A Low Viscosity Lunar Magma Ocean Forms a Stratified Anorthitic Flotation Crust With Mafic Poor and Rich Units

• Published in: Geophysical research letters Vol. 44; no. 22; pp. 11,282 - 11,291
• Main Authors: Dygert, Nick, Lin, Jung‐Fu, Marshall, Edward W., Kono, Yoshio, Gardner, James E.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 28.11.2017; American Geophysical Union
• Subjects: Anorthosite; Crystallization; Diapirs; Flotation; flotation crust; Fractionation; GEOSCIENCES; High pressure; Lava; Lunar crust; lunar magma ocean; Magma; Melts; Moon; Oceans; Paris‐Edinburgh; Plagioclase; Purity; Remote sensing; Resurfacing; Silicates; Surfacing; viscometry; Viscosity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1002/2017GL075703

Much of the lunar crust is monomineralic, comprising >98% plagioclase. The prevailing model argues the crust accumulated as plagioclase floated to the surface of a solidifying lunar magma ocean (LMO). Whether >98% pure anorthosites can form in a flotation scenario is debated. An important determinant of the efficiency of plagioclase fractionation is the viscosity of the LMO liquid, which was unconstrained. Here we present results from new experiments conducted on a late LMO‐relevant ferrobasaltic melt. The liquid has an exceptionally low viscosity of 0.22−0.19+0.11 to 1.45−0.82+0.46 Pa s at experimental conditions (1,300–1,600°C; 0.1–4.4 GPa) and can be modeled by an Arrhenius relation. Extrapolating to LMO‐relevant temperatures, our analysis suggests a low viscosity LMO would form a stratified flotation crust, with the oldest units containing a mafic component and with very pure younger units. Old, impure crust may have been buried by lower crustal diapirs of pure anorthosite in a serial magmatism scenario. Key Points New measurements establish the low viscosity of a lunar magma ocean‐relevant ferrobasalt at lunar P‐T conditions (0.22–1.45 Pa s) Plagioclase would float on a low‐viscosity lunar magma ocean to produce a crust with impure older units and very pure younger units Very pure crust in shallow impact basins and the lunar highlands is consistent with resurfacing of the Moon by lower crustal diapirs Plain Language Summary This manuscript reports the first experimental measurements of the viscosity of a late lunar magma ocean liquid. Sample studies and remote sensing suggest the Moon's crust is effectively composed of one mineral (plagioclase). The crust is thought to have formed by flotation of buoyant plagioclase on a crystallizing lunar magma ocean; whether such a pure crust can form in a flotation scenario is debated. The purity of a flotation crust crucially depends on the viscosity of the magma ocean which was previously unconstrained. Experiments were conducted at high pressure‐temperature conditions at Argonne National Laboratory to characterize viscosity. Measured viscosities are as low as or lower than geologically‐relevant silicate melts measured previously. In a two part analysis, we consider the effect of the low viscosity lunar magma ocean on the purity of the crust. We conclude that the oldest flotation crust will contain a significant mafic component, while younger portions of the flotation crust will be pure, resulting in a stratified crust with mafic poor and rich units. The high purity of the present‐day lunar crust surface can best be explained by ancient resurfacing of the crust by the rise of buoyant lower crustal diapirs.