Mafic mineralogy assemblages at the Chang'e-4 landing site: A combined laboratory and lunar in situ spectroscopic study

Context. Chang'e-4 (CE-4) provides unprecedented information about lunar materials exposed by the South Pole-Aitken (SPA) basin. Diverse results have been obtained from previous interpretations of CE-4 visible and near-infrared (VNIR) spectra. Some studies suggest that materials at the CE-4 landing site are dominated by olivine and orthopyroxene, but others argue that only a small amount of olivine should be exposed at the CE-4 landing site. Aims. Laboratory spectroscopy studies using the Engineering Model of CE-4 Visible and Near-infrared Imaging Spectrometer (VNIS) are critical in constraining the accurate mineral proportions and composition of soils and boulders at the CE-4 landing site. Methods. VNIR spectra of nine lunar analogs - prepared by mixing orthopyroxene (OPX), clinopyroxene (CPX), olivine (OL), and plagioclase - were acquired using the CE-4 VNIS Engineering Model. The spectral indices model and modified Gaussian model were developed to estimate CPX/(CPX+OPX) and OL/(OL+CPX+OPX) and are applicable to the in situ spectra acquired by the Yutu-2 VNIS spectrometer. Results. The lunar rocks and regolith at the CE-4 landing site excavated by the Finsen impact are CPX-rich with limited OL (CPX:OPX:OL = 56:29:17). The mineral chemistries of the four lunar rocks show Mid-Ca, Fe pyroxene, and Mid-Mg OL (Fo(60-79)), providing critical constraints for mineral compositions in the SPA compositional anomaly. These rocks exhibit high M1 intensity ratios, indicating that they were crystallized at a high temperature (980-1300 degrees C) and a rapid-cooling magmatic system produced by impact melt differentiation or volcanic resurfacing events.