Experimental Investigation of Mercury's Magma Ocean Viscosity: Implications for the Formation of Mercury's Cumulate Mantle, Its Subsequent Dynamic Evolution, and Crustal Petrogenesis. Issue 11 (12th November 2021)

• Record Type: Journal Article
• Authors: Mouser, Megan D.; Dygert, Nicholas; Anzures, Brendan A.; Grambling, Nadine L.; Hrubiak, Rostislav; Kono, Yoshio; Shen, Guoyin; Parman, Stephen W.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-11-12
• Subjects: Mercury -- viscosity -- Paris‐Edinburgh -- magma ocean -- viscometry -- petrogenesis; Planets -- Periodicals; Geophysics -- Periodicals; 559.9
• DOI: 10.1029/2021JE006946
• Languages: English
• ISSNs: 2169-9097

Abstract

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 would need to endure mixing processes to produce the present surface compositions after melting. Key Points: We experimentally determined the viscosity of Mercurian magma ocean analogue compositions New predictive viscosity models for Mercury analogue liquids are presented The presence of a flotation crust could cause a stratified cumulate mantle to form from a cooling magma ocean