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Influence of Elemental Composition on Fluorescence of Diagenetic Siderite

Peter Snow Mozley

The fluorescence characteristics of siderite in the Sag River and Shublik Formations (North Slope, Alaska) are extremely variable. Some of the siderite fluoresces strongly upon exposure to UV light, whereas others do not fluoresce at all. These differences in degree of fluorescence are commonly observed on the scale of a single crystal in which alternating zones of fluorescent and nonfluorescent siderite occur.

The cause(s) of fluorescence in carbonate minerals is poorly understood. Possible mechanisms for carbonate fluorescence include trace element activation (similar to that responsible for cathodoluminescence) and incorporation of fluorescent organic matter into the crystal structure. Microprobe analyses of siderite in conjunction with high-resolution back-scattered electron imaging strongly suggest that the degree of fluorescence of siderite in the Sag River and Shublik Formations is closely related to its elemental composition. In all cases, siderite that contains less than 77% FeCO3 (MgCO3 and CaCO3 substitute to varying degrees for FeCO3) fluoresces upon exposure to UV light, whereas siderite that contains greater than 77% FeCO3 s nonfluorescent.

Work is in progress to determine if the above finding applies to diagenetic siderite from other formations as well. If so, UV fluorescence petrography may be a useful means of estimating the elemental composition of diagenetic siderite.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.