Abstract
The role of lithospheric mantle metasomatized by CO2-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 (Sr-87/(86) Sr=0.7029, epsilon Nd = +5 to +6, Pb-206/(204) Pb = 20.4 and epsilon Hf = +5 to +8) indistinguishable from the host low-silica basalts and, except for Pb-207/Pb-204, 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 CO2-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 CO2-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 (similar to 5%) of amphibole participated in the melting process. Although not observed in the studied xenoliths, amphibole is associated with mantle metasomatism by carbonatitic or CO2-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 (CO2+H2O) melts are widespread within the Earth's lithospheric mantle and are a potential source for intraplate alkaline basaltic magmatism.