Pre-eruptive conditions beneath Medicine Lake Volcano, California, during the Pleistocene epoch

Abstract

This investigation uses the variations in composition of lavas and mineralogical evidence to characterize the preeruptive conditions beneath Medicine Lake Volcano (MLV) during the Pleistocene epoch. Prior to this investigation, the Pleistocene lavas of MLV had not been extensively studied because detailed time relations among them were not known. Recent work by Donnelly-Nolan has provided a time framework for relating these lavas, and has afforded us the opportunity to perform this study. Whole rock analyses for major elements, trace elements, and rare earth elements were performed, as well as electron microprobe analyses of individual minerals in the lavas. These analyses indicate that the lavas were formed by the mixing of three components. The first component was formed by fractional crystallization of a high-H20 magma (water content of 3-4 wt%) which was emplaced in the shallow crust, at approximately 1 kbar of pressure and temperatures between 980-1300K. The emplacement and crystallization of the first component supplied the heat necessary to melt the granitic crust under MLV. The crustal melt that resulted formed the second component. The third component was a fresh input from the mantle, whose injection into the shallow crust mixed all three components together. The pre-eruptive environment of these lavas was more oxidizing than those of other MILV lavas, such as the Holocene-aged Glass Mountain rhyolite.