Water influences geodynamic processes such as melting, deformation and rheology. Yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savaiʻi and 2 dunite xenoliths from Taʻū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), major and trace element concentrations. Clinopyroxenes from the Taʻū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savaiʻi peridotites are highly depleted harzburgites (melt depletion ≥17%). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element (LREE) enrichments. However, despite metasomatism the H2O concentrations in olivines (0 to 4 ppm H2O) and orthopyroxenes (17 to 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than other incompatible trace elements in these samples. Transects across mineral grains show generally homogenous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai‘i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere.

The viscosity of magma plays a crucial role in the dynamics of the Earth: from the crystallization of a magma ocean during its initial stages to modern-day volcanic processes. However, the pressure-dependence behavior of viscosity at high pressure remains controversial. In this study, we report the results of first-principles molecular dynamics simulations of basaltic melt to show that the melt viscosity increases upon compression along each isotherm for the entire lower mantle after showing minima at ~6 GPa. However, elevated temperatures of the magma ocean translate to a narrow range of viscosity, i.e., 0.01–0.03 Pa.s. This low viscosity implies that the crystallization of the magma ocean could be complete within a few million years. These results also suggest that the crystallization of the magma ocean is likely to be fractional, thus supporting the hypothesis that present-day mantle heterogeneities could have been generated during the early crystallization of the primitive mantle.

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