New Group Paper: Evidence of chemical heterogeneity within Lunar anorthosite parental magmas

The Moon’s crust is very old. Formed over 4.3 billion years ago, it represents the primary lunar crust. Made up of a rocks called anorthosite (> 95 modal % Ca-rich plagioclase; see below), it makes up the light-grey areas seen on the Moon’s surface. Understanding how these anorthosites formed is important for understanding planetary crustal formation models and modification process; a record of which is no longer accessible on geologically active bodies such as the Earth or Mars.

62275

Sample 62275: A lunar anorthosite collected during Apollo 16

Models of how this crust was formed was first developed in the 1970’s following the return of anorthosite fragments during Apollo 11. These models state that the moon had an initial magma ocean. During crystallization of this magma ocean, dense minerals such as olivine and pyroxene sank forming the Moon’s mantle, while the lighter minerals relative to the magma, such as plagioclase, floated. It was the accumulation of plagioclase by floatation that is argued to have formed the Moons primary anorthosite crust. This general model has held true over the last 50 years, however recently, the fine details about magma ocean crystallization have been actively debated (see Pernet-Fisher and Joy 2016 and Gross and Joy 2016 for a review of the different proposed models).

In our new paper, we present trace-element abundances for plagioclase crystals from a suite of lunar anorthosites sampled by Apollo 16. Trace-elements (elements that are present within a mineral < 1000 ppm) are useful geochemical tools when ratioed against each other. This is because trace-element ratios are not affected by magmatic crystallization and thus can provide information about the chemistry of the crystalizing magma ocean.

Canonical models of magma ocean solidification do not predict variations on trace-element ratios; however, this is not what we observe. Some trace-element ratios display significant variations that cannot be accounted for the crystallization of olivine, pyroxene, and plagioclase from a magma ocean. To account for these differences, we propose that overturn events occurred within the lunar mantle. During an overturn event hot mafic cumulates will be brought the base of the lunar anorthositic crust, triggering decompression melting (see figure below). These melts will be compositionally different to the composition of the lunar magma ocean. Numerical modelling suggests that mixtures of these decompression generated melts with the magma ocean are able to account for the range of trace-element ratios we observed in the anorthosite samples.

fan model

Schematic illustrating how melts generated by decompression melting might mix with a plagioclase-saturated magma ocean.

The full citation for the new paper is below and it can be accessed here:

Pernet-Fisher, J. F., Deloule, E., & Joy, K. H. (2019). Evidence of chemical heterogeneity within lunar anorthosite parental magmas. Geochimica et Cosmochimica Acta.

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Further Lunar Sample Resources

NASA Apollo sample curation office https://curator.jsc.nasa.gov/lunar/index.cfm

LPI Lunar Sample resources http://www.lpi.usra.edu/lunar/samples/

Virtual Microscope http://www.virtualmicroscope.org/content/moon-rocks

Lunar Meteorite List  http://meteorites.wustl.edu/lunar/moon_meteorites_list_alumina.htm

Why we should go back and explore the Moon in the future https://earthandsolarsystem.wordpress.com/2017/05/12/the-moon-putting-an-end-to-been-there-done-that/

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About John Pernet-Fisher

John is a researcher within the Isotope Geochemistry and Cosmochemistry group, and has a love of all things lunar!
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