Determination of Mineral Abundances in Ordinary Chondrites Using Powder X-ray Diffraction: Applications to Parent Body Processes and Asteroid Spectroscopy

The ordinary chondrites, the most abundant group of meteorites, are divided into three chemical groups (H, L, and LL), which are distinguished based on variations in bulk composition and iron content. Although ordinary chondrites are relatively abundant, our understanding of their formation has been hampered by an inability to accurately measure the abundances of minerals that they contain. Here I use power x-ray diffraction (XRD) to quantify the modal abundances of 48 unbrecciated ordinary chondrite falls, which represent the complete petrologic range of equilibrated ordinary chondrites (types 4-6).

Although the degree of metamorphism varies within each ordinary chondrite group, many details regarding the geochemical and thermal changes that take place during this process are not well understood. Using XRD-measured modal mineral abundances and chemical analyses, we evaluate the redox state and peak metamorphic temperatures in ordinary chondrite parent bodies in order to develop quantitative thermal evolution models of ordinary chondrite parent asteroids. Modal abundances and mineral compositions measured in this study indicate that progressive oxidation of the ordinary chondrites likely occurred during metamorphism. Results also suggest that plagioclase crystallizes early in the metamorphic sequence, indicating that peak temperatures derived using the two-pyroxene geothermometer are more accurate than those calculated using the plagioclase geothermometer.

I also utilize ordinary chondrite modal abundances to develop new calibrations for deriving mineralogy from meteorite and asteroid spectra. Most previous studies examining the mineralogical characteristics of VISNIR asteroid spectra have focused on olivine and pyroxene proportions in single-pyroxene mixtures; therefore, current calibrations for deriving composition mineral abundances may be poorly suited for asteroids containing more than one pyroxene. It is important that calibrations are correct, because mineral abundances derived from visible/near-infrared (VISNIR) spectra are used to classify asteroids, identify meteorite parent bodies, and understand the structure of the asteroid belt. Mineral abundances derived using my calibrations correlate well with measured data for all ordinary chondrites groups, and also appear to be valid for other meteorites and asteroids with mineral abundances and chemistries similar to those of the ordinary chondrites.