A Differential Scanning Electron Microscope Color-Cathodoluminescence Approach to Provenance Determination of Quartz in Devonian Shales of the Eastern United States

Juergen Schieber
Indiana University, Bloomington, IN

Quartz silt in Devonian black shales of the eastern US is of two principal sources (1) terrigenous, and (2) authigenic. The latter component derives from dissolution of radiolarians that are part of the original sediment budget. Except as inclusions in concretions, radiolaria do not survive early diagenesis and undergo dissolution in pore waters. Resultant silica either “bleeds out” into the overlying water mass or is reprecipitated in void spaces within the sediment (especially in organic cysts).

Authigenic quartz grains typically have rounded-lobate outlines and range from sand to silt size, whereas detrital quartz falls into the same size range but has more angular outlines. On a textural level, however, the authigenic/detrital distinction is blurred by authigenic overgrowth on detrital quartz that makes it very difficult to assess the relative proportions of the detrital vs authigenic component. To add further complexity, CL colors of authigenic quartz (orange-red) overlap significantly with those of detrital quartz (yellow-orange) under normal operating conditions.

Exploring a range of operating parameters shows that authigenic quartz requires approximately 5 times the excitation energy of detrital quartz (largely derived from slates) in order to luminesce with comparable color and intensity. Imaging first under low energy conditions (CL from detrital quartz only) and then under high energy conditions (added CL from authigenic quartz), the respective CL images can be compared to determine the detrital vs authigenic quartz component. With image processing software one can also compare orange band pixel proportions to arrive at a comparable result.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005