Ij. Parkinson et al., ANCIENT MANTLE IN A MODERN ARC - OSMIUM ISOTOPES IN IZU-BONIN-MARIANAFORE-ARC PERIDOTITES, Science, 281(5385), 1998, pp. 2011-2013
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.