Hydrous minerals are sinks for first row transition elements in the mantle: An experimental partitioning study

Abstract

Pyroxenites containing hydrous phases are increasingly recognised as contributing significant melt fractions to many primitive melts such as alkaline basalts, basanites, and lamproites. Although hydrous ultramafic rocks are minor components in the mantle, their low-temperature solidi compared to other mantle lithologies drives high melt productivity, making them significant contributors during fractional melting of heterogenous mantle domains. These hydrous ultramafic rocks contain phlogopite, amphibole, and apatite, and upon partial melting they produce distinct alkaline melts enriched in rare earth and high field strength elements when compared to basalts derived from peridotites. First row transition elements (FRTEs) are widely used indicators for identifying dry pyroxenite mantle lithologies in mantle sources, yet the partitioning behaviour of FRTEs between hydrous phases and alkaline melts remains poorly constrained. We present the first complete set of experimentally constrained partition coefficients for FRTEs between phlogopite, amphibole, and apatite and alkaline melts. Our results demonstrate that several FRTEs, Cr and Ni are compatible to highly compatible in phlogopite whilst Sc, Cr, Co and Ni are highly compatible in amphibole. We further demonstrate that coupling rare earth element concentrations with first-row transition elements is a powerful tool for discerning the mantle sources for basanites and lamproites globally.