De Leeuw, G.A.M., Hilton, D.R., Fischer, T.P., Walker, J.A., 2005: Along and across-arc profiles in He-C systematics: new data from El Salvador and Honduras, Goldschmidt Conference, Moscow, Idaho:
We sampled volcanic and geothermal gases in El Salvador and Honduras for their CO2 and He abundances and isotope ratios to investigate along and across-arc (depth) controls on the release of volatiles from the subducting slab. Results show a typical volcanic arc-signature for El Salvador with 3He/4He ratios of 6.0 – 7.6 RA, δ13C values of –1.3‰ to –3.2‰ and CO2/3He ratios of 8 – 25 × 109. In Honduras, we find similar values for volatiles collected in the Sula Graben (3He/4He: 4.5 – 6.0 RA, δ13C values of –2.2 to –3.7‰ and CO2/3He ratios of 9 – 29 × 109) even though the sample locations lie ~200 km from the volcanic front. All other sample locations in Honduras show lower 3He/4He ratios (0.6 – 3.5 RA), lower δ13C values (< –6.4‰) and either higher or lower CO2/3He ratios (2 – 258 × 109): characteristics consistent with fractional degassing of CO2 and/or interaction with crustal reservoir rocks.
Samples collected along the volcanic front in El Salvador and from the Sula Graben are used to calculate relative proportions of mantle wedge, and subducted marine carbonate and sedimentary organic carbon contributing to the CO2 flux in the volcanic arc and back-arc. Given their similar He-C characteristics, the two sample suites show similar results for mantle (5 – 19%), carbonate (77 – 90%) and organic sediment (3 – 11%) contributions to the CO2 output flux. Our results imply that devolitalization of carbon from the subducting slab extends to distances up to 200 km behind the volcanic front. We contrast these results with trace element/isotope data of alkaline lavas erupted at similar distances behind the arc that lack evidence for a slab-derived component [Patino et al. (1997); Walker et al. (2000)].