Tracing crustal and slab contributions to are magmatism in the Lesser Antilles island are using helium and carbon relationships in geothermal fluids

We report helium and carbon isotope and CO2/He-3 ratios from a regional sur vey of geothermal fluids from the Lesser Antilles island are, an are system where there is compelling geochemical evidence for the superimposition of a crustal component onto mantle-derived magmas. A predominant mantle helium isotope signature is observed throughout the are. The highest ratios coinc ide with MORE helium (similar to 8R(A) where R-A = air He-3/He-4) and occur towards the centre of the are (the islands of Martinique, Dominica. Guadel oupe, and Montserrat). In the south and north of the are (Grenada, St. Vinc ent, St. Lucia in the south and Nevis and Saba in the north) He-3/He-4 rati os are lower and Lie between 4.9 and 6.8R(A). This regional variation is al so apparent in the carbon isotope systematics: the central portion of the a re (Martinique to Montserrat) have delta(13)C(CO2) values between -2 parts per thousand and -4 parts per thousand (vs. PDB), heavier than the range ob served in MORE (-4 to -9 parts per thousand). The south of the are (Grenada to St.Lucia) is characterized by MORB-like carbon isotope ratios (centred on -6 parts per thousand). CO2/He-3 ratios are significantly higher than th e MORE value (similar to 2 x 10(9)) for the entire are. The values in the c entral islands fall close to 10(10) whereas the southern volcanoes have hig her ratios between 10(10)-10(13). Assuming the Lesser Antilles mantle wedge has a MORE-like helium and carbon composition our data can be explained by mixing of mantle wedge carbon wit h at least two other carbon components: an isotopically-heavy marine limest one endmember of slab-derivation and an isotopically-lighter component repr esenting either slab-derived organic carbon and/or an upper crustal compone nt with a large fraction of organic carbon. The helium-carbon systematics o f the central portion of the are are consistent with a large slab-derived m arine limestone input to the carbon inventory, and we calculate a non-mantl e:mantle carbon flux of 5.7:1. MORE-like helium isotope ratios, which are s ensitive to perturbation by crustal additions to degassed magmas, imply a r elatively minor role for upper crustal contributions in this part of the ar e although it could reach a maximum of 20% of the total carbon flux if the light-C component is solely of crustal origin. Higher CO2/He-3 ratios in th e southern islands coupled with lighter delta(13)C imply (1) an increase in the flux in the non-mantle contribution relative to the central are, and ( 2) an enhanced role for an isotopically-light carbon component in this part of the are. Compared to the central islands, the increase in the non-mantl e carbon flux in the southern islands is by a factor of 4.2. Assuming that the isotopically-light carbon has a delta(13)C of -10 parts per thousand, t hen its southern are flux is a factor of 9.1 times that of the central are. Lower He-3/He-4 ratios in the southern are indicate that the origin of thi s carbon component is likely to be the uppermost crust, in this case, simil ar to 50% of the total carbon lost via the southern Lesser Antilles would b e of crustal-derivation, This conclusion is compatible with the increased a vailability of carbon both in the southernmost forearc sediments and are cr ust. Our approach of combining helium and carbon systematics serves to emph asize the importance of volatile provenance on mass balance considerations of the terrestrial carbon inventory at convergent margins, Copyright (C) 19 98 Elsevier Science Ltd.