Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unradiogenic 187Os/188Os ratios (0.1193 to 0.1273), which give Proterozoic model ages of 820 to 1230 million years ago. If these peridotites are residues from magmatism during the initiation of subduction 40 to 48 million years ago, then the mantle that melted was much more depleted in incompatible elements than the source of mid-ocean ridge basalts (MORB). This result indicates that osmium isotopes record information about ancient melting events in the convecting upper mantle not recorded by incompatible lithophile isotope tracers. Subduction zones may be a graveyard for ancient depleted mantle material, and portions of the convecting upper mantle may be less radiogenic in osmium isotopes than previously recognized.

In the model age calculation, the peridotite is assumed to contain no Re, and therefore the 187Os/188Os ratio of the peridotite has not changed because the Re was extracted during an ancient melting event. The age is calculated from the time in the past that the measured 187Os/188Os ratio intersects a chondritic Re-Os evolution curve with a present-day 187Os/188Os ratio of 0.12757 and a 187Re/188Os ratio of 0.3972 (Walker et al., 1989, doi:10.1016/0016-7037(89)90240-8). Recent work suggests that the putative primitive upper mantle has an 187Os/188Os ratio of 0.1290 and a 187Re/188Os ratio of 0.423 (Meisel er al., 1996, doi:10.1038/383517a0). Using these parameters gives slightly older (100 Ma) model ages but does not affect our conclusions. For Al2O3 data see Parkinson and Pearce (1998) dataset: doi:10.1594/PANGAEA.772276.

Supplement to: Parkinson, Ian J; Hawkesworth, Chris J; Cohen, Anthony S (1998): Ancient mantle in a modern arc: Osmium Iiotopes in Izu-Bonin-Mariana forearc peridotites. Science, 281(5385), 2011-2013