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

We present whole rock 187 Os/ 188 Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5–17 wt.%) and for the subducting basalt and sediments. 187 Os/ 188 Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between 187 Os/ 188 Os and Os concentrations and between 187 Os/ 188 Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. 87 Sr/ 86 Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with 187 Os/ 188 Os. The assimilant is inferred to have a MORB-like 87 Sr/ 86 Sr with high Sr (>700 ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ∼2.5) and 187 Os/ 188 Os > 0.2. Such compositional features are likely to correspond to a plagioclase-rich early-arc cumulate. Given that assimilation affects lavas that were last stored at more than 5 kbar, assimilation must occur in the middle-lower crust. Only a high MgO picrite from Grenada escaped obvious assimilation (MgO = 17% wt.%) and could reflect mantle source composition. It has a very radiogenic 87 Sr/ 86 Sr (0.705) but a 187 Os/ 188 Os ratio that overlaps the mantle range (0.127). 187 Os/ 188 Os and 87 Sr/ 88 Sr ratios of the sediments and an altered basalt from the subducting slab vary from 0.18 to 3.52 and 0.708 to 0.714. We therefore suggest that, unlike Sr, no Os from the slab was transferred to the parental magmas. Os may be either retained in the mantle wedge or even returned to the deep mantle in the subducting slab. [ABSTRACT FROM AUTHOR]