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Science 25 September 1998:
Vol. 281. no. 5385, p. 1921
DOI: 10.1126/science.281.5385.1921a
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Volcanic arcs form above regions where oceanic crust is subducted into the mantle. Heating of the crust as it sinks into the mantle releases fluids, and these fluids in turn flux melting in overlying parts of the mantle to produce the magmas that rise to form volcanoes. This process depletes the mantle in elements that are fractionated into the melt. One of these is rhenium; the isotope rhenium-187 decays to osmium-187. Thus, melting events, which strip away rhenium, can be recorded in osmium isotopic anomalies. Parkinson et al. (p. 2011) have examined the osmium isotopic composition of pieces of the mantle preserved in part of the Izu-Bonin-Mariana arc system, an arc that first formed about 40 to 50 million years ago in the western Pacific Ocean. Surprisingly, the data show that these pieces of mantle seem to record a much earlier melting event, dating to about 1 billion years ago. In contrast, most oceanic crust is younger than about 150 million years ago because it is continually destroyed by subduction. These data may imply that the upper mantle is quite heterogeneous or that subduction zones may harbor very old pieces of the mantle.




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