¹Kevin Robertson,¹Ralph Milliken,¹Carle Pieters,¹Leif Tokle,¹Leah Cheek,¹Peter Isaacson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114836]
¹Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI 02912 2, United States of America
Copyright Elsevier

The primary objective of this work is to constrain the physical and chemical effects of ilmenite on the spectral properties of high‑titanium lunar basalts. Here we use a combination of electron probe microanalysis, x-ray diffraction and VIS-NIR spectroscopy to characterize a suite of Apollo 17 High-Ti lunar basalts and a suite of synthetic laboratory derived binary mixtures to further understand the mixing systematics of opaque Fesingle bondTi oxides in a transparent silicate matrix. We demonstrate how the texture (particle size and shape) and composition (Fesingle bondMg content) of ilmenite exhibit strong controls over the spectral parameters of bulk basalts in the VIS-NIR wavelength range. We show that the presence of fine-grained ilmenite as opposed to coarse-grained ilmenite in high-Ti basalts causes suppression of pyroxene, olivine, and plagioclase absorption features, lowers reflectance values, and induces a stronger ‘red’ spectral slope at wavelengths >1.8 μm. These effects are considerably stronger for samples with fine-grained ilmenite crystals compared with coarse-grained samples that have similar ilmenite abundance. In addition, we show that minor variations in the Mg2+ content of ilmenite significantly alters the strength of the 1 μm feature and the slope above 1.8 μm, which similarly affects the spectral parameters of the bulk basalt. Our results indicate that lunar basalts with lower abundance of fine-grained ilmenite with slightly higher Mg content could exhibit a spectrum with an apparently stronger ilmenite signature when compared to basalts with more abundant, coarser ilmenite. Results presented here suggest that the accuracy of spectral mixing models of remotely sensed data on the Moon would improve by incorporating additional spectral end-members of ilmenite to better represent the compositional variability.