Browsing by Author "Le Roux, Petrus"
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- ItemOpen AccessCirculation of copper and copper alloys in hinterland southern Africa: material evidence from Great Zimbabwe (1000-1700CE)(2022) Mugabe, Bedone; Chirikure, Shadreck; Le Roux, PetrusThis dissertation seeks to establish the possible provenance of copper and copper alloys used at Great Zimbabwe (1000-1700CE) to infer local and regional circulation circuits. It employs archaeological, archival, and geochemical methods to characterize samples of 26 wound wire bracelets, bangles, anklets, necklaces, and other forms of expressive and decorative wear excavated from the southern terrace, (Hill Complex) at Great Zimbabwe. The samples were macroscopically, microscopically, chemically and isotopically characterized to understand the technique of manufacture, chemical composition, lead isotopic ratios and the starting composition of uranium and thorium in relation the model ages. The Pb isotopic data calculated 238U/204Pb (μ), 232Th/ 238U(κ) ratios and estimated model ages (T) were compared with published lead isotopic ratios from various ore samples from central and southern Africa. Within opportunities afforded by the historical record and limitations imposed by small sample sizes especially of the comparative geology, indications are that the studied objects were produced using ores matching the Cu-Ni mineralization at Empress Mine (in Zimbabwe), Cu Ag and Cu Pb Zn Ag mineralization associated with the Kundelungu Plateau (bordering Zambia and Democratic Republic of Congo). This suggests that Great Zimbabwe participated directly and indirectly in the vibrant micro, regional and interregional circulation of copper and other resources. Future work, must however, add more trace element data from the objects and increase the number of samples from ancient mines and other sites to develop a fuller picture of the movement of copper in Iron Age southern Africa through space and time.
- ItemOpen AccessLead isotopes as a palaeodietary tracer in southwestern South Africa(2018) Scott, Mari; Le Roux, Petrus; Sealy, Judith; Pickering, RobynThis thesis evaluates the utility of lead (Pb) isotopes, in combination with strontium (Sr) isotopes, as a geochemical tracer for studying the palaeodiets and palaeo-landscape usage in southwestern South Africa. Isotopes of light elements, carbon (C), nitrogen (N), oxygen (O), and sulphur (S), are widely used as (palaeo) environmental tracers, but do not yield information on the geological substrates on which individuals have lived. Sr isotopes in bones and teeth are useful in distinguishing between areas of distinct bedrock geology, however, the efficiency of Sr is limited at near-coastal areas, which forms a major part of this study area. This is because Sr has a relatively high concentration and long residence time in seawater. In addition, coastal soils contain not only aerosol-derived marine Sr, but frequently also include fragments of shells and other marine carbonates, so their 87Sr/86Sr is like the ocean. This study analysed Pb and Sr concentrations and isotopic compositions of animals and plants derived from the various geological substrates of southwestern South Africa. In order to do this, a detailed Sr-Pb separation scheme was developed, involving the separation and pre-concentration of Sr and Pb from a single digested sample by means of ion-exchange chromatography. Elemental concentrations were measured with a Thermo X-series II quadrupole ICP-MS instrument. Sr concentrations ranged between 111 ppm and 1862 ppm, while Pb concentrations were lower, ranging between 0.012 ppm and 2.30 ppm. Isotopic ratios were determined by means of a Nu Instruments high resolution multi-collector inductively-coupled plasma mass spectrometry (HR-MC-ICP-MS). Samples were introduced into the system as solutions, producing an order of magnitude more precise results than laser ablation analysis on the same material. Sr isotopes are useful for distinguishing between individuals living in near-coastal environments and those living further inland, while Pb isotopes could differentiate between granites and shales/sandstones. Pb isotopes proved to be a valuable palaeodietary tracer and can be used in combination with Sr isotopes to extent our knowledge of palaeo-landscape usage at coastal-marine environments.
- ItemOpen AccessOrigin and evolution of parental magmas associated with Pliocene-Quaternary low-silica volcanism within the Altiplano-Puna Volcanic Complex of northern Chile(2020) González Maurel, Osvaldo Patricio; Le Roux, Petrus; Riquelme, RodrigoThe Andean continental arc is built upon the thickest crust on Earth, whose eruption products reflect varying degrees of crustal assimilation. Physical interactions between mafic and felsic magmas for this arc are therefore difficult to recognize due to the differentiation of mantle-derived magma during ascent through the thickened crust and a corresponding lack of erupted primitive lavas. However, a rare concentration of less evolved rocks is located marginal to the partially molten Altiplano-Puna Magma Body (APMB) in the Altiplano-Puna Volcanic Complex of northern Chile, between 21º10'S and 22º50'S. To unravel the petrogenesis of these less evolved eruptives and their spatiotemporal assessment, this work makes use of new major and trace element data, and Sr and Nd isotope ratios of fourteen Pliocene to Quaternary volcanoes. Whole-rock compositional and Sr and Nd isotope data reveal a large degree for compositional heterogeneity (e.g. SiO2 = 53.2 to 63.2 wt%, MgO = 1.74 to 6.08 wt%, Cr = 2 to 382 ppm, Sr = 304 to 885 ppm, 87Sr/86Sr = 0.7055 to 0.7088, and 143Nd/144Nd = 0.5122 to 0.5125). The least evolved products erupted along the periphery of the APMB and are likely equivalent to the replenishing magmas that thermally sustain the large APMB system. Here it is found that the investigated mafic to intermediate eruptives reflect mafic melt injections that underplate the APMB and escape along the sides of the large felsic body to avoid significant compositional modifications during ascent (e.g. La Poruña, San Pedro, Palpana, Chela volcanoes). Investigating these magmas therefore assist in assessing the evolution of the APMB through space and time. Additionally, individual volcanoes demonstrate that contamination of parental melts was caused by different differentiation processes (e.g. magma mixing, fractional crystallization, crustal assimilation) as a function of their spatiotemporal framework relative to the APMB and its melt fraction variation from peripheral (ca. 4 vol%) to central (up to 25 vol%) portions. New insight into the petrological processes that governed the magmatic evolution of La Poruña, one of the centres that exhibit the least evolved eruptions in the area, is presented. La Poruña is a 100 ka scoria cone, composed of pyroclastic material and an extensive basaltic-andesite to andesite lava flow, with magmatic evolution related to the neighbouring larger San Pedro stratovolcano. New petrography, geochemistry and radiogenic isotope data describe a set of porphyritic mafic samples, comprising olivine- and pyroxene-rich rocks, with well-defined major element compositional trends, as well as trace and rare earth element characteristics, that reflect magmatic differentiation at midupper crustal levels. Additionally, magma mixing as well as assimilation and fractional crystallization processes acted on these La Poruña magmas. A remarkable compositional feature is the unusual reversed isotopic behaviour of increasing silica with decreasing 87Sr/86Sr compositions related to later magmatic evolution involving selective assimilation during turbulent ascent at shallow crustal levels prior to eruption, therefore differing from the broadly accepted Central Andean magmatic model. In order to robustly model magma evolution and assimilation at subduction zones such as the Andes, the compositions of parental magmas feeding crustal magma reservoirs need to be defined. Here, new olivine and clinopyroxene oxygen isotope data from six volcanoes located at the margins of the giant APMB is presented, as these data provide robust constraints on parental magma genesis and further contamination processes. Existing olivine and pyroxene 18O values for the Central Andes are highly variable and potentially not representative of sub-arc parental compositions. However, new olivine (n = 6) and clinopyroxene (n = 12) 18O values obtained by Laser Fluorination (LF) analysis display a narrow range, with averages at 6.0‰ ± 0.2 (2σ S.D.) and 6.7‰ ± 0.3 (2σ S.D.), consistent with a common history for the investigated minerals. Additionally, the first in-situ Secondary Ionisation Mass Spectrometry (SIMS) oxygen isotope data for mafic mineral phases from La Poruña and Palpana is presented in order to ensure a robust parental melt δ18O composition, since unlike in LF analysis fractures or mineral/melt inclusions can be avoided. Crystal cores show a density peak at 5.6‰ for both La Poruña and Palpana, whereas rims have either relatively high (6.0‰) or low (5.4‰) peaks. Intra-crystal differences (up to ± 1.1‰) in 18O values between crystal cores and rims accurately record crustal assimilation that postdate cores formation at 13 to 19 km depth. Based on the more robust dataset of LF results, a 18O value for sub-arc, parental melts of ca. 7.0‰ ± 0.2 (2σ S.D.) can be estimated. SIMS core data, on the other hand, indicate a lower, but less well-constrained parental melt value of 6.3‰ ± 0.4 (2σ S.D.).Parental melts feeding the APMB and associated volcanic centres are proposed to form in the felsic continental crust following assimilation of up to 28 vol% high 18O-high 87Sr/86Sr basement rocks by mantle-derived magmas.
- ItemOpen AccessOxygen and radiogenic isotope constraints on the generation of coexisting silica-undersaturated and oversaturated felsic magmas of the Ditrau Alkaline Massif (Romania)(2018) Odri, Agnes; Harris, Chris; Le Roux, PetrusThe Ditrău Alkaline Massif (DAM) is a Mesozoic igneous complex (~200-230 Ma) was generated in a continental rift environment. This study has investigated the relationship between the silica-undersaturated and oversaturated Ditrău rocks based on oxygen, Sr, Nd and Pb isotope data. Different models have been proposed previously to explain the coexisting silica-undersaturated and oversaturated rocks in general: (1) both silicaundersaturated and oversaturated magmas evolve from the same mantle-derived, undersaturated melt, with the oversaturated rocks being produced by crustal assimilation (Foland et al., 1993); (2) both are derived from the same mantle source, but do not evolve from the same parental melt (Giret and Lameyre, 1985); (3) the sources are heterogeneous on a scale that allows undersaturated and oversaturated melts to be produced (Harris, 1995). The previous petrogenetic model of the DAM emphasises the importance of fractional crystallisation of a silica-undersaturated, mantle-derived magma (e.g. Morogan et al., 2000). The Ditrău rocks show variable initial 87Sr/86Sr ratios up to 0.72372, whereas ɛNdi values are from +5.5 to +0.8. High-temperature equilibrium O-isotope fractionations between minerals are generally preserved, but there was some sub-solidus O-isotope re-equilibration during slow cooling. Quartz, zircon and amphibole were used as a proxy for the magma δ18O values. The estimated magma δ18O values for Ditrău rocks are from 5.7 to 11.7‰. Isotope data are consistent with the involvement of both mantle and crustal sources in the formation of the DAM. The Nd and O isotope values of quartz monzonite can be explained by the subequal contribution of mantle-derived camptonite and the lower crustal partial melts generated by the intrusion of hot, mantle magmas. Both O and Nd isotope values show the dominantly mantle origin of hornblendites, diorites and nepheline syenites. The Nd and O isotope variations imply the involvement of 20-65% upper crustal melts into the re-injected camptonite magma to generate the Ditrău syenites, quartz syenites and granites. The isotope results of the DAM require the combination of the previously proposed models to explain its evolution and the relationship of the silica-undersaturated and oversaturated rocks.
- ItemOpen AccessPetrogenesis of lavas from Volcano Azufre, Northern Chile: evidence for crustal input(2019) Lister, James; Le Roux, Petrus; Harris, ChrisMagmatism that occurs within the Central Volcanic Zone (CVZ) of the Andes is heavily influenced by processes of differentiation and assimilation that occur within the extremely thick continental crust. These processes, that result in crustal contamination of mantle derived magmas, are not uniform across the region and are determined locally. Data from Azufre is combined with that of previously studied volcanoes of the CVZ, and specifically the Altiplano-Puna Volcanic Complex (APVC) in northern Chile, to further understand such processes that have occurred in this region. Factors that need to be taken into consideration when examining volcanoes of the APVC are the aforementioned thickness of the continental crust of the Andes, which differentiates the Andes from other volcanic arcs globally, and the presence of a zone of partial melt, the Altiplano-Puna Magma Body (APMB), below the APVC as they could both have had a major effect on erupting lavas. Elemental concentration and isotope data are used here to examine the role that the APMB has had on the evolution of Azufre. This data set is also compared to those of previously studied volcanoes of the APVC to further provide understanding as to the interaction between the APMB and the APVC. Major element data reveals well defined sub-alkaline trends, with samples predominantly being andesitic in composition. 87Sr/86Sr ratios of 0.7067 - 0.7075 and δ 18O values of 7.85 - 9.72 are high in comparison to those of other Central Andean lavas, however in comparison to other erupted lavas of the APVC they are low. A transition from the edges of the APVC towards the centre show an increase in 87Sr/86Sr and δ 18O. Conversely 143Nd/144Nd ratios decrease towards the centre, with Azufre displaying ratios of 0.5123 - 0.5124. These changing ratios from the edge to centre of the APVC correspond with changing thickness of the APMB below the APVC and are therefore viewed to represent the amount of interaction between parental magmas of the APVC with the partial melt of the APMB, primarily with increasing 87Sr/86Sr indicating increasing amounts of interaction. δ 18O data indicates that there are multiple sources of O enrichment, evidenced by two opposite data arrays, with one array indicating assimilation by magmas of high δ 18O crust and the second array indicating assimilation of low δ 18O crust. Modelling of δ 18O suggests that the magmas which have assimilated low δ 18O crustal material have interacted with between -2.01 and 3.49 . Low LREE/HREE ratios and a negative Eu anomaly support the idea that assimilation of crustal material occurred at shallow crustal levels by indicating shallow plagioclase fractionation and evolution in a garnet free environment, most likely within the APMB.
- ItemOpen AccessPetrogenesis of the peralkaline granite (and associated syenite) dykes of the Straumsvola Complex, Western Dronning Maud Land, Antarctica(2015) Dreyer, Tanya Shayna; Harris, Chris; Le Roux, PetrusThe Straumsvola nepheline syenite complex in Western Dronning Maud Land, Antarctica consists of an outer massive and inner layered nepheline syenite that is itself intruded by ijolite. The complex and its immediate country rock was intruded by numerous dykes. A small proportion of these dykes are peralkaline syenite and microgranite, and these are found only to intrude the nepheline syenite and not the country rock. The presence of peralkaline granite dykes intruding a silica-undersaturated complex is unexpected, given the thermal divide that exists at low pressures between silica-under and -oversaturated phases. Major and trace element variations in the dykes are found to be consistent with fractional crystallisation of a parental peralkaline magma of trachyte composition.