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

We report helium and carbon isotope and CO₂/³He ratios from a regional survey of geothermal fluids from the Lesser Antilles island arc, an arc 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 arc. The highest ratios coincide with MORB helium (~8R(A) where R(A) = air ³He/⁴He) and occur towards the centre of the arc (the islands of Martinique, Dominica, Guadeloupe, and Montserrat). In the south and north of the arc (Grenada, St. Vincent, St. Lucia in the south and Nevis and Saba in the north) ³He/⁴He ratios are lower and lie between 4.9 and 6.8R(A). This regional variation is also apparent in the carbon isotope systematics: the central portion of the arc (Martinique to Montserrat) have δ¹³C(CO₂) values between -2‰ and -4‰ (vs. PDB), heavier than the range observed in MORB (-4 to -9‰). The south of the arc (Grenada to St.Lucia) is characterized by MORB-like carbon isotope ratios (centred on -6‰). CO₂/³He ratios are significantly higher than the MORB value (~2 X 10⁹) for the entire arc. The values in the central islands fall close to 10¹⁰ whereas the southern volcanoes have higher ratios between 10¹⁰-10¹³. Assuming the Lesser Antilles mantle wedge has a MORB-like helium and carbon composition our data can be explained by mixing of mantle wedge carbon with at least two other carbon components: an isotopically-heavy marine limestone endmember of slab-derivation and an isotopically-lighter component representing either slab-derived organic carbon and/or an upper crustal component with a large fraction of organic carbon. The helium-carbon systematics of the central portion of the arc are consistent with a large slab-derived marine limestone input to the carbon inventory, and we calculate a non-mantle:mantle carbon flux of 5.7:1. MORB-like helium isotope ratios, which are sensitive to perturbation by crustal additions to degassed magmas, imply a relatively minor role for upper crustal contributions in this part of the arc although it could reach a maximum of 20% of the total carbon flux if the light-C component is solely of crustal origin. Higher CO₂/³He ratios in the southern islands coupled with lighter δ¹³C imply (1) an increase in the flux in the non-mantle contribution relative to the central arc, and (2) an enhanced role for an isotopically-light carbon component in this part of the arc. Compared to the central islands, the increase in the non-mantle carbon flux in the southern islands is by a factor of 4.2. Assuming that the isotopically-light carbon has a δ¹³C of -10‰, then its southern arc flux is a factor of 9.1 times that of the central arc. Lower ³He/⁴He ratios in the southern arc indicate that the origin of this carbon component is likely to be the uppermost crust: in this case, ~50% of the total carbon lost via the southern Lesser Antilles would be of crustal-derivation. This conclusion is compatible with the increased availability of carbon both in the southernmost forearc sediments and arc crust. Our approach of combining helium and carbon systematics serves to emphasize the importance of volatile provenance on mass balance considerations of the terrestrial carbon inventory at convergent margins.