Isotopic evidence for the relative roles of fractional crystallization, crustal assimilation and magma supply in the generation of large volume rhyolitic eruptions

The mechanisms responsible for the generation of large volumes of silicic magma remain poorly understood. Of particular interest are the relative roles of fractional crystallization, crustal assimilation and magma supply and the location and mechanism for crustal assimilation. This research is aimed at determining (1) whether large volume rhyolites can be produced mainly by fractional crystallization of andesitic magma, and (2) whether Nd, Sr and O isotopic compositions of continental arc-type lavas can be related to the magma supply and crystal thermal structure sufficiently well to help evaluate the explosive potential of silicic magma systems.; Chapter 2 presents the results of a detailed isotopic study of the Chalupas caldera, in the Andes of Ecuador. Sr, Nd and O isotope ratios of the Chalupas lavas and tuffs are shifted from typical oceanic arc values, indicating that they have been affected by crustal assimilation. A narrow range in isotopic values over a wide range in silica content (55–72% SiO₂) indicates that high silica magmas were derived from lower silica magma by fractional crystallization in the absence of crustal assimilation. Our results show that the Chalupas rhyolite approximates an example of a large volume silicic system that evolved by fractional crystallization of andesite. Crustal assimilation may increase the probability for large volume silicic eruptions, but they can be generated by fractional crystallization if the magma supply is sufficiently high.; Chapter 3 describes an isotopic study of the Clear Lake volcanic field (CLVF), in northern California. New Sr and Nd data are consistent with more silicic magmas being generated by fractional crystallization of crustally-contaminated basaltic andesites. The isotopes ratios of erupted lavas show an increase in the mantle component over time as eruption rates increase, suggesting an increasing rate of supply of magma from the mantle. Although the eruption rate has been low during the most recent period of eruption, the isotopic composition of young lavas suggests a continued increase in the magma supply rate. The idea that the CLVF could be entering a more vigorous stage of magmatism is supported by very high heat flow in the Geysers area and a strong 3He signal in the hydrothermal fluids. (Abstract shortened by UMI.)...