RECONCILING THE FORSTERITE CONTENT OF LUNAR TROCTOLITES WITH THE MAGMA OCEAN AND CUMULATE MANTLE OVERTURN HYPOTHESES

Several key aspects concerning the primary differentiation and large-scale magmatic evolution of the Moon are constrained by the petrogenesis of pristine lunar sample suites. In this study, we revisit the petrogenesis of lunar highlands magnesian-suite samples (Mg-suite; > 4.1Ga), which are comprised primarily of primitive troctolites and norites, and perhaps also gabbronorites. In particular, lunar troctolites contain the most forsteritic olivine compositions (Fo# = Mg# = Mg/(Mg + Fe)x100 = up to Fo96) among all lunar samples, suggesting the Mg-suite primary magma is derived, at least in part, from ultramafic cumulates produced during primary differentiation of a global lunar magma ocean (LMO).

There are two types of pristine lunar troctolites as distinguished by spinel chemistry: “common” lunar troctolites +/- chromite, and the volumetrically minor pink spinel-bearing troctolite clasts (PST). Future petrogenetic models should note however, that the most forsteritic troctolites (Fo# > 89) among Mg-suite samples are comprised of the volumetrically minor PST clasts with a majority containing Fo90-92. Common lunar troctolites contain Fo ≤ 89 with a majority containing Fo87-88.

Recent experimental evidence indicates PST mineralogy is best explained by reaction between MgO-rich primary liquids and anorthite (Prissel et al., 2016). The formation of common lunar troctolites can be attributed to equilibrium crystallization prior to reaction with anorthite. Previously, models of equilibrium crystallization were ruled out due to the pairing of forsteritic olivine with anorthitic plagioclase (i.e., the so-called “Mg# problem”). However, we demonstrate that equilibrium crystallization models of MgO-rich primary liquids can reconcile the forsterite content, as well as the presence of chromite, observed in common lunar troctolites.

If the Mg-suite parent is derived from primitive ultramafic cumulates of the LMO, Mg-suite samples may be used to constrain the onset and duration of cumulate mantle overturn. Further, the simplified equilibrium crystallization model presented here also supports the hypothesis that gabbronorites are not co-genetic to troctolites and norites (Lindstrom et al., 1989). If so, the differentiation trends and diversity of lunar highlands lithologies may need to be redefined.