Are Volcanic CO2 Emission Fluxes Under-estimated? The Groundwater Volcanic CO2 Component at Lassen Volcano, California

Remote sensing techniques applied to estimate volcanic degassing rates integrate species-specific concentrations in volatile plumes released from volcanic centers to the atmosphere. Such plumes represent integrated point source emissions which are usually concentrated on summit degassing sites although flank degassing emanations can also contribute to the total volatile load of the integrated plume. At individual volcanoes, however, what is less well-documented (and quantified) is the volcanic volatile inventory that is not released directly to the atmosphere but instead interacts with local and regional groundwater so that various volatile species either dissolve or are hydrolised. In such cases, volcanic volatiles can be dispersed within groundwater catchments to be released to the atmosphere only where groundwater discharges at the surface. To investigate the scale of CO₂ release via groundwater discharge(s) versus direct emanation to the atmosphere, we report reconnaissance He-CO₂ results (isotope and relative abundance) from Lassen Volcano in the southern Cascades. This volcano has been passively degassing for almost a century since its last major eruption, and has a well-characterised regional groundwater regime which has been the focus of previous volatile investigations. We report helium and carbon isotopes (³He/⁴He, δ¹³C) and relative abundance (CO₂/³He) characteristics of 13 groundwater samples collected in 2010 between 20 and 50 km to the NNW, W and SSE of Lassen Peak. Helium isotopes (³He/⁴He) range from 0.73 to 2.57 R_A (where R_A = air ³He/⁴He), with all ratios greater than radiogenic production values (~0.02 R_A) indicating the presence of a small but discernible mantle-derived helium input. Carbon isotopes (δ¹³C) vary between -13.9 and -27.7 ‰ (vs. PDB) - all values are significantly lower than mantle values (~ -6‰) consistent with addition of organic-derived CO₂. CO₂/³He values fall between 4.2 x 10⁹ and 5.7 x 10¹², all above typical MORB-like values (2 x 10⁹) and trending towards crustal values (5-50 x 10¹²). The total CO₂ groundwater load comprises organic, carbonate-related and endogenic (magmatic) CO₂, with the magmatic component identified and resolved using a combination of major element chemistry and modeling of binary mixing with an isotopically-low (organic) endmember. Using mean concentrations of magmatic CO₂ together with estimated water discharge rates for the Lassen watershed, the estimated flux of groundwater-related magmatic degassing is ~4.2 x 10⁶ kg/yr, occurring up to ~50km from the volcanic edifice. This estimate represents between 11-57% of the 1998-2000 baseline CO₂ flux from Lassen Peak (Gerlach et al., USGS Prof. Paper, 1750, 2008) indicating that groundwater can play a major role in release of volcanic CO₂ from this volcano. Continuing studies, aimed at refining CO₂ flux estimates for both groundwater and high-temperature (summit) localities, are under-way.