New approach to quantification of metamorphism using ultra-small and small angle neutron scattering

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Elsevier; Cambridge Publications
Small- and Ultra-Small Angle Neutron Scattering (SANS and USANS) provide powerful tools for analysis of porous rocks because neutrons probe both the surface and the interior of the material providing bulk statistical information over a wide range of length scales. For monomineralic materials scattering contrast arises from the difference between the scattering length density of the rock and the pores (taken to be zero). Pore-grain interfaces are best described by self-similar fractals with non-universal dimensions (2 < D < 3). This leads to a non-integer power-law as a function of the scattering I(Q) = I(1)Q-a + B where B is the incoherent background. For a volume or mass fractal scatterer a = D; if only the surface is a fractal, then a = 6-D. In this manner, surface fractals (a>3) and mass fractals (a<3) are easily distinguished. Non-fractal “fuzzy” interfaces (a>4) may also be observed. We have used SANS and USANS to characterize samples from two transects (contact out to ~1700 m) from the contact metamorphosed Hueco limestone at Marble Canyon, TX. Significant changes in a number of scattering parameters are observed as a function of distance, including regions of the aureole outside the range of classic reaction petrology. Our modeling suggests that changes in surface free energy, pore volume and hydrogen content, among other variables, can be quantified. Both the mass and surface fractals Dm and Ds can be fitted as a function of the surface fractal correlation length as: log10(D/r) = -0.9648 * log10r(Å) + 0.30103 r2 = 0.9999, s = 0.038 although metamorphism does not necessarily proceed linearly along this curve with increasing grade. Nonetheless, the ability to measure these variables shows that these techniques provide a novel approach to the analysis and study of metamorphism.
Small angle scattering, Metamorphism, Porous materials, Rocks, Neutrons, Scattering lengths
Anovitzi, L. M., Lynn, G. W., Cole, D. R., Rother, G., Allard, L. F., & Hamilton, W. A., Porcar, L., & Kim, M. H. (2008). New approach to quantification of metamorphism using ultra-small and small angle neutron scattering. 18th Annual V.M. Goldschmidt Conference (Goldschmidt 2008) - "from Sea to Sky", 13th - 18th July 2008. Vancouver, Canada: University of British Columbia. In Geochimica et Cosmochimica Acta, 72(12), A28. doi:10.1016/j.gca.2008.05.004