HELIUM ISOTOPE AND GAS-DISCHARGE VARIATIONS ASSOCIATED WITH CRUSTAL UNREST IN LONG VALLEY CALDERA, CALIFORNIA, 1989-1992

The onset of anomalous seismic activity in 1989 beneath Mammoth Mounta in on the southwestern rim of the Long Valley caldera, California, was followed within approximately 4 months by a large increase in He-3/He -4 in vapor discharged from a fumarole on the north side of the mounta in. The helium isotopic ratio at this vent rose to a maximum of 6.7 R( A) in July 1990 and subsequently declined to values near 5 R(A). Poten tial sources of the He-3-rich vapors include degassing of fresh magma, degassing from fresh surfaces generated in newly fractured igneous ro cks, and volatile release from a He-3-rich ps chamber situated above p reviously emplaced intrusives. The magnitude of the increase in helium isotopic composition (from 3.8 to 6.7 R(A)), the persistence of relat ively high values (>5 R(A)) 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 fumaro le indicate that magmatic intrusion did in fact begin in 1989 beneath Mammoth Mountain. Seismic activity and limited measurements of extensi onal deformation at the surface suggest that the depth of intrusion ma y be as shallow as 2 km, consistent with the prompt appearance of incr eased He-3/He-4 ratios in the fumarolic ps, and that the intrusive pro cess may have persisted for approximately 1 year. In contrast, a simil ar combination of magmatic intrusion and anomalous seismic activity be neath the resurgent dome-south moat region during the 1989-1991 period resulted in at most relatively small changes in He-3/He-4 in fumaroli c discharge at the southern edge of the resurgent dome. The more subdu ed response may result from a combination of greater intrusive depths and greater dilution of He-3-rich inputs to thermal fluid reservoirs i n the shallow hydrothermal system in this area compared with Mammoth M ountain.