The relative importance of crustal contamination versus subducted slab contributions to arc magmatism has been investigated through a regional survey of helium and carbon relations along the strike of the 850 km Lesser Antilles Magmatic Arc System (LAMAS). We have utilised both geothermal fluids (fumaroles and hot springs) and phenocryst-bearing young lava flows. Coverage extends from Grenada in the south of the arc to Saba in the north and encompasses both presently and recently active volcanoes.
There is a predominant mantle He contribution throughout the arc. Highest 3He/4He ratios are coincident with MORB helium (8RA where RA = air 3He/4He) and occur in the north of the arc - on the islands of Martinique, Dominica, Guadeloupe and Montserrat. In the south, from Grenada to St. Lucia, 3He/4He ratios are lower and lie between 4-7RA. There is a strong
negative correlation between fluid 3He/4He ratios and CO2/3He. At currently-active Montserrat (8.1RA), CO2/3He ratios fall in the range 109-1010; however, most of the MORB-like 3He/4He ratios of the northerly islands have CO2/3He ratios in the range 1-5 x 1010. In contrast, the southern islands - characterised by lower 3He/4He ratios - have distinctly higher CO2/3He values of 1011-1012. Carbon isotope analyses give values of 13C -2 - -4 vs. PDB for the northern islands, while in the southern part of the arc the values range from -2 - -6 vs. PDB, where the MORB range lies between -5 - -8. Carbonates have values around 0 and Organic Carbon has values around -30.
Clear regional trends are apparent for the LAMAS. Assuming that the Lesser Antilles mantle is characterised by MORB-like helium then results described here point towards an enhanced role for crustal contamination processes in the south of the arc. Presently-available evidence suggests that St. Lucia is the locus of these processes. In this case, CO2/3He ratios > 1011 in the south of the arc may indicate release of crustal carbon in this region - possibly superimposed upon a slab carbon contribution. For the Northern part of the arc evidence suggests that the magma source is contaminated by carbonate from either the slab and/or subducted carbonate bearing sediments, which contained little or no He. In this part of the arc the contribution of upper crustal contamination appears to be close to non existent.