• Nuclear, Particle, Astroparticle and Cosmology (NUPAC) Seminars

October 27, 2026 2:00 PM
PAIS 3205

Host:

Mousumi Roy

Presenter:

Colton Martin (UNM)

Current knowledge of Earth’s upper mantle composition primarily comes from understanding the origin of volcanic rocks. Typically, as melting occurs within the mantle, basaltic magma is produced and, being more buoyant than the surrounding material, migrates upwards through a network of plumbing systems. En route to the surface, these basaltic melts can interact with their surroundings, allowing their composition to change various ways, one of which is increased silica content. Frequently, basaltic melts that are low in silica content are considered to be “primitive” and are assumed to have a geochemical fingerprint that closely resembles their source material within the upper mantle. While primitive basalts provide insight to the heterogeneity of Earths upper mantle through their unique geochemical signatures (e.g. differences in the trace element compositions), this is contingent on such melts remaining unmodified throughout transport. Data from primitive basalts erupted at both continental and oceanic settings, however, suggest that the parental melts of these rocks underwent significant trace element modification during transport, between initial production within the mantle and eruption at the surface. Although processes which produce this alteration remain elusive, in this study we focus on the diffusive exchange of trace elements between rising melts and their surrounding wallrock.

Experimentally-determined trace element diffusivities within many minerals are typically very low, and it has been long assumed that diffusive equilibration during transport is therefore too slow during melt-migration. However, studies have suggested that grain boundaries within the surrounding wallrock may not only preferentially sequester trace elements, but also have much higher diffusivity than the mineral grains themselves. In light of these findings, recent studies show that diffusion through a polycrystalline aggregate results in anisotropic diffusion and reassess the role of diffusive exchange during melt-rock interactions. We develop an idealized 2D grain scale model to further investigate the relative importance of diffusive exchange during melt-migration. Comparing synthetic to observed whole-rock compositions from Saint Paul’s Rocks (SPR), we explore whether diffusion of trace elements during melt-migration may explain observations seen in major and trace element geochemistry. Our models suggest that within an upper mantle environment, diffusive exchange between migrating melts and surrounding wallrock is feasible on geological timescales and may play a greater role in the distribution of trace elements than previously believed. This study is a first corroboration of the idea that trace element compositions of primitive basalts should not be taken at face value, but rather, are highly nuanced and should be approached with caution.