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He-CO₂ Systematics in groundwaters at Mount Lassen Volcano , Northern Califronia


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, [delta]¹³C) and relative abundance (CO₂³He) 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 ([delta]¹³C) vary between -3.9 nd -7.7 ⁰/₀₀ (versus V-PDB). CO₂/³He values fall between 4.2 x 10⁹ and 5.7 x 10¹² 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 CO₂ release from Lassen. Only measuring carbon flux from the summit region of volcanoes may seriously underestimate the transfer rates of endogenic CO₂. Taken together, these data indicate large-scale transport and release of endogenic carbon upwards of 50 kilometers from the volcanic edifice

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