Investigations of CO2 Emissions From Holocene Volcanic Features of Mono Lake and the Mono Craters Volcanic Chain in Eastern California

Magmatic CO2 emissions have been well studied at the Long Valley caldera in eastern California, but not along the much younger and voluminous Mono Craters volcanic chain 10 to 20 km to the north. A June 2007 field investigation to identify and quantify emissions of magmatic CO2 focused on some of the youngest eruptive features in the chain and on Paoha Island in Mono Lake. North Coulee (NC) is one of the larger late-Holocene volcanic features in the Mono Craters chain and is composed primarily of a glassy rhyolite flow that is partially blanketed on the west by tephra deposits. There is no soil development at NC, and vegetation is generally limited to a few scattered pine trees. Portions of NC contain discrete zones of discolored altered rock that provide evidence of former fumarolic activity; however in this study, only one small area displayed temperatures greater than 40° C. CO2 flux was measured at 224 sites over a ~ 244, 000 m2 area that included many of the alteration zones. The CO2 flux at most sites was anomalously high, with an average flux over the entire study area of 109 gm-2d-1. The estimated discharge of CO2 is around 27 t d-1. Biogenic CO2 emissions at NC should be essentially zero because of the scarcity of soil and vegetation. Given that 63 sites had fluxes below detection limits this supposition is well supported. Additional gas chemistry and isotope work will help determine if all the CO2 is derived from magma. If so, then the magmatic CO2 output from NC is comparable to the CO2 output from the Long Valley hydrothermal system. Diffuse CO2 emissions were also measured at two locations on Paoha Island: around the young cinder cone on the north end of the island, and over dacite flows adjacent to Hot Springs Cove (HSC) on the east. The survey revealed no detectable flux of CO2 around the cinder cone. At HSC, CO2 emissions were localized around steam vents and were generally low. The average flux from 25 sites was less than 8 gm- 2d-1 with only 4 sites displaying an anomalous flux greater than 15 gm-2d-1. These data are in agreement with results from gas samples collected from bubbling springs along the shore of HSC, which contain predominantly C1 through C6 hydrocarbons and N2 with less than 4% CO2.