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Imaging of cathodoluminescence zoning in calcite by scanning electron microscopy and hyperspectral mapping

Martin R. Lee, Robert W. Martin, Carol Trager-Cowan and Paul R. Edwards
Imaging of cathodoluminescence zoning in calcite by scanning electron microscopy and hyperspectral mapping
Journal of Sedimentary Research (March 2005) 75 (2): 313-322

Abstract

Imaging of cathodoluminescence (CL) emission from carbonate minerals by scanning electron microscopy (SEM) is problematic owing to the slow rate at which CL decays from each point as the electron beam is scanned over an area (the phenomenon of phosphorescence). Using SEM-CL and a newly developed electron probe-based technique of hyperspectral mapping, we have evaluated methods that have been proposed to overcome phosphorescence. With the dwell-time technique, the duration of time that the electron beam is static is increased such that CL emission from a given point makes a negligible contribution to the total signal from subsequent points. The dwell-time required to form sharp SEM-CL images of calcite that has a high intensity of luminescence at orange wavelengths is equal to or greater than 6.4 milliseconds, where-as calcite that luminesces predominantly at ultraviolet to blue wavelengths can be imaged using submillisecond dwell times. By using longer dwell times (<1000 ms), CL and X-ray spectra can also be acquired from each point to form hyperspectral maps. The limited-wavelength imaging technique employs optical filters to excise slowly decaying long-wavelength emission so that the image is formed only using the more rapidly decaying ultraviolet to blue wavelengths, allowing shorter dwell times to be used. Both techniques have some disadvantages. The main drawback of the dwell-time technique is the long period of time required to acquire high-resolution images whereas the success of limited-wave-length imaging may depend on the sensitivity of the CL detector being used. CL images and emission spectra acquired by hyperspectral mapping also show that there is a nonlinear relationship between luminescence intensity variations at ultraviolet to blue wavelengths and intensity variations at orange wave-lengths, indicating that short-wavelength emission is an imperfect proxy for zoning at longer wavelengths. Additionally, we have been unable to identify the controls on spatial variations in the intensity of ultraviolet to blue luminescence, although our data discount Fe (super 2+) concentrations as being the sole determinant. By using hyperspectral mapping in combination with electron-probe micro-analysis to obtain CL and X-ray spectra from the same micrometer-sized volume of a material, it is now possible to understand and quantify the controls on activation and quenching of CL in calcite and other minerals such as apatite and zircon.


ISSN: 1527-1404
EISSN: 1938-3681
Serial Title: Journal of Sedimentary Research
Serial Volume: 75
Serial Issue: 2
Title: Imaging of cathodoluminescence zoning in calcite by scanning electron microscopy and hyperspectral mapping
Affiliation: University of Glasgow, Centre for Geosciences, Glasgow, United Kingdom
Pages: 313-322
Published: 200503
Text Language: English
Publisher: Society of Economic Paleontologists and Mineralogists, Tulsa, OK, United States
References: 15
Accession Number: 2005-030986
Categories: Geochemistry of rocks, soils, and sedimentsSedimentary petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Secondary Affiliation: University of Strathclyde, GBR, United Kingdom
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute. Reference includes data supplied by SEPM (Society for Sedimentary Geology), Tulsa, OK, United States
Update Code: 200511
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