Abstract
AbstractOlivine‐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 Δ17O value of −1.19 ± 0.10‰, 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 187Os/188Os (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.
Highlights
Melted “primitive” achondrite meteorites offer insights into the earliest stages of planetary differentiation, as they provide evidence for limited melting and melt segregation in early-formed asteroids
The uneven distribution is consistent with Lewis Cliff (LEW) 88763 representing a fragment of a larger meteoroid that broke up during entry, or preferred aerodynamic orientation during atmospheric entry
For Northwest Africa (NWA) 6693 to be a direct melt product of LEW 88763. This is because LEW 88763 has experienced partial melting that is too limited to generate a NWA 6693-like composition, and instead, LEW 88763 represents the least-modified FeO-rich achondrite source composition recognized to date
Summary
Melted “primitive” achondrite meteorites offer insights into the earliest stages of planetary differentiation, as they provide evidence for limited melting and melt segregation in early-formed asteroids. While many of these meteorite groups are defined, by among other things, the high Mg/(Mg+Fe) of their silicates (e.g., winonaites, acapulcoite-lodranites, ureilites; e.g., Mittlefehldt et al 1998), an increasing array of meteorites are being recognized with more ferroan compositions, suggestive of evolution from distinctive sources These FeO-rich meteorite types include the brachinites, brachinite-like achondrites, and GRA 06128/9 meteorites (Day et al 2012), the recrystallized primitive achondrite Tafassasset (Gardner-Vandy et al 2012) and the poikilitic cumulate NWA 6693 (Warren et al 2013). High-FeO partially melted achondrite meteorites are important materials for understanding inherent variation in oxygen fugacity and volatile abundances in the early solar system, as well as for examining the nature of precursor “chondritic” parental materials forming planets and planetesimals
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