Isotopic hyperbolas constrain sources and processes under the Lesser Antilles arc

The chemical and isotopic characteristics of island arc lavas are usually attributed to the combined effects of subducted materials and processes occurring in the mantle wedge. Most island arcs have compositions displaced towards continental crust values relative to MORB, none more so than the Lesser Antilles arc. Not only do lavas from the Lesser Antilles arc contain a high proportion of crustal material but they also display a very large range of chemical and isotopic compositions. Lavas on Martinique Island cover most of the variability known for the entire arc, and volcanism occurred almost continuously for the last 25 Ma. It is therefore an ideal target to establish the causes of the chemical and isotopic variability and to evaluate whether a secular relationship exists. We measured Pb, Sr, Nd and Hf isotopic ratios on 30 well-dated Martinique samples whose ages range between 25 Ma and the present. Samples were chosen to cover all volcanic phases, from island formation to the most recent eruptions. Our results cover most of the isotopic range defined by the entire arc, from fairly unradiogenic to highly radiogenic values; but more importantly, they define two distinct hyperbolas in isotopic space. One array is defined by old lavas (25 to 7.1 Ma), and the other by recent lavas (5.1 Ma to present). The hyperbolas are compatible with simple two-component mixtures but are not reproduced by AFC models. Using these hyperbolas, we can constrain the composition of the mixing end-members. Both depleted sources correspond to depleted mantle contaminated by a radiogenic Sr component, and the two enriched components resemble local sediments with little to no relative fractionation of Sr, Nd, Hf and Pb. We conclude on this basis that sediments were added to the mantle wedge by melting and not dehydration. Our interpretation for Martinique can be extrapolated to the entire Lesser Antilles arc because the isotopic arrays of Martinique and other islands are indistinguishable. More generally, if the same occurs under most other island arcs, the chemical and isotopic characteristics of arc volcanism might be directly linked to the amount and nature of subducted sediments.