Petrogenesis and shock metamorphism of the enriched lherzolitic shergottite Northwest Africa 7755

Abstract

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non-poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non-poikilitic lithologies, Fa 30-39 and Fa 37-40 , respectively. Pyroxene in the non-poikilitic lithology is systematically Fe-richer than that in the poikilitic lithology. The chromite grains in non-poikilitic lithology are highly Ti-richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non-poikilitic lithologies support a two-stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shockinduced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiow€ ustite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine-grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiow€ ustite, Fe-sulfide, Fe-phosphide, and Ca-phosphate minerals. The coexistence of bridgmanite and magnesiow€ ustite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800-2000°C. Coesite and seifertite are probably present in NWA 7755. The presence of these high-pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.

Occurrence of tuite and ahrensite in Zagami and their significance for shock-histories recorded in martian meteorites

“…Tuite was proposed to form from decomposition of apatite or solid-state phase transformation of merrillite (Murayama et al, 1986;Xie et al, 2016;Xie et al, 2002;Xie et al, 2013). Some tuite grains in Suizhou (Xie et al, 2016) and NWA 7755 (Wang et al, 2017) were found coexisting with Cl-rich apatite, with a Cl content of up to ~ 3.9 wt%, consistent with possibility that they formed through decomposition of adjacent chlorapatite. However, the ideal formula of tuite, Ca3(PO4)2, similar to extraterrestrial merrillite, does not contain Cl or other halogens (Jolliff et al, 2006;McCubbin et al, 2014).…”

“…The high-pressure minerals discovered in these extraterrestrial natural samples could reveal the impact history on their parent bodies and provide insights into the phase transformation mechanisms in the interior of Earth (Petrova and Grokhovsky, 2019). To date, high-pressure polymorphs of olivine (Binns et al, 1969;Chen et al, 2004;Ma et al, 2016;Miyahara et al, 2008;Putnis and Price, 1979), pyroxene (Chen et al, 1996;Sharp et al, 1997;Tomioka and Fujino, 1999;Tschauner et al, 2014), feldspar (Fritz et al, 2020;Gillet et al, 2000;Liu, 1978;Ma et al, 2018;Tschauner et al, 2021), zircon (Glass et al, 2002;Xing et al, 2020), silica (El Goresy et al, 2004;El Goresy et al, 2000b;Hu et al, 2020;Miyahara et al, 2014;Ohtani et al, 2011;Sharp et al, 1999), chromite (Chen et al, 2003;Ma et al, 2019), and Ca phosphate (Wang et al, 2017;Xie et al, 2002;Xie et al, 2013) have been reported from various meteorites displaying shock effects.…”

“…Tuite, a high-pressure polymorph of Ca-phosphate with the structure of γ-Ca3(PO4)2, is of great significance as a host for larger lithophile elements in the deep Earth (Skelton and Walker, 2017;Xie et al, 2002). It has been reported in shocked chondrites (Hu and Sharp, 2016;Xie et al, 2016;Xie et al, 2002), martian meteorites (Baziotis et al, 2013;Boonsue and Spray, 2012;Fritz and Greshake, 2009;Wang et al, 2017), and iron meteorites (Litasov and Podgornykh, 2017). Tuite was proposed to form from decomposition of apatite or solid-state phase transformation of merrillite (Murayama et al, 1986;Xie et al, 2016;Xie et al, 2002;Xie et al, 2013).…”

Elucidation of impact event recorded in the lherzolitic shergottite NWA 7397

“…Basaltic and olivine‐phyric shergottites include many kinds of high‐pressure minerals (e.g., Beck et al., 2004; Sharp et al., 2019). In contrast, only a few lherzolitic shergottites contain high‐pressure minerals, although shock‐induced melting textures and maskelynite are pervasively found in them (Imae & Ikeda, 2010; Mikouchi, 2005; Ozawa et al., 2011; Wang et al., 2017). Regions of apatite (or merrillite) grains were transformed into the high‐pressure polymorph tuite (Fig.…”

Early Impact Events on Chondritic Parent Bodies: Insights From NWA 11004, Reclassified as an LL7 Breccia

“…NWA 11004 contains 3‐ to 5‐mm‐sized poikilitic pyroxene oikocrysts enclosing 40‐ to 400‐μm‐sized olivine chadacrysts. The poikilitic texture occurs in shergottites (e.g., Wang et al, 2017), pyroxene phenocrysts in chondrules (e.g., Scott & Taylor, 1983), and some OC impact melt rocks (e.g., Mittlefehldt & Lindstrom, 2001; Wu & Hsu, 2019), all of which crystallized from melts. Pyroxene and associated olivine grains crystallized from OC impact melt rocks usually show (1) a pronounced positive linear trend between TiO 2 and Al 2 O 3 in pyroxene (e.g., L‐impact melt rock PAT 91501, Mittlefehldt & Lindstrom, 2001; L‐melt rock NWA 11042, Wu & Hsu, 2019) and (2) relatively ferroan‐rich silicates at the upper end of the corresponding chondritic compositional range (e.g., olivine Fa 25.2–26.8 for L‐impact melt rock PAT 91501 vs. olivine Fa 22–26 for typical L chondrites; Brearley & Jones, 1998, Mittlefehldt & Lindstrom, 2001).…”