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New insights into the Re-Os systematics of sub-continental lithospheric mantle from in situ analysis of sulphides |
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| Authors: | Olivier Alard William L Griffin Norman J Pearson |
| Institution: | a ARC National Key Center for the Geochemical Evolution and Metallogeny Of Continents (GEMOC), School of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia b Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK c CSIRO Exploration and Mining, P.O. Box 136, North Ryde, NSW 1069, Australia d Laboratoire de Minéralogie, Muséum National, d’Histoire Naturelle de Paris, 61 rue Buffon, 75005 Paris, France |
| Abstract: | The Os isotopic compositions of mantle rocks generally are considered to be established during melt-depletion events and to be robust to subsequent disturbances (e.g. metasomatism). Consequently, Os isotopes are used to date the main melting event that a mantle section has undergone, i.e. transformation of fertile asthenospheric material into a depleted, buoyant lithosphere. However, Os resides almost entirely in Fe-Ni-Cu sulphides, which can be very mobile under mantle conditions. In situ laser ablation multi-collector ICP-MS measurement of Re/Os isotopic ratios in sulphides from spinel peridotite xenoliths demonstrates that whole-rock Os-isotope signatures record the mixing of multiple sulphide populations. Sulphides residual after melting events have unradiogenic Os isotopic compositions reflecting ancient melt depletion; those introduced by later metasomatism events contain more radiogenic Os. Therefore, the whole-rock Os isotopic signature can be strongly altered by metasomatic processes, and studies of mantle-derived xenoliths show that such disturbance is quite common in the lithospheric mantle. Because melt-depletion ages estimated from individual sulphide inclusions are systematically older than those obtained from whole-rock analysis, caution is essential in the interpretation of the Os model ages derived from whole-rock analysis, and their use and abuse in geodynamic models. This work suggests that sulphide could become a key phase in unravelling the formation and evolution of the lithosphere. |
| Keywords: | Re/Os siderophile elements sulfides mantle metasomatism in situ |
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