Two‐dimensional X‐ray diffraction as a tool for the rapid, nondestructive 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.

ENSO and internal sea surface temperature variability in the tropical Indian Ocean since the Maunder Minimum

“…Representative samples for thin-section, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were selected from all corals based on the X-ray images. The 2-D-XRD system Bruker D8 ADVANCE GADDS at the Rheinisch-Westfaelische Technische Hochschule (RWTH) Aachen was used for non-destructive XRD point-measurements directly on thin-section blocks with a calcite detection limit of ∼ 0.2 % (Smodej et al, 2015). For each coral sample diagenetic modifications were analyzed using one thin-section, one sample for SEM, one 2D-XRD measurement and one powder-XRD measurement.…”

Diagenetic Screening in Porites Fossil Corals from South Pagai, Kendari, and Banten Bay, Indonesia

“…The diffractometer settings were adjusted to achieve the best diffractogram separation between calcite (HMC/LMC) and aragonite. HMC and LMC therefore could be identified with a diffractometer setup based on the calcite reflection peak area [13]. The samples for which the 2D-XRD point test detected a calcite content of around 1% were selected for petrographic thin section and SEM.…”

“…The calcite content in all coral samples was quantified along the growth axis using 2D-XRD, following the methods discussed in [13]. The coral growth axis was recognized from an X-radiograph ( Figure 2).…”

“…XRD is the most convenient and widespread method for mineralogy assessment [11,13]. However, screening for diagenesis cannot rely on XRD alone since it is still problematic to detect the occurrence of secondary aragonite [6,14,18,19,20].…”

“…Nevertheless, conventional XRD preparation is known to have the disadvantage of destroying the coral samples, as it requires a small proportion of the sample in powder form. To avoid this problem, 2D-XRD can be used directly on coral slabs to detect calcite rapidly and nondestructively [13].…”