MICRO-REFLECTANCE IR SPECTROSCOPY AS A TOOL FOR BETTER UNDERSTANDING REMOTE IR SPECTRA OF PLANETARY BODIES

Infrared (IR) spectroscopy is an effective method for correlating meteorites with possible remote parent bodies, or vice versa. Thermal IR (TIR) is ideal for searching for the meteorite parent bodies, dust, and exo-planets because 1) Si-O bonds in silicate minerals have characteristic TIR absorptions and, in general, mineral spectra mix linearly to produce a diagnostic rock spectrum; 2) TIR may be linked to petrology and geochemistry and; 3) modern, remote TIR instruments have high spectral resolution and are targeting many solar system objects. To make valid comparisons between meteorites and remote spectra, it is necessary to use TIR databases with appropriate mineralogy, but some minerals are difficult to analyze with standard bulk TIR techniques. Thus, we are developing new micro-specular reflectance IR (μ-R-IR) methods. The advantage of μ-R-IR is that spectra can be collected on small phases (<10 μm) and on chemically zoned minerals in variable crystallographic orientations.

Our results for olivine (San Carlos), diopside (Harcourt), enstatite (Bamble), labradorite, (Lake St. John) and anorthoclase (Larvik) show that μ-R-IR spectra have band positions that are near-identical to database emission TIR spectra (Christensen et al. 2000 JGR), similar to silicic glasses (Lee et al. 2010 JGR). The intensities differ due to 1) a lack of crystallographic orientation effects in the emission spectra, which are present in the μ-R-IR spectra; and 2) possible variations in composition in the μ-R-IR data. Nonetheless, multiple μ-R-IR spectra, collected over a range of orientations, are similar to emission spectra.

We have made μ-R-IR measurements of phases on a cut face from the D'Orbigny angrite, an early solar system basaltic meteorite. As expected, spectra for each major mineral (fassaite, olivine and anorthite) show variations in band intensity – likely related to both composition and crystallographic orientation – but the band positions are consistent. The bulk TIR spectra of D'Orbigny meteorite will likely differ from Angra dos Reis angrite due to significant amounts of olivine and plagioclase. In sum, the μ-R-IR technique represents a previously underutilized technique for exploring the solar system and unraveling the effects of mineralogy, chemistry, and crystallographic orientation on TIR spectra.