Rate of magma supply beneath Mammoth Mountain, California based on helium isotopes and CO2 emissions

Mammoth Mountain, California, has exhibited unrest over the past ~30 years, characterized by seismicity over a broad range of depths, elevated ³He/⁴He ratios in fumarolic gas, and large-scale diffuse CO₂ 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 ³He/⁴He ratios and CO₂ emissions to estimate that of the ~5.2 Mt of CO₂ released from Mammoth Mountain between 1989 and 2016, 1.6 Mt was associated with active intrusion and degassing of ~0.05–0.07 km³ of basaltic magma. Intrusion at an average rate of ~0.002–0.003 km³/year into a postulated zone of partial melt at ~15-km depth could occur without detection by local Global Navigation Satellite System stations.