Os isotopes in mantle-derived minerals: what can they really tell us?

The refinement of analytical techniques of laser ablation is now opening a wider window of possibilities for measuring Os isotopes in situ on discrete mantle-derived phases such as platinum-group minerals (PGM) and base-metal sulfides (BMS). A growing body of work based on the Re-Os isotopic study of these mineral phases from a wide range of ultramafic rocks, such as abyssal peridotites and mantle-derived xenoliths, has shown that the uppermost part of the lithospheric mantle is significantly heterogeneous at different length scales in terms of Os isotopes. Heterogeneities in 187 Os/188 Os of PGM at the km, hand-sample, thin-section, and even single-grain scales have recently been recognized in podiform chromitites hosted in both ophiolites and in the subcontinental mantle. Such a degree of heterogeneity is thought to reflect the creation and recycling of crustal/upper mantle materials, involving partial melting, melt percolation and metasomatism that imply the mobilization of melts/fluids within the upper lithosphere. As PGM from chromitites show a range of values and degree of heterogeneity in 187 Os/188 Os identical to those observed in BMS from their respective country-rock peridotites, melting, melt-rock reaction and/or pooling processes in the mantle do not significantly disturb the original Os isotopic signatures of the mantle. On the other hand, the observation of larger ranges of 187 Os/188 Os values in the secondary PGM of metamorphosed podiform chromitites suggests the disturbance of the Re-Os isotopic system by hydrothermal alteration, which has significant implications forthe interpretation of Os-isotope data in mantle-derived rocks.