On the evolution of the lunar magma ocean

Thesis

The Earth and the Moon are observed to be almost indistinguishable over a range of refractory isotopes systems. This thesis aims to test the hypothesis that the lunar magma ocean (LMO) possessed the same elemental composition as that of the terrestrial mantle. In particular, I explore whether the LMO’s differentiation would result in liquid and crystal phases that can reproduce the Apollo observations of the lunar surface. To achieve this, I have conducted a detailed series of experiments which track the evolution of a hypothetical lunar magma ocean of terrestrial mantle composition. I have broken the differentiation of the LMO into discrete steps, with the bulk composition of the next step taken as the melt composition of the previous step. The final experimental liquid, representing ∼98% crystallisation of the LMO, is highly enriched in Fe (>30 wt.%) and Ti (∼3 wt.%). To explore the petrogenesis of the mare basalts and the Ferroan Anorthosite Suite (FAS), I have simulated the remelting of the lunar cumulate mantle using the MELTS program. By recombining experimental phases and melts in different proportions, I am able to prove that the Apollo observations are compatible with a LMO of terrestrial mantle composition. The mafic crystals present in the ferroan anorthosite suite are best matched to experimentally produced crystals rather than melt, indicating inefficient flotation rather than trapped melt has contributed to the ferroan anorthosite suite.

Mineral/melt Rare Earth element (REE) partitioning occurring throughout the evolution of the lunar magma ocean has been experimentally determined, and predict heavy REE depletion of final liquids, due to high-temperature pyroxene crystallisation. Consequently, the ’bow shaped’ trend witnessed in the mare basalts results from a lunar mantle possessing pyroxene enriched in heavy REE and a light REE enriched KREEP basalt. I explore the implications of different compositions on the lunar moment of inertia, finding that an overturn of the lunar mantle is more significant than any conceivable change in composition.

Authors: Baker, E

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Lunar; Moon; Mare Basalts; Magma Ocean; Geology; Ferroan Anorthosite Suite; Experimental Petrology; Rare Earth Elements
• Subjects: Moon; Geology