A refined technique for measuring crystal size distributions in thin section |
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| Authors: | Tony D Peterson |
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| Institution: | (1) Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A-0E8, Canada, CA |
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| Abstract: | Log-normal size distributions are of the form n=noe−L/α, where n=number density, L=crystal length, and α is a constant. A method for measuring three-dimensional log-normal crystal
or grain size distributions (CSDs) in thin section has been deduced from computer experiments, in which 2D sections were cut
through assemblages of 3D solids. The size ranges and distributions studied were appropriate for igneous microphenocryst to
megacryst populations. Conversion from 2D to 3D is based on an exact correction for spheres of uniform diameter. Cumulate
numbers of polygons with length≥L (N2D) are converted to N3D by the equation:
ln(N3D)=ln(N2D/L·S])+ln(γ)−β/L·S]
The number density is then obtained as n=−dN/dL. The parameters S and γ correct the measured lengths and no (no=number density at L=0) respectively, and are functions of crystal shape. The parameter β is a weak function of the degree
of spatial orientation of the crystals. Highly symmetrical shapes such as cubes, octahedra, and elongated prisms can be accurately
measured when randomly oriented; however, rectangular solids with a≠b≠c cannot be accurately measured because they produce
bimodal length distributions in cross section. Strongly oriented textures (trachytic or lineated) can be accurately measured
regardless of crystal shape. New CSD data from alkaline rocks and a kimberlite give examples of CSDs modified by megacryst
retention, xenocryst addition, phenocryst accumulation, and groundmass nucleation.
Received: 25 November 1994 / Accepted: 14 November 1995 |
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