Neodymium and Strontium Isotope Investigation of the Precambrian Kalkkloof Paleosol, South Africa
Year: 2005
Type: article
Abstract: 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 Kalkkloof paleosol samples (Watanabe et al., 2003), indicating formation under high-O₂ conditions. In this study, I applied the samarium-neodymium (Sm-Nd) and rubidium-strontium (Rb-Sr) isotope systems to samples of the Kalkkloof paleosol and parent material. The goals of this study are to constrain the age of pedogenesis of the Kalkkloof paleosol, and to determine the extent to which rare earth elements (REE, including Ce) and other elements were mobilized during and after pedogenesis. Titanium-normalized concentration patterns for Sm and Nd are consistent with accumulation of REE in the lower portions of the weathering profile during its formation. Isotopic analysis of eight whole-rock Kalkkloof samples, including the parent ultramafic material, indicates that the Sm-Nd and Rb-Sr systematics have been disturbed by at least two geological events subsequent to formation of the ultramafic parent, and no meaningful ages were obtained. These events are likely to include weathering and formation of the paleosol around 2.5 Ga ago, and later metamorphism associated with intrusion of the Bushveld igneous complex around 2 Ga ago. The four samples with the highest concentrations of Nd (>1 ppm) have ∈Nd(2.5 Ga) values that are consistent with REE fractionation during weathering around 2.5 Ga ago. These four samples also contain significant Ce anomalies. Preservation of REE systematics could result if the REE were concentrated in relatively resistant trace phases such as phosphates. Thus, the data are consistent with a high-O₂ atmosphere (leading to Ce oxidation) when the paleosol formed >2.3 Ga ago. To a first approximation the Rb-Sr data are consistent with the multi-stage history suggested by the Sm-Nd data. The Rb-Sr results further suggest that the Kalkkloof rocks were at least mildly affected by metamorphism and/or weathering events that ended no earlier than about 1.7 Ga ago.
Author: Katherine Mary Walden
Language: English
Subfield: Geochemistry and Petrology
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W65747317