Multiple Mantle Sources of High‐Magnesium Terranes on Mercury

Abstract

AbstractThe composition and interior structure of Mercury's mantle are poorly known but critical to understanding its magma ocean differentiation, mantle dynamics, and crustal petrogenesis. Various crustal geochemical terranes are identified on Mercury based on their Mg/Si and Al/Si values, but element abundances exhibit a weak correlation with surface reflectances. The high‐magnesium region (HMR), which includes end‐members for both reflectance spectral units (high‐reflectance plains and low‐reflectance materials (HRP and LRM)) and geochemical terranes (Mg and Al contents), contains important information about Mercury's mantle composition and dynamics. We propose that the northwestern boundary of HMR can be extended to the Sobkou basin and its ejecta deposits. Within the resolution limit of element maps, we notice that mantle materials beneath the Sobkou basin have an elevated content of magnesium, and a large area of HRP‐like materials in HMR exhibit Mg/Si greater than 0.5. Our results suggest that HMR was formed by partial melting of heterogeneous mantle sources in terms of compositions, supporting the existence of stratified and/or lateral geochemical diversities in the mantle. While typical HRP on Mercury, including the Borealis Planitia and Caloris interior plains formed by effusive volcanism around 3.7 Ga, exhibit a lower Mg/Si than the global average, the Rachmaninoff interior plains emplaced by volcanism at 1.7 Ga exhibit a Mg/Si greater than 0.5. This observation indicates that local thermal anomalies may be prolonged in the mantle, inducing a higher degree of partial melting. We also report a weak correlation between the global LRM and magnesium content in the crust.