IS MAFIC RECHARGE NECESSARY TO TRIGGER ANDESITIC VOLCANIC ERUPTIONS? NEW INSIGHTS FROM FELDSPAR AND PYROXENE THERMOBAROMETRY

Injection of mafic magma into the base of crustal reservoirs has long been proposed as a critical mechanism for triggering intermediate to felsic volcanic eruptions (Kent et al, 2010; de Silva et al, 2008; Tepley et al, 2000); however recent work at Augustine and Merapi (Putirka, 2016) identifies thermobarometric evidence for pronounced isobaric cooling between the final recharge event and eruption. Our research tests the extrapolability of these findings by employing the feldspar thermometer of Putirka (2008), clinopyroxene thermometer of Putirka (2008), and clinopyroxene barometer Neave and Putirka (2017) to reconstruct pre-eruption magma chamber conditions for a series of eight andesite flows at Brokeoff Volcano, Lassen Volcanic Center, California. Results from feldspar thermometry indicate that forty-three of the feldspar transects from Brokeoff Volcano preserve evidence for cooling between the final heating event and the crystal rim, with an average temperature change of -47°C and a maximum temperature change of -145°C. Only ten profiles recorded evidence of warming immediately prior to eruption, with an average temperature increase 39°C and a maximum temperature increase of 67°C. These observations suggest that while heat from the arrival of mafic recharge may be necessary to overcome rheologic barriers and produce an eruptible magma, the arrival of the recharge magma alone is insufficient to control the timing of volcanic eruptions. These findings are consistent with those of Putirka (2016) and highlight the importance of sub-mineral scale analysis in understanding volcanic process.