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Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis

Harvey, J.; Dale, C.W.; Gannoun, A.; Burton, K.W.

Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis Thumbnail


Authors

J. Harvey

C.W. Dale

A. Gannoun

K.W. Burton



Abstract

Analyses of enriched mantle (EM)-basalts, using lithophile element-based isotope systems, have long provided evidence for discrete mantle reservoirs with variable composition. Upon partial melting, the mantle reservoir imparts its isotopic fingerprint upon the partial melt produced. However, it has increasingly been recognised that it may not be simple to delimit these previously well-defined mantle reservoirs; the “mantle zoo” may contain more reservoirs than previously envisaged. Here we demonstrate that a simple model with varying contributions from two populations of compositionally distinct mantle sulphides can readily account for the observed heterogeneities in Os isotope systematics of such basalts without additional mantle reservoirs. Osmium elemental and isotopic analyses of individual sulphide grains separated from spinel lherzolites from Kilbourne Hole, New Mexico, USA demonstrate that two discrete populations of mantle sulphide exist in terms of both Re–Os systematics and textural relationship with co-existing silicates. One population, with a rounded morphology, is preserved in silicate grains and typically possesses high [Os] and low [Re] with unradiogenic, typically sub-chondritic 187Os/188Os attributable to long term isolation in a low-Re environment. By contrast, irregular-shaped sulphides, preserved along silicate grain boundaries, possess low [Os], higher [Re] and a wider range of, but generally supra-chondritic 187Os/188Os ([Os] typically ⩽ 1–2 ppm, 187Os/188Os ⩽ 0.3729; this study). This population is thought to represent metasomatic sulphide. Uncontaminated silicate phases contain negligible Os (<100 ppt) therefore the Os elemental and isotope composition of basalts is dominated by volumetrically insignificant sulphide ([Os] ⩽ 37 ppm; this study). During the early stages of partial melting, supra-chondritic interstitial sulphides are mobilised and incorporated into the melt, adding their radiogenic 187Os/188Os signature. Only when sulphides armoured within silicates are exposed to the melt through continued partial melting will enclosed sulphides add their high [Os] and unradiogenic 187Os/188Os to the aggregate melt. Platinum-group element data for whole rocks are also consistent with this scenario. The sequence of (i) addition of all of the metasomatic sulphide, followed by (ii) the incorporation of small amounts of armoured sulphide can thus account for the range of both [Os] and 187Os/188Os of EM-basalts worldwide without the need for contributions from additional silicate mantle reservoirs.

Citation

Harvey, J., Dale, C., Gannoun, A., & Burton, K. (2011). Osmium mass balance in peridotite and the effects of mantle-derived sulphides on basalt petrogenesis. Geochimica et Cosmochimica Acta, 75(19), 5574-5596. https://doi.org/10.1016/j.gca.2011.07.001

Journal Article Type Article
Publication Date Oct 1, 2011
Deposit Date Sep 1, 2011
Publicly Available Date Apr 12, 2013
Journal Geochimica et Cosmochimica Acta
Print ISSN 0016-7037
Electronic ISSN 1872-9533
Publisher Meteoritical Society
Peer Reviewed Peer Reviewed
Volume 75
Issue 19
Pages 5574-5596
DOI https://doi.org/10.1016/j.gca.2011.07.001
Public URL https://durham-repository.worktribe.com/output/1505615

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Copyright Statement
NOTICE: this is the author’s version of a work that was accepted for publication in Geochimica et Cosmochimica Acta. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Geochimica et Cosmochimica Acta, 17 (19), 2011, 10.1016/j.gca.2011.07.001





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