Geochemical Studies of Primitive Melt Inclusions from Mt. Shasta

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Shimizu EAR-0106578 This project is an investigation of melting and melt migration processes operating for the Cascadia subduction zone using the geochemical analyses of primitive melt inclusion hosted in high-Mg olivine phenocrysts from Mt. Shasta, northern California. The Cascadia system represents one of the hottest subduction systems where contributions of slab-derived melt components could be tested quantitatively. A detailed geochemical study of primitive melt inclusions as proposed here has never been carried out for a subduction zone system. The emphasis of the proposed work is: (1) to focus on primitive melts so that new insights into processes of melt generation could be gained; (2) to focus on melt inclusions so that a representative spectrum of melt composition can be captured, and pre-eruptive volatile contents can be determined; and (3) to characterize an endmember case for hot young subduction system. The primary objective of the proposed work is to establish a geochemical characterization of melts involved in all major eruptive stages that created edifices of Mt. Shasta. The proposed work will be carried out in the following steps: (1) documentation of geochemical variabilities among melt inclusions in each sample; (2) clarification of the relationships between melt inclusions and host rock; (3) evaluation of geochemical variabilities among separate eruptive units within each eruptive stage; (4) assessment of temporal geochemical trends for the major eruptive stages. Melt inclusions in olivines will be homogenized using a Sobolev-type heating stage, and will be analyzed for major elements with an electron probe. Ion probe analyses will be made in a sequence beginning with B and Pb isotope compositions, followed by trace element analysis for light elements (Li, Be, B, H2O ), REE (La, Ce, Nd, Sm, Dy, Er, Yb), HFSE (Ti, Zr, Nb), alkaline earths (Ba, Sr) and Y.