Walden, Katherine Mary
(2005)
Neodymium and Strontium Isotope Investigation of the Precambrian Kalkkloof Paleosol, South Africa.
Master's Thesis, University of Pittsburgh.
(Unpublished)
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.
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Details
| Item Type: |
University of Pittsburgh ETD
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| Status: |
Unpublished |
| Creators/Authors: |
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| ETD Committee: |
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| Date: |
9 June 2005 |
| Date Type: |
Completion |
| Defense Date: |
18 April 2005 |
| Approval Date: |
9 June 2005 |
| Submission Date: |
21 April 2005 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Dietrich School of Arts and Sciences > Geology and Planetary Science |
| Degree: |
MS - Master of Science |
| Thesis Type: |
Master's Thesis |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
Archean atmosphere; paleosol; ultramafic weathering profile |
| Other ID: |
http://etd.library.pitt.edu/ETD/available/etd-04212005-204922/, etd-04212005-204922 |
| Date Deposited: |
10 Nov 2011 19:40 |
| Last Modified: |
15 Nov 2016 13:41 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/7414 |
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