Lower crustal assimilation in oceanic arcs: insights from an osmium isotopic study of the Lesser Antilles

We investigated the whole rock 187Os/188Os of the most mafic lavas along the Lesser Antilles arc (MgO=17-5 wt.%) and of the subducting slab to test: (1) the mobility of Os in the subduction zone and the potential of magmatic 187Os/188Os to reflect the slab composition, (2) the sensitivity of lava 187Os/188Os to crustal assimilation. 187Os/188Os varies from 0.127-0.202 in the lavas. A correlation between 187Os/188Os and indexes of differentiation such as Os and MgO shows that assimilation, rather than source variation, is responsible for the range of 187Os/188Os observed. Magmatic compositions from all islands plot along a single trend indicating the presence of a common assimilant in the basement of every island. 87Sr/86Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187Os/188Os. The assimilant displays a MORB-like 87Sr/86Sr with high Sr (> 700 ppm), low L/M-HREE (La/Sm < 1.6) and 187Os/188Os > 0.2. Such a composition could correspond to a plagioclase rich cumulate. Given that assimilation affects lavas that were last stored at more than 5 kbar [1], assimilation must occur in the middle-lower crust. In the whole arc, only a Grenada high MgO picrite escaped assimilation (MgO = 17%) and could represent the source characteristics. It displays a very radiogenic 87Sr/86Sr (0.705) but 187Os/188Os overlapping the mantle range (0.127). 187Os/188Os and 87Sr/86Sr of the sediments and basalt from the subducting slab vary from 0.18-3.52 and 0.708-0.714. We therefore suggest that, unlike for Sr, no Os from the slab was involved in the primitive magma source. The later may be either retained in the mantle or in the subducting slab. Finally, unlike the other lavas of the arc, Grenada picrites are silica undersaturated which suggests that, before assimilation, primitive magmas from across the arc could be of similar nature. [1] Heath, E., Macdonald, R., Belkin, H., Hawkesworth, C., Sigurdsson, H., 1998. Magmagenesis at Soufriere Volcano, St Vincent, Lesser Antilles Arc. Journal of Petrology 39, 1721-1764.