Laser ablation ICP-MS age determination of detrital zircon populations in the Phanerozoic Cape and Lower Karoo Supergroups (South Africa) and correlatives in Argentina.
- Authors: Vorster, Clarisa
- Date: 2014-01-14
- Subjects: Karoo Supergroup , Cape Supergroup , Zircon - South Africa , Zircon - Argentinia , Inductively coupled plasma mass spectrometry , Geochronometry
- Type: Thesis
- Identifier: uj:7897 , http://hdl.handle.net/10210/8789
- Description: Ph.D. (Geology) , The successions of the Cape- and Karoo Supergroups preserve an integrated history of sedimentation along the paleo-Pacific margin of Gondwana from the Paleozoic to the Early Mesozoic. The Cape- and Karoo Supergroups have been well studied with regard to stratigraphy, sedimentary facies and depositional environment. However, the nature and location of their source regions, especially for the changeover from deposition within an Atlantic-type continental margin basin for the successions of the Cape Supergroup to an Andean-type continental foreland basin for some of the units of the Karoo Supergroup, remains poorly understood. In order to shed light on the nature of these source regions, a comprehensive U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was launched. A representative number of samples from the upper and lower successions of the Table Mountain- Bokkeveld- and Witteberg Groups of the Cape Supergroup as well as the Dwyka and Ecca Groups of the Karoo Supergroup were collected throughout the western, southwestern and southern Cape region. A few samples of the Dwyka Group were also collected within the more eastern outcrop regions of the succession located in Kwazulu-Natal. The sedimentary rocks of the Natal Group and Msikaba Formation have long been regarded as coeval with the Cape Supergroup. Similar to the successions of the Cape- and Karoo Supergroups, very little is known about their sedimentary source regions. Also, their relative age of sedimentation remains poorly constrained. The U-Pb detrital zircon study of the successions of the Cape- and lower Karoo Supergroups was thus extended so as to include the successions of the Natal Group and Msikaba Formation. The detrital zircon age populations of the successions of the Natal Group and Msikaba Formation would not only improve the present understanding with regards to the sedimentary source regions to these units but would also facilitate the evaluation of possible correlations between these units and the stratigraphic units of the Cape Supergroup. Samples of both the lower Durban Formation and the upper Mariannhill Formation of the Natal Group and the Msikaba Formation (which is presently regarded as being part of the Cape Supergroup) were therefore collected within their respective outcrop regions in the Kwazulu-Natal area. The similarities in litho- and bio-stratigraphy between the successions of the Cape- and Karoo Supergroups and those of the Ordovician to Early Permian successions of the Ventania System and the Ordovician to Silurian successions of the Tandilia System in Argentina have long been recognized. Although the detrital zircon populations of some of the formations within these Systems have been evaluated in the past, it is yet to be determined whether these successions and those of the Cape- and lower Karoo Supergroups have certain source regions in common. In order to facilitate such a comparison, samples of selected units of the Ventania System were therefore collected near Sierra de la Ventania, while a sample of the Balcarce Formation of the Tandilia System was obtained near Mar del Plata. The detrital zircon age populations of the successions of the Ventania and Tandilia Systems were also further evaluated in the light of establishing or confirming a time-correlation between these formations and those of the Cape- and lower Karoo Supergroups. U-Pb age determination of the detrital zircons population of the samples was conducted by means of Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS). Although LA-ICP-MS is a routine, well-established technique where the U-Pb age determination of detrital zircons is concerned, it was yet to be established at the centralized analytical facility of the University of Johannesburg, SPECTRUM, using the instrumentation currently available (i.e. 213nm Nd:YAG laser coupled to Quadrupole-based ICP-MS). The U-Pb age determination of detrital zircons was therefore preceded by a fair amount of instrument optimization and method development. Well studied shortcomings of U-Pb detrital zircon dating by LA-ICP-MS such as laser induced elemental fractionation, mass discrimination effects and as well as the possible occurrence of minor common-Pb needs were addressed and corrected for. The detrital zircon populations of successions in the Cape Supergroup have a distinct major Neoproterozoic to Early Cambrian age component, which can be attributed to an input of detritus from successions related to the Pan-African Orogeny in South Africa, such as the Gariep- and Saldania Belts located towards the north of the Cape Basin. A substantial amount of Mesoproterozoic detrital zircon grains is also present in all the samples from the successions of the Cape Supergroup. These grains of Mesoproterozoic age were probably derived from the Namaqua-Natal Metamorphic Province, which is also regarded as the source of some minor amounts of Paleoproterozoic detrital zircon grains. The near absence of Archean grains from the detrital zircon populations of the successions of the Cape Supergroup is notable, and is thought to be due to the Namaqua-Natal Metamorphic Province acting as a geomorphological barrier at the time of their deposition. The minor Paleozoic (Ordovician to Carboniferous) detrital zircon populations in the samples from the formations of the Cape Supergroup increase progressively upwards through the succession. ....
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Precise 87Sr/86Sr isotope ratio measurement by quadrupole based ICP-MS after ion exchange separation of Rb for provenance determination of agricultural products
- Authors: Vorster, Clarisa
- Date: 2010-04-08T08:33:41Z
- Subjects: Wine and wine making , Strontium isotopes , Inductively coupled plasma mass spectrometry , Ion exchange chromatography , Provenance trials
- Type: Thesis
- Identifier: uj:6750 , http://hdl.handle.net/10210/3158
- Description: M.Sc. , The strontium isotope ratio 87Sr/86Sr in the wine and soil of four wine-producing regions in South Africa has been determined using quadrupole ICP-MS. Isotope ratio measurements with fairly good precision had been obtained and 87Sr/86Sr ratio values in wine and soil samples were statistically evaluated to determine whether the regions investigated could be distinguished. Statistical interpretation of data revealed that the Robertson wine-producing region could be distinguished from Stellenbosch, Swartland and Walker Bay wine regions on the basis of their 87Sr/86Sr ratio. By using a combination of 87Sr/86Sr and multielement data, canonical discriminant functions with very good classification abilities could be obtained to correctly distinguish between wine and soil samples originating from a specific production region. Due to the isobaric overlap of strontium and rubidium at mass 87, a very good strontium-rubidium separation method had to be developed. The chelating properties of EDTA and DCTA in combination with ion exchange chromatography were explored in order to achieve such a separation. Due to the advanced rubidium separation ability of the developed DCTA elution method, it has been employed in all wine and soil sample preparations. Since chromatographic ion exchange procedures are not capable of removing all the rubidium from the sample matrices, 87Sr/86Sr ratio values were mathematically corrected for residual rubidium using an 88Sr correction. Since strontium isotope ratio measurements require very good precision, it is traditionally determined using TIMS. However, due to the ease of operation, shorter analysis time and widespread availability of quadrupole ICP-MS, the technique was evaluated for its merit for determining strontium isotope ratios. The optimization of acquisition parameters for the determination of the 87Sr/86Sr ratio in wine and soil samples with good precision of below 0.1% RSD has been conducted. Furthermore, selected sample matrix components were assessed as possible matrix interferences as well as for their ability to affect the precision of measurements. It was concluded that with adequate optimization of acquisition parameters, proper sample preparation and reliable mathematical correction of data, 87Sr/86Sr ratio measurements with reasonable precision could be obtained using quadrupole ICP-MS.
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