Detailed mineralogy and petrology of highly shocked poikilitic shergottite Northwest Africa 6342

Abstract

Northwest Africa (NWA) 6342 is an intermediate, poikilitic shergottite, found in Algeria in 2010. It is comprised of two distinct petrographic areas; poikilitic domains with rounded Mg‐rich olivine chadacrysts enclosed by large low‐Ca pyroxene oikocrysts, and a nonpoikilitic domain mainly comprised of subhedral olivine and vesicular recrystallized plagioclase. Oxygen fugacity conditions become more oxidizing during crystallization from the poikilitic to the nonpoikilitic domain (QFM−3.0 to QFM−2.2). As such, it is likely that NWA 6342 experienced a two‐stage (polybaric) crystallization history similar to that of the enriched poikilitic shergottites. NWA 6342 also experienced relatively high levels of shock metamorphism in comparison to most other poikilitic shergottites as evidenced by the fine‐grained recrystallization texture in olivine, as well as melting and subsequent crystallization of plagioclase. The recrystallization of plagioclase requires an extended period of postshock thermal metamorphism for NWA 6342 and similarly shocked intermediate poikilitic shergottites NWA 4797 and Grove Mountains 99027 most likely due to launch from Mars. The similarities in petrology, chemistry, and shock features between these three meteorites indicate that they have similar crystallization and shock histories; possibly originating from the same source area on Mars.

Geochemical and mineralogical classification of four new shergottites: NWA 10441, NWA 10818, NWA 11043, and NWA 12335

“…[2019] and references therein) shergottites. Light gray dashed lines represent olivine compositions from other poikilitic shergottites (e.g., Kizovski et al., 2019; Rahib et al., 2019).…”

What Martian Meteorites Reveal About the Interior and Surface of Mars

“…Oxygen fugacity data sources for olivine-phyric (early crystallizing assemblages only) and basaltic shergottites as summarized in Castle and Herd (2017) and updated in Herd (2019), except for additional estimates, which are calculated using Fe-Ti oxide data from Ferdous et al (2017), Hui et al (2011), and Ikeda et al (2006). Poikilitic shergottite data representing the early crystallizing assemblages are from Rahib et al (2019), Kizovski et al (2019), and Walton et al (2012). Exponential lines-of-best-fit are shown for each set of data: solid black line = poikilitic; dashed gray line = olivine-phyric; dashed black line = basaltic.…”

“…2016a). Third are the poikilitic shergottites that contain olivine chadacrysts (up to 1.8 mm) enclosed by large pyroxene oikocrysts (from 3 to 10 mm in length), with later‐crystallizing olivine, pyroxene, and maskelynite (Figure 3c; Combs et al., 2019; Howarth et al., 2014; Kizovski et al., 2019; Rahib et al., 2019; Walton et al., 2012). Poikilitic shergottites were previously termed lherzolitic shergottites (e.g., Mikouchi & Kurihara, 2008).…”

“…The presence of crystalline plagioclase in several meteorites (NWA 4480, NWA 10416; Walton et al., 2014, NWA 8159; Sharp et al., 2019, and NWA 12241; Udry et al., 2020) suggests that they were subjected to lower shock pressures than other meteorites, in which all of the plagioclase has been converted to diaplectic glass. Most shergottites show higher shock pressures >19 GPa (Baziotis et al., 2013; Fritz et al., 2005) and rarely up to 70–90 GPa (Kizovski et al., 2019). Nakhlites have undergone limited shock and contain crystalline plagioclase, although they can contain twinned augite (Treiman, 2005).…”

“…Second in abundance are the olivine-phyric shergottites, which are porphyritic and contain olivine phenocrysts (sometimes megacrystic with sizes up to 2.5 mm) with later-crystallizing olivine, pyroxene, and maskelynite ( Figure 3b; grains in the groundmass of ∼0.25 mm; e.g., Balta et al, 2015;Basu Sarbadhikari et al, 2009;Basu Sarbadhikari et al, 2016;Chen et al, 2015;Dunham et al, 2019;Goodrich, 2002;Liu et al, 2016a). Third are the poikilitic shergottites that contain olivine chadacrysts (up to 1.8 mm) enclosed by large pyroxene oikocrysts (from 3 to 10 mm in length), with later-crystallizing olivine, pyroxene, and maskelynite ( Figure 3c; Combs et al, 2019;Howarth et al, 2014;Kizovski et al, 2019;Rahib et al, 2019;Walton et al, 2012). Poikilitic shergottites were previously termed lherzolitic shergottites (e.g., Mikouchi & Kurihara, 2008).…”