Neodymium and Strontium Isotope Investigation of the Precambrian Kalkkloof Paleosol, South Africa

摘要: The Precambrian Kalkkloof paleosol, South Africa, developed on an Archean ultramafic complex sometime before ∼2.3 billion years (Ga) ago. This weathering profile is of great interest because it formed during a time when many workers believe atmospheric oxygen levels were rising to near-present day concentrations, and cerium (Ce) anomalies have been measured in paleosol samples (Watanabe et al., 2003), indicating formation under high-O₂ conditions. In this study, I applied the samarium-neodymium (Sm-Nd) rubidium-strontium (Rb-Sr) isotope systems parent material. goals study are constrain age pedogenesis determine extent which rare earth elements (REE, including Ce) other mobilized after pedogenesis. Titanium-normalized concentration patterns for Sm Nd consistent with accumulation REE lower portions its formation. Isotopic analysis eight whole-rock samples, material, indicates that Sm-Nd Rb-Sr systematics disturbed by at least two geological events subsequent parent, no meaningful ages obtained. These likely include around 2.5 Ga ago, later metamorphism associated intrusion Bushveld igneous 2 four highest concentrations (>1 ppm) ∈Nd(2.5 Ga) values fractionation also contain significant Ce anomalies. Preservation could result if concentrated relatively resistant trace phases such as phosphates. Thus, data atmosphere (leading oxidation) >2.3 To first approximation multi-stage history suggested data. results further suggest rocks mildly affected and/or ended earlier than about 1.7