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

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.