Ten samples from a suite of spinel peridotite xenoliths found in alkali basalts from the Anakies and Mt Porndon, Newer Volcanics Province, western Victoria have been analysed for Re and Os concentrations and Os isotopic composition by isotope dilution N-TIMS following a low-blank, Carius tube digestion/equilibration procedure. These subcontinental lithospheric mantle (SCLM) xenoliths were chosen because they display a negative correlation between whole rock concentrations of compatible elements (MgO, Ni) and moderately incompatible elements (V, Sc, Al), suggesting that they represent residues from variable degrees of partial melting or mixtures of refractory harzburgites and picritic or basaltic melts.
All of the spinel lherzolite-harzburgite xenoliths analysed from Mt Porndon have high Os concentrations and low Re/Os ratios (0.93-3.72 ppb and 0.0066-0.0701, respectively), within the range of refractory, melt-depleted peridotite xenoliths from the Archaean Kaapvaal Craton (Walker et al., 1989), southern Africa and have gOs values ranging from -0.7 (spinel lherzolite) to -4.3 (harzburgite). Os isotopic composition is positively correlated with Al2O3 and CaO. In contrast to the oldest exposed crust in Victoria, represented by Cambrian greenstone belts, one harzburgite xenolith has
a significantly older late Proterozoic Re-Os model age
(TRD = 760 Ma; TMA = 900 Ma). This harzburgite has also undergone post melt depletion LREE-enrichment (147Sm/144Nd = 0.1078) and the minimum TDM model age for this later event is also late Proterozoic (650Ma). Thus crust formed during this Proterozoic mantle melting and SCLM formation event probably underlies Victoria. This is supported by the presence of one population of inherited zircons in Palaeozoic granites in S.E. Australia with U-Pb ages of 1000Ma. Most peridotite xenoliths, however, have younger minimum TRD model ages of 300-700 Ma. This range in ages could be due to development of residues with variable Re/Os during a single Proterozoic melting event. Alternatively the age range could suggest that growth of SCLM was episodic and coupled to crustal growth, with younger TRD ages being correlated with magmatism during the development of the Cambrian-Devonian Lachlan Fold Belt in S.E. Australia. To date, no peridotites have yielded model ages similar to possible older crustal components infered from Nd TDM model ages of 1300-2300 Ma for Palaeozoic granites and U-Pb ages for a second population of inherited zircons (1150-3350 Ma). The preservation of Palaeozoic and Proterozoic Re-Os TMA model ages in a volcanically active region indicates that Cainozoic basaltic volcanism (Older and Newer Volcanics) associated with separation of the southeastern Australian margin from Antarctica and the Lord Howe Rise has not completely reworked the continental lithosphere.
Re-Os isotopic analyses of two spinel lherzolites from the Anakies have yielded near-chondritic Re/Os ratios (0.0645 to 0.0710) and gOs values (+1 to +2), consistent with the derivation of these xenoliths from recently accreted SCLM, potentially a result of conductive cooling of the asthenosphere during modern lithospheric extension. Re-Os isotopic data for another fertile spinel lherzolite from the Anakies document the presence of enriched components within the SCLM. A clinopyroxene banded and fertile lherzolite (A13) has given both very radiogenic (+8.5 to +13) as well as sub-chondritic (-1.8) gOs values, suggesting that banding may be the result of intermingling/mixing of old, refractory harzburgite with ancient melts (basalts or picrites). The high gOs value of lherzolite A13 is not supported by measured Re/Os, demonstrating that the source of this xenolith has experienced a multi-stage history of enrichment/depletion. Ancient subduction-related recycling of Re-rich oceanic crust into the upper mantle or intra-mantle melt metasomatism have both been invoked to explain similar enriched Os isotopic compositions reported for some HIMU ocean-island basalts (Hauri and Hart, 1993). Modelling suggests that enrichment of the SCLM with basaltic melt can yield the required high gOs value of the most radiogenic lherzolite if melt addition occurred at ca. 500-600 Ma. This time period is consistent with a TCHUR model age of 560 Ma for a Victorian garnet metapyroxenite mantle xenolith (with high gOs (+4.7)), suggesting that basaltic melts traversed the SCLM during the Cambrian and may have been temporally associated with greenstone belt development in southeastern Australia.
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