Project Title: Lunar Meteorites and the Early Evolution of the Lunar Crust
Description: The lunar crust is dominated by anorthosite, which forms much of the lunar highlands, the bright and heavily cratered areas that you can see on the moon with the naked eye. This is at least 4.35 billion years old (Barboni et al. 2017) and is thought to have formed over a ~200 million year period. The anorthosites are thicker on the farside (~30-50 km) versus the nearside of the moon where they reach zero thickness in the Mare impact basins, the dark areas visible on the moon’s surface. The anorthosites are thought to have formed from a planetary-scale, magmatic fractionation process as suggested by the original lunar magmatic ocean (LMO) model (Smith et al. 1970; Wood et al. 1970). The LMO model suggests an early, 1000km deep, molten lunar surface developed and was enriched in plagioclase. Being less dense, the plagioclase floated to form the anorthosites of the lunar highlands. There are now several models supported by increasing volumes of data that suggest more complex formation mechanisms. The onion-skin model suggests a LMO with limited circulation under an initially thin, quenched crust. This produced rapid accumulation of anorthosites and minor, interstitial, mafic liquids. Tidal overturning of the plagioclase cumulates then prolonged the crystallization times (Elkins-Tanton et al. 2011). A second model, which attempts to also explain the asymmetric nature of the lunar crustal thicknesses, suggests that circulation may be the result of a giant impact which caused early differentiation of the nearside and farside lunar surfaces (Aria et al. 2008). A third model suggests that serial magmatism, and recycling of crystallizing plagioclase into a pre-existing quenched crust, produced the variety of assemblages found in many lunar samples (Gross et al. 2014). Another recent model suggests that modification of the initial crust formed from the LMO was aided by widespread early bombardment. This in turn created a series of magma seas that differentiated to result in the observed compositional variations in the lunar highlands (Vaughan et al. 2013). In this study, we will examine a small suite of lunar meteorite samples which has recently been obtained, including several pieces of NWA 11474 and NWA 12593. These meteorites are lunar breccias and contain clasts of anorthosite and mafic minerals. The first part of the project will involve a detailed documentation of the variations in mineral compositions in these samples. This will allow the range in lunar anorthosites represented to be determined. Preliminary crystallization temperatures will also be calculated using this data. Minor elements such as Sr, Ba and rare-earth elements including Eu will also be measured. Published distribution coefficients for trace-elements in plagioclase crystallizing from a LMO (Sun et al. 2017) will be used to determine whether these samples crystallized from a single-composition magma or represent more than one phase of crystallization from several magmas. Results will be compared with other studies to help refine the models of lunar crustal formation. Students interested in this project should contact Dr. Richard Cox (richard.cox@dal.ca).
