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

Several recent studies have explored the relationship between the injection of hot recharge magma into colder magma reservoirs and the timing of volcanic eruptions. This study contributes to that discussion by applying mineral-melt equilibria geothermometers to mineral compositional profiles to determine if mineral compositions record a heating event immediately preceding eruption. If the arrival of hot recharge magma does provide the immediate trigger for eruptions, as hypothesized by some researchers, then thermal profiles should indicate on a rim-ward heating trend. Alternatively, an absence of a heating trend or the presence of a post-heating cooling trend in the mineral thermal profiles would suggest that processes other than recharge contribute to controlling the timing of eruptions. We test these hypotheses by applying feldspar, orthopyroxene, clinopyroxene, and olivine thermometry to ninety-two mineral compositional profiles derived from ten andesite flows collected from Brokeoff Volcano, Lassen Volcanic Center, California. Seventy-four (81%) of the resulting thermal profiles record the arrival of hot recharge magma followed by a period of post-recharge cooling back toward pre-recharge temperatures, while only seventeen (19%) of the thermal profiles record heating immediately prior to eruption. The overwhelming majority of thermal profiles calculated for nine of the ten andesite flows exhibit evidence for post-recharge cooling, with only one andesite 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 a significant role in controlling 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.