INFRARED PETROGRAPHY OF THIN SECTIONS

Fourier Transform Infrared Spectroscopy (FTIR) and infrared microscopy are powerful, and low-cost, tools for examining water, hydroxyl groups, and carbonate in minerals. Combined with an XY stage, samples may be mapped in resolutions to 30-50 µm. We have developed techniques that allow analysis of minerals in commercially manufactured thin sections without the loss of petrographic context. Minerals with symmetry hexagonal and higher may analyzed quantitatively with polarized radiation. Minerals of lower symmetry also yield useful information.

The Davie County “Orbicular Diorite” is an enigmatic rock showing green orbs of amphibole in a white matrix of microcline and plagioclase, textually dissimilar to other orbicular granites. X-ray maps and petrography show 3-5 distinctly different amphiboles in the orbs. Ca-Mg-Fe composition of the amphibole is reflected in the OH-stretching regions of FTIR spectra. Quantification of OH in clinoamphiboles formally requires three polarized spectra taken in three directions normal to one another. Our analysis of oriented single crystals shows that the various OH absorbances all behave similarly with respect to orientation, so that the different moieties can be treated as percentages of the total stretching region. Preliminary results suggest that the rock records interaction between a hydrous basaltic magma and the surrounding Churchill granite.

Hydrogen defects in quartz have been intensely studied for industrial purposes and to examine hydrolytic weakening under metamorphic conditions. Recently studies have used FTIR to establish provenance of sediments. We conducted reconnaissance analysis of quartz as a possible forensic tool. Systematic analysis of quartz in a weathering sequence of Rolesville granite shows that water from fluid inclusions is the first lost during weathering, then hydrogen associated with Li and B defects. Quartz transported in active sand-sized sediments appears to have little or no hydrogen. Results show that the hydrogen signature in quartz from a single geographic location may have little forensic value, but much greater promise as tool for examining the role of water in granites, and the respective roles of thermal and mechanical weakening during weathering.

This work was supported by National Science Foundation Grant EAR-0929898.