Geochemistry of intermediate olivine‐phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine‐phyric shergottite Northwest Africa 2990

Abstract

Abstract-The newly found meteorite Northwest Africa 6234 (NWA 6234) is an olivine (ol)-phyric shergottite that is thought, based on texture and mineralogy, to be paired with Martian shergottite meteorites NWA 2990, 5960, and 6710. We report bulk-rock major-and trace-element abundances (including Li), abundances of highly siderophile elements, Re-Os isotope systematics, oxygen isotope ratios, and the lithium isotope ratio for NWA 6234. NWA 6234 is classified as a Martian shergottite, based on its oxygen isotope ratios, bulk composition, and bulk element abundance ratios, Fe ⁄ Mn, Al ⁄ Ti, and Na ⁄ Al. The Li concentration and d 7 Li value of NWA 6234 are similar to that of basaltic shergottites Zagami and Shergotty. The rare earth element (REE) pattern for NWA 6234 shows a depletion in the light REE (La-Nd) compared with the heavy REE (Sm-Lu), but not as extreme as the known ''depleted'' shergottites. Thus, NWA 6234 is suggested to belong to a new category of shergottite that is geochemically ''intermediate'' in incompatible elements. The only other basaltic or ol-phyric shergottite with a similar ''intermediate'' character is the basaltic shergottite NWA 480. Rhenium-osmium isotope systematics are consistent with this intermediate character, assuming a crystallization age of 180 Ma. We conclude that NWA 6234 represents an intermediate compositional group between enriched and depleted shergottites and offers new insights into the nature of mantle differentiation and mixing among mantle reservoirs in Mars.

Lithium isotope constraints on crust–mantle interactions and surface processes on Mars

“…The currently available Li isotope data for bulk analyses of martian meteorites appear to reflect magmatic fractionation (Bridges et al, 2005;Magna et al, 2006;Reynolds et al, 2006;Seitz et al, 2006;Filiberto et al, 2012). In contrast, intra-mineral variations in Li abundances and isotope compositions in shergottites and nakhlites have been interpreted in the context of the presence of magmatic water and subsequent degassing McSween et al, 2001;Beck et al, 2004;Herd et al, 2005), or have been related to kinetic isotope fractionation by diffusion (Beck et al, 2006b;Chaklader et al, 2006;Treiman et al, 2006).…”

“…NWA 4925 (d 7 Li indicated with an arrow) is omitted for a better clarity of the figure. Lithium concentrations and isotope compositions for Dhofar 019 and NWA 6234 shergottite are fromReynolds et al (2006) andFiliberto et al (2012), respectively. Reported are martian meteorites (see…”

“…Elevated Ba/La in NWA 6234 is not coupled with a light Li isotope signature, providing limited evidence for terrestrial modification of this meteorite. Lithium and other trace element data for NWA 6234 is fromFiliberto et al (2012), data for Dho 019 is from elsewhere…”

Gabbroic Shergottite Northwest Africa 6963: An intrusive sample of Mars

“…Lherzolitic shergottites are coarse-grained olivine-pigeonite cumulate rocks and are intrusive in nature (Goodrich 2002;Bridges and Warren 2006). Shergottites are also classified based on rare earth element bulk chemistry and isotopic signatures into enriched, intermediate, and depleted categories (Jones 1989(Jones , 2003Treiman 2003;Bridges and Warren 2006;Filiberto et al 2012). Shergottite NWA 6963 has previously been classified as an enriched basaltic shergottite (Meteoritical Bulletin 2013; Wilson et al 2012), but based on mineral abundances, textural analysis, and quartz-alkali feldspar intergrowths, we suggest it represents an intrusive rock similar to a terrestrial gabbro.…”

Crystallization history of enriched shergottites from Fe and Mg isotope fractionation in olivine megacrysts

“…Some olivine-phyric shergottites are strongly (e.g. Zipfel et al, 2000;Shirai and Ebihara, 2004;Aoudjehane et al, 2012) or moderately (Filiberto et al, 2012;Gross et al, 2013) depleted in light rare earth elements (LREE) relative to heavy rare earth elements (HREE). Other shergottites are not depleted in LREE and are characterized by higher initial 87 Sr/ 86 Sr and lower initial e 143 Nd.…”