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 the origins of such rocks are critical to a broader understanding Earth's interior. Resolving debate regarding the dynamical origin of intraplate basalts requires constraints on the relationship between the compositions of magmas and the temperatures, pressures, and compositions of their sources. Previous experimental studies have failed to reproduce major element compositions of alkalic oceanic island basalts (OIB) from partial melts of peridotite+/-CO2. 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. Evaluation of these possibilities requires new experiments to determine the compositions and proportions of liquids generated during incipient partial melting of garnet peridotite+/-CO2 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.

This project has a goal to determine the compositions of partial melts of fertile garnet peridotite at and near its solidus from 0 to 5% melting at pressures of 3 to 7 GPa. It is planned to employ modified iterative sandwich experiments (MISE), which are designed to determine accurately the composition of very low melt fraction melts. These techniques will allow determination of small-degree melts of garnet peridotite, which may be important sources of intraplate magmatism, but for there are few experimental data, owing to difficulties in analyzing high temperature, high MgO partial melts. The team will also use the MISE method to determine the influence of CO2 on near-solidus melting of garnet peridotite by conducting experiments with peridotite with CO2 concentrations (0.1 wt.%) close to those likely in natural alkali basalt source regions.