Limited recycling of crustal osmium in forearc mantle during slab dehydration

Elevated (super 187) Os/ (super 188) Os ratios compared to ambient oceanic mantle, i.e., (super 187) Os/ (super 188) Os > 0.13, have been reported for both arc lavas and mantle wedge xenoliths, which have been attributed to the addition of crustal Os through slab dehydration or melting. By contrast, much lower (super 187) Os/ (super 188) Os ratios of spinels from Izu-Bonin-Mariana (western Pacific Ocean) boninites indicate that slight or no crustal Os was transferred from the slab to the forearc mantle. Here we report Os isotopic compositions of peridotites from New Caledonia (southwest Pacific Ocean) ophiolites, which represent relics of a forearc mantle. Some New Caledonia peridotites are characterized by Os concentrations of <1 ppb, yet have (super 187) Os/ (super 188) Os ratios comparable to those of the ambient oceanic mantle (i.e., (super 187) Os/ (super 188) Os < 0.13). This confirms that little crustal Os was transported to the forearc mantle via slab dehydration. Contrasting Os isotopes between forearc peridotites and mantle wedge xenoliths may reflect the changing behavior of Os in diverse agents released from the descending slab as a function of depth; this is mainly controlled by the stability of sulfides in the slabs. During dehydration at shallow depths, sulfides keep stable and thus little Os is transported to the overlying mantle. In comparison, sulfides become unstable and tend to break down at deeper depths where slab melting or supercritical fluid generation occurs, and thus Os behaves like a mobile element.