2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 12
Presentation Time: 11:00 AM

Gas Sources and Source Volumes at Mammoth Mountain Volcano CA

EVANS, William C., U.S. Geological Survey, 345 Middlefield Rd. M/S 434, Menlo Park, CA 94025, FARRAR, Chris D., U.S. Geological Survey, Carnelian Bay, CA 96140, HUNT, Andrew G., U.S. Geological Survey, Denver, CO 80225 and KENNEDY, B. Mack, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, wcevans@usgs.gov

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 RA to an average of 5.5 RA during 1990-present suggests that He emissions after 1989 are an equal proportion of gas from a new intrusion of magma (3He/4He ~7 RA) and gas long stored in a postulated, pre-existing reservoir a few km deep (3He/4He ~4 RA). 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.