THE PLAGIOCLASE-BEARING POIKILITIC SHERGOTTITE NORTHWEST AFRICA 12241. A. Udry1, A. Ostwald1, G. H. Howarth2, M. Paquet3, L. Forman4, J. M. D. Day3, and A. Taylor1, 1Department of Geosciences, University of Nevada Las Vegas, 4505 S. Maryland Pkwy, Las Vegas, NV 890154 (arya.udry@unlv.edu), 2Department of Geological Sciences, University of Cape Town, Rondebosch 7701, South Africa, 3Scripps Institution of Oceanography, University of California San Diego, La Jolla CA 92093-0244, 4Space Science & Technology Cen-tre Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia.

Introduction: Poikilitic shergottites are gabbro-

ic/lherzolitic lithologies and currently consist of 30 paired groups [e.g., 1–5]. Poikilitic shergottites have a bimodal texture that allows us constraints to be placed on magmatic processes from the crust-mantle bounda-ry to the shallow crust [1]. We examined the newly recovered sample Northwest Africa (NWA) 12241, which was classified as an olivine gabbro in the Mete-oritical Bulletin. This meteorite is, in fact, one of the few martian shergottites that contains plagioclase and not solely the high pressure phase maskelynite, and has all of the textural and geochemical characteristics of an ‘intermediate’ poikilitic shergottite.

Methods: We conducted quantitative textural anal-yses, bulk major and trace composition analyses, and mineral major element analyses following the same procedure and in the same laboratories as [1]. Electron Backscatter Diffraction (EBSD) analyses were con-ducted at the John de Laeter Centre at Curtin Universi-ty, Western Australia. We collected high resolution maps of plagioclase grains to understand the crystallin-ity of this phase. These data were obtained using step sizes of 0.1–0.5 µm.

Results and Discussion: Mineralogy and petro-genesis. Northwest Africa 12241 has a bimodal tex-ture, including: 1) the early-stage poikilitic texture consisting of pyroxene oikocrysts containing olivine and chromite chadacrysts, and 2) a later-stage non-poikilitic texture, containing olivine, pyroxene, plagio-clase, chromite, merrillite, and ilmenite, similar to oth-er poikilitic shergottites [1]. Modal abundances of all phases fall within the range of the other poikilitic sher-gottites [1]. Pyroxene oikocrysts have low-Ca cores with sharp boundaries to high-Ca rims. The low-Ca poikilitic pyroxenes have compositions of En55–70Fs25–

27Wo5–18, whereas high-Ca poikilitic pyroxene rims have compositions of En44–54Fs17–20Wo26-39. Non-poikilitic pyroxenes are distinct grains and are more Ca-rich and more Mg-poor than the pyroxene oiko-crysts, with compositions of En51Fs11Wo37 – En64Fs26Wo10. Olivine chadacrysts within pyroxene oikocrysts have a compositions of Fo64–67 and non-poikilitic olivine have compositions of Fo60–67. Lath-shaped plagioclase grains, which have with lengths, up to ~ 750 µm long, are present in NWA 12241 and dis-play undulose extinction and polysynthetic twinnings. Birefringent and amorphous plagioclase have stoichi-ometric composition (5 cations based on 8 O) of An53–

59Ab46–40Or2–1. Some plagioclase grains show a sharp transition from birefringent to isotropic in the same grains, although most plagioclase grains are birefrin-gent. Note that we do not define amorphous plagio-clase as maskelynite as NWA 12241 plagioclase is stoichiometric. Mineral compositions in NWA 12241 and minerals in other intermediate poikilitic sher-gottites are similar in both poikilitic and non-poikilitic textures [1–5].

We conducted olivine-pyroxene-spinel oxybarom-etry and olivine-spinel geothermometry calculations on both NWA 12241 poikilitic and non-poikilitic textures in eight mineral assemblages [6,7,http://melts.ofm-research.org/CORBA_CTserver/Olv_Spn_Opx/index.p hp]. Poikilitic textures have ƒO2 ranging from -4.3 to -3.6 relative to the Fayalite-Magnetite-Quartz (FMQ) buffer and non-poikilitic textures have ƒO2 from FMQ -3.2 to -1.7. The oxygen fugacities of NWA 12241 are similar to the intermediate poikilitic shergottites Lewis Cliff (LEW) 88516 and Allan Hills (ALH) 77005 with increasing ƒO2 from poikilitic to non-poikilitic tex-tures.

Oxygen fugacities and mineral compositions of NWA 12241 indicate similar petrogenesis as the other intermediate and enriched poikilitic shergottites, start-ing with formation of poikilitic textures at crust-mantle boundary depths and formation of non-poikilitic tex-tures during ascent [1–3]. The change from poikilitic to non-poikilitic texture formation is accompanied by degassing and/or autoxidation as evident from the in-crease of melt ƒO2.

Emplacement of NWA 12241. We conducted quan-titative textural analyses of the olivine population (n =

Figure 1: Crystal Size Distribution plot of all poikilitic sher-gottites. Circle = enriched, triangle = intermediate sher-gottites. Other data from [1].

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Figure 3: Forescatter electron image (left) and band contrast (BC) image (right) of plagioclase grain, from EBSD analyses. Bright regions (BC map) = good quality EBSD patterns, crystalline; black regions = no EBSD patterns, amorphous.

50 µm

Plag

664) in both poikilitic and non-poikilitic zones of NWA 12241. The CSD plot of the NWA 12241 olivine population has a negative and concave-up overall slope of -3.54 ± 0.16 mm-1 and intercept of 4.42 ± 0.11, with steeper slope for poikilitic olivine (-3.84 versus -3.51 mm-1) (Fig. 1). The CSD data have similar best-fit rati-os and olivine grain sizes to poikilitic shergottites, with CSD slopes most similar to the enriched poikilitic shergottite NWA 10169 [1], although the olivine popu-lation is smaller than the other poikilitic shergottites.

Parental melt compositions. Northwest Africa 12241 has bulk Mg# of 62, and major element compo-sitions are similar to other poikilitic shergottites [1,8]. The bulk (La/Yb)CI of NWA 12241 is 0.66 and its rare earth element (REE) pattern shows enrichment in La (Fig. 2). This increase is likely due to terrestrial con-tamination [8]. Northwest Africa 12241 has V/Sc of 8.3, Nb/Y of 0.09, and Zr/Y of 2.9, values similar to most intermediate shergottites [9]. Based on its trace element composition, we classify NWA 12241 as an intermediate poikilitic shergottite (Fig. 2) [1,9], as also suggested by its εNd and εHf measured by [10].

Figure 2: CI-normalized rare earth element patterns of NWA 12241 compared to other poikilitic shergottite. Dark grey envelope = enriched poikilitic shergottites and light grey envelope = intermediate poikilitic shergottites. See [2] for references.

Shock history of NWA 12241. The studied thin sections of NWA 12241 does not contain strong evi-dence for shock metamorphism, as no shock melt and no olivine darkening. We observed birefringent, and isotropic and amorphous plagioclase in NWA 12241, with amorphization not evenly distributed throughout plagioclase grains (Fig. 3). No previously studied poi-kilitic shergottites contain plagioclase, except the new-ly recovered depleted poikilitic shergottite Asuka (A) 12325 [11,12]. Similar to NWA 12241, the depleted shergottites NWA 8159 and NWA 7635 contain bire-fringent plagioclase and amorphous plagioclase [13,14]. Note that plagioclase was also found in the intermediate and depleted olivine-phyric shergottites NWA 4480 and NWA 10146, respectively [15,16].

Based on the presence of birefringent plagioclase with intermediate chemistry, Northwest Africa 12241 underwent an ejection event involving shock pressures <25 GPa [17]. However, Sims et al. [17] suggest that maskelynite cannot be used to constrain peak shock

pressures as amorphization can be heterogeneous in one sample, as is the case for NWA 12241, with amor-phization occurring at pressures between 8 GPa and 11 GPa and 17.6 GPa for anorthite with strain rate at 10−3

s−1. In addition, maskelynite can retain crystallinity up to pressures of 45 GPa [18]. Similar plagioclase tex-tures and crystallinity are observed in NWA 8159, which has estimated shock pressures between 15 and 23 GPa [13]. Thus, the plagioclase textures imply that NWA 12241 also underwent shock pressures of 15–23 GPa. The range of shock metamorphism observed in poikilitic shergottites is large, as this group includes highly shocked meteorites, such as NWA 4797 and NWA 6342, which have possibly undergone shock pressures of 75 – 90 GPa [19,20].

Conclusions: According to its bulk and mineral compositions as well as its texture, NWA 12241 formed and emplaced similarly to the other poikilitic shergottites. However, due to the presence of birefrin-gent plagioclase, this meteorite possibly originated from a different ejection event and thus a different location on Mars, as also suggested for the depleted poikilitic shergottite Asuka 12325 [12].

References: [1] Rahib et al. (2019) GCA, 266, 463–496. [2] Combs et al. (2019) GCA, 266, 435–462. [3] Ho-warth et al. (2014) Meteoritics & Planet. Sci., 49, 812–830. [4] Lin (2013) Meteoritics & Planet. Sci., 48, 1572–1589. [5] Usui (2010) GCA, 74, 7283–7306. [6] Sack and Ghiorso (1989) Contrib. Miner. Petrol., 102, 41–68. [7] Sack and Ghiorso (1991) Am. Mineral., 76, 827–847. [8] Crozaz et al. (2003) GCA, 67, 4727–4741. [8] Tait and Day (2018) EPSL, 484, 99–108. [9] Lapen et al. (2019) LPSC XLX, Abstract #2605. [10] Debaille et al. (2019) 10th Symposium on Polar Science, Abstract #0219. [11] Takenouchi et al. (2019) 10th Symposium on Polar Science, Abstract #0089. [12] Herd et al. (2017) GCA, 218, 1–26. [13] Lapen et al. (2017) Science Advances, 3: e1600922. [14] Irving et al. (2016) LPSC XLVII, Abstract #2330. [15] Walton et al. (2016) LPSC XLVII, Abstract #1639. [16] Sims et al. (2019) EPSL, 507, 166–174. [17] Ostertag (1983) JGR, 88, B364–B376. [18] Kizovski et al. (2019) Meteoritics & Planet. Sci., 54, 768–784. [19] Walton et al. (2012) Meteoritics & Planet. Sci., 47, 1449–1474.

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