ANCIENT MANTLE IN A MODERN ARC - OSMIUM ISOTOPES IN IZU-BONIN-MARIANAFORE-ARC PERIDOTITES

Mantle peridotites drilled from the Izu-Bonin-Mariana forearc have unr adiogenic Os-187/Os-188 ratios (0.1193 to 0.1273), which give Proteroz oic 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 deplet ed 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 recor ded by incompatible Lithophile isotope tracers. Subduction zones may b e a graveyard for ancient depleted mantle material. and portions of th e convecting upper mantle may be less radiogenic in osmium isotopes th an previously recognized.