Browsing by Author "Harris, Chris"
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- ItemOpen AccessCratonic eclogite xenoliths - formation and evolution of the subcontinental lithospheric mantle(2018) Radu, Ioana-Bogdana; Harris, Chris; Cottin, J-Y; Moine, B NThe most extensive evidence for the evolution of the primitive crust is preserved at the base of the continental roots (140 to 200 km depth). However, this is completely inaccessible and hard to evaluate, except through the study of mantle xenoliths brought to the surface by kimberlite magmas. Most mantle xenoliths undergo kimberlitic/carbonaceous metasomatism prior to their entrapment, altering their primary composition. Despite a very complex history and a very low abundance (180 nodules), sampled in four main localities from the Siberian (Obnazhennaya, Udachnaya) and South African (Jagersfontein, Roberts Victor) cratons. Major and trace element compositions have been analysed in representative bimineralic (garnet (gt)-omphacite (cpx)), coesite-, kyaniteand corundum-bearing eclogites. Metasomatized eclogites (Type I) have a higher alkali- (Na2O in garnet and K2O in omphacite) and LREE, LILE-content. Non-metasomatized (Type II) eclogites include two main compositional groups, derived from different protoliths. The first group (Type IIA) is typically more magnesian, enriched in LREE and has δ18O values from 3.73 to 7.50 ‰, with positive Sr anomalies and corresponding to a low pressure-temperature layer. The whole-rock composition is consistent with a basaltic protolith. The second group (Type IIB) is more sodic and aluminous, depleted in LREE and has δ18O values of 2.35 to 3.59 ‰, corresponding to equilibrium at high pressure and temperature. The whole-rock trace element composition is consistent with a pyroxenitic protolith. Eclogites that contain coesite, kyanite and corundum (coe-ky-cor) are typically characterized by jadeite-rich clinopyroxenes with positive Eu and Sr anomalies and grossular-rich garnets with corresponding positive Eu and negative Sr anomalies. Additionally, corundum-bearing samples are overall LREE-depleted. Pressure-temperature estimates indicate coe-ky-cor-bearing eclogites equilibrated in the lowermost part of the cratonic keel, and the reconstructed whole-rock trace element composition corresponds to a very depleted gabbroic protolith. Corundum-bearing eclogites often show topotactic exsolution textures hosted in omphacite, consisting of a more calcic garnet ± kyanite/zoisite. Based on Si, Al and Mg diffusion profiles it can be inferred that exsolution was controlled by chemical exchange in an isochemical system. Similarly, an interpretation can be made for the variable HREE fractionation of intermediate composition between the exsolutions and the host omphacite, as evidence for intermineral partitioning. The formation of zoisite (1.7 % H2O) from a precursor clinopyroxene requires a significant amount of hydrogen (as H+ , OH- or H2O) incorporated in omphacite at mantle conditions. Calculated water content of omphacite, based on Fourier transform infrared spectrometry (FTIR) analyses, varies from ~930—1410 to ~1100—1680 ppm by weight H2O, according to different absorption coefficients. Primary garnets are typically anhydrous (<7 ppm H2O), whereas coarse exsolutions contain between 165—1950 ppm H2O. Reconstructed estimates for whole-rock water content (~310—890 ppm H2O) for the Obnazhennaya eclogites are significantly higher than those of the surrounding peridotites. The variability of δ18O in garnet among 41 xenoliths, shows a bi-modal distribution with median values at 3.57 ‰ and 5.68 ‰ and strong correlation (r = 0.96) between garnet and omphacite. The δ18O values and the reconstructed whole-rock trace element compositions indicate an oceanic crustal protolith for mantle eclogite xenoliths. This is consistent with the subduction of a hydrothermally altered, basaltic to websteritic sequence of an incompatible-element-depleted oceanic crust. Furthermore, although eclogites represent a small fraction of the upper mantle, they may be an important water reservoir at the base of the cratonic root. High water content in the lithospheric mantle would have major consequences for the longevity of the cratonic keel, forphysical and chemical properties in minerals, partial melting, mantle rheology and electrical conductivity and the global water cycle.
- ItemOpen AccessGeochemical and isotopic constraints on the source regions of phanerozoic carbonatites and associated alkaline rocks from the Zandkopsdrift complex of Namaqualand, South Africa, and the Marinkas Quellen, and Dicker Willem complexes in Namibia(2020) Ogungbuyi, Ibiyemi Prisca; Janney, Philip Edward; Harris, ChrisThis is a study of Phanerozoic carbonatites and related alkaline silicate rocks from Zandkopsdrift in Namaqualand, South Africa (55 Ma), and Marinkas Quellen (529 Ma), and Dicker Willem (49 Ma) in southern Namibia, all emplaced within the Proterozoic Namaqua Natal province. The aims of this thesis are to characterise their mantle source region, including the timing and nature of the metasomatism that affected their sources, and to constrain their petrogenesis, particularly the relationships between the carbonatites and associated silicate igneous rocks. These associated silicate igneous rocks include olivine melilitite, aillikite and alkaline lamrophyre at Zandkopsdrift, nepheline syenite and trachyte at Marinkas Quellen, and ijolite and trachyte at Dicker Willem. At both Marinkas Quellen and Dicker Willem, the trachytes appear to be derived primarily from fenitised country rock. The major and trace element characteristics, enrichment in LREE and other incompatible elements, large Zr-Hf-Ti depletions and high Zr/Hf ratios all appear to have been inherited from a metasomatised mantle source region rather than being the result of residual source mineralogy. The δ18O and δ13C values of carbonate in the study locations vary significantly. The δ13C values (-3.9 to -8.8 ‰) are within the range of mantlederived carbonatites, whereas the δ18O values are often significantly higher (+8.64 to +22.22 ‰, versus SMOW) for “mantle-derived” carbonatites. The higher δ18O values observed are most likely attributable to low-temperature, post-emplacement alteration by hydrous fluids. O-isotope thermometry of the silicate mineral pairs (clinopyroxene, amphibole, and biotite) gives close to magmatic temperatures (≈800 oC), whereas the equilibration temperature of calcite-silicate mineral pairs is lower. The carbonatite and associated igneous rock samples contain unradiogenic Sr and mildly radiogenic Nd isotope compositions below and above Bulk Earth/CHUR values respectively. This suggests that carbonatite magmas were generated from sources with long-lived Rb/Sr lower than, and Sm/Nd higher than, the primitive mantle. In ƐHf(t)-ƐNd(t) space, the carbonatites and associated silicate rocks plot as much as 8 ƐHf units below the terrestrial ƐNd-ƐHf array, indicating mixing of a source with moderate ƐNd and exceptionally unradiogenic Hf isotope compositions. The radiogenic Pb isotope composition of the carbonatites (206Pb/204Pbi ratios from 18.06 to 22.38), is consistent with a source having high U/Pb, akin to the HIMU mantle end member. The radiogenic isotopes of the carbonatites and the alkaline silicate rocks seem most consistent with a dominantly asthenospheric source, with minor contributions from lithospheric sources. There is little evidence supporting the derivation of carbonatites at the three complexes from parental hybrid carbonate-silicate magmas. Rather, the evidence seems most consistent with deriving the carbonatites directly from very low-degree mantle melts, which subsequently become variably differentiated, first by melt-rock interaction in the mantle and subsequently by fractional crystallization. The associated silica undersaturated silicate igneous rocks appear most likely to represent relatively primitive (melilitites) to differentiated (nepheline syenites) melts of metasomatic wehrlites that were formed by carbonatite-peridotite melt-rock interaction.
- ItemOpen AccessGeochemistry, stable isotopes and fluid inclusion studies of the Otjikoto gold deposit, Central Namibia(2018) Negonga, Lisias; Greyling, Lynnette; Harris, ChrisThe Otjikoto gold deposit is located within the Damara Orogenic Belt which is part of the system of Pan-African mobile belts. The Damara Belt is a northwest trending orogenic belt which formed through sequences of spreading, rifting, subduction and from continental collision between the Congo, Kalahari and Rio Delta Plata Cratons between 900 Ma and 450 Ma. Mineralisation at Otjikoto is hosted by a group of metamorphic rocks which consist of biotite schists from the Okonguarri Formation, albitites from the Oberwasser member and marble units from the Okawayo member. The gold mineralization occurs in a sheeted vein network system with veins which are positioned parallel towards the foliation of the albitites and hornfels of the Oberwasser Member. The vein types include both quartz and carbonate hosted vein types. The vein mineralogy includes magnetite, pyrite and pyrrhotite which are associated with amphibole, quartz, almandine garnets, and free gold. A fluid inclusion study carried out on the vein quartz showed the co-existence of the aqueous, carbonic and aqueouscarbonic fluid inclusions within the Okonguarri Formation. The aqueous inclusions have moderate to high salinities with salinities ranging from 30.0 to 44.5 wt. % NaCl eq. in aqueous inclusions with halite crystals and from 0.9 to 22.9 wt. % NaCl eq. in inclusions without halite crystals. The total homogenisation temperatures, Th tot (L), ranged from 151 to 488 °C in the aqueous-carbonic inclusions and from 156 to 444 °C in the aqueous inclusions. The melting temperature of CO2 (TmCO2) ranged from -60.3 to -57.5 °C in carbonic inclusions and from - 60.2 to -57.2 °C in aqueous-carbonic inclusions. It is proposed that the aqueous and carbonic inclusions (CO2-CH4) from the Okonguarri Formation are the end members of fluid immiscibility. The presence of CO2 and CH4 is indicated by the low TmCO2 values and may be justified by the decomposition of the carbonaceous metasediments which are found in the stratigraphy at Otjikoto. The δ 18O values range from of 8.35 ‰ to 8.75 ‰ for the magnetite and 17.51 ‰ to 18.47 ‰ for the vein quartz samples. Analysis of the microprobe data on the amphiboles shows that the analysed grains are made up of anthophyllites. Microprobe data of the chlorites showed that the majority of the chlorites are made up of ripidolites. Temperatures of chlorite formation were calculated to be in the range of 221°C to 358 °C by using chlorite geothermometric equations. Major element variation diagrams show a strong negative correlation of TiO2, Al2O3, MgO, Na2O and FeOt vs SiO2. Variation diagrams were also plotted for the trace elements vs Zr. The Sr, Y, La and Ce vs Zr plots show a strong positive correlation.
- ItemOpen AccessGroundwater hydrogen, oxygen, and strontium isotope variation of Hout Bay Valley, Cape Town(2022) Daws, Warrick; Harris, Chris; Sealy, JudithAccessible groundwater can be an alternative water supply to meet water demands and elevate stress from existing water supply systems in semi-arid regions. Monitoring stable isotope compositions (δD and δ18O) in the hydrosphere can aid hydrogeologists in understanding groundwater processes. The δD and δ18O of 148 water samples (groundwater, rainwater, and stream water) from Hout Bay, a coastal valley 20 km south-west of Cape Town are considered in this study. Groundwater samples from 65 boreholes/wellpoints is presented over two distinct phases (February/March 2020, and November 2020) with groundwater electrical conductivity and groundwater temperature measurements. Hout Bay groundwater δD and δ18O range from -16.5‰ to -6.4‰ and -3.90‰ to -2.14‰ respectively and show no strong correlation with seasonality or elevation above sea level. Electrical conductivity of Hout Bay 2020 groundwater is relatively low ranging from 126 µS/cm to 2370 µS/cm. Hout Bay monthly rainwater amount, δD, and δ18O were measured from March 2020 to February 2021. The weighted mean δD and δ18O values of Hout Bay monthly rainwater from March 2020 to February 2021 are - 6.2‰ and -2.38‰ respectively. The δD and δ18O values of Hout Bay rainwater cluster around the local meteoric water line established for UCT, and show a strong amount effect (r = -0.92 for δD vs amount). Three possible explanations are proposed for the δD and δ18O difference between Hout Bay monthly rainwater weighted mean from March 2020 to February 2021, and Hout Bay February/March 2020 and November 2020 groundwater. The first is that intense rainfall events preferentially recharge groundwater with low δD and δ18O values. The second explanation is that Hout Bay groundwater might have retained more negative δD and δ18O values from rainwater recharge prior to March 2020. Lastly, recharge from mountain peaks at higher elevation than the Hout Bay groundwater sample locations add a component of more negative δD and δ18O to Hout Bay groundwater than proximal rainwater. Nine samples from Hout Bay February/March 2020 groundwater sample collection were analysed for strontium isotope composition (87Sr/86Sr) to determine the applicability of strontium to trace the source of groundwater. The 87Sr/86Sr ratios in Hout Bay groundwater vary from 0.710 to 0.731. There is a strong negative correlation between 87Sr/86Sr and δ18O (r = -0.91). The low end of the 87Sr/86Sr data array appears to be of marine origin but the corresponding δ18O values of ~-2.9‰ do not reflect that of seawater. Minor interactions of Hout Bay groundwater with Cape Granite produce relatively high 87Sr/86Sr ratios while lower 87Sr/86Sr ratios similar to that of modern seawater are attributed to dissolution of the marine sediments of the Quaternary Witzand Formation and marine aerosols in sea spray. Higher rainwater amounts are proposed to increase 87Sr/86Sr ratios of higher elevation groundwater sample locations by diluting the dissolution of marine aerosols in rainwater.
- ItemOpen AccessHeuweltjies : a window into the palaeoclimate and palaeovegetation of the Western Cape?(2006) Potts, Alistair; Midgley, Jeremy; Harris, ChrisLate Pleistocene terrestrial climate information in the Western Cape. South Africa may be preserved in oxygen and carbon isotope ratios in paedogenic calcrete that bas Conned in zeuweltjies (large circular earth mounds). He11weltjles are common landscape features and their potential as palaeoclimatic archives is not known, simply because it has never been investigated. Samples were taken from three depth-profiles down a single hemvcltjie near Worcester. Carbon and oxygen isotope ratios were detennincd. In total, fifty-four samples were analysed. The o13C values vary from -7.4%. to -4.9%0 relative to PDB. whereas 15180 vary from 28.05'160 to 30.91%0 relative to SMOW.
- ItemOpen AccessInvestigating recharge rates in Table Mountain springs using oxygen and hydrogen isotopes(2018) Halenyane, Keolebogile; Harris, ChrisThis thesis investigates the monthly variation in hydrogen and oxygen isotope composition of four springs at the foot of Table Mountain, and by comparing this variation with that of local rainfall, estimates are made of their recharge rate. The oxygen, hydrogen, and EC data for Main spring, De Waal spring, Albion spring, and Newlands springs for the years 2013 to 2016 are presented. The oxygen and hydrogen isotope composition of monthly rainfall collected nearby at the University of Cape Town (UCT) over the same period are also presented. The rainfall data ranges in δD and δ¹⁸O values from -57 to +18 ‰ and -8.1 to +3.19 ‰, respectively, with most δD and δ¹⁸O values between -60 and -20‰, and -10 and +4‰. The data shows seasonal pattern, with lower δD and δ¹⁸O values in winter and higher in summer. The UCT monthly rainfall samples define a meteoric water line whose equation is δD = 6.03*δ¹⁸O + 7.07, which is similar to previous versions of the equation from the 1995 to 2008 data. The range of recorded EC values for the springs are distinct, the average EC values for each of the spring is; Albion spring 207 μS/cm, Newlands spring 128 μS/cm, Main spring 171 μS/cm and De Waal 166 μS/cm. The isotope compositions of the springs are also well grouped, with each spring plotting in a distinct field on a δD and δ¹⁸O plot. Main spring has the lowest δD and δ¹⁸O values and De Waal spring has the highest δD and δ¹⁸O values. The springs' average δD and δ¹⁸O values for the three years sampled are -7.7 and -2.74‰ (Albion), -6.48 and -2.46‰ (De Waal), -11.52 and -3.48‰ (Main) and -8.49 and -3.14‰ (Newlands). The d-excess of the rainfall range between -11.56 and +33.12 and for the springs is between 0.17 and 27.97. The summer months (low rainfall) have lower d-excess values than the winter months (higher rainfall). There is a positive relationship between the rainfall d-excess, springs d-excess and the rainfall amount, as the amount of rainfall increases the d-excess values of both the rainfall and springs increase. The change in the springs d-excess values clearly mimics that of the rainfall. The similar pattern changes of the d-excess values and δD and δ¹⁸O of the rainfall is reflected in the spring water, suggesting that recharge is occurring rather rapidly and from the d-excess values, the recovery seem to occur for some springs from month to month. De Waal spring mimics the rainfall d-excess very closely, the changes are almost at the same time, and therefore the De Waal spring is recharged the fastest, recovery occurs from month to month. Newlands d-excess does not change at the same time at the rainfall's d-excess, Newlands is the slowest spring to recover, with recharge occurring at least within three months. Main spring and Albion spring fall in between the fastest and slowest recovering spring. The springs are recharged at different rates but with the general average of recharge rate of one to three months of recovery. The difference in δD and δ¹⁸O values between the springs is best explained in terms of difference in average altitude of the potential recharge areas. Average altitude of potential recharge estimated from the topographic map is as follows: Albion spring 286 m, Newlands spring 359 m, Main Spring 518 m and for the De Waal spring 335 m. There is a strong negative correlation between δD and average altitude (r = -0.90) and for δ¹⁸O and average altitude (r = -0.82). In 2012 the annual weighted mean δ¹⁸O value of rainfall on top of Table Mountain was -4.0 ‰, therefore De Waal spring (δ¹⁸O -2.46 ‰) has a small component of mountain rainfall compared to Main spring (δ¹⁸O -3.48 ‰), with Albion spring and Newlands spring in between.
- ItemOpen AccessAn isotope study of the felsic units of the Bushveld Large Igneous Province, South Africa(2010) Fourie, Duane; Harris, ChrisAn O, H, Sr and Nd isotope study was carried out on the ~ 2059 Ma Bushveld granites and granophyres. A small number of Rooiberg Group felsites were also studied.
- ItemOpen AccessA low - δ 18 O intrusive breccia from the Koegel Fontein complex, South Africa(2016) Olianti, Camille Andrea Elisa; Harris, ChrisThe ~135 Ma Koegel Fontein complex, 350 km north of Cape Town, has rocks with abnormally low δ 18 O values, as low as - 5.2 ‰. The rocks with the lowest δ 18 O values belong to an intrusive breccia and have a fine - grained, black matrix. The silica content of these breccia rocks ranges between 44 - 68 wt %, their whole - rock δ 18 O values vary between - 5.2 ‰ and +1.8 ‰ , and their water content is between 0.2 - 1.1 wt %. The major and trace element composition of the breccia rocks is consistent with them containing variable proportions of bentonite, alkali basalt, gneiss, and epidote - and xenolith - rich material. Mapping indicates that the initial intrusion was an alkali basalt dyke and a bostonite dyke that were then intruded by two breccia plugs that extended along the planes of weakness created by the pre - existing dykes, forming two sub - parallel breccia dykes. The water content of the breccia rocks is low (average <1.0 wt %) and is consistent with interaction with fluids at a relatively high temperature (> 300°C). These fluids interacted with the rocks from both dykes and are responsible for their 18 O - depletion. On the basis of the O - and H - isotope composition of the breccia rocks, the δ 18 O value calculated for the fluids in equilibrium with the breccia rock s is approximately - 10.8 ± 0.2 ‰. This low δ 18 O value of the meteoric fluids is too low for the low latitude and warm climate at ~135 Ma and indicates that 18 O - depletion at Koegel Fontein predates the intrusion of the complex in the Cretaceous. It is possible that low - δ 18 O fluids circulated through the country rock during the Pan African when crustal reworking enabled deep circulation of surface water. This period of crustal reworking coincides with a 'Snowball Earth' event and would have been accompanied by 18 O - depleted meteoric water. In the Cretaceous, it is assumed that a mantle plume heated the crust, causing dehydration before partial melting of the crust. The initial low - δ 18 O fluids are effectively metamorphic fluids in equilibrium with the rock. It i s proposed that the initial loss of these fluids became explosive. These fluids migrated along the pre - existing alkali basalt and bentonite dykes, incorporating fragments of alkali basalt, bostonite, and country rock gneiss. This xenolith - rich fluidised material was then emplaced rapidly into the crust
- ItemOpen AccessMineralogy and provenance of the TiO₂ - ilmenite heavy mineral sand deposit of Nataka(2016) Elias, Sílvio José; Harris, ChrisThe Nataka heavy mineral sand deposit occurs along the northeast Mozambique coastline. It comprises a regional Pleistocene elliptical structure extending from Somalia, passing through Kenya, Tanzania, Madagascar and Mozambique, to Richards Bay in South Africa. The deposit consists of fine- to medium- grained, unconsolidated red sediments, hosting heavy minerals. The deposit mineral assemblage is made up of non-valuable phases comprising mostly magnetite, hematite, chromite, monazite, and the valuable phases dominated by ilmenite (50.91 wt. %), with additional zircon and rutile (9.96 and 3.52 wt. % respectively). The total heavy minerals comprise about 5% volume, of which 2 % are valuable heavy minerals making up about 445 Mt (million tonnes) probable resources. This study focuses on the mineralogical characterization of ilmenite from the Nataka deposit, alongside with sediment provenance. Mineralogical and chemical characterisation of ilmenite undertaken on 32 samples from 16 selected drill holes using a combination of QEMSCAN and EPMA revealed that the ilmenite has undergone different stages of alteration, at distinct environment conditions, yielding products spanning from hydrated ilmenite to leucoxene. The alteration dominantly involved groundwater, which was oxidizing and acidic, hence the predominance of ilmenite-pseudorutile alteration. Long exposure to direct sunshine has been hypothesized as a different process that might have favoured the direct alteration of ilmenite to leucoxene and of pseudorutile to leucoxene, on a smaller scale. The major impurities in the ilmenite are Al and Si, which are enriched in the advanced ilmenite alteration products (leucoxene), where they fill pores and cracks. Chromium impurities occur as discrete grains of chrome spinel. The compositional variety of magnetite, Cr-spinel, tourmaline, zircon and rutile indicate major contribution from granitoid terranes, subjected to granulite metamorphic facies (750 to 1000 :C), and minor contribution from mafic plutonic intrusions. The granitoid field as a sediment source area if analysed in conjunction with zircon ages (1100 – 900 Ma, 900 – 700 Ma, and 650 – 500 Ma), and zircon δ¹⁸O (7.07 ‰) is consistent with preferential sourcing from the proximal Mesoproterozoic Nampula Complex, with some contribution from igneous plutonic rocks from Xixano, Lalamo and Montepuez Complexes.
- ItemOpen AccessA multi-proxy study of the Kalkkop impact Crater Lake deposits in south-central South Africa: implications for late neogene climate evolution(2014) Mthembi, Ponani; Harris, Chris; Roberts, DaveThe Kalkkop Crater Lake deposit formed as a result of a meteorite impact, and is situated ~51 km southwest of Graaff-Reinet in the Southern part of South Africa. The structure is about 650 m wide, and contains 90 m of lake deposits that are mainly carbonate precipitates. A 90 m sediment core from the Kalkkop Crater Lake was investigated using sedimentology, palynology and geochemical analysis. The KK1 core, along with the other two boreholes (KK2 and KK3), comprise of finely laminated and massive, structureless carbonates interrupted by debris flows from the crater rim.
- 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.
- ItemOpen AccessPost-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry(2021-02-19) Clark, Martin D; Kovaleva, Elizaveta; Huber, Matthew S; Fourie, Francois; Harris, ChrisBetter characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.
- ItemOpen AccessThe potential for using stable isotopes for solving urban geohydrological problems in the Cape Town area(1997) Oom, Bruce Michael; Harris, ChrisThis thesis investigates the potential for using the stable isotopes of hydrogen and oxygen from surface water and shallow aquifer water in solving urban geohydrological problems in the Cape Town metropolitan region. Three different water groups were chosen for detailed analysis, namely groundwater, springwater and reservoir water destined to join the mains water supply system. The stable isotope composition of groundwater and springwater were assumed to be fairly constant throughout the year, and were thus sampled only once. The reservoir waters were subject to evaporation effects, which result in isotope fractionation. The reservoir waters were sampled at the end of summer (9th April 1996), and near the end of winter (14th August 1996), in order to identify any seasonal isotope variations in the reservoir water isotope values. Reservoir water was sampled before and after the mains water treatment process, to identify any effect water treatment had on the isotope composition of the water. The two effects of water treatment and seasonal variation were determined simultaneously. Chemical analyses (pH, electrical conductivity, alkalinity, major anions and cations) of many of the water samples were performed to investigate the potential for correlating isotope values with chemical concentrations, and for using chemical analyses to complement or supplement isotope studies.
- ItemOpen AccessSpatial analysis of the O-, H- and Sr- isotope composition of Cape Town groundwater(2022) Finlayson, Rosy; Harris, Chris; Sealy, Judith[Abstract has scientific formulas not accommodated by DSpace]
- ItemOpen AccessStable isotope hydrology of the Table Mountain group(2014) Diamond, Roger Edward; Harris, ChrisRain was collected from 2010 to 2012 at 15 locations around the Cape Fold Belt, at the same time as samples from rivers, springs, seeps and boreholes, totalling 435 samples. Precipitation ranged from -75 ‰ to +40 ‰ for δD and -12 ‰ to +8 ‰ for δ¹⁸O , showing seasonal patterns, with lower δ values in winter and higher in summer. Certain anomalous δ values can be attributed to individual weather events, such as thunderstorms. Using weighted data, the meteoric water line is δD = 6.15 δ¹⁸O + 8.21, which is similar to previous equations. The best fit line for groundwater δ values is δD = 7.09 δ¹⁸O + 10.08, the steeper gradient and higher intercept reflecting the predominance of heavy rainfall events with lower δ values in recharge, known as selection. The range of -47 ‰ to 0 ‰ for δD and -8 ‰ to -1 ‰ for δ¹⁸O values for all groundwater data is about half that of the rain values, due to the averaging effect from mixing during groundwater flow. Rainfall isotope composition is negatively correlated with continentality, as defined by the product of distance to the Atlantic and the closest coast. Isotope composition of rainfall is also strongly negatively correlated with altitude. Sites that are elevated within the landscape have a reduced altitude effect, such as tall peaks, whereas mountain valleys display enhanced altitude effects. Temporal and spatial variations in the strength of the amount effect reveal meteorological variability and emphasise the need for long term monitoring.
- ItemOpen AccessA stable isotope investigation into fluid-rock interaction during regional metamorphism in western dronning maud land, East Antarctica(2001) Johnstone, Warren Peter; Harris, ChrisThe Sverdrupfjella Group in western Dronning Maud Land forms part of a 1200 Ma to 900 Ma orogenic belt which experienced a thermal overprint at around 500 Ma. Although the degree of tectonic reworking during this later event remains uncertain, evidence for late fluid alteration is widespread. In this study, the high-grade metamorphic rocks which make up the Sverdrupfjella Group were sampled in the central Kirwanveggen area. The stable isotope and whole-rock composition of these rocks have been determined, in order to test whether zones of intense fluid-rock interaction can be used as evidence for Pan African overprinting of Grenvillian orogenesis in western Dronning Maud Land.
- ItemOpen AccessA stable isotope study of the hydrological systems in the Naukluft region in Namibia(2010) Naude, Kate; Harris, Chris; Miller, Jodie; Rowe, ChristieThe Naukluft Region is situated +- 200 km southwest of Windhoek in Namibia and includes the Naukluft Nappe Complex (NNC), a series of nappe stacks of severely thrusted and folded limestones and dolomites of Neoproterozoic Damara orogen. Although it is a very arid (<200 mm/yr) part of the country, it is also one of the most important tourist destinations, because of its varied geomorphology, spectacular scenery and fragile vegetation biomes. It is the availability of fresh water that that will limit the growth and development of both the agricultural and tourism industries in the region. In this detailed stable isotope study (0180, oD and Ol3C) of the precipitation, surface- and ground- water in the region, numerous possibilities for recharge and aquifer characterization are provided.
- ItemOpen AccessStable isotopes of the thermal springs of the Cape Fold Belt(1997) Diamond, Roger Edward; Harris, ChrisThe Cape Fold Belt is a 250Ma orogenic belt comprised of rocks of the Cape Supergroup, an Ordovician to Devonian sedimentary sequence. The mountainous areas, which reach over 2000m, are composed of the faulted and highly jointed quartzites and sandstones of the Table Mountain Group, which acts as the main deep aquifer. It is from the secondary porosity of this aquifer that over ten thermal springs issue forth, ranging in temperature from ~27°C to 64°C. Samples of the integrated total monthly rainfall were taken for several months at each of four stations around the Cape Fold Belt. Values ranged from δ¹⁸O=- 8.3⁰/₀₀ and 6δ=-37⁰/₀₀ to 1.2⁰/₀₀ and 7⁰/₀₀, respectively. Integrated recharge values based on a full year of observation at UCT, Cape Town, are δ¹⁸O =- 3. 7o/oo and 6δ = -10⁰/₀₀. Eleven thermal springs were sampled in 1995, four of which were sampled once a month for several months, over the same period that the rain was being sampled. The average discharge values for the four springs sampled monthly are, for δ¹⁸O and δD respectively, at Malmesbury - 3.9⁰/₀₀, -18⁰/₀₀; at Citrusdal -4.9⁰/₀₀, - 20⁰/₀₀; at Brandvlei - 5.6⁰/₀₀, - 30⁰/₀₀ and at Calitzdorp -7.3⁰/₀₀,-40⁰/₀₀. A meteoric water line for the Cape Mediterranean climate area was calculated by the general form of a structural regression, using the monthly data weighted by rainfall amount. It has the equation δD = 7.38δ¹⁸O + 18.6. Using the same calculation technique, but not weighting the data, the data for the thermal springs yield a water line with the equation δD = 8.32δ¹⁸O + 16.5. The difference in gradient suggests that the springs are recharged from a colder and isotopically more fractionated weather system, such as during a previous colder climate regime, or at high altitude. There is no evidence for isotope exchange between the groundwater and host rocks; rather, the shift of the spring water line to less negative δ¹⁸O values suggests evaporation prior to recharge. Oxygen and hydrogen isotope ratios from the discharged spring water are clearly more negative than those expected for rain falling at the spring, which can be explained by recharge at much higher altitudes. The springs are therefore believed to be recharged high on mountains in the near vicinity. A continental effect was observed in the spring data, with respect to the distance from the west coast, which is the direction from which weather systems approach. Most of the thermal springs of the Cape Fold Belt seem to be recharged at high altitude in nearby mountains, whereafter the water is heated by geothermal gradient upon reaching depths of two to three kilometres via the secondary porosity of the Peninsula Formation, finally reaching the surface by means of various faults, which allow passage through the impermeable Cedarberg Formation. Isotopically light carbon is released at some springs in the form of CO₂ and CH₄ (total carbon δ¹³C≈- 21⁰/₀₀). These gases could come from near surface bog environments, however, at Malmesbury, where H₂S is also released, a possible geological source is indicated for the CO₂ and the CH₄.