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

The oxygen isotopic composition (delta(18)O) 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 6 180 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) microcrystalline quartz. Thus, its delta(18)O value must be interpreted as that of a mixture, although the mixture consists mainly of cryptocrystalline quartz. As the amount of Si-OH-exchangeable oxygen in the cement would bias the results by an amount less than the analytical error (0.09parts per thousand), it is unnecessary to go through an equilibration procedure prior to isotopic analysis. The delta(18)O value of the cement sample is between 24.8parts per thousand and 25.8parts per thousand relative to V-SMOW. Assuming an Eocene/Oligocene temperature of 15-20 degreesC in agreement with published regional temperature estimates, the inferred delta(18)O value of the quartz-forming solution was between -6.9parts per thousand and -12.2parts per thousand. This is lower than the minimum delta(18)O value of -6.70parts per thousand estimated for the modem groundwater near the sample site. A plausible paleoenvironmental scenario is that silcrete formed in a climate colder and wetter than the modem one, from shallow groundwater frequently recharged, in a setting of slow tectonic uplift. (C) 2004 Elsevier B.V. All rights reserved.