Near Solidus Partial Melting of Garnet Peridotite and the Origin of Alkali Olivine Basalt

Alkali basalts and related rocks (basanites, nephelinites) are common products of magmatic processes beneath oceanic islands and seamounts, but their origin is poorly understood. Because such rocks hold key information about geochemical and geodynamical processes in Earth's mantle, constraints on their genesis are critical to our broader understanding of the interior of the planet. Presently, there remains considerable debate and uncertainty regarding the thermal vigor of the mantle sources of oceanic island basalts (OIB) and the possibility that recycled materials - ancient crust that has been returned to the mantle via subduction, as well as volatile components such as H2O and CO2 - was involved in their genesis. Resolving these debates requires constraints on the relationship between the compositions of magmas and the temperatures, pressures, and compositions of their sources.

This proposal will support an extension of recent experimental work by the team that has shown that small-degree partial melts of 'typical' garnet peridotite - the material thought to constitute most of the upper mantle - cannot account for the compositions of oceanic island basalts. This means either that experiments have not been conducted under the right conditions or that an additional (pyroxenitic or metasomatized peridotitic) lithology is required in the source of OIB. They will continue to use an iterative experimental method ('MISE'), developed and improved over the last funding period, to determine the composition of small degree partial melts of garnet lherzolite and to explore the effects of pressure, melt fraction, and source enrichment in CO2, K2O, H2O and FeO on the compositions of near-solidus partial melts. These new experiments will allow for a better determination of the compositions and proportions of liquids generated during incipient partial melting of garnet peridotite. If alkali basalts originate from small-degree melting of garnet peridotite, and if the temperature and pressure of melting can be constrained by major element chemistry of plausible parental liquids, the experiments will relate the petrologic character of OIB to the depth of melting and mantle potential temperature in their source. A second set of experiments will be aimed at understanding the sources and conditions of melting of basalts from oceanic island based on their trace element compositions. In particular, first row transition elements (FRTE) have emerged as important probes of the origins of OIB. Concentrations of Ni and Ti as well as Mn/Fe and Zn/Fe ratios are distinct in OIB as compared to MORB and may require contributions from pyroxenite or from compositionally modified peridotite. Consequently, it is proposed to measure partition coefficients between garnet lherzolite minerals (olivine, pyroxenes, and garnet) of FRTE from small degree partial melting experiments and from crystallization experiments at 3-5 GPa.