The origin of hydrous amphibole in the subcontinental lithospheric mantle beneath the Southern Alps of New Zealand

Abstract

Peridotite xenoliths provide valuable insight into lithospheric mantle conditions, composition, and evolution. The origin of amphibole in the lithospheric mantle and whether amphibole melts to produce alkaline intraplate magmas is a highly debated topic. Large areas of the lithospheric mantle forming Earth’s youngest continent, Zealandia, have chemical compositions comparable to Archean mantle lithosphere but Re-Os isotope and bulk rock data indicate that lithosphere stabilisation occurred in the Mesozoic. Some areas of this refractory lithospheric mantle have been metasomatized, with one of the clearest occurrences being MARID-like veins in xenoliths in alkaline intraplate magmas in the Southern Alps of New Zealand. These xenoliths contain abundant veinlets composed of amphibole, phlogopite, clinopyroxene and apatite in rocks that have average olivine compositions exceeding Mg# 92 and spinel Cr# 70. The latter indicates that these peridotites have undergone >25% partial melting prior to metasomatism.Using a combination of quantitative scanning electron beam methods, trace element and in-situ laser ablation inductively coupled mass spectrometry (LA-ICP-MS) analysis, and conventional 87Sr/86Sr isotope analysis by solution, we seek to establish the origin of hydrous phases in this mantle lithosphere. The benefit of inspecting formerly highly depleted peridotites is that the chemistry of the metasomatic agent, which is typically enriched in incompatible elements, is less diluted than in the cases where melts infiltrate fertile lithosphere. Although minor Fe-diffusion has occurred within the studied host rock, the bulk compositions of the veins are picro-basaltic. The mica separates, measured by solution chemistry, are today more radiogenic than the in-situ diopside and amphibole analyses, however, we find that the age-corrected ~25 Ma, 87Sr/86Sr initials fall in a tight cluster of very depleted mantle-like ratios from 0.7027 to 0.7056. Although the fluids appear to have sub-alkaline bulk compositions, the amphibole trace elements are enriched in HFSE and lack depleted Nb components.The data suggests that these basaltic veins are not arc-related and do not derive from melting of subducted sediment, but also have no direct genetic link to the host alkaline melts. If this latter interpretation is correct, then the injection of hydrous veins was not part of a continuous process that resulted in alkaline magmatism, although they may have been subsequently melted to give rise to alkaline magmas with depleted mantle-like isotopic characters.