SPECTROSCOPY AND ELECTRON MICROSCOPY OF TERRESTRIAL CARBONATES: MINERALOGY, MICROTEXTURES, AND IMPLICATIONS FOR THE STABLE ISOTOPE RECORD OF THE PLIO-PLEISTOCENE MEADE BASIN (SW KANSAS)

The stable isotope composition of terrestrial carbonates, whether conventional isotope ratios or measures of the temperature-dependent clumping of ¹³C and ¹⁸O, is a widely-used tool in paleoenvironmental reconstruction. Recognition of carbonates that retain original mineralogy and microtexture is critical to the use of carbonate geochemistry in paleoenvirornmental studies. These carbonates often form initially as micrite, but soil parent material grain size can influence grain size of pedogenic carbonates and post-depositional processes can lead to recrystallization and/or formation of isotopically and texturally distinct secondary carbonates (e.g., spar). Alteration is typically assessed by macroscopic inspection of hand samples, light microscopy of thin sections to identify spar, and cathodoluminescence to detect multiple episodes of carbonate dissolution and (re)precipitation. Here we report on the mineralogy and microtextures of Miocene to Pleistocene carbonates formed in lacustrine, palustrine, and soil environments in the Meade Basin (SW Kansas) using a combination of bulk powder X-ray diffraction, Fourier transform infrared spectroscopy mapping of thick sections, scanning electron microscopy, and electron back scatter diffraction. Most samples are composed of calcite, but XRD and EBSD indicate that several samples, including early diagenetic burrow cements and laterally discontinuous micritic beds, are dolomite; FTIR maps of these samples indicate that dolomite is pervasive and not just localized detrital grains. Non-carbonate mineralogies include dispersed quartz, feldspar, plagioclases, and minor rutile and Fe-oxides. SEM and EBSD indicate carbonate in several samples comprises ~10x5 μm grains without euhedral faces, regardless of carbonate mineralogy. In one sample, these grains are in a carbonate matrix too fine to resolve by SEM, and in another a small population of larger grains has grown by recrystallization of smaller grains, but no grains exhibit clear preferred crystallographic orientations. Our results suggest 1) the mineralogies and textures of most of our samples are primary, 2) alteration evidenced by CL variability and recrystallization may not overprint original isotopic signals, and 3) the carbonates retain an informative paleoenvironmental archive.