Volcanic Hazards and CO₂ Emissions:
Mammoth Mountain Long Valley Caldera, California

Collabortors:

(1) Prof. Don DePaolo, Center for Isotope Geochemistry, Lawrence Berkeley National Laboratory and Dept. of Geology and Geophysics, University of California, Berkeley.

(2) Mike Sorey, Bill Evans, and Chris Farrar, U.S. Geological Survey, Menlo Park, CA.

(3) Andrea Cook and Laura Hainsworth, Lawrence Livermore National Laboratory.

(4) John Rogie, Pennsylvania State University.

Crustal Unrest and CO₂ and He Emissions From a Reservoir of Magmatic Gas Beneath Mammoth Mountain

The Mammoth Mountain volcano, host of one of the largest recreational ski areas in California, has a history of volcanism over the past 50,000-200,000 years. The volcano is located on the southwestern rim of the 760,000-year-old Long Valley Caldera in eastern California and forms the southern end of the Inyo Craters volcanic chain that has produced intermittent rhyolitic and phreatic eruptions over the last 40,000 years, most recently about 600 years ago.

Carbon dioxide and helium with isotopic compositions indicative of a magmatic source are discharging at anomalous rates from Mammoth Mountain. The gas is released mainly as diffuse emissions from normal-temperature soils, but some gas issues from steam vents or leaves the mountain dissolved in cold groundwater. The rate of gas discharge increased significantly following a 6-month period of persistent earthquake swarms and associated strain and ground deformation that has been attributed to dike emplacement beneath the mountain (Figure 4).

Figure 4: Temporal variations in the helium isotopic composition, normalized to the ratio in air (R/Ra), in gas discharged from a fumarole near the summit of Mammoth Mountain demonstrating the dramatic increase in magmatic helium following the onset of seismic activity and dike intrusion beneath Mammoth Mountain in 1989. The high rate of magmatic helium discharging from the fumarole has persisted until the present time. For a continuos update of our monitoring of gas discharges from this site visit: http://quake.wr.usgs.gov/VOLCANOES/LongValley/HeliumDischarge.html

Soil Gas CO₂ and Helium Emissions

Anomalous discharge of CO₂ and magmatic helium from soils first occurred during the winter of 1990 and was followed by observations of several areas of tree kill and/or heavier than normal needlecast the following summer. Subsequent measurements have confirmed that the tree kill areas are associated with CO₂ concentrations of 30-90% in soil gas and gas flow rates of up to 31,000 g m^-2 d^-1 at the soil surface. The carbon and helium isotopic compositions are indistinguishable from that in the Mammoth Mountain fumarole. We estimate that the total diffuse soil gas CO₂ flux from the mountain is approximately 520 tonnes/day. For additional information visit: http://quake.wr.usgs.gov/VOLCANOES/LongValley/gasses.html

Groundwater CO₂ and Helium Emissions

Numerous cold springs and wells around the flanks of Mammoth Mountain contain significant quantities of dissolved CO₂. The isotopic composition of the carbon and the helium associated with the dissolved CO₂ is similar to that found in the tree-kill areas and the Mammoth Mountain fumarole (Figure 5).

Figure 5: The dissolved helium and neon in the CO₂-rich cold groundwaters appear to be a mixture of water in equilibrium with air (air-saturated water ,ASW) and a gas component that is similar in composition to the soil gas in the tree-kill zones. This suggests that some of the cold groundwaters may recharge through the zones of diffuse soil CO₂ flux.

Some of the CO₂-rich groundwaters yield apparent ¹⁴C ages of 30,000-40,000 years. The very old apparent ages reflect a component of deep-sourced carbon that is depleted (i.e. dead) in radioactive ¹⁴C. Wells operated by the Mammoth Mountain ski area that were drilled prior to 1989 also contain high concentrations of dissolved CO₂. This suggests that anomalous concentrations of CO₂ may have been discharging from Mammoth Mountain prior to the 1989 period of unrest and dike intrusion, but the overall rate of discharge has increased significantly since 1989. We estimate that presently 30-50 tonnes/day of CO₂ dissolved in cold groundwater flows off the flanks of the mountain. For further information visit: http://quake.wr.usgs.gov/VOLCANOES/LongValley/CO2Dissolved.html

Isotopic and chemical analyses of soil, fumarolic and dissolved gas demonstrate a remarkable homogeneity in composition, suggesting that the CO2 and associated helium and an excess nitrogen component may be derived from a common gas reservoir whose source is associated with some combination of magmatic degassing related to the 1989 dike intrusion and thermal metamorphism of metasedimentary rocks. Furthermore, N₂/Ar ratios and nitrogen isotopic values indicate that the Mammoth Mountain gases are derived from sources separate from those that supply gas to the hydrothermal system within the Long Valley caldera. Various data suggest that the Mammoth Mountain gas reservoir is a large, low-temperature cap over an isolated hydrothermal system, that it predates the 1989 intrusion, and that it could remain a source of gas discharge for some time.

Related Publications

Sorey, M.L., Kennedy, B.M., Evans, W.C., Farrar, C.D., and Suemnicht, G.A., Helium isotope and gas discharge variations associated with crustal unrest in Long Valley caldera, California, J. Geophys. Res., 98, 15,871-15,889, 1993.

Farrar, C.D., Sorey, M.L., Evans, W.C., Howle, J.F., Kerr, B.D., Kennedy, B.M., King, C-Y, and Southon, J.R., Forest-killing diffuse CO₂ emissions at Mammoth Mountain as a sign of magmatic unrest, Nature, 376, 675-678, 1995.

Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth, L.J., and Hausback, B., Carbon dioxide and helium emissions from a reservoir of magmatic gas beneath Mammoth Mountain, California, J. Geophys. Res., 103, 15,303-15,323, 1998.

Sorey, M.L., Ebans, W.C., Kennedy, B.M., Rogie, J., and Cook, A., Magmatic gas emissions from Mammoth Mountain, California Geology, 52, 4-16, 1999.

Funding

This project was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Engineering, and Geosciences Division of the U.S. Department of Energy (http://www.er.doe.gov/production/bes/bes.html).

Related Web Sites:

http://quake.wr.usgs.gov/VOLCANOES/LongValley.html