Neodymium and Strontium Isotope Investigation of the Precambrian Kalkkloof Paleosol, South Africa by Katherine M. Walden Submitted to the Graduate Faculty of Arts and Sciences in partial fulfillment of the requirements for the degree of Master of Science University of Pittsburgh 2005 UNIVERSITY OF PITTSBURGH FACULTY OF ARTS AND SCIENCES This thesis was presented by Katherine M. Walden It was defended on 4-18-2005 and approved by Brian W. Stewart Thomas H. Anderson Charles E. Jones Rosemary C. Capo Neodymium and Strontium Isotope Investigation of the Precambrian Kalkkloof Paleosol, South Africa Katherine Walden, MS University of Pittsburgh, 2005 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 ii 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. iii TABLE OF CONTENTS 1. KALKKLOOF PALEOSOL: INTRODUCTION AND BACKGROUND ........................... 1 1.1. Introduction..................................................................................................................... 1 1.2. Background..................................................................................................................... 3 1.2.1. Area Geology.......................................................................................................... 3 1.2.2. Kalkkloof Paleosol.................................................................................................. 8 1.2.3. Kalkkloof Paleosol age constraints......................................................................... 9 1.2.4. Geochemistry of rare earth elements in paleosols ................................................ 11 1.2.5. Sm-Nd system....................................................................................................... 13 1.2.6. Rb-Sr system......................................................................................................... 15 2. SAMPLE PREPARATION METHODS AND RESULTS.................................................. 17 2.1. Sample Preparation and Isotopic Results...................................................................... 17 2.1.1. Sample Chemistry................................................................................................. 17 2.1.2. Mass spectrometry ................................................................................................ 18 2.2. Radiogenic isotope results ............................................................................................ 19 2.2.1. Sm-Nd System ...................................................................................................... 19 2.2.2. Rb-Sr system......................................................................................................... 26 3. DISCUSSION AND CONCLUSIONS ................................................................................ 32 3.1. Discussion..................................................................................................................... 32 3.1.1. Sm-Nd systematics and REE mobility during pedogenesis.................................. 32 3.1.2. Modeled Sm-Nd evolution of Kalkkloof paleosol samples.................................. 33 3.1.3. Rb-Sr systematics and Sr isotopic evolution ........................................................ 36 3.2. Conclusions................................................................................................................... 39 BIBLIOGRAPHY......................................................................................................................... 41 iv LIST OF TABLES Table 1-1: Major events affecting the Kalkkloof paleosol and protolith...................................... 10 Table 2-1: Kalkkloof Sm-Nd isotope data.................................................................................... 20 Table 2-2: : Kalkkloof Rb-Sr isotope data.................................................................................... 26 v LIST OF FIGURES Figure 1-1: Location map of the Kalkkloof paleosol...................................................................... 4 Figure 1-2: Geologic map showing the Barberton Greenstone Belt, Transvaal Sequence, Archean granite-gneiss, and the Kalkkloof area.................................................................................... 5 Figure 1-3: Idealized cross-section of the Kalkkloof paleosol area showing the unconformity between the Archean units and the Transvaal Sequence ........................................................ 7 Figure 2-1: Percent increase in Sm and Nd (normalized to Ti) relative to Kalkkloof paleosol parent material. ..................................................................................................................... 21 Figure 2-2: Sm/Nd isochron plot for Kalkkloof paleosol samples. ............................................. 23 Figure 2-3: ∈Nd (T) vairations with depth..................................................................................... 25 Figure 2-4: Percent increase in Rb and Sr (normalized to Ti) relative to Kalkkloof paleosol parent material. ..................................................................................................................... 27 Figure 2-5: Rb-Sr isochron diagram for Kalkkloof samples. ....................................................... 29 Figure 2-6: Plot of 87Sr/86Sr(T) variations with depth in the Kalkkloof profile............................ 31 Figure 3-1: ∈Nd values of Kalkkloof samples corrected to 2.5 Ga plotted against Nd concentration. Note the range of εNd values converges with higher Nd concentrations. Open circles are samples with Ce anomalies.................................................................................. 33 Figure 3-2. Model for ∈Nd evolution. ........................................................................................... 35 Figure 3-3. Model for Sr evolution............................................................................................... 38 vi 1. KALKKLOOF PALEOSOL: INTRODUCTION AND BACKGROUND 1.1. Introduction Paleosols, weathering or soil profiles that have been preserved in the geologic record, are products of interactions among the Earth’s atmosphere, hydrosphere, lithosphere and biosphere. The mineralogy and chemistry of paleosols can provide information about atmospheric composition and terrestrial environmental conditions at the time they formed (Holland, 1984; Ohmoto, 1996; Rye and Holland, 1998). During weathering, rocks are physically disrupted and chemically altered or dissolved. Certain elements tend to exhibit mobile behavior during weathering (e.g., alkali and alkaline earth elements) whereas others tend to remain in place (e.g., high charge/radius ions such as Al3+, Ti4+, and the REE3+). Mobile ions can be leached from the upper horizons of a weathering profile and redeposited into lower horizons during chemical weathering. In detail, ion mobility depends on many factors including temperature, Eh, pH, and atmospheric composition, and any element can be mobilized during soil formation given the right conditions. Elements such as iron, vanadium, and cerium behave differently in oxidized and reduced states, and therefore may provide information about oxygen levels in the atmosphere at 2+ the time of formation of the paleosol. For example, Fe in the presence of O2 becomes oxidized to a 3+ state and is then relatively immobile. In a reducing environment, iron