Experimental Investigation of Mercury's Magma Ocean Viscosity: Implications for the Formation of Mercury's Cumulate Mantle, Its Subsequent Dynamic Evolution, and Crustal Petrogenesis

Abstract

Mercury has a compositionally diverse surface that was produced by different periods of igneous activity suggesting heterogeneous mantle sources. Understanding the structure of Mercury's mantle formed during the planet's magma ocean stage could help in developing a petrologic model for Mercury, and thus, understanding its dynamic history in the context of crustal petrogenesis. We present results of falling sphere viscometry experiments on late-stage Mercurian magma ocean analogue compositions conducted at the Advanced Photon Source, beamline 16-BM-B, 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 0.6-3.9 (sulfur-bearing; 2.6-6.2 GPa) and 0.6-10.9 Pa•s (sulfur-free; 3.2-4.5 GPa) at temperatures of 1600-2000°C. We present new viscosity models that enable extrapolation beyond the experimental conditions and evaluate grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, suggesting endmember outcomes for Mercury's mantle structure. With a graphite 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. Without a flotation crust, crystals may remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. In the context of these endmember models, the surface could result from dynamical stirring or mixing of a mantle that was initially heterogeneous, or potentially from different extents of melting of a homogeneous mantle.Plain Language Summary This study explores the solidification of Mercury's magma ocean via viscosity experiments on a late-stage magma ocean liquid. Remote sensing from an orbital mission to Mercury revealed the planet has multiple geologic provinces including Borealis Planitia and the Intercratered Plains-Heavily Cratered Terrain, which differ compositionally. We measured the viscosity of compositions analogous to Mercury's late-stage magma ocean using high-pressure and high-temperature experiments in a Paris-Edinburgh press at the Advanced Photon Source, beamline 16-BM-B, Argonne National Laboratory. The viscosities of the experimental liquids are comparable to an andesite, which has an intermediate silica content relative to the range of igneous rocks. Using the experimental data, we developed predictive models so that the viscosity of the compositions can be estimated at different conditions. Additional calculations determined how crystals would grow in the magma ocean and if they would sink, float, or be entrained in the liquid. If Mercury evolved to have a more compositionally homogeneous mantle, the different surface compositions could potentially have been produced through different extents of mantle melting. If the mantle evolved to be compositionally heterogeneous, mineral layers in the mantle w...

Multiple Mantle Sources of High‐Magnesium Terranes on Mercury

“…If a graphitic primary crust was formed, magma ocean crystallization should have formed compositional stratification in the mantle, with ultramafic materials at the base and increasingly incompatible‐element‐enriched materials near the surface, for example, a basal layer of dunite, overlain by layers of harzburgite, lherzolite/wehrlite, and gabbro (Brown & Elkins‐Tanton, 2009; McCoy et al., 2018; Mouser et al., 2021). Notably, a lherzolite layer may not be a direct product of magma solidification but could be formed very early in the history by gravitational overturn between a pyroxenite layer and the underlying harzburgite (Mouser et al., 2021). This event possibly induced widespread volcanism across the planet, forming secondary crust that has largely been destroyed by subsequent impacts and volcanism.…”

“…While different geochemical terranes on Mercury are usually interpreted as an indicator of a compositional heterogenous mantle (Frank et al., 2017; McCoy et al., 2018; Peplowski & Stockstill‐Cahill, 2019), it is currently unknown whether or not the differentiation of a hypothesized magma ocean had formed compositionally layering in the mantle (Mouser et al., 2021). Possible primordial compositional layering in the mantle, as well as other compositional heterogeneities introduced by late‐accreted large impactors (Rivera‐Valentin & Barr, 2014), might have been largely preserved, since convective cells in the mantle of Mercury may have limited sizes (due to the thin silicate shell) to homogenize mantle materials (Hauck et al., 2013).…”

“…The primordial graphite was perhaps mixed with early volcanic products by impact bombardment, explaining both the stratigraphy and reflectance spectra of LRM (Klima et al., 2018; Murchie et al., 2015). If a graphitic primary crust was formed, magma ocean crystallization should have formed compositional stratification in the mantle, with ultramafic materials at the base and increasingly incompatible‐element‐enriched materials near the surface, for example, a basal layer of dunite, overlain by layers of harzburgite, lherzolite/wehrlite, and gabbro (Brown & Elkins‐Tanton, 2009; McCoy et al., 2018; Mouser et al., 2021). Notably, a lherzolite layer may not be a direct product of magma solidification but could be formed very early in the history by gravitational overturn between a pyroxenite layer and the underlying harzburgite (Mouser et al., 2021).…”

“…We compare our results to existing models of mantle structures of Mercury (i.e., compositionally-layered and homogenous mantle structures; Mouser et al, 2021) to evaluate their consistencies and further implications to mantle structures and dynamics. Our observations suggest that compositional layering in the mantle of Mercury, if existed, should feature highly uneven boundaries and/or local thermal anomalies (Figure 7a).…”

“…As revealed by the X‐Ray Spectrometer (XRS; Schlemm et al., 2007) and the Gamma‐Ray and Neutron Spectrometer (Goldsten et al., 2007) onboard the MErcury Surface, Space ENvironment, GEochemical, and Ranging (MESSENGER) spacecraft (Solomon et al., 2001), the crust of Mercury is comprised of several geochemical units that have different abundances of rock‐forming elements (Lawrence et al., 2017; McCoy et al., 2018; Peplowski & Stockstill‐Cahill, 2019; Peplowski et al., 2015; Vander Kaaden et al., 2017; Weider et al., 2015). Petrological modeling revealed that the crustal geochemical diversity of Mercury could be formed by different degrees of partial melting of a common mantle reservoir, and/or melting of a geochemically heterogeneous (vertically and/or laterally) mantle (Beuthe et al., 2020; Charlier et al., 2013; McCoy et al., 2018; Mouser et al., 2021; Namur & Charlier, 2017; Namur et al., 2016; Weider et al., 2015). However, the composition(s) and interior structures of the mantle of Mercury are not confirmed (Mouser et al., 2021; Namur et al., 2016).…”