The Osmium Mantle Evolution Curve and Limits on Accretion and Differentiation of the Earth from Archean (3.4 - 3.8 Ga) Ultramafic Rocks

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Bennett, Victoria, Nutman, Allen, Esat, Tezer
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Pergamon-Elsevier Ltd
• Subjects: Keywords: Archean; crustal evolution; isotopic composition; mantle chemistry; osmium isotope; Australia; Greenland
• Online Access: http://hdl.handle.net/1885/71962; https://doi.org/10.1016/S0016-7037(02)00862-1; https://openresearch-repository.anu.edu.au/bitstream/1885/71962/5/MigratedxPub2976_RSD_2002.pdf.jpg; https://openresearch-repository.anu.edu.au/bitstream/1885/71962/7/01_Bennett_The_Osmium_Mantle_Evolution_2002.pdf.jpg

Initial 187Os/188Os isotopic compositions for geochronologically and geologically well -constrained 3.8-Ga spinel peridotites from the Itsaq Gneiss Complex of southern West Greenland and chromite separates from 3.46-Ga komatiites from the Pilbara region of Western Australia have been determined to investigate the osmium isotopic evolution of the early terrestial mantle. The measured compositions of 187Os/188Os(0) = 0.10262 ± 2, from an olivine separate, and 0.10329 ± 3, for a spinel separate from ∼3.8-Ga peridotite G93/42, are the lowest yet reported from any terrestrial sample. The corrections for in situ decay over 3.8 Ga for these low Re/Os phases are minimal and change the isotopic compositions by only 0.5 and 2.2% for the spinel and the olivine, respectively, resulting in 187Os/188Os(3.8Ga) = 0.1021 ± 0.0002 and 0.1009 ± 0.0002, respectively. These data extend direct measurement of Os isotopic compositions to much earlier periods of Earth history than previously documented and provide the best constraints on the Os isotopic composition of the early Archean terrestrial mantle. Analyses of Pilbara chromites yield 3.46-Ga mantle compositions of 0.1042 ± 0.0002 and 0.1051 ± 0.0002. These new data, combined with published initial Os isotopic compositions from late Archean and early Proterozoic samples, are compatible with the mantle, or at least portions of it, evolving from a solar system initially defined by meteorites to a modern composition of 187Os/188Os(0) = 0.1296 ± 0.0008 as previously suggested from peridotite xenolith data (Meisel et al., 2001); the associated 187Re/188Os(0) = 0.435 ± 0.005. Thus, chondritic 187Os/188Os compositions were a feature of the upper mantle for at least 3.8 billion years, requiring chondritic Re/Os ratios to have been a characteristic of the very early terrestrial mantle. In contrast, nonchondritic initial compositions of some Archean komatiites demonstrate that Os isotopic heterogeneity is an ancient feature of plume materials, reflecting the development of variable Re/Os ...