Helium isotopes in geothermal and volcanic gases of the Western United States, II. Long Valley Caldera

Quantifying the flux of hydrothermal fluids into Mono Lake by use of helium isotopes

“…Helium isotope samples were collected from the Paoha Island thermal area in Mar 1996. The 3 He : 4 He isotope ratio of one hydrothermal spring and one gas vent was 2.7 R a , which is in good agreement with measurements reported earlier by Welhan et al (1988). The helium concentration of the spring water was 4.5 ϫ 10 Ϫ6 cc STP g Ϫ1 , nearly 200 times its concentration in the lake.…”

“…Prior to the 1995-1996 winter, this stratification was caused by the seasonal thermocline; subsequently, the stratification was caused by a salinity gradient that developed after the onset of meromixis. Welhan et al (1988) showed that ratios of helium isotopes in hydrothermal springs vary geographically within Mono Basin. The Navy Beach springs, south of the lake, have the highest 3 He : 4 He isotope ratio (5.2-5.8 R a ) whereas a small spring near Black Point had the lowest ratio (1.8 R a ).…”

Magmatic gas emissions at Holocene volcanic features near Mono Lake, California, and their relation to regional magmatism

“…No other hydrocarbons were detected. The R C /R A value was 5.40, similar to the value of 5.15 measured in 1987 and the average value of 5.16 from 15 gas samples collected at nearby South Mono Lake spring (now dry), from 1983 to 1988 (Welhan et al, 1988;Hilton, 1996). Gas from the springs on Paoha Island was dominated by CH 4 with elevated concentrations of C 2 -C 6 hydrocarbons but contained only~3% CO 2 .…”

Evolution of the geothermal fluids at Los Azufres, Mexico, as traced by noble gas isotopes, δ18O, δD, δ13C and 87Sr/86Sr

“…Goff et al (1991) interpreted these radiogenic Sr isotopic ratios as having been derived by leaching of metamorphic units of the basement by uprising thermal waters. The high He isotopic ratios point to a pure mantle He source located deep beneath Long Valley caldera (e.g., Welhan et al, 1988;Sorey, 1985). We could speculate that in Los Azufres the mantle He component comes from deeper "primitive" parent magmas below the caldera.…”