Constraining ordinary chondrite composition via near-infrared spectroscopy

Abstract

Ordinary chondrites comprise a significant fraction (~75%) of meteorites that fall on the Earth. A key goal in small body science is to link meteorites to their parent bodies in order to understand their formation conditions early in our Solar System history. Dunn et al. (2010a) provided a robust set of equations for deriving olivine/pyroxene chemistry and abundance ratio from visible and near-infrared (0.35–2.5 μm) spectra of silicate-rich asteroids. These equations were calibrated to X-ray diffraction (XRD) and electron microprobe measurements as ground truth for the spectrally derived values. The small body community employs a range of methods to extract spectral band parameters from telescopic spectra of S-/Q-type asteroids and use the Dunn et al. (2010a) equations to constrain mineral chemistry and abundance. The goal of this work is to understand how the changing of polynomial order and method of extracting spectral band parameters from spectra of ordinary chondrite meteorites affects the precision of derived olivine and pyroxene chemistry and abundance compared with laboratory XRD and microprobe values. Based on our analysis, we find that 2nd order polynomials provide good agreement with the linear relationship found by Dunn et al. (2010a), but with a systematic offset. We also find that Band I center values derived from differing polynomial orders cannot be used for extracting mineral chemistry with Dunn et al. (2010a) equations. We find that the Band Area Ratio (BAR) values are independent of polynomial order and the olivine to pyroxene abundance ratio extracted from BAR is immune to changing polynomial order. Of the four published methods for extracting spectral band parameters (Sanchez et al. 2015, Dunn et al. 2010a, Spectral Analysis Routine for Asteroids, or SARA, Modeling for Asteroids, or M4AST), Dunn et al. (2010a)'s method most successfully reproduces both olivine and pyroxene chemistry, followed by Sanchez et al. (2015). SARA most successfully reproduces the olivine to pyroxene abundance ratio, very closely followed by the other three methods. We find systematic underestimation of ordinary chondrite Band I centers compared to Dunn et al. (2010a) and the resulting chemistry derived from them. To account for this underestimation, we have developed a correction factor for band parameters extracted using other methods that must be added to Band I centers for asteroids that fall in H, L, LL chondrite zones when using Dunn et al. (2010a) equations.