Silicate glasses and sulfide melts in the ICDP-USGS Eyreville B core, Chesapeake Bay impact structure, Virginia, USA

Abstract

Optical and electron-beam petrography of melt-rich suevite and melt-rock clasts from selected samples from the Eyreville B core, Chesapeake Bay impact structure, reveal a variety of silicate glasses and coexisting sulfur-rich melts, now quenched to various sulfi de minerals (??iron). The glasses show a wide variety of textures, fl ow banding, compositions, devitrifi cation, and hydration states. Electron-microprobe analyses yield a compositional range of glasses from high SiO 2 (>90 wt%) through a range of lower SiO 2 (55-75 wt%) with no relationship to depth of sample. Some samples show spherical globules of different composition with sharp menisci, suggesting immiscibility at the time of quenching. Isotropic globules of higher interfacial tension glass (64 wt% SiO 2 ) are in sharp contact with lower-surface-tension, high-silica glass (95 wt% SiO 2 ). Immiscible glass-pair composition relationships show that the immiscibility is not stable and probably represents incomplete mixing. Devitrifi cation varies and some low-silica, high-iron glasses appear to have formed Fe-rich smectite; other glass compositions have formed rapid quench textures of corundum, orthopyroxene, clinopyroxene, magnetite, K-feldspar, plagioclase, chrome-spinel, and hercynite. Hydration (H 2 O by difference) varies from ~10 wt% to essentially anhydrous; high-SiO 2 glasses tend to contain less H2O. Petrographic relationships show decomposition of pyrite and melting of pyrrhotite through the transformation series; pyrite? pyrrhotite? troilite??? iron. Spheres (~1 to ~50 ??m) of quenched immiscible sulfi de melt in silicate glass show a range of compositions and include phases such as pentlandite, chalcopyrite, Ni-As, monosulfi de solid solution, troilite, and rare Ni-Fe. Other sulfi de spheres contain small blebs of pure iron and exhibit a continuum with increasing iron content to spheres that consist of pure iron with small, remnant blebs of Fe-sulfi de. The Ni-rich sulfi de phases can be explained by melting and/or concentrating targetderived Ni without requiring an asteroid impactor source component. The presence of locally unaltered glasses in these rocks suggests that in some rock volumes, isolation from postimpact hydrothermal systems was suffi cient for glass preservation. Pressure and temperature indicators suggest that, on a thin-section scale, the suevites record rapid mixing and accumulation of particles that sustained widely different peak temperatures, from clasts that never exceeded 300 ?? 50 ??C, to the bulk of the glasses where melted sulfi de and unmelted monazite suggest temperatures of 1500 ?? 200 ??C. The presence of coesite in some glass-bearing samples suggests that pressures exceeded ~3 GPa. ?? 2009 Geological Society of America.