ABSTRACT: Diagenetic Phase Composition by an X-Ray Microchemical Method

MELANCON, GINA M., Basin Research Institute, Baton Rouge, LA, and RAY E. FERRELL, Louisiana State University, Baton Rouge, LA

The electron microprobe provides many opportunities for microchemical analysis of the minerals in sedimentary rocks. In this application, X-ray intensities are collected by EDS techniques and processed to provide maps of element variability that are then overlain to chart the distribution of minerals. The size of the area covered, the grid spacing, the elements selected, and the counting time at a grid node may be varied in order to provide results that are most suitable to the samples being analyzed and the objectives of the analysis.

A JEOL 733 Super Probe and the XPHASE program supplied by Tracor-Northern were used to study samples of sandstones and shales embedded in epoxy and prepared as polished thin sections. A smooth, scratchless surface is essential for good analytical results. Data were obtained at 15KV with variable beam currents for Na, Mg, Al, Si, K, Ca, Ti, Mn, and Fe. Phases were defined by the common intersection of pixels representing up to four elements and maps of the overlays illustrate spatial arrangements of the mineral phases. For example, illite was identified at the locations where K, Al, Si, and Fe were present; chlorite was represented by the common occurrence of Mg, Al, Si, and Fe; while quartz was simply identified by the presence of Si. When minerals with the same combinations of elemen s were present, the X-ray intensities attributed to one of the minerals could be removed before processing for the other. In some cases, it was difficult to distinguish illite and an Fe-stained K-feldspar. Maps of mineral distribution could also be processed to provide estimates of mineral abundances.

The results are particularly helpful in recognizing and quantifying mineral abundances in fine-grained materials. In diagenetic studies, one can identify and quantify volumes of pore-filling kaolinite, the extent of illite altering to chlorite, and the composition of expandable clays lining pores and coating grains, etc. Feldspar alteration and the identity of carbonate cements may also be established. Point-to-point spatial resolution is limited to approximately 5 um, but the combination of X-ray and backscattered electron results may reduce this limit.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)