A feldspar-nepheline achondrite clast in Parnallee

概要

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A feldspar-nepheline clast (FELINE) has been identified in Parnallee (LL3.6). Plagioclase is An_<83-87>, Ab_<17-13> and nepheline contains 0.24-3.12wt% Cl. The calculated bulk composition is mildly alkaline, with 3.5wt% Na_2O. Plagioclase has heavy REE depletion and a positive Eu anomaly (Eu/Eu^*=65). Nepheline has lower total REE than plagioclase. On a three isotope plot, the oxygen isotope composition of FELINE falls near the Carbonaceous Chondrites Anyhdrous Minerals Line, beneath the Terrestrial Fractionation Line. This suggests that the parental material had carbonaceous chondrite affinities. It was derived from a melt with moderately enriched LREE and Eu (13.5×CI), which probably underwent an influx of Na-, Cl-rich fluids during crystallisation. This LREE-enrichment suggests that Ca-pyroxene crystallised in the parent body residue during a melt extraction event. REE abundances and the oxygen isotope signature are consistent with an origin as a lost plagiophile melt fraction complementary to the ureilites. FELINE provides further evidence that achondritic fragments with an igneous, exotic origin are an important component of chondritic meteorites.
国立極地研究所の論文