Differentiation processes in FeO-rich asteroids revealed by the achondrite Lewis Cliff 88763

James Day (1) , Christopher A. Corder (2) , Douglas Rumble Iii (3) , Nelly Assayag (3) , Pierre Cartigny (4) , Laurence A. Taylor,

1 Geosciences Research Division [La Jolla]
2 Geophysical Laboratory [Carnegie Institution]
3 IPGP - Institut de Physique du Globe de Paris
4 Planetary Geosciences Institute [Knoxville]

Olivine-dominated (70-80 modal %) achondrite meteorite Lewis Cliff (LEW) 88763 originated from metamorphism and limited partial melting of a FeO-rich parent body. The meteorite experienced some alteration on Earth, evident from subchondritic Re/Os, and redistribution of rhenium within the sample. LEW 88763 is texturally similar to winonaites, has a Delta O-17 value of -1.19 +/- 0.10 parts per thousand, and low bulk-rock Mg/(Mg+Fe) (0.39), similar to the FeO-rich cumulate achondrite Northwest Africa (NWA) 6693. The similar bulk-rock major-, minor-, and trace-element abundances of LEW 88763, relative to some carbonaceous chondrites, including ratios of Pd/Os, Pt/Os, Ir/Os, and Os-187/Os-188 (0.1262), implies a FeO-and volatile-rich precursor composition. Lack of fractionation of the rare earth elements, but a factor of approximately two lower highly siderophile element abundances in LEW 88763, compared with chondrites, implies limited loss of Fe-Ni-S melts during metamorphism and anatexis. These results support the generation of high Fe/Mg, sulfide, and/or metal-rich partial melts from FeO-rich parent bodies during partial melting. In detail, however, LEW 88763 cannot be a parent composition to any other meteorite sample, due to highly limited silicate melt loss (0 to << 5%). As such, LEW 88763 represents the least-modified FeO-rich achondrite source composition recognized to date and is distinct from all other meteorites. LEW 88763 should be reclassified as an anomalous achondrite that experienced limited Fe, Ni-FeS melt loss. Lewis Cliff 88763, combined with a growing collection of FeO-rich meteorites, such as brachinites, brachinite-like achondrites, the Graves Nunataks (GRA) 06128/9 meteorites, NWA 6693, and Tafassasset, has important implications for understanding the initiation of planetary differentiation. Specifically, regardless of precursor compositions, partial melting and differentiation processes appear to be similar on asteroidal bodies spanning a range of initial oxidation states and volatile contents.

Mots clés: basaltic meteorites; brachinites; chondrites; early solar-system; highly siderophile elements; northwest-africa-011; oxygen-isotope; parent body; partial melt residues; ureilites

Domaines: Planète et Univers [physics]

James Day, Christopher A. Corder, Douglas Rumble Iii, Nelly Assayag, Pierre Cartigny, et al.. Differentiation processes in FeO-rich asteroids revealed by the achondrite Lewis Cliff 88763. Meteoritics and Planetary Science, 2015, 50, pp.1750-1766. ⟨10.1111/maps.12509⟩. ⟨insu-01447679⟩