The origin of bimodal volcanism has been a particularly persistent problem in igneous petrology. This project seeks to understand the petrologic processes responsible for bimodal basalt-rhyolite volcanic suites. Some petrogenetic studies have assumed such volcanic suites formed by mechanisms operating in a single, large, long-lived magma chamber, where complex AFC processes are required to explain the compositional variations in bimodal suites. Here it is suggested that bimodal suites are produced by repeated injection of mafic magma into the crust. These small volume bodies cause partial melting of the crust. Basalt-rhyolite magmas are generated in a series of small, unconnected reservoirs. Lavas erupted over a large area within a short time may be related only by their common anatectic origin. Repeated injection of mafic magma, and subsequent crustal partial fusion and melt crystallization will generate significant changes in the character of the crust beneath long-lived volcanic systems. Recent geophysical studies and excellent geologic constraints make this volcano ideal for testing this proposed model. Holocene lavas will be used to test the generation of this volcanic suite in small unconnected magma chambers. Then the applicability of the crustal distillation model will be tested by comparing differences in the Holocene and Pleistocene volcanic suites to determine if temporal changes in lava characteristics agree with predictions. Confirmation of this model could dramatically affect petrologists' perceptions of magma chambers and quantitative modelling of geochemical data from temporally and spatially related lavas.
