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

Abstract

In Mono Lake, concentrations of 3 He and 4 He are greater than atmospheric equilibrium values, indicating a subsurface helium source. This assertion is supported by vertical concentration profiles that show maximum ␦ 3 He values (ϩ68%) occurring within the pycnocline. The slope of the regression between 3 He and 4 He concentrations is near 2.8 R a (R a is the atmospheric helium isotope ratio), indicating that the subsurface source has a component of mantle helium. Hydrothermal springs and gas vents from Paoha Island have similar ratios, which suggests that discharge from this thermal system is the most important source. A 3 He mass balance indicated that below the deep pycnocline at 17 m, ϳ25 cc STP of 3 He accumulates each year (ϳ90% from the influx of hydrothermal water and ϳ10% from the in situ decay of tritium). Although it is not retained because of gas transfer across the air-water interface, ϳ70 cc STP of 3 He is injected each year into the upper 17 m. When 3 He is used as a tracer, the influx of hydrothermal water below the deep pycnocline was estimated to be 0.045 Ϯ 0.025 m 3 s Ϫ1 . Because the hydrothermal water is fresher than Mono Lake, this influx causes the salinity of the monimolimnion to decrease by ϳ0.1 salinity units per year and may play a small but important role in the salinity budget of this layer, which is presently denser than and isolated from the surface water.

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

“…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 . Clark and Hudson (2001) report a R C /R A of 2.7 for two samples from this hot spring area.…”

“…The very high He concentrations in these gases mainly reflect CO 2 loss, since the spring and lake waters through which the bubbles rise are alkaline fluids that convert CO 2 into dissolved carbonate species. The few percent of residual CO 2 in the hot-spring gases provides little information on magmatic carbon input, but the reported 3 He/ 4 He value for the hot springs of 2.7 R A (Clark and Hudson, 2001) demonstrates a magmatic gas component.…”

Noble Gases in Lakes and Ground Waters

“…2). The 3HefHe ratio has been employed as an indication of a mantle component (e.g., Sano et al 1990;Collier et al 1991;Giggenbach et al 1991;Igarashi et al 1992;Kipfer et al 1994;Aeschbach-Hertig et aL 1996bClark and Hudson 2001). In the lakes studied, the 3HefHe ratio varies between 7.4xl0-6 and 10.3xlO-6 (Table 5).…”

“…1994;Aeschbach-Hertig et aI. 1996bClark and Hudson 2001). Because typical mantle helium isotope ratios (about IQ-5, e.g., Mamyrin and Tolstikhin 1984) are much larger than R m and Re q , the mantle-derived 3He may become the dominant 3He-component, rendering it difficult or even impossible to calculate 3Hetri.…”

Hydrological and chemical budgets in a volcanic caldera lake: Lake Kussharo, Hokkaido, Japan

“…The isotope-based approach is now popular for the assessment of the hydrological and chemical budgets of lakes (Clark and Hudson, 2001;Gibson et al, 2002). Motz et al (2001) evaluated the annual groundwater output or leakage to an aquifer, distributed below the lake basin, by assessing the hydrological budget of a lake and the variation in the surrounding groundwater level.…”