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A new approach to quantification of metamorphism using ultra-small and small angle neutron scattering
Authors:Lawrence M Anovitz  Gary W Lynn  Gernot Rother  William A Hamilton  Man-Ho Kim
Institution:a Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA
b Chemical Sciences Division, MS 6110, P.O. Box 2008, Bldg. 4500S, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6110, USA
c Neutron Scattering Science Division, MS 6393, P.O. Box 2008, Bldg. 7962, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6393, USA
d Materials Science and Technology Division, MS 6064, P.O. Box 2008, Bldg. 4515, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6064, USA
e Bragg Institute, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia
f Large Scale Structures Group, Institut Laue-Langevin, 6 rue Jules Horowitz, B.P. 156, F-38042 Grenoble CEDEX 9, France
g Materials Science and Technology Research Division, Korean Institute of Science and Technology (KIST), P.O. Box 131, Cheongryang, Seoul 130-650, Republic of Korea
Abstract:In this paper we report the results of a study using small angle and ultra-small angle 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 m. Scattering curves obtained from these samples show mass fractal behavior at low scattering vectors, and surface fractal behavior at high scattering vectors. 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 metamorphic petrology. A change from mass-fractal to non-fractal behavior is observed at larger scales near the outer boundary of the aureole that implies significant reorganization of pore distributions early in the metamorphic history. Surface fractal results suggest significant smoothing of grain boundaries, coupled with changes in pore sizes. A section of the scattering curve with a slope less than −4 appears at low-Q in metamorphosed samples, which is not present in unmetamorphosed samples. A strong spike in the surface area to volume ratio is observed in rocks near the mapped metamorphic limit, which is associated with reaction of small amounts of organic material to graphite. It may also represent an increase in pore volume or permeability, suggesting that a high permeability zone forms at the boundary of the aureole and moves outwards as metamorphism progresses. Neutron scattering data also correlate well with transmission electron microscopic (TEM) observations, which show formation of micro- and nanopores and microfractures during metamorphism. The scattering data are, however, quantifiable for a bulk rock in a manner that is difficult to achieve using high-resolution imaging (e.g. TEM). Thus, neutron scattering techniques provide a new approach to the analysis and study of metamorphism.
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