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Silicate Liquid Immiscibility in a Lunar Basalt

This work is in preparation for publication in Meteoritics and Planetary Science. If you would like to know more about this project, feel free to contact me.


The sample 12032,366-18 is an unusual basalt with KREEP components and a highly- silica-rich mesostasis. I performed experiments that indicate that the mesostasis likely formed through SLI, which has implications for the formation of late-stage, accessory phases like apatite or zircon. I propose to continue this investigation and to submit a manuscript on these analyses and experiments of 12032,366-18. This project is innovative; mimicking a lunar basalt and using experimental petrology to gain insight into SLI and late-stage crystallization makes this work one of the first analog experiments, combining the study of natural SLI and experimental SLI.

Basaltic volcanism is a process that occurs on all terrestrial bodies in the Solar System (Basaltic Volcanism Study Project, 1981; Melosh, 2011). The Apollo missions brought back many lunar basalts that help elucidate the variety of volcanism that occurs on the Moon and on other terrestrial bodies. Studying this Apollo basalt, 12032,366-18, helps to better understand late-stage KREEP-bearing (potassium, rare earth element, and phosphorous-bearing) basalts, and the impact of silicate liquid immiscibility (SLI) on the evolution of lunar magmas (Warren and Wasson, 1979; Veksler et al., 2007).

The last crystallizing portions of igneous rocks are enriched in incompatible elements (Yoder, 1979). These late crystallizing portions are sometimes called mesostasis, and are where you’d expect to find important accessory phases such as apatite and zircon (Yoder, 1979). These phases are important for understanding the volatile content of the Moon, and for geochronology. Determining where these phases occur within a rock will help future sample return missions to the Moon and other terrestrial bodies.

SLI is a process by which a magma spontaneously splits into two liquids, an iron-enriched Lfe and a silica-enriched Lsi (Figure 1; Roedder, 1951; Longhi, 1990; Thompson et al., 2007; Veksler et al., 2007; Charlier and Grove, 2012; Hamann et al., 2017). These two liquids evolve away from each other as the temperature lowers (Roedder, 1951). These two liquids have very different viscosities, due to their varying SiO2 content, and different densities, due to their varying FeO content. Additionally, minor and trace elements partition preferentially into one liquid or the other (Veksler et al., 2007). The partition of elements and the differences between the two liquids could potentially cause macro-scale heterogeneities in the mineral assemblage that results from the mesostasis, if the two liquids separate over some distance (Yoder, 1979, Veksler et al., 2007).

Sample 12032,366-18 was collected during the Apollo 12 mission, which landed in western Oceanus Procellarum at 3.0oS, 23.4oW. Ejecta from both Kepler Crater and Copernicus Crater may reach the Apollo 12 landing site and a bright ray from Copernicus directly superposes the landing area, allowing for material from beneath Mare Cognitum to be present at the Apollo 12 site (Barra et al., 2006).

The basalt in this study is referred to as a KREEP-bearing basalt. KREEP is thought to form in either of two methods: partial melting of the Moon’s mantle, or extensive fractional crystallization of a magma ocean (Warren and Wasson, 1979). In this particular sample, the presence of KREEP components may indicate the specific extent to which the magma that formed 12032,366- 18 was differentiated. On the basis of the 40Ar-39Ar dating, we can infer that 12032,366- 18 came to the site as ejecta from a relatively young basalt flow, possibly as young as 2.7 Ga, with a disturbance in the range of 500-700 Ma (Jolliff et al., 2005; Barra et al., 2006). This age supports the idea that the basalt fragment could be ejecta from an impact event such as Kepler. From orbital data, it is clear that rocks in Oceanus Procellarum are enriched in Th (Jolliff et al., 2000), by studying sample 12032,366-18 we can learn about the processes associated with the apparent enrichment of thorium (and KREEP elements in general) in mare basalts of Oceanus Procellarum that were not directly sampled by Apollo missions, and the effect that SLI may have had on these magmas. High degrees of magmatic differentiation may have led to localized production of felsic materials by SLI on a scale larger than mesostasis (μm to cm), and perhaps even on a scale that led to silicic segregation on a larger scale (km), such as excavated by Aristarchus Crater or erupted at the Mairan Domes (Jolliff, 2004).

"You develop an instant global consciousness, a people orientation, an intense dissatisfaction with the state of the world, and a compulsion to do something about it. From out there on the moon, international politics look so petty." – Edgar Mitchell