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Identification of cathodoluminescence activators in zoned alkali feldspars by hyperspectral imaging and electron-probe microanalysis

Lee, Martin R. and Parsons, Ian and Edwards, Paul R. and Martin, Robert W. (2007) Identification of cathodoluminescence activators in zoned alkali feldspars by hyperspectral imaging and electron-probe microanalysis. American Mineralogist, 92 (2-3). pp. 243-253. ISSN 0003-004X

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Abstract

Cryptoperthitic alkali feldspars within the Proterozoic Klokken syenite have been pervasively altered deuterically to patch perthites whose constituent subgrains display very fine-scale zoning in optical cathodoluminescence (CL). Electron-probe analyses and hyperspectral maps were acquired from several areas of patch perthite to potentially identify the centers responsible for CL emission and to access information that the subgrains might provide about the late-stage geological evolution of the pluton. Each hyperspectral map is composed of tens of thousands of CL spectra covering a wavelength range of 350-850 nm, and images can be formed from any desired wavelength band. The patch perthite subgrains have two emission bands: one in the blue (similar to 460 nm) and the other at red to infrared wavelengths (similar to 690-725 nm). Cathodoluminescence images formed using both bands show that the optically visible zoning is developed only at blue wavelengths. Comparisons of emission intensities with electron-probe analyses show that the blue band is activated by Ti3+ and less than similar to 25 ppm of the trace element is required for optically detectable luminescence. Adjacent Ab- and Or-rich subgrains can have identical patterns of zoning at blue wavelengths, indicating that they crystallized from the deuteric fluid simultaneously, but in most cases, differences in zoning within any one area of patch perthite indicate that subgrains grew at different times and slowly relative to the frequent changes in concentrations of Ti within the deuteric fluid. The red to infrared CL emission band is inferred to be activated by Fe3+. Ab-rich feldspar with the greatest intensities of long-wavelength emission and FeO concentrations crystallized from a late-stage and relatively low-T fluid that obtained a proportion of its Fe from mafic grains. The generally poor correlation between variations in intensities of the two emission bands within any one subgrain indicates that concentrations of Ti and Fe in the deuteric fluid varied independently of each other, and one trace element has no sensitizing or quenching effect on emission from the other center. The maximum length of zones within individual subgrains (similar to 0.5 mm) and maximum separation of subgrains with similar zoning patterns (similar to 1 mm) helps to constrain the length and interconnectivity of the thin films of deuteric fluid that mediated alteration.

Item type: Article
ID code: 30535
Keywords: cathodoluminescence, feldspar, deuteric alteration, alkali feldspar, electron microscopy, igneous petrology, microporosity, luminescence, microscopy, dependence, chemistry, minerals, trace, Physics, Geochemistry and Petrology, Geophysics
Subjects: Science > Physics
Department: Faculty of Science > Physics
Related URLs:
    Depositing user: Pure Administrator
    Date Deposited: 18 May 2011 15:02
    Last modified: 05 Sep 2014 08:25
    URI: http://strathprints.strath.ac.uk/id/eprint/30535

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