TEXTURAL AND COMPOSITIONAL CHARACTERISTICS OF METALLIC SPHERULES IN IMPACT MELT FROM MONTURAQUI CRATER, CHILE

Samples of impactite from the small (~350 m diameter) Monturaqui crater in northern Chile contain Fe-Ni metallic spherules sourced from the iron meteorite impactor. Textural characterization and quantification was done using SEM and µCT analysis. Two spherule textural types are distinguished, and their different size distributions indicate different origins. The smaller spherical spherules (mostly < 100 µm in diameter) follow a power-law size distribution, while larger objects are mostly irregular-shaped blebs. These textural types are analogous to the small (nm – 50 µm) immiscible spherical metal droplets and large (150 – 500 µm) irregular partly-fused fragments of the iron meteorite projectile observed in highly shocked ejecta pieces in the MEMIN high-velocity impact experiment (Ebert et al., 2013, Meteoritics & Planetary Science 48, 134-149). Compositions of both textural types were determined using in-situ methods (electron microprobe, LA-ICP-MS), as well as solution ICP-MS on individual spherules separated from impact melt glass using Electric Pulse Disintegration. The spherules are enriched in highly siderophile elements (PGEs, Re, Au) together with Ni, Co, Cu and Mo, relative to Fe, while W is depleted. There is limited fractionation between different PGEs, and all spherules have similar chondrite-normalized patterns that are also broadly similar to those of the weathered fragments of the iron meteorite impactor. These data can be explained by a selective oxidation model in which Fe (and W) is progressively lost through oxidation to the surrounding silicate melt.