Unravelling the effects of primary melt depletion and metasomatism in the lithospheric mantle

Details Unravelling the effects of primary melt depletion and metasomatism in the lithospheric mantle Unravelling the effects of primary melt depletion and metasomatism in the lithospheric mantle

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja....13845h&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #13845

Computer Science

Scientific paper

Osmium behaves as a highly compatible element during mantle melting and is preferentially retained in the residue, whereas Re is moderately incompatible and enters the melt. The Re-Os isotope system thus offers unique information on the timing and nature of melt depletion in the mantle, that is often considered to be immune from secondary metasomatic processes. For the system to be perturbed either Re or "exotic" Os must be added during some later event. However, the Os budget in mantle rocks is almost entirely controlled by sulphide, and thus the integrity of Os is fundamentally dependent upon the behaviour of sulphide. In this study we have reexamined the petrology and geochemistry of a classic suite of spinel lherzolite xenoliths from the Massif Central, France. These samples preserve a range of fertility (0.85 - 3.61 CaO wt %) attributed to variable melt extraction. Equilibration temperatures of 900^oC-1100^oC, and minimum equilibration pressures of 1.3Gpa (from geobarometry and fluid inclusions) indicate that these xenoliths last equilibrated under upper mantle conditions. Whole rock Re-Os data yields a range of 187Os/188Os ratios from 0.115 - 0.125 which correlate well with indices of fertility, consistent these rocks having been simply affected by melt depletion. However, A detailed study of secondary melt inclusions within the silicate phases has revealed that not only are the xenoliths contaminated by exotic, small percentage highly silicic melts but also that sulphide blebs are found in co-genetic relationships with CO_2 rich, silica rich and composite melt inclusions. That, at least some, of the Os in these rocks is hosted in metasomatic sulphide is consistent with in-situ Os isotope data from Mont Briancon (1). This shows that within a single rock only sulphides trapped in olivine preserve unradiogenic compositions, whereas interstitial, metasomatic sulphides are more radiogenic. Preliminary Re-Os data for olivine also yields unradiogenic values relative to the whole rock, which suggests that this phase may also have been affected by metasomatism. In this case, radiogenic values in the whole-rock samples reflect metasomatic enrichment, not primary depletion. (1) Alard, O. et al. Earth Planet. Sci. Letts., 203, 651-663 (2002).

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