LUNAR SURFACE MG # DISTRIBUTION AND MAGMA OCEAN CRYSTALLIZATION

Recent results from the Japanese Kaguya (SELENE) mission [1], and analysis of lunar meteorites [2,3] suggest that the lunar highlands are much more heterogeneous than previously thought. Specifically, there is a longitudinal gradient in magnesium number (Mg#) that needs to be explained. During magma ocean crystallization, the Mg# of anorthosite varies with time as iron and magnesium fractionate between liquid and solid phase. Understanding the observed hemispherical Mg# trend is thus linked to the problem of understanding the time evolution of nearand farside crustal thickness. In this presentation, we review the petrological character of the Mg# variation trend and report possible scenarios to explain its existence. Introduction. The lunar nearside and farside differ in terms of volcanic activity, crustal thickness, elemental abundances, but the highlands have long been thought to be homogeneous on both hemispheres. Recent analysis using Kaguya (SELENE) spectral profiler showed that there is a significant difference in magnesium content relative to iron (Mg#) between the two hemispheres, as can be seen in Fig. 1 [1]. Iron is more incompatible than magnesium (i.e., stays preferentially in the melt), therefore a higher Mg# implies crystallization from a less evolved magma ocean. However, such an observation could be explained by different scenarios: (1) a homogeneous primordial magma ocean followed by an asymmetric crustal crystallization, (2) a poorly mixed magma ocean followed by a uniform crust formation, or (3) a symmetric crust composition, but an asymmetric mixing process that resurfaced different portions of the crust on each hemisphere. We approach this problem from two perspectives. From petrological considerations, we study the link between magma ocean properties and anorthosite composition. On the other hand, we also consider an energy balance approach to understand which environmental conditions could produce a thermal evolution consistent with observations. Magma ocean crystallization sequence. The sequence of crystallizing phases in a lunar magma ocean has been studied both through direct experiments and thermodynamic considerations. Here we use the software package MELTS to understand how the Mg# content of anorthosite evolves with magma ocean crystallization. Figure 2 (top) shows the anorthosite Mg# as a function of magma ocean crystallization state (in “percent solid”, PCS, vol%) for different relative plagioclase fractions, defining anorthosite as a pure mixture of plagioclase and clinopyroxene. From these curves, it is possible to compute what PCS distribution is required to reproduce a given Mg# distribution (Fig. 2, bottom). We do this by inverting the functions in Fig. 2 (top) and asking which range of PCS would produce the Mg# distribution of Fig 1. This result depends on a minimal amount of assumptions, namely a given crystallization sequence. We can therefore use this to get an insight into the timing of crystallization and depth of origin of the crust. It is important to stress here that we can only make statements about the part of the crust that is sampled by remote sensing. For example, the nearside Mg# distribution can be best explained by a narrow range of PCS, while the farside is explained by a broader, almost uniform distribution. In this study, we focus on scenario (1), which can be explained either by asymmetric crystallization, where the early crust is formed first on the farside [3], or formed everywhere but transported preferentially to the farside [e.g., 4]. A scenario where the crust is formed symmetrically, but partly removed on the nearside is possible but not studied here. We now investigate models that can explain a crystallization sequence such as that in Fig. 2 (bottom). FIG. 1: Magnesium number (Mg# = Mg/(Mg+Fe) in mol%) distribution on the lunar far (top) and nearside (bottom) using data from Ohtake et al (2012). The distributions are normalized per hemisphere. 1434.pdf Lunar and Planetary Science XLVIII (2017)