SILICATE GLASSES AND SULFIDE MELTS IN THE ICDP-USGS EYREVILLE CORE-B, CHESAPEAKE BAY IMPACT STRUCTURE

Optical and electron beam petrography of melt-rich suevite and melt-rock clasts has revealed a wide variety of glasses and co-existing sulfur-rich melts, now quenched to a variety of sulfide minerals (± iron). The glasses show a wide variety of textures, compositions, devitrification, and hydration states. Flow banding with intricate swirls shows that the glasses were dynamically mixing and of low viscosity. Some samples show spherical globules of different composition with sharp menisci suggesting immiscibility at the time of quenching. Electron microprobe analyses, using 15 KeV accelerating voltage, 10 nA current, 20 µm beam spot, and 10 s counting times on peak, show a compositional range from high SiO₂ (>90 wt%) to a range of lower SiO₂ (55 to 75 wt%) glasses. For example, isotropic globules of higher interfacial tension glass (wt% 64 SiO₂, 15 Al₂O₃, 5 FeOT, 2 MgO, 1.3 CaO, 1.4 Na₂O, and 2.5 K₂O) are in sharp contact with lower surface tension, high-silica glass (wt% 95 SiO₂, 1 Al₂O₃, 1 FeOT, 0.1 MgO, 0.2 Na₂O). Devitrification 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, and plagioclase. Hydration (H₂O by difference) varies from about 10 wt% to essentially anhydrous; high SiO₂ glasses tend to contain less H₂O. Petrographic relationships show decomposition of target rock pyrite forming pyrrhotite and its subsequent melting. Spheres (1 to 50 µm) of this quenched immiscible sulfide melt in glass show a range of composition and include phases such as pentlandite, chalcopyrite, and Ni-As, but pyrrhotite is predominant. Some sulfide spheres contain small blebs of pure iron, and other spheres consist of pure iron with small, remnant blebs of Fe-sulfide. The presence of locally unaltered glasses in these rocks suggests that in some rock volumes, isolation from post-impact hydrothermal systems was sufficient for glass preservation.