Recognizing recycled osmium

Understanding the behavior of noble metals like Os in Earth systems is important from both scientifi c and economic standpoints. The abundances of noble metals in the mantle have signifi cant implications for our understanding of the Earth’s evolution. The relative abundances of these strongly siderophile elements in the mantle potentially preserve a fi ngerprint of core formation, which likely scavenged much of the Earth’s budget of these elements, as well a record of the latest stages of accretion history, during which time the Earth’s mantle may have been re-enriched in these elements (e.g., Walter et al., 2000). Understanding processes that can modify these signatures in the Earth’s mantle is fundamental to such models. In addition, documenting environments under which noble metals are mobile, and those under which they become scavenged and concentrated, are critical for understanding the formation of precious metal ore deposits and developing strategies for prospecting. For example, many economic Au deposits are associated with magmatism at convergent margins, yet the source of the Au and the behaviors of other noble metals are generally not well constrained (McInnes et al., 1999). The paper by Suzuki et al. (2011, p. 999 in this issue of Geology) contributes important new results regarding the mobility of Os in subduction settings. The Re-Os isotope system, involving the decay of