Other
Scientific paper
Jun 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007e%26psl.258..132d&link_type=abstract
Earth and Planetary Science Letters, Volume 258, Issue 1-2, p. 132-146.
Other
12
Scientific paper
We report helium and carbon isotope and relative abundance data of fumaroles, hot springs, water springs, mud-pots and geothermal wells from El Salvador and Honduras to investigate both along and across-arc controls on the release of CO2 from the subducted slab. El Salvador localities show typical volcanic front volcanic gas signatures, with 3He/4He ratios of 5.2 7.6 RA, δ13C values of - 3.6‰ to - 1.3‰ and CO2/3He ratios of 8 25 × 109. In Honduras, we find similar values only for volatiles collected in the Sula Graben region located ˜ 200 km behind the volcanic front. All other areas in Honduras show significantly lower 3He/4He ratios (0.7 3.5 RA), lower δ13C values (< - 7.3‰) and more variable CO2/3He ratios (6.2 × 107 2.0 × 1011): characteristics consistent with degassing-induced fractionation of CO2 and He and/or interaction with crustal rocks. The provenance of CO2 released along the volcanic front is dominated by subducted marine carbonates (L = 76 ± 4%) and organic sediments (S = 14 ± 3%), with the mantle wedge (M) contributing 10 ± 3% to the total carbon flux. The L/S ratio of the El Salvador volatiles (average = 5.6) is comparable to volcanic front localities in Costa Rica and Nicaragua [A.M. Shaw, D.R. Hilton, T.P. Fischer, L.A. Walker, G.E. Alvarado, Contrasting He C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones. Earth Planet. Sci. Lett. 214 (2003) 499 513] but is approximately one-half the input value of sediments at the trench (L. Li, G.E. Bebout, Carbon and nitrogen geochemistry of sediments in the Central American convergent margin: Insights regarding subduction input fluxes, diagenesis, and paleoproductivity, J. Geophys. Res. 110 (2005), doi: 10.1029/2004JB003276). We use the L/S ratio of El Salvador geothermal fluids, together with estimates of the CO2 output flux from the arc, to constrain the amount and composition of subducted sediments involved in the supply of CO2 to the arc. For the El Salvador segment of the volcanic front, a ˜ 180 m continuous section of the incoming sedimentary pile — with the uppermost ˜ 42 m removed by under-plating, is required. Significantly, there is no need for oceanic basaltic basement to supply CO2 to El Salvador — or any other part of the volcanic front. This new approach, combining provenance characteristics of CO2 from the slab (L/S ratio) and CO2 flux estimates of the volcanic output, allows a more realistic estimate of the recycling efficiency of slab-derived sedimentary CO2 through the Central American Volcanic Arc to the atmosphere. Furthermore, the low L/S ratio (4.8) of Sula Graben samples from behind the front in Honduras is inconsistent with continued supply of slab-derived sedimentary CO2 following volatile loss at sub-arc depths, thereby pointing to ancient enrichment and/or lateral entrainment processes controlling CO2 in the mantle wedge below Honduras.
de Leeuw A. M. G.
Fischer Tobias P.
Hilton David R.
Walker Joe A.
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