Platinum-osmium isotope evolution of the Earth's mantle: Constraints from chondrites and Os-rich alloys

Separation of a metal-rich core strongly depleted the silicate portion of the Earth in highly siderophile elements (HSE). including Pt, Re, and Os. To address the issues of how early differentiation, partial melting, and enrichment processes may have affected the relative abundances of the HSE in the upper mantle, Os-187/Os-188 and Os-186/Os-188 data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that Os-187 and Os-186 are decay products of Re-187 and Pt-190, respectively, these ratios can be used to constrain the lone-term Re/Os and Pt/Os of mantle reservoirs in comparison to chondrites. Because of isotopic homogeneity, H-group ordinary and other equilibrated chondrites may be most suitable for defining the initial Os-186/Os-188 of the solar system. The Os-186/Os-188 ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 +/- 0.0000016 (2 sigma). Using the measured Pt/Os and Os-186/Os-188 for each chondrite, the calculated initial Os-186/Os-188 at 4.567 Ga is 0.1198269 +/- 0.0000014 (2 sigma). This is the current best estimate for the initial Os-186/Os-188 of the bulk solar system. The mantle evolution of Os-186/ Os-188 can be defined via examination of mantle-derived materials with well-constrained ages and low Pt/Os. Two types of mantle-derived materials that can be used for this task are komatiites and Os-rich alloys. The alloys are particularly valuable in that they have little or no Re or Pt, thus, when formed, evolution of both Os-187/Os-188 and Os-186/Os-188 ceases. Previously published results for an Archean komatiite and new results for Os-rich alloys indicate that the terrestrial mantle evolved with Pt-Os isotopic systematics that were indistinguishable from the H-group ordinary and some enstatite chondrites. This corresponds to a Pt/Os of 2.0 +/- 0.2 for the primitive upper mantle evolution curve. This similarity is consistent with previous arguments, based on the Os-187/Os-188 systematics and HSE abundances in the mantle, for a late veneer of materials with chondritic bulk compositions controlling the HSE budget of the upper mantle. It is very unlikely that high pressure metal-silicate segregation leading to core formation can account for the elemental and isotopic compositions of HSE in the upper mantle. (c) 2006 Elsevier Inc. All rights reserved.