PII S0016-7037(00)00566-4 Osmium isotopic compositions of mantle xenoliths: A global perspective THOMAS MEISEL, 1, *RICHARD J. WALKER, 2 ANTHONY J. IRVING, 3 and JEAN-PIERRE LORAND 4 1 General and Analytical Chemistry, University of Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria 2 Isotope Geochemistry Laboratory, Department of Geology, University of Maryland, College Park, MD 20742, USA 3 Department of Geological Sciences, University of Washington, Seattle, WA 98195, USA 4 Laboratoire de Mine ´ralogie, Muse ´um National d’Histoire Naturelle, Paris, France (Received December 1, 1999; accepted in revised form August 14, 2000) Abstract—The Re-Os isotopic systematics of spinel- and garnet-bearing mantle xenoliths from North and Central America, Europe, southern Africa, Asia, and the Pacific region were determined to define more precisely the isotopic composition of a hypothetical primitive upper mantle (PUM). When plotted against Al 2 O 3 or other melt depletion indicators, the 187 Os/ 188 Os ratios of all xenolith suites display positive trends toward a uniform composition at a fertile mantle composition. A 187 Os/ 188 Os value for PUM of 0.1296  0.0008 (level of confidence 95%) was defined on the basis of 117 spinel-bearing xenoliths from this work and data from the literature, including data for massif peridotites. The 187 Os/ 188 Os ratio of the PUM is similar to the range of compositions defined by ordinary and enstatite chondrites, not carbonaceous chondrites. Spinel-bearing mantle peridotites sampled by volcanism and peridotite massifs appear to have been extracted from a common fertile source (PUM) between 1 and 2 Ga ago. These peridotites now form part of the subcontinental lithospheric mantle underlying continental crust of similar or greater formation age. Copyright © 2001 Elsevier Science Ltd 1. INTRODUCTION Xenoliths derived from the mantle and brought to the surface via volcanism are among the few types of materials that permit direct study of the chemical composition of the Earth’s upper mantle. By examining the isotopic composition of mantle sam- ples, a time dimension is added to the estimate of the chemical composition of domains within the mantle. This is important for understanding the chemical evolution of the terrestrial man- tle from a primitive composition, created during Earth’s early history, to the present differentiated states of the mantle and crust. Because of these aspects, mantle xenoliths have been vigorously studied for major and trace element compositions, mineral compositions, and lithophile isotope systems such as Rb-Sr, U-Pb, and Sm-Nd (Menzies et al., 1987; McDonough and Frey, 1989). The addition of the Re-Os isotope system ( 187 Re3 187 Os +  - ;  = 1.666  10 -11 a -1 ; Smoliar et al., 1996) to the study of xenoliths has proven important because parent and daughter elements have contrasting geochemical behaviors during partial melting processes in the mantle (e.g., Morgan, 1986). Although Re is a mildly incompatible element, Os usually remains compatible in the residue (Shirey and Walker, 1998). This characteristic of the Re-Os isotope system can provide unique information about the timing of melt depletion in the mantle. The system is normally immune to subsequent process- ing within the mantle because the Os model dating technique used for peridotites is based on the loss of the parent isotope and the resulting retardation of the growth of radiogenic 187 Os (e.g., Walker et al., 1989). For the system to be disturbed, either Re must be added during some later event, or Os with a different isotopic composition must be added to the system. Evidence from previous studies suggests that these situations are rare (Walker et al., 1989; Pearson et al., 1995b) and re- stricted to specific tectonic settings such as mantle lithosphere above plumes (e.g., Chesley et al., 1999) or above subducted mantle wedges (Brandon et al., 1996). In addition to the normal closed-system behavior within the mantle, the high abundance of Os in most ultramafic xenoliths relative to silica-rich crustal sediments and S-saturated basaltic and alkaline melts makes the Os isotope system robust to late-stage contamination that can hamper the usefulness of the lithophile isotope systems for the study of mantle evolution. Over the previous decade, a number of studies have exam- ined in detail the Os isotopic compositions of peridotitic xeno- liths from a variety of mantle domains, including domains underlying southern Africa (Pearson et al., 1995a; Olive et al., 1997; Walker et al., 1989), Tanzania (Chesley et al., 1999), the Siberian craton (Pearson et al., 1995b), the Wyoming craton of North America (Carlson and Irving, 1994), the Canadian Cor- dillera (Peslier et al., 2000), the Baikal rift (Pearson et al., 1998), and southern Australia (Handler et al., 1997). The Os isotopic systematics of peridotitic xenoliths associated with ocean islands (Hassler and Shimizu, 1998) and arc systems (Brandon et al., 1996; Parkinson et al., 1998) have also been examined. Here, Re-Os isotopic data for a suite of 39 previ- ously well-characterized peridotite xenoliths from worldwide localities are reported. The purpose of this study is to expand the Re and Os concentration and the Os isotopic database for well-characterized upper mantle samples from a variety of different settings and to begin the task of synthesizing the rapidly expanding Os isotopic database for upper mantle peri- dotites to understand the causes of the global variations. Twenty of the samples examined here were collected during the course of the Basaltic Volcanism Study Project (BVSP; assembled by A.J.I. in 1978; Project, 1981). The BVSP was an important advance in the understanding of the mantle because *Author to whom correspondence should be addressed (meisel@ unileoben.ac.at). Pergamon Geochimica et Cosmochimica Acta, Vol. 65, No. 8, pp. 1311–1323, 2001 Copyright © 2001 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/01 $20.00 + .00 1311