MAGMA CHAMBER DYNAMICS AND ERUPTIVE MECHANISMS IN THE CASCADE ARC: INSIGHTS FROM MELT INCLUSIONS AND TITANIUM-IN-QUARTZ THERMOBAROMETRY

Trace element concentrations in quartz phenocrysts, integrated with melt inclusion chemistry have been used to reconstruct late stage thermobarometric dynamics in large caldera magmatic systems. However, this technique has not been used to examine magma chambers of arc volcanoes. In this study, we use these data to examine the late stage magma chamber dynamics and crystallization conditions for eruptive products from Mount St. Helens and the Lassen Volcanic Center of the Cascade Volcanic Arc.

In order to constrain the thermal and barometric histories we use scanning electron microscope-cathodoluminescence (SEM-CL) imagery of quartz phenocrysts from samples associated with multiple eruptive events (both effusive and explosive) over the last 588 ka for the Lassen Volcanic Center and 270 ka for Mount St. Helens. These ages encompass the life span of these volcanoes. SEM-CL imagery reveals otherwise unobservable growth zones in quartz phenocrysts based on changes in CL intensity or brightness. CL intensity is directly related to Ti concentration, which has a positive correlation to magma temperatures and a negative correlation to pressures during growth. Therefore, CL-defined growth zones suggest changes in crystallization conditions throughout crystal growth prior and leading to eruptions.

Within each sample, several quartz populations have been established based on morphology of dissolution, growth zone textures, and relative intensity of CL-defined zones. Populations include homogenous, oscillating growth zones, CL-dark cores abruptly transitioning to CL-bright rims, and multiple abrupt growth zones progressively increasing in CL-intensity from core to rim. In the homogenous and oscillating phenocrysts examined thus far, we observed a positive correlation between CL intensity and Ti concentration within a range of 70 to 100 ppm. These results indicate crystal growth conditions between 1.5 kbar at ~700° C to 5 kbar at ~800° C with oscillating changes in conditions rather than progressive. Further CL and thermobarometric investigation of additional quartz phenocryst populations will reveal whether different quartz populations reflect different thermobarometric histories during crystal growth prior to eruptions.