Rate of Magma Supply Beneath Mammoth Mountain, California, Based on Helium Isotopes and CO 2 Emissions

Mammoth Mountain, California, has exhibited unrest over the past ~30 years, characterized by seismicity over a broad range of depths, elevated 3 He/ 4 He ratios in fumarolic gas, and large‐scale diffuse CO 2 emissions. This activity has been attributed to magmatic intrusion, but minimal ground deformation and the presence of a shallow crustal gas reservoir beneath Mammoth Mountain pose a challenge for estimating magma supply rate. Here, we use the record of fumarolic 3 He/ 4 He ratios and CO 2 emissions to estimate that of the ~5.2 Mt of CO 2 released from Mammoth Mountain between 1989 and 2016, 1.6 Mt was associated with active intrusion and degassing of ~0.05–0.07 km 3 of basaltic magma. Intrusion at an average rate of ~0.002–0.003 km 3 /year into a postulated zone of partial melt at ~15‐km depth could occur without detection by local Global Navigation Satellite System stations.