Helium-Carbon Relationships in Geothermal Fluids of Guatemala: Mapping the Subducted Sedimentary Component

As part of the Central America MARGINS initiative, we report a comprehensive study of helium isotope (³He/⁴He), carbon isotope (δ ¹³C(CO₂)) and He-C abundance relationships of geothermal fluids from 7 volcanic centres of Guatemala. Guatemala was selected to provide a contrast to on-going studies in Costa Rica (see next abstract) as the arc crust in this region is thicker and older, and (unlike Costa Rica) the entire sedimentary sequence on the down-going plate (both hemipelagic muds and underlying pelagic carbonates) likely contributes to the slab flux. The Guatemalan volcanic centres of Amatitlan-Pacaya, Tecuamburro, Moyuta, Zunil and V. Fuego have ³He/⁴He values which fall within the range normally associated with subduction zones (5 - 8 R_A where R_A = ³He/⁴He of air). Slightly lower ³He/⁴He values (< 3 R_A) are found at San Marcos and L. Atitlan but sampling in these regions was less extensive. CO₂/³He ratios vary between 5 and 81 (x 10⁹) with δ ¹³C values scattering between -1.4 and -5.5 ‰ . Although the helium in Guatemala is overwhelmingly of mantle wedge derivation, He-C results are consistent with the bulk of the CO₂ having a slab origin. Using the approach of Sano and Marty (Chem. Geol., 1995) i.e. volcanic gases can be approximated by a 3-component mixture of MORB (M: δ ¹³C = -6.5 ‰ ), slab-derived marine carbonate/limestone (L: δ ¹³C = 0 ‰ ) and (organic) sedimentary (S) endmember components, the calculated L/S ratio for Guatemala is 8 and 5 for sedimentary endmember values of -30 and -20 ‰ respectively. These values are higher than worldwide arc averages and imply preferential incorporation of carbonate into the slab flux compared to the hemipelagic muds. However, both values are lower than those found for Costa Rica which indicates that possible loss of subducting sediments (e.g. by underplating) is less pronounced in Guatemala compared with contiguous arc segments to the south.