Oldest high-Ti basalt and magnesian crustal materials in feldspathic lunar meteorite Dhofar 1428

1,2Zhuqing Xue,1,3Long Xiao,2Clive R.Neal,4Yigang Xu
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.06.022]
1State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
2Department of Civil Engineering and Geological Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A
3State Key Laboratory of Lunar and Planetary Science, Macau University of Science and Technology, Avenida Wai Long, Taipa, Macau, China
4State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China
Copyright Elsevier

We conducted a thorough analysis of the feldspathic breccia meteorite Dhofar 1428 with the aim of better understanding the composition and evolution of lunar crust. This sample comprises a heterogeneous array of lithic fragments including magnesian and ferroan anorthositic granulites, mafic granulites/granulitic breccia, basalts, and different kinds of impact melt rocks. In which, a high-Ti basalt clast comprising large zoned pyroxene was observed. Based on equilibrium melt calculations of mineral zonations from this basalt, Mg-pyroxene cores were interpreted to be formed from a light rare earth element (LREE) enriched liquid, whereas the Fe-pyroxene rims grew from an LREE-depleted magma. We propose that LREE-depleted signature of Fe-pyroxene results from co-crystallization with apatite. The Mg-pyroxenes suggest that enriched liquids with higher REE contents and different REE patterns relative to KREEP existed within lunar interior. Oscillating Ti/Al ratios across pyroxene in this basalt may indicate several magma recharge events or crystal movement within a zoned magma chamber. This feature illustrates that magmas were derived from a variety of sources around the time of formation of this basalt. In situ U-Pb dating was conducted on apatite grains within this basalt, the excellent consistence between the U-Pb Concordia age (3941±24 Ma, 2σ) and 207Pb/206Pb isochron age (3934±24Ma, 2σ) indicates the most likely crystallization age of this high-Ti basalt at ∼3940 Myr, making it one of the oldest high-Ti basalts formed on the Moon.
Magnesian anorthositic granulites are mineralogically and geochemically similar to those trace element-poor magnesian anorthositic granulites in many lunar meteorites. These magnesian granulites cannot form from simple mixing of pristine Ferroan Anorthosite and lithologies from the Mg-Suite, and do not have any affinities with KREEP or the Procellarum KREEP Terrane, and they could be important components of farside highlands.

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