A Mössbauer spectroscopy and X-ray diffraction study of ordinary chondrites: Quantification of modal mineralogy and implications for redox conditions during metamorphism

Abstract

We present a method that combines Mössbauer spectroscopy and X-ray diffraction to quantify the modal mineralogy of unequilibrated ordinary chondrites (UOCs). Despite being a fundamental tool in the interpretation of geological systems, there are no modal mineralogical data available for these meteorites. This is due to their fine-grained nature, highly heterogeneous silicate mineralogy, and the presence of poorly characterized phases. Consequently, it has not been possible to obtain accurate modal mineralogy by conventional techniques such as point counting. Here we use Mössbauer spectroscopy as a preliminary identification technique and X-ray diffraction provides the quantification for a suite of recent UOC falls. We find the most primitive UOCs to contain a significant amount of phyllosilicate material that was converted during metamorphism to form ferromagnesian silicates. A complete suite of Antarctic samples is analyzed by each method to observe mineralogical trends and these are compared with trends shown by recent falls. The fact that mineralogical relationships shown by finds and falls are in agreement allows us to be confident that we are observing the products of pre-terrestrial alteration. Mössbauer spectroscopy reveals evidence of steadily increasing reduction with metamorphism in the UOCs. Because this technique allows comparisons to be made between UOCs and EOCs, our reduction sequence can be combined with other evidence showing progressive oxidation in the EOCs. This yields an integrated model of changing redox conditions on equilibrating ordinary chondrite parent bodies.