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

Silicic lavas have erupted repeatedly in the Mono Basin over the past few thousand years, forming the massive domes and coulees of the Mono Craters chain and the smaller island vents in Mono Lake. We report here on the first systematic study of magmatic CO2 emissions from these features, conducted during 2007–2010. Most notably, a known locus of weak steam venting on the summit of North Coulee is actually enclosed in a large area (~0.25km2) of diffuse gas discharge that emits 10–14t/d of CO2, mostly at ambient temperature. Subsurface gases sampled here are heavily air-contaminated, but after standard corrections are applied, show average δ13C-CO2 of −4.72‰, 3He/4He of 5.89RA, and CO2/3He of 0.77×1010, very similar to the values in fumarolic gas from Mammoth Mountain and the Long Valley Caldera immediately to the south of the basin. If these values also characterize the magmatic gas source at Mono Lake, where CO2 is captured by the alkaline lake water, a magmatic CO2 upflow beneath the lake of ~4t/d can be inferred. Groundwater discharge from the Mono Craters area transports ~13t/d of 14C-dead CO2 as free gas and dissolved carbonate species, and adding in this component brings the estimated total magmatic CO2 output to 29t/d for the two silicic systems in the Mono Basin. If these emissions reflect intrusion and degassing of underlying basalt with 0.5wt.% CO2, a modest intrusion rate of 0.00075km3/yr is indicated. Much higher intrusion rates are required to account for CO2 emissions from Mammoth Mountain and the West Moat of the Long Valley Caldera.