Case study of magmatic differentiation trends on the Moon based on lunar meteorite Northwest Africa 773 and comparison with Apollo 15 quartz monzodiorite

• Published in: Geochimica et cosmochimica acta 2014-05, Vol.133, p.97-127
• Main Authors: Fagan, Timothy J., Kashima, Daiju, Wakabayashi, Yuki, Suginohara, Akiko
• Format: Article
• Language: English
• Subjects: Basalt; Crystallization; Feldspars; Iron; Liquids; Pyroxenes; Silicon dioxide; Trends
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2014.02.025

Pyroxene and feldspar compositions indicate that most clasts from the Northwest Africa 773 (NWA 773) lunar meteorite breccia crystallized from a common very low-Ti (VLT) mare basalt parental magma on the Moon. An olivine cumulate (OC), with low-Ca and high-Ca pyroxenes and plagioclase feldspar formed during early stages of crystallization, followed by pyroxene gabbro, which is characterized by zoned pyroxene (Fe#=molar Fe/(Fe+Mg)×100 from ∼35 to 90; Ti#=molar Ti/(Ti+Cr)×100 from ∼20 to 99) and feldspar (∼An90–95Ab05–10 to An80–85Ab10–16). Late stage lithologies include alkali-poor symplectite consisting of fayalite, hedenbergitic pyroxene and silica, and alkaline-phase-ferroan clasts characterized by K-rich glass and/or K,Ba-feldspar with fayalite and/or pyroxene. Igneous silica only occurs with the alkaline-phase-ferroan clasts. This sequence of clasts represents stages of magmatic evolution along a ferroan–titanian trend characterized by correlated Fe# and Ti# in pyroxene, and a wide range of increase in Fe# and Ti# prior to crystallization of igneous silica. Clasts of Apollo 15 quartz monzodiorite (QMD) also have pyroxene co-existing with silica, but the QMD pyroxene has more moderate Fe# (∼70). Thus, in AFM components (A=Na2O+K2O, M=MgO, F=FeO), the QMD clasts are similar to the terrestrial calc-alkaline trend (silica-enrichment at moderate Fe#), whereas the ferroan–titanian trend is similar to the terrestrial tholeiitic trend (silica-enrichment only after strong increase in Fe#). However, the variations in SiO2-contents of QMD clasts are due to variable mixing of SiO2-rich and FeO-rich immiscible liquids (i.e., not a progressive increase in SiO2). Immiscibility occurred after fractionation of a KREEP-rich parent liquid. A third trend is based on zoning relations within the NWA 773 OC, where pyroxene Ti# increases at constant Fe# with proximity to intercumulus, incompatible element-rich pockets rich in K,Ba-feldspar and Ca-phosphates. This type of fractionation (increasing refractory trace elements at constant Fe#) in a cumulate parent rock may have been important for generating lunar rocks that combine low Fe# with high incompatible trace element concentrations, such as KREEP basalts and the magnesian suite. MELTS (Ghiorso and Sack, 1995; Asimow and Ghiorso, 1998) models of one VLT, one low-Ti and two high-Ti mare basalts and one KREEP basalt all show evolution from low to high Fe# residual liquids during fractional crystallization; however strong enr