Helium-carbon dioxide systematics in groundwaters at Mount Lassen Volcano, Northern California

Although CO2 degassing from volcanic centers has been well documented, emissions from the flanks and areas surrounding volcanoes are not well quantified. To further investigate mechanisms of volcano degassing we have selected Mount Lassen, in the Cascades range, as a type locality where degassing occurs both proximal and distal to the volcano summit. Our aim is to quantify distal magmatic CO2 release via groundwaters to compare to magmatic carbon fluxes centered on the summit region.;We report new helium and carbon isotopes (3He/ 4He, delta13C) and relative abundance (CO2/ 3 He) characteristics of 13 groundwater samples collected between 20 and 50 km from Lassen Peak. All of the sampled groundwaters have 3He/4He values greater than radiogenic production values (∼0.02 RA) indicating a discernible presence of mantle-derived helium addition. Carbon isotopes (delta13C) vary between -13.9 and -27.7 ‰ (versus V-PDB). CO2/3He values fall between 4.2 x 109 and 5.7 x 10 12; all above typical MORB-like values (2 x 109). Using groundwater chemistry, we resolve endogenic CO2 for contributions from organic and carbonate derived CO2. Then we combine our estimate of average endogenic CO2 contributions with estimated groundwater flow rates to derive an endogenic carbon flux of 1.18 x 107 kg year-1. This value represents 25% of the total CO 2 release from Lassen. Only measuring carbon flux from the summit region of volcanoes may seriously underestimate the transfer rates of endogenic CO 2. Taken together, these data indicate large-scale transport and release of endogenic carbon upwards of 50 kilometers from the volcanic edifice.