Viscosity of the Mercurian Magma Ocean: Implications for the Mineralogical Stratigraphy of Mercury’s Juvenile Mantle and Crustal Petrogenesis

Abstract:

Mercury has a compositionally heterogeneous surface produced by different periods of igneous activity during the planet’s history, suggesting heterogeneous mantle sources. Understanding the structure of Mercury’s mantle formed during its magma ocean stage could help in developing a petrologic model for Mercury, and thus, its dynamic history in the context of crustal petrogenesis. Here we present results of falling sphere viscometry experiments on late stage Mercurian magma ocean compositions. Experiments were conducted using the Paris-Edinburgh press at the Advanced Photon Source, Argonne National Laboratory. Owing to the presence of sulfur on the surface of Mercury, two compositions were tested, one with sulfur and one without. The liquids have viscosities of 0.6-10.9 Pas, similar to an andesite at the tested conditions (1.4-6.2 GPa, 1600-2000°C).

Using the experimental results, we parameterized models that enable extrapolation of our results beyond the experimental conditions and evaluated grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, which suggests endmember outcomes for Mercury’s mantle structure. Without a flotation crust, crystals would largely remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. With a flotation crust, crystallization of the mantle would be fractional with negatively buoyant minerals sinking to form a stratified cumulate pile according to the crystallization sequence. If Mercury had a flotation crust, the compositional variation on the surface could be attributed to magmas from distinct mantle sources formed by partial mixing of an initially stratified mantle, perhaps after convective stirring or mixing after formation of density driven Rayleigh-Taylor instabilities. If there was no flotation crust, Mercury’s mantle would likely be homogeneous, in which case the surface volcanics could be produced by different degrees of partial melting of the homogeneous mantle. This work contributes predictions for the mantle’s initial state after the magma ocean state, such that new constraints on the compositions and distribution of volcanic provinces by BepiColombo may provide deeper insights into Mercury’s history.