Petrology and mineralogy of Chang’e-6 basalts sampled from the South Pole-Aitken Basin

Introduction: On June 25th, 2024, China’s Chang’e-6 (CE-6) mission successfully returned the first lunar farside sample from the South Pole–Aitken (SPA) basin [1]. Orbital investigations have revealed hemispheric asymmetries between the lunar farside and nearside, including disparities in crustal thickness, magmatic activity, and geochemical compositions [2-4]. The origin of these fundamental dichotomies remain a subject of debate [5-10]. Notably, all lunar samples collected prior to CE-6 mission, ranging from Apollo 11 to Chang’e-5 (CE-5), were exclusively sourced from the nearside. Comparative studies of lunar samples from both hemispheres could provide key constraints on our understanding of the lunar evolution. Therefore, CE-6 basalts offer a unique opportunity to investigate the composition and evolution of the previously inaccessible farside of the Moon.

Samples and methods: The basalt fragments analysed in this study were from the scooped samples of CE-6 mission allocated by the China National Space Administration (CNSA). The petrography was carried out on a Zeiss Supra 55 field emission scanning electron microscopy (SEM). The major element compositions of plagioclase, pyroxene, olivine, ilmenite and spinel were analysed with a JEOL JXA8230 electron probe. The bulk chemistry of CE-6 lunar basaltic fragments were determined by X-ray fluorescence spectrometry (XRF) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP–MS), respectively.

Results and discussion: The CE-6 basalt fragments studied here are composed of clinopyroxene, plagioclase, and ilmenite, as well as minor amounts of silica, fayalite, ulvöspinel, troilite, phosphates, and Zr-bearing minerals. According to their petrographic characteristics, the basalt fragments can be classified into four distinct textural subtypes: vitrophyric, porphyritic, subophitic, and poikilitic (Figure 1).

The vitrophyric clasts consist of glass and needle-like microcrystals of plagioclase, pyroxene, and ilmenite, which are typically less than 10 μm in size (Figure 1a). The phenocrysts are mainly clinopyroxene (Wo_29.7-36.8En_29.0-36.1Fs_29.2-36.2), ranging from 50 to100 μm, accompanied by a minor amount of slender plagioclase.

The porphyritic clasts display coarse-grained  clinopyroxene and plagioclase (An_88.7-92.3) phenocrysts (approximately 50 ´ 300 μm) embedded within fine-grained matrix (<10 μm, Figure 1b). The matrix is primarily composed of acicular plagioclase (An_76.3-85.2), interstitial clinopyroxene and tiny ilmenite (<5 μm). In contrast to the phenocrysts, the clinopyroxene within the matrix exhibits higher FeO but lower MgO and Cr₂O₃ contents. Ilmenite needles commonly intersect the matrix plagioclase and pyroxene, indicating a late-stage crystallisation phase.

The subophitic clasts show a diverse range of grain sizes from 20 to 300 μm and consist mainly of plagioclase, clinopyroxene, ilmenite, with minor ulvöspinel, troilite, olivine and cristobalite (Figure 1c). Both clinopyroxene and olivine have compositional zoning, with Mg-rich cores and Fe-rich rims. Plagioclase shows euhedral to subhedral shape with anorthite-rich composition (An_83.6-92.0).

The poikilitic clasts are composed of clinopyroxene, plagioclase, ilmenite, and accessory ulvöspinel, troilite, and a mesostasis phase including K-feldspar, fayalite, cristobalite, baddeleyite, tranquillityite, zirconolite, and phosphates (Figure 1d). Plagioclase is anorthite-rich composition (An_81.9-94.3). Clinopyroxene displays a large compositional range (Wo_8.5-38.9En_0.2-55.7Fs_20.8-89.8), systematically characterized by Mg-rich cores and Fe-rich rims. Small amounts of Fe-rich olivine (fayalite, Fo_1.6) associated with cristobalite, baddeleyite, tranquillityite, zirconolite, and phosphates occur as mesostasis phases representing the late-stage crystallisation products.

The major mineral compositions of basalt fragments with different textures show that the anorthite content in plagioclase varies from 81.1 to 94.3, with an average composition of An_87.5Ab_12.1Or_0.4 (n = 305). Pyroxene in the basalt is predominantly augite, with an average composition of Wo_27.4En_28.7Fs_43.9 (n = 354). Pigeonite is less abundant, with an average composition of Wo_15.6En_28.5Fs_55.9 (n = 169). The composition of ilmenite is homogeneous, with average contents of 51.7% TiO₂ and 47.0% FeO. According to the Fe# vs. Ti# correlation diagram of pyroxene, the basalts with porphyritic, subophitic and poikilitic textures in the CE-6 samples are all classified as low-Ti basalts (Fig. 2). Although the composition of vitrophyric pyroxene falls within the range of the high-Ti basalts, petrographic evidence suggests that the ilmenite crystallized after the clinopyroxenes and plagioclase phenocrysts, which follows the typical crystallization sequence of low-Ti basalts [11]. Therefore, we conclude that the CE-6 basalts with different textures in this study are all low-Ti basalts.

References: [1] Li et al., 2024, National Science Review, 11, nwae328. [2] Zuber et al., 1994, Science, 266, 1839-1843. [3] Jolliff et al., 2000, Journal of Geophysical Research, 105, 4197-216. [4] Wieczorek et al., 2013, Science, 339, 671-675. [5] Loper and Warner, 2002, Journal of Geophysical Research, 107, 13-11-13-17. [6] Zhong et al., 2000, Earth and Planetary Science Letters, 177, 131-140. [7] Parmentier et al., 2002, Earth and Planetary Science Letters, 201, 473-480. [8] Zhu et al., 2019, Journal of Geophysical Research, 124, 2117-2140. [9] Jones et al., 2022, Science Advance, 8, eabm8475. [10] Zhang et al., 2022, Nature Geoscience, 15, 37-41. [11] Shearer et al., 2006, Reviews in Mineralogy and Geochemistry, 60, 365-518.

Figure 1: BSE image of typical basaltic fragments with various textures.

Figure 2: Ti# versus Fe# diagram of the pyroxene from CE6 basalt fragments. Fields represent variation in Fe# and Ti# in Apollo very low-Ti, low-Ti, and high-Ti mare basalts.