Studies of the evolution of felsic magma systems: I. Zircon in historic eruptions, Iceland; II. Modeling magma chamber evolution leading to the Peach Spring Tuff supereruption, Arizona, Nevada and California
Carley, Tamara Lou
This two-part study is focused on the evolution of silicic magma in two very different settings. The first part is a study of zircon to understand the generation, storage, and evolution of felsic magmas erupted in historical times in Iceland. Icelandic zircon compositions, which are correlated with proximity to the main axial rift, are distinct from those of continental zircon. Icelandic zircon exhibit generally low U (<200 ppm), low U/Th (<1), low Hf (<10,000 ppm) and uniformly high Ti (>10 ppm). Ti concentrations imply that zircon grew at relatively high temperatures (800-950 °C). U-Th ages demonstrate the range of ages is far less than is common in continental settings, but zircon growth predates eruptions by thousands to tens of thousands of years. These magmas did not evolve by monotonic fractionation, but experienced zircon entrainment and open-system magma recharge. The second part of this study is a modeling project focused on better understanding the conditions that lead to the evolution and destabilization of the Peach Spring Tuff (AZ, CA, NV) in particular, and large (super-eruption) silicic magmas in general. Analyses of pumice and fiamme from intracaldera and outflow exposures reveal systematic heterogeneity in the underlying chamber at the time of eruption. Whole-rock compositions and modeling results are compatible with differentiation in a shallow magma reservoir at ~200-300 MPa. Destabilization of this large chamber appears to be strongly related to the timing of water saturation relative to other phases and to the pseudo-invariant, both of which are correlated with pressure and water content. Changes in total magma volume and density are modest for systems with high initial water contents (7 wt. %) and significant for systems with low initial water (2-3 wt. %). In the latter case, crystallization and bubble exsolution likely promote magma destabilization and eruption.