Stable-isotope geochronology of the Australian regolith

Abstract

Australian regolith profiles can be assigned to one of three post-Palaeozoic age groups on the basis of the oxygen-isotope composition of authigenic clay minerals developed in the profile. Systematic variations in the isotopic composition of meteoric waters, and therefore of the authigenic regolith minerals that formed in equilibrium with them, are a result of the continent's drift from high to low latitudes and changes in global climate. Residual clays (collected in situ from regolith profiles) of post-mid Tertiary age have δ 18 O values between +17.5 and +21.3%., with the exception of samples from northern Australia (north of approximately 20°S), which have anomalously low values because meteoric waters in northern Australia are largely derived from monsoonal rain. The high δ 18 O, values of post-mid Tertiary residual clays are consistent with Australia's northward drift to low (warm) latitudes during this time, compared with the much higher latitudes it occupied for most of its post-Palaeozoic history. Late Tertiary residual clays have δ 18 O values that are indistinguishable from all other post-mid Tertiary clays, indicating that further subdivision of the sample groups is not feasible on a continent-wide scale. Comparison with hypothetical “modern” clays in equilibrium with modern meteoric waters confirms that there has been very little change in the isotopic composition of meteoric waters in Australia since the mid-Tertiary. Pre-mid Tertiary clays have lower δ 18 O values, between +10.0 and +17.5%., which are consistently lower than post-mid-Tertiary clays. δ 18 O values of less than +14 to +15%. are thought to reflect weathering events during much earlier geologic periods, perhaps early or mid Mesozoic, when Australia was at high latitude and the Australian climate was humid and cool to cold. Several workers have documented the existence of very old regolith profiles and land surfaces in Australia and support for the above hypothesis comes from three analyses of clays from profiles of stratigraphically demonstrable pre-late Mesozoic age which have δ 18 O values between +10.2 and +12.3%.. Regolith profiles composed of low- 18O clays (< +15%.) are widespread in Australia, and it is possible that a much greater part of the modern landscape than previously recognized developed in the early- or mid-Mesozoic. The δ 18 O values of transported (i.e., sedimentary) kaolinites reveals that the major source for many Tertiary kaolinitic sediments was this postulated pre-late Mesozoic low- 18O regolith. The stableisotope evidence that much of the Australian regolith formed in comparatively cold conditions suggests that, contrary to some traditional interpretations, lateritization and deep weathering phenomena are not solely the result of weathering in tropical or sub-tropical climates. The hydrogen-isotope composition of the clays range from −115 to −50%.; however, it is inferred that the majority of clays have undergone some post-formational hydrogen-isotope exchange which renders their δD composition unsuitable for dating purposes.