Methodological improvements for investigating silcrete formation: petrography, FT-IR and oxygen isotope ratio of silcrete quartz cement, Lake Eyre Basin (Australia)

Abstract

The oxygen isotopic composition (δ18O) of the cement of a silcrete sample from the “Cordillo silcrete” (Late Eocene/Early Oligocene) of Lake Eyre Basin (Australia) was investigated. The massive structure of the silcrete outcrop and the absence of pedogenic features suggest that this silcrete formed by precipitation from groundwater. In order to avoid errors inherent in estimates of the cement δ18O value from isotopic analyses of bulk material and mass balance considerations, methodological improvements are described. The silcrete cement is physically separated from the detrital phases. Its purity is checked using cathodoluminescence (CL) spectroscopy. Optical microscopy, X-ray diffraction (XRD) analyses, and Fourier transform infrared (FT-IR) spectroscopy show the cement is made of three phases: (i) overgrowth; (ii) crypto-crystalline quartz; and (iii) micro-crystalline quartz. Thus, its δ18O value must be interpreted as that of a mixture, although the mixture consists mainly of crypto-crystalline quartz. As the amount of Si–OH-exchangeable oxygen in the cement would bias the results by an amount less than the analytical error (0.09‰), it is unnecessary to go through an equilibration procedure prior to isotopic analysis. The δ18O value of the cement sample is between 24.8‰ and 25.8‰ relative to V-SMOW. Assuming an Eocene/Oligocene temperature of 15–20 °C, in agreement with published regional temperature estimates, the inferred δ18O value of the quartz-forming solution was between −6.9‰ and −12.2‰. This is lower than the minimum δ18O value of −6.70‰ estimated for the modern groundwater near the sample site. A plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modern one, from shallow groundwater frequently recharged, in a setting of slow tectonic uplift.