Spectral reflectance properties of HED meteorites+CM2 carbonaceous chondrites: Comparison to HED grain size and compositional variations and implications for the nature of low-albedo features on Asteroid 4 Vesta

We have studied the spectral reflectance properties of the CM2 xenolith-bearing howardite PRA 04401, intimate and areal mixtures of a eucrite (Millbillillie) and a CM2 chondrite (Murchison), and a mineralogically diverse suite of 12 HED meteorites. The main goal is to ascertain whether the presence of CM2-type material can be discriminated from grain size variations in HEDs. This project was motivated by the presence of CM2 material in a number of howardites. This study is of high relevance to Vesta because of the detection of low-albedo features on its surface by the Framing Camera (FC) aboard NASA's Dawn spacecraft. The addition of CM2 material and increasing HED grain size both lead to decreasing overall albedo. However these two processes can be recognized by how they affect pyroxene band depths and shapes and, to a lesser extent, band widths. Pyroxene band depths and widths decrease with increasing CM2 abundance and increase with increasing HED grain size, as do various FC reflectance ratios. HED pyroxene absorption bands appear to reach saturation when grain size is in the region of 60-170. μm, with band I saturating at smaller grain sizes than band II. Band I centers are largely insensitive to CM2 abundances or HED grain size variations, while Band II center positions increase by up to 40. nm with increasing CM2 abundance, and vary non-systematically by up to 22. nm as a function of grain size. The variation with CM2 abundance is probably due to the red-sloped nature of the CM2 spectrum and the broad and shallow nature of the pyroxene band II absorption. Band area ratios show less consistent behavior, likely due to CM2-induced slope changes in the mixtures and band saturation effects in the HEDs. Because HED pyroxene is so strongly featured, CM2 abundances must be well in excess of 80. wt.% to allow for the appearance of their much weaker CM2 phyllosilicate absorption bands. CM2 material may also cause a reddening of spectral slope and a shifting of pyroxene band minima to shorter wavelengths, although CM2 chondrites can exhibit a range of spectral slopes. The presence of submicron opaques (specifically chromite) in HED pyroxenes can lead to large variations in all spectral parameters. Discriminating the spectrum-altering effects of CM2 material from HED grain size variations is possible, with the confidence in the interpretation increasing as a larger range of spectral parameters are applied to the analysis.