Osmium isotopic compositions of mantle xenoliths: A global perspective

The Re-Os isotopic systematics of spinel- and garnet-bearing mantle xenolit hs from North and Central America, Europe, southern Africa, Asia, and the P acific region were determined to define more precisely the isotopic composi tion of a hypothetical primitive upper mantle (PUM). When plotted against A l2O3 or other melt depletion indicators, the Os-187/Os-188 ratios of all xe nolith suites display positive trends toward a uniform composition at a fer tile mantle composition. A Os-187/Os-188 value for PUM of 0.1296 +/- 0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing x enoliths from this work and data from the literature, including data for ma ssif peridotites. The Os-187/Os-188 ratio of the PUM is similar to the rang e of compositions defined by ordinary and enstatite chondrites, not carbona ceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism an d peridotite massifs appear to have been extracted from a common fertile so urce (PUM) between 1 and 2 Ca ago. These peridotites now form part of the s ubcontinental lithospheric mantle underlying continental crust of similar o r greater formation age. Copyright (C) 2001 Elsevier Science Ltd.