Re-os isotope constraints on the age of the Lithospheric mantle beneath western Greenland.

Abstract

Alkaline magmatic activity across Western Greenland (W.G.) provides a record of lithosphere evolution over the last 600 Ma. Ultra Mafic Lamprophyre (UML) magmatism in particular has erupted an exceptional inventory of mantle xenoliths allowing a detailed look at the lithospheric root beneath both the exposed craton and the re-worked Archean terrane (RAT) of the Kangerlussuaq region, S.W Greenland. Samples from within the re-worked Archean terrane more commonly have diopside plus garnet, i.e. they are more lherzolitic whereas highly depleted harzburgites and dunitic lithologies are prevalent amongst xenoliths erupted through the undisturbed Archean craton. W. Greenland peridotite xenoliths show a large range in bulk composition. The peridotites are MgO - rich (44-51 wt%), with most samples overlapping the range for cratonic peridotites and being generally more MgO-rich than non-cratonic or massif peridotites. In contrast SiO(_2) contents are relatively low (36-43 wt%), being at the lower end of the range for both cratonic and non-cratonic/massif peridotites, indicating that the W.G. mantle has not experienced significant SiO(_2) enrichment. The low SiO(_2) of most residual, minimally metasomatised harzburgites indicate particularly othopyroxene poor protoliths. This contributes to growing evidence that the Kaapvaal cratonic lithosphere is the exception rather than the rule in terms of craton lithosphere bulk compositions and evolution. A selection of dunitic, harzburgitic and lherzolitic lithologies have been analysed for Re-Os isotopes and PGE abundances. Os ranges from 0.86 to 5.02 ppb, scattering around the typical average for cratonic residues. Re contents range from 0.009 to 0.290 ppb, and are all less than primitive mantle. However, in extended PGE patterns Re can be seen to be anomalously high in numerous samples and has obviously experienced secondary enrichment, possibly by UML infiltration. Pt and particularly Pd demonstrate their incompatible behaviour during high degrees of melting, with Pt as low as 0.035 and Pd depleted to 0.007 ppb, comparable to highly depleted residues from other cratons (Pearson et al, 2004). The effects of metasomatism are also seen on Pd in some samples but this element is less disturbed than Re and, for some samples, (Pd/Ir) n correlates well with indices of melt depletion such as AI(_2)O(_3) consistent with high degrees of melting. Some disturbance relates to metasomatic introduction of clinopyroxene in certain rocks. Re-depletion ages for the most residual W.G. peridotites, corrected to the 600 My eruption age for the host UML, range from 1.8 to 3.0 Ga, with Archean depletion ages clearly evident in the lithosphere beneath the RAT of the Nagssugtoqidian. Archean (>2.5 Ga) ages are found in even the deepest derived (> 200 km) samples indicating that the basal lithosphere is of this age. The ages for W.G. peridotites together with pressure and temperature investigations by Sand (2007) indicate clearly that an Archean keel existed beneath the region that extended well into the diamond stability field at 600 Ma, both on the recognised craton and beneath the region of re-worked Archean crust suggesting that further exploration in this area can be targeted right across the region where UML's occur. The validity of these findings is emphasised by the recent new finds of macro-diamonds in the Kangerlussuaq region (Hudson Resources Website).