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

• Main Author: Webb, Michelle
• Format: Thesis
• Language: unknown
• Published: 2007
• Subjects: Greenland; Kangerlussuaq
• Online Access: http://etheses.dur.ac.uk/2306/; http://etheses.dur.ac.uk/2306/1/2306_316.pdf

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 ...