A layer of neutrally buoyant olivine in the early stages of a deep lunar magma ocean and a possible consecutive overturn
Abstract
The moon is thought to have undergone a completely molten stage during its accretion, the lunar magma ocean. In order to understand the evolution and first differentiation of the lunar magma ocean, we performed a series of consecutive liquidus experiments at pressures of the lower half of the lunar magma ocean. In these experiments, we determined the liquidus, crystallized some amount of minerals (typically 10-20%) and then stepped to a new bulk composition representing the residual liquid after fractionation of these minerals. Mineral and melt densities were then calculated in order to decide whether minerals would float or sink. The bulk lunar composition used in this study (Taylor 1982) results in extensive early olivine crystallization with high XMg (94-90) for all experimental pressures, the liquidus temperature slightly decreasing from 1900 to 1850°C from 4.5 to 3.5 GPa. Crystallization begins at the core-mantle boundary, but calculations indicate that olivine initially floats and becomes neutrally buoyant at 3.5-3.7 GPa, leading to a stable olivine layer of several 100 km thickness at this depth. This layer should rapidly compact yielding two chemically separated magma reservoirs. Olivine crystallization is followed by orthopyroxene (1650°C, twm_fr2), minor garnet (1600°C, twm_fr3), clinopyroxene and spinel (1550°C, twm_fr3) in the lower magma ocean. Despite continuously decreasing XMg and increasing Ca/Al, further experiments indicate that the more extensively fractionated lower magma should become finally buoyant, possibly causing an overturn of the previously layered structure. Additionally, few centrifuge assisted experiments at 2.5-3.5 GPa were performed showing decreasing olivine-melt density contrasts with increasing pressure. Slightly higher pressures would be necessary to positively prove the neutral buoyancy of olivine at 3.6 GPa, currently we are improving the piston cylinder on the centrifuge to reach 4 GPa. Taylor, Stuart Ross. Planetary science: a lunar perspective. Vol. 3303. Houston: Lunar and Planetary Institute, 1982.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V33E0560K
- Keywords:
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- 1060 Planetary geochemistry;
- GEOCHEMISTRY;
- 3630 Experimental mineralogy and petrology;
- MINERALOGY AND PETROLOGY;
- 6299 General or miscellaneous;
- PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS;
- 5480 Volcanism;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS