GEOCHEMICAL CHARACTERISTICS OF MELTED DOMAINS IN SUEVITES OF THE EYREVILLE DRILL CORE B; CHESAPEAKE BAY IMPACT STRUCTURE, USA

Suevites occurring in the Eyreville drill core B contain up to 25 vol% of components in the size range from 0.1 to 3 cm that is either glass, or hydrated and/or altered melt material (25 samples, optical microscopy). The irregular shaped clasts are brown and/or green, often showing differently colored bands or cores as well as schlieren, yet only few domains are completely isotropic. Most of the fragments contain < 10-µm–sized inclusions which are currently investigated by DEGAS. Some also show plagioclase microcrysts. Using electron microprobe analyses (15 kV, 5 nA, beam Ø < 10 µm) of such clasts in four suevite samples (depth between 1400 and 1439 m) we distinguish, (i) high silica, (ii) medium iron-medium alumina, and (iii) high calcium “glasses” which are all altered to different degrees. These types have the following major element characteristics (average values or ranges in wt%): (i) 92 % total, 75 to 87 % SiO₂, 5 % Al₂O₃, other elements ≤ 1 %; (ii) 90 % total, 64 to 86 % SiO₂, 6 to 10 % Al₂O₃, 3 to 5 % FeO_tot, other elements ≤ 1 %; (iii) 90 % total, 70 % SiO₂, 8 % Al₂O₃, 5 % FeO_tot, 4 to 8 wt% CaO, other elements ≤1 %. Some homogeneous domains of plagioclase-like composition contain drop shaped chlorite-smectite mixed layer clays (84 % total, 56 % SiO₂, 11 to 17 % Al₂O₃, 5 to 11 % FeO_tot, 2 % MgO, 2 % CaO, other elements ≤1 %). Clay minerals also form sharp menisci. Primary quartz and alkalifeldspar (partly with ilmenite inclusions) clasts display corroded irregular grain shapes due to the impact related thermal overprint. In some melted domains columnar alkalifeldspar accumulate to aggregates up to a diameter of 50 µm. Pre-impact calcite and dolomite are partly molten and survived as rounded or smeared grains. Iron sulfides form melt drops and hypidiomorph crystals respectively. Sphene, ilmenite, and zircon are accessory minerals. The different melt types have to be ascribed to a complex scenario. For a better understanding of the formation processes, mineral and lithic clasts as well as the melt domains will be characterized by isotopic and trace element analyses.