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Harvey, J.; Gannoun, A.; Burton, K. W.; Schiano, P.; Rogers, N. W. and Alard, O.
(2010).
DOI: https://doi.org/10.1016/j.gca.2009.09.031
Abstract
Spinel lherzolite xenoliths from Mont Briançon, French Massif Central, retain evidence for multiple episodes of melt depletion and melt / fluid infiltration (metasomatism). Evidence for primary melt depletion is still preserved in the co-variation of bulk-rock major elements (MgO 38.7 - 46.1 weight%; CaO 0.9 - 3.6 weight%), and many samples yield unradiogenic bulk-rock Os isotope ratios (187Os/188Os = 0.11541-0.12626). However, many individual xenoliths contain interstitial glasses and melt inclusions that are not in equilibrium with the major primary minerals. Incompatible trace element mass balance calculations demonstrate that metasomatic components comprise a significant proportion of the bulk-rock budget for these elements in some rocks, ranging to as much as 25% of Nd and 40% of Sr Critically, for Re-Os geochronology, melt / fluid infiltration is accompanied by the mobilisation of sulfide. Consequently, bulk-rock isotope measurements, whether using lithophile (e.g. Rb-Sr, Sm-Nd) or siderophile (Re-Os) based isotope systems, may only yield a perturbed and / or homogenised average of these multiple events. Osmium mass balance calculations demonstrate that bulk-rock Os in peridotite is dominated by contributions from 2 populations of sulfide grain: (i) interstitial, metasomatic sulfide with low [Os] and radiogenic 187Os/188Os, and (ii) primary sulfides with high [Os] and unradiogenic 187Os/188Os, which have been preserved within host silicate grains and shielded from interaction with transient melts and fluid. The latter can account for > 97% of bulk rock Os and preserve geochronological information of the melt from which they originally precipitated as an immiscible liquid. The Re-depletion model ages of individual primary sulfide grains preserve evidence for melt depletion beneath the Massif Central from at least 1.8 Gyr ago despite the more recent metasomatic event(s).
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About
- Item ORO ID
- 18541
- Item Type
- Journal Item
- ISSN
- 0016-7037
- Project Funding Details
-
Funded Project Name Project ID Funding Body Not Set NER/A/S/2001/00538 NERC (Natural Environment Research Council) - Academic Unit or School
-
Faculty of Science, Technology, Engineering and Mathematics (STEM) > Environment, Earth and Ecosystem Sciences
Faculty of Science, Technology, Engineering and Mathematics (STEM) - Copyright Holders
- © 2009 Elsevier Ltd.
- Depositing User
- Colin Smith