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Chemical, Microstructural and Chronological Record of Phosphates in the Ksar Ghilane 002 2 Enriched Shergottite

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Chemical, Microstructural and Chronological Record of Phosphates in the Ksar Ghilane 002 2 Enriched Shergottite 1 Chemical, microstructural and chronological record of phosphates in the Ksar Ghilane 002 2 enriched shergottite. 3 Roszjar, J.1*, Whitehouse, M. J. 2, Terada, K. 3, Fukuda, K. 4,5, John, T. 6, Bischoff, A.7, Morishita Y. 8,9, 4 and Hiyagon, H.4 5 1*Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, 1010 6 Vienna, Austria 7 2Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden 8 3Department of Earth and Space Science, Graduate School of Science, Osaka University, 1-1 9 Machikaneyama-cho, Toyonaka 560-0043, Japan 10 4Department of Earth and Planetary Science, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, 11 Japan 12 5WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St. 13 Madison, WI 53706, USA 14 6Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstr. 74-100, 12249 Berlin, 15 Germany 16 7Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm-Str. 10, 48149 17 Münster, Germany 18 8Faculty of Science, Shizuoka University, 836 Ohya, Shizuoka, 422-8529, Japan 19 9National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8567, Japan 20 21 *Corresponding author: J. Roszjar, NHM Vienna, [email protected] 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 Keywords: Mars, shergottite, phosphates, halogens, U-Pb isotopes, stable chlorine isotopes 1 38 Abstract 39 The enriched basaltic (martian) shergottite Ksar Ghilane (KG) 002, discovered in 2010, is 40 exceptionally rich in coexisting but discrete apatite and merrillite crystals. It has been selected to 41 better constrain the formation conditions and post-crystallization processes, and thus the evolution 42 of martian rocks based on Ca-phosphates. A petrological, chemical, chronological and microstructural 43 approach using a series of high-spatial resolution techniques including Raman spectroscopy, electron 44 microscopy (SEM, EPMA, CL-imaging) and secondary ion mass spectrometry (SIMS) analysis has been 45 applied to a representative number of Ca-phosphate grains. Analytical results for apatite and 46 merrillite reveal: (i) zoning in F, Cl, Br and I concentrations, (ii) elevated Cl concentrations in the 47 range of ~11,900 to 35,300 µg/g and halogen ratios, i.e., Cl/Br and Cl/I, as well as stable chlorine 48 isotope composition, reported as δ37Cl values rel. to Standard Mean Ocean Chloride (SMOC, defined 49 as 0 ‰) with a value of +0.67 ± 0.14 ‰ (1σ), discriminating KG 002 phosphates from that of other 50 enriched and depleted shergottites. The halogen and heavier δ37Cl record, indicate a slightly higher 51 degree of ~3.5 % assimilation of Cl-rich and isotopically heavier crustal reservoir on Mars when 52 compared to other enriched shergottites. (iii) Structural investigations together with the chemical 53 and petrological context of the grains confirm the occurrence of hydroxyl-poor merrillite, indicate 54 weak if any alteration effects induced by metamictization, only minor structural modifications due to 55 shock metamorphism, and absence of replacement reactions. Therefore, igneous crystallization of 56 Ca-phosphates from a fractionated, hydrous and ferrous mantle source, rich in volatiles including the 57 halogens and Na and lithophile rare earth-elements, and absence of interaction with crustal 58 fluids/brines of the sample is deduced. (iv) The Pb isotopic composition of six apatite and three 59 merrillite grains is highly unradiogenic and the 238U-206Pb record yields a phosphate crystallization 60 time at 395 ± 240 Ma (2σ), which is similar to those of other enriched shergottites. 61 62 1. Introduction 63 Calcium-phosphates are widespread accessory phases in various types of martian rocks, such as: 64 nakhlites (i.e., olivine-bearing clinopyroxenites, exposed to the martian hydrosphere and rich in 65 secondary minerals), chassignites (i.e., dunites and olivine-chromite cumulates containing hydrated 66 phases), Allan Hills (ALH) 84001 (i.e., ultramafic, orthopyroxenite, rich in carbonates), a polymict, 67 regolith breccia (i.e., Northwest Africa (NWA) 7034 and paired samples) and the largest suite among 68 martian rocks, namely shergottites. The latter can be subdivided based on their petrology into: (a) 69 basaltic (i.e., mafic) shergottites, by far the largest group of martian meteorites and of volcanic 70 origin, (b) lherzolitic (i.e., wehrlitic) shergottites, characterized by a cumulate texture and thus of 71 plutonic origin, and (c) olivine-phyric/picritic (i.e., permafic) or olivine-orthopyroxene-phyric 72 shergottites that are extrusive rocks of magmatic origin. All of these groups share diagnostic isotopic 73 and petrological features indicative of a martian origin (e.g., McSween et al., 1985). Shergottites can 74 be further subdivided into enriched, intermediate, and depleted rock types according to their bulk 75 rock light rare earth element (LREE) depletion, which is generated by different degrees of partial 76 melting of the martian mantle (e.g., Bridges and Warren 2006, and references therein). 77 Apatite group minerals (Ca5(PO4)3(Cl,F,OH) and merrillite (Ca18Na2Mg2(PO4)14), if present, are late- 78 stage crystallizing phases and thus major sinks for incompatible trace elements, such as rare earth 79 elements (REE), but also for the actinides U and Th, making them well-suited for combined chemical 80 and chronological investigations (e.g., Ward et al., 2017 and references therein). Most Ca-phosphates 81 in martian rocks are considered to be of magmatic origin, crystallizing prior to degassing of the host 82 rock and thus can be used as a probe for the content of volatile elements and water in the martian 83 mantle sources (e.g., McCubbin et al., 2012, 2014; Shearer et al., 2015; Ward et al., 2017). Some Ca- 2 84 phosphates, however, may be affected by the assimilation of (Cl-rich) crustal component(s) on Mars 85 during latest stages of basaltic crystallization, or interaction with Cl-rich crustal fluids/ brines 86 (Howarth et al., 2016; McCubbin et al., 2016; Williams et al., 2016; Bellucci et al., 2017). 87 Furthermore, martian apatite is the prime carrier phase for the stable halogens (F, Cl, Br, I) and OH 88 (e.g., McCubbin et al., 2012, 2014, 2016; Usui et al., 2012 and references therein; Roszjar et al., 2014; 89 Bellucci et al., 2017). In addition, hydrous amphibole (i.e., kaersutite), biotite, and glass (e.g., 90 Watson et al., 1994), but also nominal-“anhydrous” merrillite, if present, can take up a certain 91 amount of volatiles in its crystal structure (e.g., Roszjar et al., 2014; Shearer et al., 2015; Bellucci et 92 al., 2017; Sarafian et al., 2017; this study), and therefore contribute to the bulk budget of hydrophilic 93 and incompatible elements of the host rock. Thus, a detailed study of Ca-phosphates using high- 94 spatial resolution techniques can further our understanding on the uptake mechanisms and 95 evolution of volatile (with halogens in particular) and trace elements in the source region(s) of 96 martian meteorites. Combined with a chronological record, it enables better evaluation of the 97 volatile component of the martian mantle and crust and its evolution over time. For this purpose a 98 case study of the petrological, structural, chemical, isotopic and chronological record of apatite and 99 merrillite grains in the enriched, basaltic shergottite Ksar Ghilane (KG) 002, first described in Llorca et 100 al. (2013) has been carried out to better constrain the role of martian phosphates. This sample has 101 been selected because of its high modal abundance of ~3.4 vol.% of Ca-phosphates, exceeding those 102 of other martian meteorites and the size of the grains reaching up to ~2 mm (i.e., merrillite). 103 Since the suite of martian meteorites, except NWA 7034 and paired samples, are strongly affected 104 by impact(s) as well as thermal metamorphism (e.g., McSween and Harvey, 1993), these two findings 105 in common makes radiometric dating, i.e., using 147Sm-143Nd and 176Lu-176Hf, 87Rb-87Sr, 235/238U-207/206Pb 106 and the interpretation of the volatile budget of martian rocks and their source region(s) challenging. 107 In addition to observed trace elemental and stable isotope heterogeneity between phosphates 108 derived from various martian source regions (e.g., Bellucci et al., 2017), discrepancies between 109 chronometers for single martian rocks, notably shergottites, often exist. Discrepancies between 110 various isotope systems and/or bulk rock vs. internal isochrons (i.e., analyses of separated mineral 111 fractions and/or single grains from a single sample), and high-spatial resolution analyses of single 112 crystals, with a significant role of phosphate have been discussed (e.g., Bouvier et al., 2008). It is 113 known that Ca-phosphates have a relatively high closure temperature of 500 - 600°C for the U-Pb 114 system (Cherniak et al., 1991). In addition, if disturbance occurred, the degree of disturbance can be 115 assessed by using the two U decay series (238U-206Pb and 235U-207Pb), providing not only formation age 116 but also alteration age. Using these great advantages of Ca-phosphates, we address the role of 117 martian Ca-phosphates in both the volatile budget, potential mantle-crust interaction and 118 chronology of the host rock. 119 120 2. Methods 121 Optical microscopy, SEM, and CL investigations 122 Textural and mineralogical investigations of merrillite and apatite grains, representative for 123 the Ca-phosphate population in the enriched basaltic shergottite KG 002 were performed first by 124 optical and electron microscopy on two thin sections (PL 11154 and PL 11156, WWU Münster 125 collection). A JEOL JSM 6610-LV scanning electron microscope (SEM) equipped with energy 126 dispersive spectrometers (EDS; INCA, Oxford Instrument) at the Interdisciplinary Center for Electron 127 Microscopy and Microanalysis (ICEM) at the WWU Münster and the central research laboratory 128 department at the NHM Vienna (EDS; Quantax Bruker) were used for detailed petrographic 129 investigations.
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