Carbon dioxide and helium emissions from a reservoir of magmatic gas beneath Mammoth Mountain, California

Abstract

Abstract. Carbon dioxide and helium with isotopic compositions indicative of a magmatic source (b 13C = -4.5 to -5%o, 3He/4He = 4.5 to 6.7 R^) are discharging at anomalous rates from Mammoth Mountain, on the southwestern rim of the Long Valley caldera in eastern California. 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 in 1989 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. An increase in the magmatic component of helium discharging in a steam vent on the north side of Mammoth Mountain, which also began in 1989, has persisted until the present time. Anomalous CO 2 discharge 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 kills are associated with CO 2 concentrations of 30-90% in soil gas and gas flow rates of up to 31,000 g m '2 d 4 at the soil surface. Each of the tree-kill areas and one area of CO 2 discharge above tree line occurs in close proximity to one or more normal faults, which may provide conduits for gas flow from depth. We estimate that the total diffuse CO 2 flux from the mountain is approximately 520 t/d, and that 30-50 t/d of CO 2 are dissolved in cold groundwater flowing off the flanks of the mountain. Isotopic and chemical analyses of soil and fumarolic gas demonstrate a remarkable homogeneity in composition, suggesting that the CO 2 and associated helium and excess nitrogen may be derived from a common gas reservoir whose source is associated with some combination of magmatic degassing 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, lowtemperature 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.

Airborne detection of diffuse carbon dioxide emissions at Mammoth Mountain, California

“…California [Sorey et al, 1998]. Areas of tree kill ( The conventional airborne method for measuring volcanic CO2 emissions is not entirely applicable at Mammoth Mountain.…”

Geochemical evidence for a magmatic CO2degassing event at Mammoth Mountain, California, September–December 1997

“…Mammoth Mountain (Figure 1), a 200-kyr-old dacitic volcano that produced a sequence of small phreatic blasts as recently as 700 _+ 30 years ago, lies at the southern end of the Mono-Inyo Craters volcanic chain on the southwestern rim of Long Valley caldera in central eastern California [Miller, 1985;Bailey, 1989;Sorey et al, 1998]. Areas of CO2-induced tree kill began to appear on Mammoth Mountain in 1990 after a 6-month earthquake swarm in 1989 associated with shallow magmatic intrusion under the volcano [Hill et al, 1990 granitic rocks, provide beds through which CO2 can easily diffuse and spread both laterally and vertically.…”

Natural and industrial analogues for release of CO2 from storagereservoirs: Identification of features, events, and processes and lessonslearned

“…Here, soil gas CO 2 concentrations measured using infrared gas analyzers are commonly >30 vol.% and fluxes measured using the accumulation chamber method are commonly >500 g m -2 d -1 [Farrar et al, 1995[Farrar et al, , 1999Rahn et al, 1996;McGee and Gerlach, 1998;Sorey et al, 1998;Gerlach et al, 2001]. For reference, background CO 2 concentrations and fluxes outside of the tree-kill area are usually < 1 vol.% and 25 g m -2 d -1 , respectively.…”

“…Circle size is proportional to magnitude in three steps: 1.2≤M≤1.9, 2.0≤M≤2.9, and 3.0≤M≤3. Sorey et al [1998], and plexus of dikes and sills. Orange dots, black dots, and triangles show hypocentral locations of BF earthquakes, locations of 1990-1999 LP earthquakes recorded by NCSN seismic network , and 1997 LP earthquakes recorded by temporary array of 3-component digital seismic stations [Foulger et al, 1998] …”

“…One or more local LP earthquakes occurred concurrently with each of the VLP events and spasmodic bursts accompanied the July and August 2000 VLP events. The VLP earthquakes were loosely constrained to a source volume centered ~3 km beneath the summit of Mammoth Mountain, in the vicinity of a CO 2 reservoir proposed by Sorey et al [1998] (see conceptual model section below). These data may reflect the transport of a slug of fluid (most likely CO 2 or a CO 2 -rich hydrous phase) through a nearvertical, northwest-striking crack [e.g., Hill and Prejean, 2005].…”