Abstract
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Abstract In this paper we report the results of a study using neutron scattering techniques (SANS and USANS) to examine the evolution of carbonates during contact metamorphism. Data were obtained from samples collected along two transects in the metamorphosed Hueco limestone at the Marble Canyon, Texas, contact aureole. These samples were collected from the igneous contact out to ~1700 meters. Significant changes are observed in the surface and mass fractal dimensions as well as the correlation lengths (pore and grain sizes), surface area to volume ratio and surface Gibbs Free energy as a function of distance, including regions of the aureole outside the range of classic reaction petrology. Strong linear correlations exists between the log of the pore correlation length, r, and the ratios of the surface and mass fractal dimensions to that length, Ds/r and Dm/r. These ratios do not, however vary linearly with distance from the contact. A zone of �fuzzy� scattering appears at low-Q in metamorphosed samples that is not present in unmetamorphosed samples. A strong spike in the surface area to volume ratio is observed in rocks just outside the mapped metamorphic limit associated with reaction of small amounts of organic materials to graphite. It is unclear whether this also represents an increase in pore volume or permeability, but may represent a high permeability zone that moves outwards as metamorphism progresses.
Neutron scattering data also correlate well with TEM observations, which show formation of micro- and nanopores and microfractures during metamorphism. The scattering data are, however, quantifiable for a bulk rock. Thus, neutron scattering techniques provide a new approach to the analysis and study of metamorphism.
