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

摘要

We present whole rock ¹⁸⁷Os/¹⁸⁸Os data for the most mafic lavas along the Lesser Antilles arc (MgO = 5-17 wt.%) and for the subducting basalt and sediments. ¹⁸⁷Os/¹⁸⁸Os ratios vary from 0.127 to 0.202 in the arc lavas. Inverse correlations between ¹⁸⁷Os/¹⁸⁸Os and Os concentrations and between ¹⁸⁷Os/¹⁸⁸Os and indices of differentiation such as MgO suggests that assimilation, rather than source variation, is responsible for the range of Os isotopic variation observed. ⁸⁷Sr/⁸⁶Sr, La/Sm and Sr/Th are also modified by assimilation since they all correlate with ¹⁸⁷Os/¹⁸⁸Os. The assimilant is inferred to have a MORB-like ⁸⁷Sr/⁸⁶Sr with high Sr (>700 ppm), low light on middle and heavy rare earth elements (L/M-HREE; La/Sm ~2.5) and ¹⁸⁷Os/¹⁸⁸Os > 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 ⁸⁷Sr/⁸⁶Sr (0.705) but a ¹⁸⁷Os/¹⁸⁸Os ratio that overlaps the mantle range (0.127). ¹⁸⁷Os/¹⁸⁸Os and ⁸⁷Sr/⁸⁶Sr 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.