Masters Thesis

Constraining internal eruption trigger mechanisms for flows at Brokeoff Volcano, Lassen Volcanic Center, California

The injection of mafic magma into cooler crustal reservoirs has long been proposed as a trigger for volcanic eruptions. If the arrival of hot recharge magma does provide the immediate eruption trigger, then mineral thermal profiles should reflect a final heating event immediately prior to eruption. Recent work at the Chaos Crags member of the Lassen Volcanic Center, California, identifies evidence for pronounced post-recharge cooling and crystallization of mafic enclaves, suggesting that time must have elapsed between the timing of recharge and eruption. This thesis tests those findings by applying feldspar, clinopyroxene, and orthopyroxene geothermometry to ninety-two mineral transects derived from crystals in ten andesite flows at Brokeoff Volcano to reconstruct pre-eruption magma chamber conditions. Seventy-four (81%) of the analyzed thermal profiles record the arrival of high-temperature recharge magma followed by post-recharge cooling back toward pre-recharge temperatures, while only seventeen (19%) thermal profiles record heating immediately prior to eruption. Most mineral thermal profiles from nine of the andesitic flows exhibit evidence for post-recharge cooling, while only one flow primarily preserves evidence for eruption immediately following recharge. This analysis suggests that while the arrival of recharge magma may be necessary to produce an eruptible magma, the increase in volume and heat flux associated with recharge may be insufficient to directly trigger some volcanic eruptions.