meteoritics & Planetary Science 48, Nr 5, 854–871 (2013) doi: 10.1111/maps.12092 Petrography, mineral chemistry, and crystallization history of olivine-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 Earth 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 Martian igneous and mantle compositions is crucial for understanding Mars’ mantle evolution, including early differentiation, mantle convection, and the chemical alteration at the surface. Primitive magmas provide the most direct information about their mantle source regions, but most Martian meteorites either contain cumulate olivine or crystallized from fractionated melts. The new Martian meteorite Northwest Africa (NWA) 6234 is an olivine-phyric shergottite. Its most magnesian olivine cores (Fo78)arein Mg-Fe equilibrium with a magma of the bulk rock 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 pyroxenes in NWA 6234, the minor components in olivine and merrillite, and phosphorus zoning of olivine, we infer that the rock crystallized completely at pressures consistent with conditions in Mars’ upper crust. The textural intergrowths of the two phosphates (merrillite and apatite) indicate that at a very 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 magnesium basaltic shergottites have yielded many insights into the numbers (Mg# = molar Mg/[Mg+Fe]) suggests that they planet’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 partial melting of the mantle. recent ages of magmatic activities on Mars (e.g., Primitive melts can provide direct information about Dreibus and Waenke 1982, 1985; Jones 1986; Waenke their mantle source regions, including compositions and 1991; Borg and Draper 2003; Treiman 2003; Agee and mineralogy (e.g., Langmuir et al. 1992; Asimow and © The Meteoritical Society, 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 olivines (e.g., Northwest Africa (NWA) 1068, Dar al Gani (DaG) SAMPLE AND ANALYTICAL TECHNIQUE 476, Sayh al Uhaymir (SaU) 005, Elephant moraine (EET) A79001 lithology-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 vein, 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 geochemistry of this meteorite. Analyses here are from oxygen 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 minerals other than apatite in chlorine compared with terrestrial basalts, 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 diopside olivine-pyroxene-spinel equilibrium from olivine-phyric (Si, Ca, Mg,), olivine (Si, Mg, Fe), oligoclase, albite, shergottites provide evidence for low oxygen fugacities jade (Na), hematite (Fe), rutile (Ti), corundum, augite (approximately 1–3 log units below the QFM buffer) in (Al), chromite (Cr), Ni-diopside (Ni), rhodochrosite Martian basalts and their mantle source region; more (Mn), orthoclase (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 iron, 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, petrology, 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-feldspar (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
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