DOES THE ARRIVAL OF RECHARGE MAGMA CONTROL THE TIMING OF VOLCANIC ERUPTIONS? EVIDENCE FOR POST-RECHARGE COOLING IN ERUPTIONS FROM BROKEOFF VOLCANO, LASSEN VOLCANIC CENTER, CALIFORNIA

A fundamental challenge in volcanology is constraining the influence of magmatic processes on the timing of volcanic eruptions. Prior studies have identified several potential internal and external trigger mechanisms that influence the timing of volcanic eruptions, several of which highlight the significant role of the intrusion of hot recharge magma in initiating eruptions. In this study we use mineral thermal profiles to test the influence of the arrival of recharge magma on the timing of volcanic eruptions. We apply feldspar, orthopyroxene, clinopyroxene, and olivine thermometry to ninety-two mineral compositional profiles derived from ten andesite flows from Brokeoff Volcano, a Pleistocene stratovolcano in the Lassen Volcanic Center. If the arrival of recharge magma is the proximal trigger mechanism, then mineral thermal profiles should reflect a heating event immediately prior to eruption. The absence of mineral record of a heating or post-recharge cooling would suggest that processes other than recharge must have controlled the timing of eruptions at Brokeoff Volcano.

Seventy-four (81%) of the resulting thermal profiles record evidence for the arrival of hot recharge magma followed by a period of post-recharge cooling, while only seventeen (19%) of the thermal profiles record heating immediately prior to eruption. The majority of thermal profiles calculated for nine of the ten andesite flows exhibit evidence for post-recharge cooling, with only one flow containing evidence for heating immediately prior to eruption.

Our findings suggest that (1) while the arrival of hot 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 eruption and that (2) post-recharge processes, such as crystallization or crystallization induced vapor saturation, likely exert significant influence on the proximal timing of eruptions. No temporal relationship was identified between sample age and the extent of heating or cooling recorded in thermal profiles, suggesting that the role of magma recharge in triggering eruptions likely did not change during the evolution of Brokeoff Volcano.