Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989-1992

Journal of Geophysical Research B: Solid Earth
By: , and 

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Abstract

The onset of anomalous seismic activity in 1989 beneath Mammoth Mountain on the southwestern rim of the Long Valley caldera, California, was followed within ∼4 months by a large increase in 3He/4He in vapor discharged from a fumarole on the north side of the mountain. The helium isotopic ratio at this vent rose to a maximum of 6.7 RA in July 1990 and subsequently declined to values near 5 RA. Potential sources of the 3He‐rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous rocks, and volatile release from a 3He‐rich gas chamber situated above previously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 RA), the persistence of relatively high values (>5 RA) over a period of 3 years, the increase in the flux of total He relative to gases in air‐saturated water, and the increases in the rates of discharge of steam and gas from this fumarole indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensional deformation at the surface suggest that the depth of intrusion may be as shallow as 2 km, consistent with the prompt appearance of increased 3He/4He ratios in the fumarolic gas, and that the intrusive process may have persisted for ∼1 year. In contrast, a similar combination of magmatic intrusion and anomalous seismic activity beneath the resurgent dome‐south moat region during the 1989–1991 period resulted in at most relatively small changes in 3He/4 He in fumarolic discharge at the southern edge of the resurgent dome. The more subdued response may result from a combination of greater intrusive depths and greater dilution of 3He‐rich inputs to thermal fluid reservoirs in the shallow hydrothermal system in this area compared with Mammoth Mountain.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Helium isotope and gas discharge variations associated with crustal unrest in Long Valley Caldera, California, 1989-1992
Series title Journal of Geophysical Research B: Solid Earth
DOI 10.1029/93JB00703
Volume 98
Issue B9
Year Published 1993
Language English
Publisher American Geophysical Union
Contributing office(s) California Water Science Center
Description 19 p.
First page 15871
Last page 15889
Country United States
State California
Other Geospatial Long Valley Caldera