A Quantitative Link Between Recycling and Osmium Isotopes

摘要

Formation, subduction, and incomplete mixing of oceanic crust produces chemical and isotopic heterogeneity in Earth's mantle (1, 2). The signature of these processes in the mantle over time and the importance of recycling in explaining the origins and compositions of volcanic rocks remain major questions. Recently, osmium isotopes have proved to be an important tracer (3, 4). Basaltic ocean crust has much higher Re/Os ratios than mantle peridotite, from which it is derived through partial melting, because osmium remains mostly in the solid (mantle residue), whereas Re preferentially enters the melt. The decay of long-lived ~(187)Re to stable ~(187)Os therefore generates increased ~(187)Os/~(188)Os ratios in such crustal rocks. Elevated ~(187)Os/~(188)Os values have been recently found in Icelandic basalts, where they are correlated with the relative abundances of other isotopes (5, 6). These data support the notion that the mantle source of Icelandic basalts contains oceanic crust that has been recycled by subduction and mantle convection (7). However, it has been difficult to assess this model because independent estimates of the proportions of components and their Os isotopic compositions were lacking.