Gas Sources and Source Volumes at Mammoth Mountain Volcano CA

A 6-month swarm of seismicity accompanied by 6-Mm³ of volumetric inflation at Mammoth Mountain in 1989 was followed by a huge diffuse efflux of CO₂ on the volcano's flanks and gas composition changes in a higher elevation fumarole. The increase in fumarolic ³He/⁴He ratio from 4.0 R_A to an average of 5.5 R_A during 1990-present suggests that He emissions after 1989 are an equal proportion of gas from a new intrusion of magma (³He/⁴He ~7 R_A) and gas long stored in a postulated, pre-existing reservoir a few km deep (³He/⁴He ~4 R_A). If this mixing ratio represents the entire gas efflux at Mammoth Mountain, then half of the ~4 Mt of CO₂ emitted over the past 18 years is derived from the new intrusion. For reasonable T and P gradients between a magma source region at 10-20 km depth (1200°C and 4 kb) and a shallow pre-existing gas reservoir (150°C and 0.4 kb), the specific volume of free CO₂ is relatively constant at 1.5±0.1 m³/t. Source volumes of ~3 Mm³ for gas released from both the pre-existing reservoir and from newly intruded magma are implied. Although the new gas may have been released gradually from magma over a period of several years, the risk of imputing inflation volumes solely to magmatic intrusion is clearly shown by the comparison here – a 6-Mm³ intrusion would be 50 vol-% gas. A more realistic magma CO₂ concentration of 0.75 wt-% would require degassing ~100 Mm³ of magma to source the new pulse of CO₂ at Mammoth Mountain. The volume of magma inferred for the cumulative (1978-present) uplift of adjacent Long Valley caldera is ~100 Mm³, but intracaldera gas emissions are minor. As that episode of uplift and the Mammoth Mountain CO₂ efflux seem to be winding down simultaneously, the possible connection is worth considering.