QUANTITATIVE MODAL ANALYSES OF METEORITE BRECCIAS

Howardite, Eucrites, and Diogenites (HEDs) comprise a clan of meteorites thought to originate from the differentiated asteroid 4 Vesta and are analogs for its surface regolith and megaregolith. Surface regolith is composed of material circulated by impact gardening on upper most portion of Vesta’s surface. Megaregolith refers to an underlying subsurface layer of impact fragmented material into which some surface regolith material may percolate. The surface regolith and megaregolith on Vesta combined are up to 1-2 km thick. Howardites are breccias primarily composed of fragments of eucrites and diogenites—groups that respectively include numerous rocks with basaltic and ultramafic bulk compositions—plus impact-derived lithologies, exogenic carbonaceous chondrite material, and mineral fragments interpreted as mantle samples. We quantify the heterogeneous source material in howardites using image-processing software typically used in remote-sensing (ENVI) to analyze image cubes composed of WDS x-ray maps of 8 elements. This method has the advantage of incorporating a greater number of elements than modal analyses that use element map color-mixing, which can only incorporate 3 elements for each resultant color to represent a unique phase. We have used this method to analyze 20 thin sections from 7 separate howardite samples, including 2 established pairing groups, an additional proposed pairing group, and four single-stone howardite finds. In 4 of these 7 howardites, modes of major components notably vary between thin sections, indicating that analyzing a single thin section is not enough material to accurately determine the component modes in a howardite. We also found in all the howardites that are analogs for vestan surface regolith, plagioclase is depleted relative to eucrite-composition pyroxene compared to unbrecciated eucrites, the primary plagioclase-bearing source lithologies. Plagioclase is preferentially comminuted relative to pyroxene, which is the other dominant mineral in these breccias and their source materials. We suggest this relative depletion in plagioclase may be indicative of regolith maturity in howardites.