FLUID INCLUSION EVIDENCE FOR POST-IMPACT HYDROTHERMAL CIRCULATION AT THE CENTRAL UPLIFT OF THE CHESAPEAKE BAY IMPACT STRUCTURE

The Chesapeake Bay impact structure formed about 35.3 m.y. ago by an impact on the continental shelf of North America. Water was about 300 m deep during the late Eocene sedimentation that buried the structure. A partially cored test hole drilled near Cape Charles, VA, situated above the central uplift, provided cores from depths of 744 to 823 m. They consist of crystalline-clast breccias, including suevite and brecciated felsic to mafic gneisses that are interpreted as basement-derived megablocks.

Where brecciated gneisses contain calcite veins, much of the calcite shows high-strain fabrics such as twinning that has been warped and offset by microfaults; very little of this calcite contains fluid inclusions. These calcite veins may predate the impact, and most if not all preexisting fluid inclusions may have been destroyed. Veins of clear, unstrained calcite are also present in some rocks. A sample of grayish-black, aphanitic impact-melt rock from a clast in suevitic breccia has a quenched groundmass and is cut by <1 mm calcite veins in which fluid inclusions are common. The vein fill is mostly sparry, and some of the smallest veins have a cross-fiber texture. Fluid inclusions (FI) occur only in the sparry calcite.

Most FI are elongate to irregular liquid (L) + vapor (V) FI that homogenize to L. A few L-only inclusions occur as well. Less abundant, larger, more irregular L+V FI are confined to certain growth zones in the sparry calcite crystals. These are prone to leakage in thin sections, because they have extensions that tend to intersect the top or bottom of the sections. Vapor-rich FI that appear to homogenize to vapor are rare. The FI confined to growth zones appear to be primary, while others could be primary or secondary.

Homogenization temperatures of reliable FI assemblages range up to 230°C. Ice melting (Tm) ranges from -1.1 to -11.5°C, with most between -1.8 and -3.1°C. Thus, most salinities are close to that of seawater (Tm = -2.0°C), yet there are some much more dilute and other much more saline FI. Hydrothermal fluids above the central uplift reached temperatures close to the boiling point of seawater (220°C at 300 m water depth). Variable FI salinities and some FI that homogenize to vapor suggest that boiling occurred. The thermal conditions described here are consistent with those inferred from secondary mineral assemblages.