This study will combine in situ ion microprobe analysis of oxygen isotope ratios and U-Pb ages on zircon crystals from high pressure metamorphic rocks of the Franciscan Complex, California. The goals of this study include: establishing the pressure-temperature-time-fluids (P-T-t-fluid) history of these rocks before, during and after burial and exhumation; determining the protoliths of the high pressure metamorphic rocks; improved reliability of geochronology; and testing competing theories if exhumation for eclogites. Samples will be chosen from well-known localities in the Franciscan, including Ward Creek, Jenner, Panoche Pass, and Santa Catalina Island. Analysis will focus on zircons and associated minerals. Special attention will be paid to intracrystalline isotope zonation and mineral inclusions. The combination of d18O and age from single spots in zircon will directly couple timing to fluid and thermal events. Our preliminary studies have already described and analyzed zircons from eclogite, blueschist, and amphibolite of the Franciscan, as well as young ocean floor samples (gabbronorite, plagiogranite, serpentinite) that may represent protoliths to high pressure rocks. Analysis of d18O will employ the new Cameca 1280 ion microprobe at the University of Wisconsin. Analysis of U-Pb age and REEs will employ the Stanford/USGS SHRIMP-RG. Ion microprobe analysis will be made using zircon crystals both in thin section and in grain mounts that have been petrologically studied for inclusions and coexisting minerals; and imaged by optics, BSE, CL, and X-rays. The broader impacts of this research include post-graduate training and installation of the new Wisconsin ion microprobe lab, which will also be employed in other studies related to societal problems such as environmental remediation, paleoclimatology, CO2 sequestration, and nutrition.
