Filiberto, Justin; Chin, Emily; Day, James M. D.; Franchi, Ian A.; Greenwood, Richard C.; Gross, Juliane; Penniston-Dorland, Sarah C.; Schwenzer, Susanne P. and Treiman, Allan H.
(2012).
Geochemistry of intermediate olivine-phyric shergottite Northwest Africa 6234, with similarities to basaltic shergottite Northwest Africa 480 and olivine-phyric shergottite Northwest Africa 2990.
Meteoritics and Planetary Science, 47(8)
pp. 1256–1273.
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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 δ 7Li 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.
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