GEMOC ARC National Key Centre

Griffin, W.L., Alard, O., Pearson, N.J. and O'Reilly, Suzanne Y.
GEMOC Macquarie

The composition of the subcontinental lithospheric mantle (SCLM) has been shown to vary in a systematic way with the age of the last major tectonothermal event in the overlying crust (Griffin et al, 1999) by the detailed mapping of lithospheric sections in terms of composition, "stratigraphy" and thermal state, using xenocrystic and xenolithic material brought to the surface in volcanic rocks (kimberlites, lamproites, basalts). This implies quasi-contemporaneous formation of the crust and its underlying mantle root, and that crust and mantle in many cases have remained linked through their subsequent history. The contrasting composition of different mantle domains requires lateral contrasts in physical properties and depth for present-day lithospheric domains.

Re-Os ages of mantle-derived peridotite xenoliths have provided some answers to the timing and formation of lithosphere terranes but the results are ambiguous. In-situ analysis of PGE abundances and Re-Os isotopes in single grains of sulfides in mantle-derived rocks (Alard et al., 2000) opens the way for more precise and less ambiguous dating of mantle events.

Sulfide phases contain nearly all of the Re and Os in mantle-derived peridotites.  Detailed studies of PGE distribution and Re-Os systematics using in-situ analysis of sulfide grains in mantle-derived peridotites have now shown that most samples contain two distinct generations of sulfide: residual ones, generally enclosed in primary silicates, are S-rich MSS, have high Os (20-1000 ppm) but low Pd/Ir (0.001-1), and older Re-Os depletion ages; in contrast, interstitial sulfides which typically occur with metasomatic phases, are S-poor and  Ni±Cu-rich, with low Os, Ir but high Pd/Ir (up to 1000), and extremely radiogenic Os.

In-situ analysis of sulfides in peridotite xenoliths shows that the two types of sulfide differ significantly in their Re-Os isotopic systematics.  Type 2 sulfides typically have Os isotopic compositions ranging from asthenospheric (0.127) to highly radiogenic (0.175) values.  Type 1 sulfides typically have less radiogenic Os, and may give realistic depletion ages; e.g. sulfide inclusions in olivine macrocrysts from Udachnaya (Siberia) kimberlite lie on an isochron with published analyses of sulfide inclusions in Udachnaya diamonds. Both types of sulfide occur in many (most?) mantle peridotites. Many published "depletion ages" for mantle-derived peridotites probably represent mixtures of sulfide generations and thus have ambiguous age significance.

Meaningful interpretation of Re-Os data in terms of mantle events requires understanding of the occurrence and mobility of sulfides in mantle peridotites.  In-situ LAM-MC-ICPMS analysis is less precise than conventional techniques, except for high-Os sulfides. Spatially resolved data from LAM-MC-ICPMS analysis allow recognition of different sulfide generations and thus greatly enhance the interpretation of depletion ages and the understanding of fundamental lithospheric terranes.

REFRENCES :

Alard O., Griffin W.L., Lorand, J.P., Pearson N.J. and O'Reilly S.Y., 2000, In-situ analysis of highly siderophile trace elements and Re-Os isotopes by LAM ­ICPMS and LAM-MC-ICPMS reveal the residual and mobile nature of mantle sulfides. Goldschmidt 2000, Journal of Conference Abstracts v.5. 128-129.

Griffin W.L., O'Reilly S.Y. and Ryan C.G. 1999, The composition and origin of subcontinental lithospheric mantle: in Y. Fei C. M. Bertka and B. O Mysen (eds) Mantle Petrology: Field observations and high-pressure experimentation: A tribute to Francis R. (Joe) Boyd, The Geochemical Society, Special Publication No. 6, p 13-43.