Mineral chemistry and 3D tomography of a Chang’E 5 high-Ti basalt: implication for the lunar thermal evolution history

Abstract

In December 2020, Chang’E-5 (CE-5), China’s first lunar sample return mission, successfully collected samples totaling 1731 g from the northern Oceanus Procellarum. The landing site was located in a young mare plain, a great distance from those of Apollo and Luna missions. These young mare basalts bear critical scientific significance as they could shed light on the nature of the lunar interior (composition and structure) as well as the recent volcanism on the Moon. In this article, we investigated a CE-5 basalt sample (CE5C0000YJYX065) using a combination of state-of-art techniques, including high resolution X-ray tomographic microscopy (HR-XTM), energy dispersive X-ray spectroscopy (EDS)-based scanning electron microscope (SEM), and electron probe microanalysis (EPMA) to reveal its 3D petrology and minerology. Our results show that this sample has a fine- to medium-grained subophitic texture, with sparse olivine phenocrysts setting in the groundmass of pyroxene, plagioclase, ilmenite and trace amounts of other phases. It has an extremely high ilmenite modal abundance (17.8 vol%) and contains a significant amount (0.5 vol%) of Ca-phosphate grains. The mineral chemistry is in excellent agreement with that of Apollo and Luna high-Ti basalts. The major phase pyroxenes also display strong chemical zoning with compositions following the trends observed in Apollo high-Ti basalts. Based on current data, we came to the conclusion that CE5C0000YJYX065 is a high-Ti mare basalt with a rare earth element (REE) enriched signature. This provides a rigid ground-truth for the geological context at the CE-5 landing site and clarifies the ambiguity inferred from remote sensing surveys.