Enclave formation, magma mixing, and eruption triggering: A case study of the Chaos Crags, Lassen Volcanic Center, California

The exact manner in which mafic recharge acts as an eruption trigger mechanism at arc volcanoes remains the subject of intense debate. This study addresses the issues of mafic recharge and magma mixing as a trigger for volcanic eruptions by examining mafic enclaves of the Chaos Crags, Lassen Volcanic Center, a type locality for mixing of two end-member magmas in the Cascade Arc.;Mineral and bulk rock compositions were examined in order to determine end-member magma compositions and pre-mixing magma densities, with specific consideration given to comparisons between cores and rims of mafic enclaves. Mineral-melt equilibria were used to calculate crystallization temperatures and pressures, which were used in conjecture with measured vesicularities to estimate pre-eruptive magmatic densities for enclaves and host lavas. The wide range of compositions, textures, crystallization P and T, and vesicularities present within Chaos Crags eruptive products strongly support that mafic enclaves of the Chaos Crags were formed by the mixing of a parental basaltic end-member magma with rhyodacitic host magma, and that crystallization of the enclaves occurred to some degree after mixing of these two magmas prior to eruption.;This study concludes that although mafic recharge and magma mixing were most likely not the proximal trigger for eruption, they were the ultimate eruption trigger for the 1,103 +/-13 years BP eruption of the Chaos Crags sequence. The proximal eruption triggering mechanism for the Chaos Crags system may be attributed to increased overpressure within the chamber caused by volatile contributions from the crystallizing mafic magma, leading to fluid saturation, rapid vesiculation, and an increase in pH 2O within the host magmas of the Chaos Crags.