SEDIMENTARY INPUT TO THE SOURCE OF LESSER ANTILLES LAVAS: A LI PERSPECTIVE

Li isotopes in compositionally diverse Martinique lavas, as well as sea floor sediments cored at the southern (DSDP Site 144) and northern part (DSDP Site 543) of the subducting slab were analyzed in order to investigate the origin of the continental crust compositional signature seen in Lesser Antilles lavas and to investigate Li cycling in arcs. Although the subducting sediments display marked mineralogical and chemical shifts from south to north, the average δ⁷Li for sediments from the two cores are indiscernible from each other (–1.0 ± 1.8, 1 σ, n = 15, δ⁷Li = –4.4 to +2.9). This is the lowest bulk δ⁷Li seen in subducting sediments from any trenches, and is significantly lower than that of MORB (δ⁷Li ~ +4 ± 1). These low δ⁷Li values reflect the dominance of terrigneous input and the influence of chemical weathering in the sediment’s continental provenance. With a few exceptions, the Li isotopic compositions of the Martinique lavas are also systematically lighter than MORB, yielding an average δ⁷Li of +1.8 ± 1.3 (1 σ, n = 24, excluding three outliers that are isotopically heavy, erupted below seawater and may have incorporated sea water Li). The δ⁷Li values in the lavas show no correlation with radiogenic isotope ratios (²⁰⁶Pb/²⁰⁴Pb, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf), Li/Y, La/Sm or SiO₂. There is also no correlation between d⁷Li and radiogenic isotopes in the subducting sea floor sediments. Thus, the low δ⁷Li in the Martinique lavas likely reflects a mantle source that incorporated isotopically light subducted sediments. A two-end-member mixing model requires a bulk sedimentary input of at least 2% by mass to the depleted mantle source, within the range of sediment input (0.1–5%) inferred from radiogenic isotope data. The Lesser Antilles is the first arc shown to have δ⁷Li systematically lower than MORB, reflecting the influence of subducted terrigneous sediments. Our data suggest that the enrichment in Li in most arc lavas is due to the addition of subducted sediment-derived Li, and that the isotopic signature of this Li can traverse the mantle wedge with little alteration. It is only in arcs such as the Lesser Antilles where the isotopic composition of subducted sediments is distinct from MORB that the slab signature is apparent in the lavas.