Publications

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2015
Day, JMD, Corder CA, Rumble D, Assayag N, Cartigny P, Taylor LA.  2015.  Differentiation processes in FeO-rich asteroids revealed by the achondrite Lewis Cliff 88763. Meteoritics & Planetary Science. 50:1750-1766.   10.1111/maps.12509   AbstractWebsite

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.

2012
Day, JMD, Walker RJ, Qin LP, Rumble D.  2012.  Late accretion as a natural consequence of planetary growth. Nature Geoscience. 5:614-617.   10.1038/ngeo1527   AbstractWebsite

Core formation should strip highly siderophile elements (HSEs) from planetary mantles according to the expected metal-silicate partitioncoefficients(1,2). However, studies of Earth(3), the Moon(4) and Mars(5) indicate mantles with HSE abundances in chondrite-relative proportions that exceed the values expected from metal-silicate partitioning. Competing hypotheses have been proposed to account for these observations, including metal-silicate partitioning at higher pressures and temperatures(6) and late accretion(7). Here we present petrological and geochemical analyses of diogenite meteorites that represent mantle and crustal materials from two or more differentiated asteroids. We find that diogenites show HSE abundances that are consistent with metal-silicate equilibration, followed by minor continued accretion. Isotope chronometry supports diogenite crystallization ages within 2-3 million years of Solar System formation, indicating that late accretion occurred earlier than postulated for Earth, the Moon and Mars. The early timing and occurrence on differentiated asteroids, as well as on the larger terrestrial planets, therefore ties late accretion to planetary growth. On asteroidal bodies, such as the diogenite parent bodies, variations in HSE compositions may reflect regional rather than global effects. In contrast, for Earth, the Moon and Mars, compositional variations in mantle materials seem to be consistent with more homogeneous distributions through prolonged melting and/or solid-state convection.