ALBERT

Description: The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole-rock and mineral major element geochemistry, whole-rock trace element chemistry and Mössbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyroxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modified by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those represented in the Mount Morning xenolith suite has contributed to the enriched trace element (and isotope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30% melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in the harzburgites (20–30%), whereas plagioclase lherzolite mineral data suggest 〈15% melt depletion. The lherzolite (spinel and plagioclase) xenolith whole-rocks have compositions indicating 〈20% melt depletion, consistent with Proterozoic to Phanerozoic cratonization ages, and have mantle-normalized trace element patterns comparable with typical depleted mid-ocean ridge mantle. All peridotite xenoliths have undergone a number of melt–rock reaction events. Melting took place mainly in the spinel peridotite stability field, but one plagioclase peridotite group containing high-sodium clinopyroxenes is best modelled by melting in the garnet field. Median oxygen fugacity estimates based on Mössbauer spectroscopy measurements of spinel and pyroxene for spinel-facies conditions in the rifted Antarctic lithosphere are –0·6 log fO 2 at Mount Morning and –1·0 ± 0·1 (1) log fO 2 for all of Victoria Land, relative to the fayalite–magnetite–quartz buffer. These values are in good agreement with a calculated global median value of –0·9 ± 0·1 (1) log fO 2 for mantle spinel-facies rocks from continental rift systems.

Description: The role of lithospheric mantle metasomatized by CO 2 -bearing melts in the genesis of HIMU-like alkaline intraplate basalts is investigated using a suite of peridotite xenoliths from New Zealand. The xenoliths have Sr–Nd–Pb–Hf isotope compositions ( 87 Sr/ 86 Sr = 0·7029, Nd = + 5 to + 6, 206 Pb/ 204 Pb = 20·4 and Hf = +5 to +8) indistinguishable from the host low-silica basalts and, except for 207 Pb/ 204 Pb, overlapping with the HIMU mantle reservoir. Laser line scans across grain boundaries in the xenoliths show, however, that the host magma contribution is restricted to minor degrees of melt infiltration along grain boundaries during ascent, with the distinctive peridotite isotopic compositions having been imparted earlier by mantle metasomatism. Two mantle metasomatic styles are distinguished from pyroxene trace element concentrations (in particular, rare earth elements, Ti, Zr and Hf) and are interpreted to be the result of reaction of peridotite with CO 2 - bearing magmas. The occurrence of two subtly chemically different but isotopically indistinguishable styles of metasomatism in rocks with the same equilibrium temperatures within the same mantle column may be due to separate volatile-rich melts formed by different degrees of melting of a deeper carbonated peridotitic ± pyroxenitic source, or due to metasomatism having been imparted to different degrees on a variably depleted protolith. In either case, the formation of the HIMU-like enriched lithospheric mantle was achieved by percolation of volatile-rich melts, which probably rose from the asthenosphere. Melt modelling of representative depleted and subsequently enriched samples shows that low-degree melting of a CO 2 -bearing melt-metasomatized peridotite could yield a melt with a trace element composition very similar to that of the Zealandia HIMU-like alkaline basalts, but only if small volumes (~5%) of amphibole participated in the melting process. Although not observed in the studied xenoliths, amphibole is associated with mantle metasomatism by carbonatitic or CO 2 -bearing melts elsewhere in the world and has been found as xenocrysts with HIMU-like isotope compositions in some Zealandia basalts. The melt modelling results also imply that amphibole could buffer the trace element budgets of a low-degree melt regardless of the source peridotite composition; therefore, provided that hydrous metasomatized lithospheric mantle can be perturbed to melt, the contribution of amphibole would explain the similarities of alkaline ocean island basalt-like magmas in continental and oceanic settings. HIMU-like reservoirs formed by percolation of young volatile-rich (CO 2 + H 2 O) melts are widespread within the Earth’s lithospheric mantle and are a potential source for intraplate alkaline basaltic magmatism.