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

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• 作者：Timothy J. Fagan ; ^{fagan@waseda.jp" class="auth_mail} ; Daiju Kashima ; Yuki Wakabayashi ; Akiko Suginohara
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：15 May 2014
• 年：2014
• 卷：133
• 期：Complete
• 页码：97-127
• 全文大小：6818 K

文摘

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 (∼An_90–95Ab_05–10 to An_80–85Ab_10–16). 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 and 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 enrichments in FeO-concentrations are limited to the VLT and low-Ti liquids. In the high-Ti liquids, crystallization of Fe–Ti-oxides prevents enrichment in FeO, and the increases in Fe# are due to depletion of MgO. Fe–Ti-oxide fractionation results in steady silica-enrichment in the high-Ti mare compositions. Intervals of FeO-enrichment on the VLT and low-Ti mare liquid lines of descent are linked to shifts from olivine to pyroxene crystallization. The onset of plagioclase feldspar crystallization limits the depletion of FeO during crystallization of one high-Ti mare basalt and of the KREEP basalt composition modeled.