Platinum–osmium isotope evolution of the Earth’s mantle: Constraints from chondrites and Os-rich alloys
- 作者：A.D. Brandon ; R.J. Walker and I.S. Puchtel
- 刊名：Geochimica et Cosmochimica Acta
- 出版年：2006
- 期刊代码：27_00167037
- 类别：ear
- 出版时间：15 April 2006
- 卷：70
- 期：8
- 页码：2093-2103
- 文件大小：255 K
摘要
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, <sup>187sup>Os/<sup>188sup>Os and <sup>186sup>Os/<sup>188sup>Os data for chondrites are compared with data for Os-rich alloys from upper mantle peridotites. Given that <sup>187sup>Os and <sup>186sup>Os are decay products of <sup>187sup>Re and <sup>190sup>Pt, respectively, these ratios can be used to constrain the long-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 <sup>186sup>Os/<sup>188sup>Os of the solar system. The <sup>186sup>Os/<sup>188sup>Os ratios for five H-group ordinary chondrites range only from 0.1198384 to 0.1198408, with an average of 0.1198398 ± 0.0000016 (2<i>σi>). Using the measured Pt/Os and <sup>186sup>Os/<sup>188sup>Os for each chondrite, the calculated initial <sup>186sup>Os/<sup>188sup>Os at 4.567 Ga is 0.1198269 ± 0.0000014 (2<i>σi>). This is the current best estimate for the initial <sup>186sup>Os/<sup>188sup>Os of the bulk solar system. The mantle evolution of <sup>186sup>Os/<sup>188sup>Os 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 <sup>187sup>Os/<sup>188sup>Os and <sup>186sup>Os/<sup>188sup>Os 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 <sup>187sup>Os/<sup>188sup>Os 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.