Nature and timing of a significant reduction event on the L‐chondrite parent asteroid

Abstract

About 17% of L6 chondrites (15/87) show significant reduction features in BSE images in thin section. Because some thin sections of these meteorites do not show reduction features, this percentage is a lower limit. Reduction features include: (1) 4–5‐μm‐thick BSE‐dark reduction rims on olivine and orthopyroxene grains and along fracture boundaries in these grains, (2) 4–12‐μm‐thick dark bands (probably poorly crystalline pyrrhotite) at the margins and along fractures in troilite grains, and (3) 2–5‐μm‐thick dark rinds of kamacite around some taenite grains. Only one of 70 L‐group chondrites (1.4%) of lower petrologic type exhibits minor reduction. The L6 chondrites showing major reduction have 40Ar/39Ar plateau ages ranging from 156 ± 1 Ma for Guangnan to 4543 ± 3 Ma for Thamaniyat Ajras. Reduction occurred after silicate, sulfide, and metal grains had attained their present sizes during parent‐body thermal metamorphism (and had been fractured by parent‐body collisions). The precise plateau age of Thamaniyat Ajras probably marks the timing of the L6 reduction event. It seems likely the reductant was a low‐viscosity fluid, plausibly CO, derived from oxidation of poorly graphitized and amorphous carbon within fine‐grained matrix. Water‐ice that had accreted to the L‐chondrite asteroid was heated and mobilized during metamorphism, causing oxidation. After peak metamorphism, ~75% of the water had been used up or lost; the remaining water facilitated continuing graphite oxidation so that, after this point, overall reduction effects exceeded those of oxidation. L chondrites of lower petrologic type were less affected by reduction due to their lower metamorphic temperatures.