Exploration of the Lesser Antilles arc signature in St Lucia using a multiscale analytical approach

The geochemical “arc signature” has been largely interpreted as reflecting a mantle source contaminated by fluids and sediments released by the subducted slab. Amongst intra-oceanic arcs, the Lesser Antilles displays some of the most extreme chemical and isotopic variations ranging from fluid-modified MORBs to compositions approaching that of continental crust [1]. The extreme range in geochemistry might result from sediments subducted into the mantle source [2], assimilation of sediments in the arc crust [1], or both. This debate, first outlined in 1984, has still not been resolved since the different tools used up to now have led to conflicting conclusions. The relative role of source contamination by subducted material and magma-arc crust interaction may be constrained by using a multiscale isotopic and chemical approach from whole rock to sub-mineral scale on stratigraphically-constrained samples from a single locality. To do this, we chose the island of St Lucia, specifically its Soufriere Volcanic Complex (SVC) since, according to preliminary data, it displays one of the largest ranges of radiogenic isotope ratios in the arc. Our preliminary whole rock trace element and isotopic data confirm that St Lucia displays most of the compositional range observed at the whole arc. There are two distinct groups: the Pre-Soufrière Volcanic Complex (SVC) and the SVC itself. The Pre-SVC group comprises the oldest, most mafic rocks of the island and has typical intra-oceanic arc signatures (e.g. 87Sr/86Sr = 0.70439-0.70413) similar to compositions observed in the north of the arc. Despite following the same differentiation trend as the Pre-SVC (suggesting a common source), the younger SVC rocks are more felsic and display extreme crustal signatures (e.g. 87Sr/86Sr = 0.70754-0.70906) more typically observed in continental arcs. At the mineral scale, plagioclase, quartz, pyroxene and amphibole separates from the SVC lavas all display strong crustal signatures with δ18O compositions much higher than the ranges in equilibrium with the mantle (9.92-11.1, 10.6-11, 9-9.5, 9.4-9.8‰VSMOW, respectively). Such high values have never been reported in intra-oceanic arcs lacking known continental basement and strongly argue for assimilation of sediments during ascent and storage of the magmas. Sr isotopic analyses of single plagioclases support crustal assimilation since different crystals within the same sample are isotopically distinct. Submineral scale analysis should refine these interpretations.