The first author wishes to thank Dr. Sherwood Wise for helpful comments and reviews of early drafts of this paper, which was submitted in partial fulfillment of the requirements for a master's degree in geology at Florida State University. We also wish to thank Robert Whitmarsh and José-Abel Flores for external reviews of the final draft. Samples were provided by NSF through the Ocean Drilling Program. This study was supported by USSAC funds to Dr. S.W. Wise. Laboratory facilities were provided by NSF grant no. DPP 94-22893.