OSMIUM ISOTOPE BINARY MIXING ARRAYS IN ARC VOLCANISM

Abstract

Os isotope ratios and Os and Re concentrations were measured in 56 lavas coming from 10 different subduction zones. Samples span a large range of major element concentrations (from basalts to dacites) and Mg# (from 0.32 to 0.81). The 10 subduction zones, namely the Lesser Antilles, Java, Papua New Guinea, the Philippines, Izu–Bonin, Kamchatka, the Aleutians, Mexico, Colombia and Peru–Chile, have a range of geodynamic settings. Measured 187Os/188Os ratios range from 0.130 to 1.524 and Os concentrations range from 0.05 to 46 ppt. Re concentrations range from 24 to 915 ppt. Os initial isotope ratios are systematically positively and linearly correlated with the inverse of Os concentrations in arc lavas from a given volcano, indicating that the Os isotopic compositions always reflect a binary mixing process. Similar mixing relationships are also seen at the sample scale. All trends converge towards unradiogenic compositions similar to those of upper mantle peridotites. These mixing relationships might be ascribed to a general contamination process; however, a single shallow-level process of crustal assimilation is hardly reconciled with the diversity of basements (from oceanic crust to continental crust compositions) of the selected arc volcanoes, the occurrence of the mixing lines for both primary and differentiated samples, and the absence of covariations between Os contents, isotope ratios, and indices of contamination and differentiation. On the other hand, because subducted components are very radiogenic and differ from one zone to another, the radiogenic components may be explained by varying amounts and natures of oceanic crust and sediments in the source of arc lavas. However, this explanation implies two disequilibrium processes, first during magma formation in order to produce heterogeneous lavas, and second during magma ascent to the surface to preserve slab signatures.