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Scientific Goals: A significant amount of thermal energy is deposited as a result of an impact event, with the major post-impact heat sources being bodies or sheets of impact melt and suevite. Heat dissipation is likely to involve hydrothermal circulation through the underlying fractured basement and the permeable crater fill material, particularly when the impact is on a continental shelf. Thus, the hydrothermal setting envisioned for the Chesapeake Bay impact crater shares several features of current models for mid-ocean ridge and seamount hydrothermal systems, and has numerous differences as well. Core samples from the impact crater can provide information about post-impact hydrothermal conditions recorded in secondary mineral chemistry and in fluid inclusions. The main scientific goal of the present proposal is to help constrain models for post-impact hydrothermal circulation at the Chesapeake Bay impact site. A related specific objective is to use fluid inclusion constraints to test whether phase separation was a process that could have generated saline brine within the crater as a result of high-temperature hydrothermal circulation. Background. Post-impact hydrothermal systems have been recognized at several impact sites. These include the Chicxulub structure in Mexico (Gonzalez-Partida et al., 2000; Zurcher and Kring, 2004), the Kardla crater in Estonia (Kirsimae et al., 2002), the Haughton structure, Canada (Osinski et al., 2001), and the Lockne marine impact structure in Sweden (Sturkell et al., 1998). New studies at the Chesapeake Bay are underway on the 2004 Cape Charles core (Vanko, in progress), and results from this and the ICDP-USGS deep hole will be compared and contrasted with those from other impact sites with evidence of past hydrothermal activity. Brine generation. High fluid salinities, elevated temperatures, and the occurrence of both liquid-dominated and vapor-dominated fluid inclusions in some secondary minerals all suggest that post-impact hydrothermal fluid flow may frequently involve phase separation, or boiling. At the Chesapeake Bay impact site, Sanford (2003) hypothesizes that hydrothermal activity accompanied by phase separation generated the brine that is still thought to exist today within the Exmore breccia. Sanford (2003) calculates that brine generated 35 Ma ago would still be present because of low groundwater velocities and minor molecular diffusion effects. Drilling the impact crater will provide specimens from deep within the impact breccia and in the basement that should contain secondary minerals and fluid inclusions. Fluid inclusion investigations can test the hypothesis of hydrothermal brine generation, and provide firm chemical and temperature constraints for the hydrothermal system. Deep-sea example. Fluid inclusions can provide excellent records of subseafloor boiling in deep-sea hydrothermal systems. One example is the deep-sea PACMANUS system in the eastern Manus Basin back-arc spreading system offshore Papua New Guinea (Vanko et al., 2004). Similar investigations of core material from the proposed deep drilling within the Chesapeake Bay impact crater will define the critical parameters of post-impact hydrothermal activity: fluid temperatures and temperature profiles; fluid chemistry; fluid-rock interaction and secondary mineralogy and mineral chemistry; nature of the heat source driving circulation; and possibly even timing and longevity of the hydrothermal circulation. Techniques and facilities: Fluid inclusion studies on core material will include heating and freezing measurements to define fluid chemical systems, obtain estimates of fluid densities, and interpret the temperatures and pressures of secondary mineral growth (Roedder, 1984). These results will be integrated with constraints from phase chemistry, isotopic ratios, and other data obtained by science team members. The fluid inclusion laboratory at Towson University contains a USGS-type gas-flow heating and freezing stage capable of temperature control between -196? and +700?C. Sample preparation equipment includes a precision low-speed saw and a polishing unit. Mineral chemical analysis is available using microprobes at the University of Maryland-College Park or the USGS-Reston, and laser-Raman microprobes at the USGS or Virginia Tech can be used to characterize Raman-active fluid inclusion contents such as hydrocarbons and many types of daughter minerals. Sample requirements: Having completed and published fluid inclusion investigations for five different Ocean Drilling Program legs (118, 140, 148, 168, 193), the PI thinks that sample requirements from drill core from the Chesapeake Bay impact site are simple to anticipate. Small (domino-sized) aliquots of core specimens containing medium-to-coarse secondary mineralization, either in veins, nodules, or as pervasive alteration, are likely to yield fluid inclusions. Mineral hosts that have yielded good data in the past include quartz, calcite, aragonite, anhydrite, gypsum, plagioclase, diopside, amphibole, and epidote. A broad spectrum of specimen types is useful for reconnaissance work, while more focused sampling of, say, an individual paragenesis may eventually be needed to answer some of the more interesting questions about fluid properties. Sources of funding: The PI is a named subcontractor in the current NSF submission by Daniel Larsen (U. Memphis). If funded, that subcontract will provide the main portion of necessary funding. Departmental funds from Towson University will also be applied, especially for travel and student involvement. Collaborators and Science Team affiliation: Prime collaborators are Daniel Larsen (Memphis), Wright Horton and Ward Sanford (USGS-Reston). The science team with the most direct relevance is probably (1) Crater Materials, although there is considerable overlap with (3) Hydrologic processes and resources, and (4) Crater mechanics and modeling. The PI and, probably, some undergraduate students, will be available to assist in the coring operations.
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