Is Ilmenite Always the Dominant Carrier of Titanium in Lunar Mare Basalts?

Introduction: The dominant cause for lunar visible color differences is variations in the degree of soil maturity and the abundance of FeO and opaque minerals, such as ilmenite. The addition of opaque minerals to a lunar soil tends to decrease the overall reflectance and continuum slope (darkens and decreases “redness”). As a soil weathers its reflectance is also lowered but its slope increases (darkens and increases “redness”). By exploiting these color trends numerous studies have attempted to correlate the abundance of TiO2 in the form of ilmenite to variations in blue to red ratios (i.e. 415 to 750 nm Clementine bandpasses) with varying degrees of success. Discrepancies between independent methods have been attributed to: unknown opaque phases in the lunar soil; overcorrection of thorium values in Lunar Prospector epithermal and thermal neutron data; mixing of highlands and mare basalt materials; and uncorrected scattered light effects. To investigate this problem we are examining the spectral differences between plausible lunar analog opaque mineral powders and their respective quenched glasses. Background: Lunar ilmenite (FeTiO3) is a potential ISRU source of oxygen and titanium metal. Additionally, ilmenite-rich soils preferentially retain solar wind volatiles H and He, that are also potential lunar resources. Understanding the distribution and abundance of ilmenite also serves to elucidate the Moon’s thermal history and formation of the crust. Additionally, titanium concentration is one of the most useful discriminators in classifying lunar mare basalts due to its substantial variation (<1 wt. % to > 14 wt. % TiO2) [1]. The prevalence of titanium in mare basalts, relative to the terrestrial basalts, is curious and not yet satisfactorily explained. The Hapke model [2] of lunar reflectance attributes the spectral properties of lunar soil to four components of the regolith: the ferrous iron (Fe) in silicate minerals and glasses; submicroscopic metallic iron (SMFe) grains produced in the maturation process; titanium in silicate minerals and glasses; and opaque phases (in the case of the Moon, often assumed to be mostly ilmenite). The presence of opaque phases causes an overall decrease in reflectance and spectral slope (less red, titanium rich basalts are sometimes referred to as “blue”). Increased maturation and added FeO cause a decrease in reflectance and an increase spectral slope (redder). Lucey et al. [3] developed a method for estimating titanium abundance using the Clementine 415/750 nm ratio versus 750nm reflectance to define a Ti spectral parameter. Lucey notes that this method relies on the spectral properties of opaque minerals and the assumption that ilmenite is the dominant opaque mineral everywhere on the Moon. The titanium abundances estimated by Lucey et al. are higher, by up to a factor of 2, than values from Lunar Prospector epithermal and thermal neutron data in some areas, as shown in Figure 1 [4]. Gillis et al. [5] modified the algorithm of Lucey et al. after observing spectral similarities of landing sites corresponding to outliers on the plot of Ti spectral parameter versus measured TiO2 content of returned samples. By separating the two trends and applying independent fits to the data they found TiO2 abundances more consistent with Lunar Prospector epithermal and thermal neutron values.