Lunar iron and optical maturity mapping: Results from partial least squares modeling of Chang'E-1 IIM data

Iron and optical maturity (OMAT) are two key geological marks of the Moon that closely related to its geochemical evolution and interactions between surface and space environment. We apply Partial Least Squares (PLS) regression to Chang'E-1 Imaging Interferometer (IIM) (32 bands between 480 and 960 nm) in mapping lunar global FeO and OMAT, and the FeO and OMAT values are derived based on reasonable spectral parameters (absorbance, band ratios, TiO2 and maturity sensitive parameters, etc.). After been calibrated by the FeO map from Lunar Prospector Gamma-Ray Spectrometer (LP-GRS), the global FeO map derived from PLS modeling shows a quantitatively more reasonable result consistent with previous remote sensing results (LP) as well as lunar feldspathic meteorite studies and Chang'E-3 landing site. Based on the new FeO map by Chang'E-1, we discover a compositional inhomogeneity across lunar highland regions, which has not been suggested by previous datasets (e.g., Clementine UVVIS). Furthermore, we suggest that at least part of the FeO enrichments in highlands would be caused by mixing of highland and mare materials. The IIM derived OMAT map does not suggest a dichotomy of the lunar highlands and mare regions, implying the compositional differences between those two terrains have been suppressed. We further check the maturity effect for the young mare basalts (<3.0 Ga), and find that (1) the OMAT values of the young basaltic units with medium and high FeO and TiO2 show a linear decrease with ages; (2) units with ultrahigh-FeO (>20 wt%) and ultrahigh-TiO2 (>10 wt%) tend to have greater OMAT values and vary little with ages; (3) this may be due to the distinct optical maturity effects of ultramafic minerals (i.e., ultrahigh Fe and Ti) and/or the spectral blue shifts of abundant ilmenite.