The scale and origin of the osmium isotope variations in mid-ocean ridge basalts

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doi: 10.1016/j.epsl.2007.05.014
Authors:Gannoun, A.; Burton, K. W.; Parkinson, I. J.; Alard, O.; Schiano, P.; Thomas, L. E.
Author Affiliations:Primary:
Open University, Department of Earth Science, Milton Keynes, United Kingdom
Université Blaise-Pascal, France
Volume Title:Earth and Planetary Science Letters
Source:Earth and Planetary Science Letters, 259(3-4), p.541-556. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X
Publication Date:2007
Note:In English. Supplemental information/data is available in the online version of this article. 78 refs.; illus., incl. 2 tables
Summary:The decay of 187Re to 187Os provides a unique tracer of recycled lithosphere in the Earth's upper mantle, and the radiogenic Os isotope compositions of many mid-ocean ridge basalts (MORB) have been taken to indicate the presence of recycled oceanic crust in the mantle source. However, others have attributed the data to contamination by seawater or an age effect (due to 187Re decay following igneous crystallisation). This study presents new Re-Os, U-Th-Ra and B isotope data for normal-MORB (N-MORB) glass (including 18 samples analysed previously). The scale of the variation in 187Os/188Os observed here in N-MORB glass ranges from 0.126 to 0.148, with a mean value of 0.133±0.009 (2σ). Previously published Os isotope data for MORB is shown to have been affected by an analytical artefact, and in particular the very radiogenic Os isotope compositions reported earlier are now shown not to exist. Nevertheless, some radiogenic Os isotope compositions are still seen in the data obtained here. Coupled Th and Ra excesses appear to rule out an age effect, at least, for those samples studied, however, δ11B covariations with 187Os/188Os are consistent with the assimilation of older oceanic crust (affected by seawater contamination). Individual sulphides (less likely to be affected by such contamination) are always less radiogenic than their glass host, with 187Os/188Os values ranging from 0.123 to 0.131. This indicates that sulphide precipitation occurred prior to contamination of the surrounding melt, but the data also suggest that low-Os sulphides have been affected by contamination through partial equilibration with that melt. Taken together, these indicate that dominant process responsible for the radiogenic Os isotope compositions of N-MORB glass is contamination by older oceanic crust. Although the influence of age effects and the presence of recycled material cannot be completely ruled out, Os-rich sulphides yield sub-chondritic Os isotope compositions which point rather to a long-term depletion in Re in the MORB source likely to have resulted from formation of the continental crust over much of Earth's history.
Sections:Geochemistry; Petrology
Subsections:Igneous petrology
Subjects:B-11/B-10; Basalts; Boron; Crust; Glasses; ICP mass spectra; Igneous rocks; Ion probe data; Isotope ratios; Isotopes; Magma contamination; Mantle; Mass spectra; Metals; Mid-ocean ridge basalts; Oceanic crust; Os-188/Os-187; Osmium; Platinum group; Recycling; Rhenium; Sea water; Spectra; Stable isotopes; Sulfides; Volcanic rocks; Atlantic Ocean; Indian Ocean; Pacific Ocean
Abstract Numbers:07M/3864
Record ID:2008016677
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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