Two-dimensional X-ray diffraction as a tool for the rapid, non-destructive detection of low calcite quantities in aragonitic corals

Abstract

Paleoclimate reconstructions based on reef corals require precise detection of diagenetic alteration. Secondary calcite can significantly affect paleotemperature reconstructions at very low amounts of ~1%. X-ray powder diffraction is routinely used to detect diagenetic calcite in aragonitic corals. This procedure has its limitations as single powder samples might not represent the entire coral heterogeneity. A conventional and a 2-D X-ray diffractometer were calibrated with gravimetric powder standards of high and low magnesium calcite (0.3% to 25% calcite). Calcite contents <1% can be recognized with both diffractometer setups based on the peak area of the calcite [104] reflection. An advantage of 2-D-XRD over convenient 1-D-XRD methods is the nondestructive and rapid detection of calcite with relatively high spatial resolution directly on coral slabs. The calcite detection performance of the 2-D-XRD setup was tested on thin sections from fossil Porites sp. samples that, based on powder XRD measurements, showed <1% calcite. Quantification of calcite contents for these thin sections based on 2-D-XRD and digital image analysis showed very similar results. This enables spot measurements with diameters of ∼4 mm, as well as systematic line scans along potential tracks previous to geochemical proxy sampling. In this way, areas affected by diagenetic calcite can be avoided and alternative sampling tracks can be defined. Alternatively, individual sampling positions that show dubious proxy results can later be checked for the presence of calcite. The presented calibration and quantification method can be transferred to any 2-D X-ray diffractometer.