HYDROTHERMAL ALTERATION WITHIN DEEP CRATER FILL MATERIAL, CHESAPEAKE BAY IMPACT STRUCTURE: A MINERAL AND FLUID INCLUSION STUDY

Hydrothermal circulation cooled the CBIS, which formed in 300 m of water. Cores from the central uplift show post-impact T exceeding the apatite fission-track closure at 100°C (Horton et al., Lunar Planet. Sci. Conf., 2006) and reaching up to 230°C based on fluid inclusions (FI) in calcite (cal) (Vanko et al., GSA Abstr. w. Prog. 38, 2006). The new ICDP-USGS Eyreville-B hole is in the annular moat 10 km away from the central uplift. Crystalline blocks and suevitic breccia were examined for post-impact hydrothermal features. All evidence indicates relatively low-T alteration at this site.

Granite overlying suevite and cataclastic rocks contains pegmatitic zones and a quartz-sulfide vein in a biotite schist enclave. Qtz from the pegmatite has mottled extinction (ext) and multiple FI generations, and some feldspar has kinked twins. The qtz vein in the enclave contains dense H₂O-CO₂ FIs with a high-T metamorphic/igneous origin. These pre-impact FIs survived the meteorite impact, so the granite is allocthonous.

Suevite, polymict suevitic breccia and associated cataclastic rocks contain both pre- and post-impact veins. Pre-impact qtz veins have mottled ext and PDFs, and typically have tiny one-phase FI. Most cal veins are later, and some cut pre-impact qtz veins. Some cal may be pre-impact, and contains fine PDFs. Cal that is most likely to be post-impact contains one-phase FI (i.e., low-T, <~80°C) with close-to-seawater-salinity.

Suevitic breccia is cut by a low-T clay + phillipsite vein, and contains a vug lined by low-T chalcedony, zeolite, and a very fine acicular mineral.

Graphitic schist below the suevitic rocks contains mosaic-textured qtz lenses, some brecciated and cal-cemented, which have mottled ext but no PDFs. Two-phase FIs from multiple generations suggest these are pre-impact FI that survived because they did not experience high shock stages. Similarly, granite from near the bottom of the hole also has qtz with mottled ext, feldspar with bent twin lamellae, and many FI in qtz.

Post-impact hydrothermal activity in the Eyreville-B core was limited to low-T interactions that formed secondary phillipsite, clay, chalcedony, and calcite. One-phase FIs in cal indicate temperatures below about 80°C and salinities near seawater.