The scale and origin of the osmium isotope variations in mid-ocean ridge basalts
The decay of <sup>187</sup>Re to <sup>187</sup>Os 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...
| Main Authors: | , , , , , |
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| Format: | Journal article |
| Language: | English |
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Elsevier
2007
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| _version_ | 1797094104849448960 |
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| author | Gannoun, A Burton, K Parkinson, I Alard, O Schiano, P Thomas, L |
| author_facet | Gannoun, A Burton, K Parkinson, I Alard, O Schiano, P Thomas, L |
| author_sort | Gannoun, A |
| collection | OXFORD |
| description | The decay of <sup>187</sup>Re to <sup>187</sup>Os 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 <sup>187</sup>Re 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 <sup>187</sup>Os/<sup>188</sup>Os 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, δ<sup>11</sup>B covariations with sup>187Os/sup>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 sup>187Os/sup>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 equilibrium 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-chondric 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. |
| first_indexed | 2024-03-07T04:09:30Z |
| format | Journal article |
| id | oxford-uuid:c758ac1e-b46f-4c01-900e-8aaa47b57441 |
| institution | University of Oxford |
| language | English |
| last_indexed | 2024-03-07T04:09:30Z |
| publishDate | 2007 |
| publisher | Elsevier |
| record_format | dspace |
| spelling | oxford-uuid:c758ac1e-b46f-4c01-900e-8aaa47b574412022-03-27T06:44:24ZThe scale and origin of the osmium isotope variations in mid-ocean ridge basaltsJournal articlehttp://purl.org/coar/resource_type/c_dcae04bcuuid:c758ac1e-b46f-4c01-900e-8aaa47b57441Earth sciencesEnglishOxford University Research Archive - ValetElsevier2007Gannoun, ABurton, KParkinson, IAlard, OSchiano, PThomas, LThe decay of <sup>187</sup>Re to <sup>187</sup>Os 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 <sup>187</sup>Re 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 <sup>187</sup>Os/<sup>188</sup>Os 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, δ<sup>11</sup>B covariations with sup>187Os/sup>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 sup>187Os/sup>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 equilibrium 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-chondric 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. |
| spellingShingle | Earth sciences Gannoun, A Burton, K Parkinson, I Alard, O Schiano, P Thomas, L The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title | The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title_full | The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title_fullStr | The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title_full_unstemmed | The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title_short | The scale and origin of the osmium isotope variations in mid-ocean ridge basalts |
| title_sort | scale and origin of the osmium isotope variations in mid ocean ridge basalts |
| topic | Earth sciences |
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