The Geology of the Moon: Geochemistry and Petrology of Lunar Basalts

Hallis, Lydia Jane (2010). The Geology of the Moon: Geochemistry and Petrology of Lunar Basalts. PhD thesis The Open University.



The aim of this research is to determine any genetic relationships between high- and lowTi lunar basalts in terms of petrology, mineralogy, geochemistry and geochronology, using a set of Apollo samples. SEM images, mineral compositions, and major- and trace-element data indicate that the high and low-Ti mare-basalts form two distinct groups, both mineralogically and chemically. Apollo 12 and 15 low-Ti basalts were produced from similar source regions, whereas the Apollo 11 and 17 basalts show greater source region compositional variation. Pb-Pb age dating of low-Ti and high-Ti samples confirm previous observations indicating the former are generally younger than the latter. Geochemical modelling suggests that the low-Ti basalts formed from < 10 % partial melting of a depleted Moon source, followed by olivine fractionation. The high-Ti basalts were probably formed as a result of < 15 % partial melting of relatively incompatible-elementenriched source regions, followed by multi-phase crystal fractionation. The REE content of Apollo 14 low-Ti, high-AI basalt 14053 can be reconstructed by ~ 10 % partial melting of a depleted Moon source, followed by olivine fractionation and 1.5 % simultaneous assimilation of a KREEPy/granitic crustal component. The REE contents of Apollo 11 high-Ti, high-K basalts match well with 10 % KREEP assimilation, along with multi-phase fractionation from a parental melt similar to Apollo 11 orange glass. Setting aside the different parental melt evolutions, bulk-rock elemental evidence clearly indicates that the high- and low-Ti mare-basalts were produced by compositionally different source regions. This conclusion is supported by different oxygen isotope compositions of the high- and low-Ti basalts. However, oxygen isotope data implies that rather than a bi-modal system, a mare-basalt compositional continuum exists. This continuum fits well with the heterogeneous cumulate source model for mare-basalt formation. Our oxygen isotope data reveal no detectable difference between the 0 of the Earth and Moon, with implications for the viability of the giant impact origin of the Moon.

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