ELEMENTAL ANALYSIS OF HOST-ROCK MATRIX MINERALS ASSOCIATED WITH SOFT-TISSUE PRESERVATION IN THE BURGESS SHALE AND KINZERS FORMATIONS

The factors essential for Burgess Shale-type preservation remain uncertain. Proposed models include the reaction or catalytic involvement of minerals in the sediment matrix (e.g., nontronite) with the organic matter, as well bacterially mediated mineral growth on organic material under certain chemical conditions of pore fluid as evidenced by authigenic minerals such as siderite. To test the validity of these existing models, or provide data on which to base new models it is essential to accurately document the mineral composition of the matrix surrounding Burgess-type fossils. The fine-grain size of host shales (5-25µm) precludes petrographic identification of minerals. XRD-based identification is problematic in that the peaks of minerals with similar atomic structures (e.g., carbonate species) may be difficult to distinguish, and peaks of trace or accessory minerals may not stand-out above background. Microprobe-based elemental mapping may provide a means to accurately identify matrix mineral grains, even when present in trace amounts. Elemental maps of bedding surfaces that contain fossils can be overlaid and mathematically combined by graphic analysis programs such as “ImageJ”. Images can be analyzed pixel by pixel for elemental combinations that are indicative of specific minerals (e.g., Fe and C for siderite, or Ca, Mg, Fe and C for ankerite). An additional benefit to this method over petrographic and powder XRD techniques is that one can be certain that the matrix analyzed is spatially associated with soft-tissue preservation. Initial results using this method on samples taken from the Burgess Shale and the Kinzers formations that host the fossils Marpolia and Margaretia show a matrix that is dominated by muscovite. Chlorite grains are of intermediate Fe-Mg composition rather than being Fe-rich. Carbonate minerals are rare to absent, and no Fe-bearing carbonate minerals have been documented.