Contrasting He^C relationships in Nicaragua and Costa Rica: insights into C cycling through subduction zones

Abstract

We report 3He/4He ratios, relative He, Ne, and CO2 abundances as well as N13C values for volatiles from the volcanic output along the Costa Rica and Nicaragua segments of the Central American arc utilising fumaroles, geothermal wells, water springs and bubbling hot springs. CO2/ 3He ratios are relatively constant throughout Costa Rica (av. 2.1U1010) and Nicaragua (av. 2.5U1010) and similar to arcs worldwide (V1.5U1010). N13C values range from 36.8x (MORB-like) to 30.1x (similar to marine carbonate (0x)). 3He/4He ratios are essentially MORBlike (8 !1 RA) with some samples showing evidence of crustal He additions ^ water spring samples are particularly susceptible to modification. The He^CO2 relationships are consistent with an enhanced input of slab-derived C to magma sources in Nicaragua ((L+S)/M = 16; where L, M and S represent the fraction of CO2 derived from limestone and/or marine carbonate (L), the mantle (M) and sedimentary organic C (S) sources) relative to Costa Rica ((L+S)/ M = 10). This is consistent with prior studies showing a higher sedimentary flux to the arc volcanics in Nicaragua (as traced by Ba/La, 10Be and La/Yb). Possible explanations include: (1) offscraping of the uppermost sediments in the Costa Rica forearc, and (2) a cooler thermal regime in the Nicaragua subduction zone, preserving a higher proportion of melt-inducing fluids to subarc depths, leading to a higher degree of sediment transfer to the subarc mantle. The absolute flux of CO2 from the Central American arc as determined by correlation spectrometry methods (5.8U1010 mol/yr) and CO2/ 3He ratios (7.1U1010 mol/yr) represents approximately 14^18% of the amount of CO2 input at the trench from the various slab contributors (carbonate sediments, organic C, and altered oceanic crust). Although the absolute flux is comparable to other arcs, the efficiency of CO2 recycling through the Central American arc is surprisingly low (14^18% vs. a global average of V50%). This may be attributed to either significant C loss in the forearc region, or incomplete decarbonation of carbonate sediments at subarc depths. The implication of the latter case is that a large fraction of C (up to 86%) may be transferred to the deep mantle (depths beyond the source of arc magmas).