Ilmenite megacryst-hosted melt inclusions from the Monastery kimberlite: implications for kimberlite origins
- Authors: Van Huyssteen, Aiden
- Date: 2021-04
- Subjects: To be added
- Language: English
- Type: Masters theses , text
- Identifier: http://hdl.handle.net/10962/178387 , vital:42935
- Description: Polymineralic inclusions encapsulating a daughter assemblage of crystalline phases (including silicates, oxides, and carbonates) and an amorphous glass phase, hosted in ilmenite megacrysts from the Monastery kimberlite, were investigated texturally and geochemically in order to constrain their melt origin, modeof formation, and evolution prior to quenching. The isolated nature of the melt inclusions within the ilmenite megacrysts provides an opportunity to study components of primary kimberlitic magma captured within the SCLM (4.5–6 GPa) that has been isolated from pervasive modifying processes that are common in kimberlites. The common daughter phase assemblage within the melt inclusions comprises serpentine, phlogopite, calcite, spinel, kassite, perovskite, ilmenite, and glass. The glass is Si-Mg-Fe-rich, with low Al2O3 contents. It is also K2O- and TiO2-free, with variably depleted REE. In composition, serpentine forms a crystalline equivalent to the glass. However, these phases are optically distinct. Serpentine represents two modes of formation: (i) discrete euhedral grains set within a glass matrix that represent a primary phase, crystallising directly from the entrapped melts, and (ii) as patches of partially crystallised glass that represent a secondary phase formed by the devitrification of the glass. Spinel and phlogopite form along early kimberlitic evolutionary trends and record the depletion of the melt in TiO2, Al2O3, and K2O, which typically decreases from the core to the rim of the crystals. Volatile and alkali-bearing minerals (calcite, apatite, phlogopite) crystallised within the melt inclusions from the captured alkali-rich carbonated-silicate kimberlite melt. The daughter mineral assemblage initially crystallised as euhedral and subhedral grains with a uniform composition under equilibrium conditions. Subsequent crystallisation formed grains that exhibit magmatic zoning due to their crystallisation in a progressively depleted melt. Lastly, the crystallisation of skeletal oxide grains occurred under disequilibrium conditions, at a stage of magma ascent with rapidly changing variables including temperature, melt viscosity, and diffusivity. Prior to complete crystallisation, the residual Si-Mg-Fe melt of this crystallisation process was quenched to form the observed glass. The phases that constitute the common daughter assemblage show large variations in modal proportions, forming a continuum from silicate-rich to carbonate-rich endmember inclusions, with certain daughter phases absent in some inclusions. This suggests that the melt was heterogenous at the time of capture and comprised immiscible silicic/oxidic and carbonate melts. Phase separation, therefore, may have started prior to capturing of magma batches as inclusions in ilmenite, but further segregation and crystallisation continued after these batches had become isolated from the megacryst matrix as melt inclusions. The immiscibility and co-existence of the silicic/oxidic and carbonate melts is preserved by textural features between calcite and glass, such as rounded globules of calcite grains set within a silicate glass matrix, calcite forming the matrix for euhedral silicate and oxide minerals, and calcite occupying the interior void of skeletal oxide grains set within a silicate glass matrix. Furthermore, spherulitic globular domains of Ca- and Ti-rich glasses set within a matrix of the Si-Mg-Fe glass suggest that the silicic/oxidic melt underwent further segregation into oxide-rich (Ca-Ti) and silicate-rich (Si-Mg-Fe-Al-K-Ti) melts, potentially crystallising the oxide and silicate minerals of the daughter assemblage, respectively. The abundance of incompatible trace elements and the Cr-poor composition of secondary low-Mg ilmenite as a daughter mineral within the melt inclusions (~1400 ppm Nb; <0.1 wt% Cr2O3; <0.1 wt% MgO), in addition to the Cr-poor composition of the other daughter phases within the inclusions (i.e. <0.1 wt% Cr2O3 for phlogopite and spinel), indicate that they crystallised from a similar melt as the Cr-poor, but high Mg-ilmenite megacrysts (~1400 ppm Nb; <0.1 wt% Cr2O3; ~10 wt% MgO). Furthermore, the melt inclusions are randomly distributed and no textural and/or geochemical evidence for melt infiltration of the ilmenite megacrysts was associated with the melt inclusions. These features are consistent with a primary origin for the melt inclusions which implies a cognate relationship between the megacrysts and the captured kimberlite melt. , Thesis (MSc) -- Faculty of Science, Geology, 2021
- Full Text:
- Date Issued: 2021-04
- Authors: Van Huyssteen, Aiden
- Date: 2021-04
- Subjects: To be added
- Language: English
- Type: Masters theses , text
- Identifier: http://hdl.handle.net/10962/178387 , vital:42935
- Description: Polymineralic inclusions encapsulating a daughter assemblage of crystalline phases (including silicates, oxides, and carbonates) and an amorphous glass phase, hosted in ilmenite megacrysts from the Monastery kimberlite, were investigated texturally and geochemically in order to constrain their melt origin, modeof formation, and evolution prior to quenching. The isolated nature of the melt inclusions within the ilmenite megacrysts provides an opportunity to study components of primary kimberlitic magma captured within the SCLM (4.5–6 GPa) that has been isolated from pervasive modifying processes that are common in kimberlites. The common daughter phase assemblage within the melt inclusions comprises serpentine, phlogopite, calcite, spinel, kassite, perovskite, ilmenite, and glass. The glass is Si-Mg-Fe-rich, with low Al2O3 contents. It is also K2O- and TiO2-free, with variably depleted REE. In composition, serpentine forms a crystalline equivalent to the glass. However, these phases are optically distinct. Serpentine represents two modes of formation: (i) discrete euhedral grains set within a glass matrix that represent a primary phase, crystallising directly from the entrapped melts, and (ii) as patches of partially crystallised glass that represent a secondary phase formed by the devitrification of the glass. Spinel and phlogopite form along early kimberlitic evolutionary trends and record the depletion of the melt in TiO2, Al2O3, and K2O, which typically decreases from the core to the rim of the crystals. Volatile and alkali-bearing minerals (calcite, apatite, phlogopite) crystallised within the melt inclusions from the captured alkali-rich carbonated-silicate kimberlite melt. The daughter mineral assemblage initially crystallised as euhedral and subhedral grains with a uniform composition under equilibrium conditions. Subsequent crystallisation formed grains that exhibit magmatic zoning due to their crystallisation in a progressively depleted melt. Lastly, the crystallisation of skeletal oxide grains occurred under disequilibrium conditions, at a stage of magma ascent with rapidly changing variables including temperature, melt viscosity, and diffusivity. Prior to complete crystallisation, the residual Si-Mg-Fe melt of this crystallisation process was quenched to form the observed glass. The phases that constitute the common daughter assemblage show large variations in modal proportions, forming a continuum from silicate-rich to carbonate-rich endmember inclusions, with certain daughter phases absent in some inclusions. This suggests that the melt was heterogenous at the time of capture and comprised immiscible silicic/oxidic and carbonate melts. Phase separation, therefore, may have started prior to capturing of magma batches as inclusions in ilmenite, but further segregation and crystallisation continued after these batches had become isolated from the megacryst matrix as melt inclusions. The immiscibility and co-existence of the silicic/oxidic and carbonate melts is preserved by textural features between calcite and glass, such as rounded globules of calcite grains set within a silicate glass matrix, calcite forming the matrix for euhedral silicate and oxide minerals, and calcite occupying the interior void of skeletal oxide grains set within a silicate glass matrix. Furthermore, spherulitic globular domains of Ca- and Ti-rich glasses set within a matrix of the Si-Mg-Fe glass suggest that the silicic/oxidic melt underwent further segregation into oxide-rich (Ca-Ti) and silicate-rich (Si-Mg-Fe-Al-K-Ti) melts, potentially crystallising the oxide and silicate minerals of the daughter assemblage, respectively. The abundance of incompatible trace elements and the Cr-poor composition of secondary low-Mg ilmenite as a daughter mineral within the melt inclusions (~1400 ppm Nb; <0.1 wt% Cr2O3; <0.1 wt% MgO), in addition to the Cr-poor composition of the other daughter phases within the inclusions (i.e. <0.1 wt% Cr2O3 for phlogopite and spinel), indicate that they crystallised from a similar melt as the Cr-poor, but high Mg-ilmenite megacrysts (~1400 ppm Nb; <0.1 wt% Cr2O3; ~10 wt% MgO). Furthermore, the melt inclusions are randomly distributed and no textural and/or geochemical evidence for melt infiltration of the ilmenite megacrysts was associated with the melt inclusions. These features are consistent with a primary origin for the melt inclusions which implies a cognate relationship between the megacrysts and the captured kimberlite melt. , Thesis (MSc) -- Faculty of Science, Geology, 2021
- Full Text:
- Date Issued: 2021-04
Understanding the contribution of Land Use/Cover (LUC) classes on soil erosion and sedimentation using sediment fingerprinting technique and RUSLE in a GIS interface at sub-catchment level
- Taeni, Thembalethu (https://orcid.org/ 0000-0001-7662-8652)
- Authors: Taeni, Thembalethu (https://orcid.org/ 0000-0001-7662-8652)
- Date: 2021-04
- Subjects: Geographic information systems , Soil erosion , River sediments
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/20920 , vital:46747
- Description: Soil erosion by water is the major source of soil degradation in the world, and South Africa (SA) is not an exception particularly in the Free State (FS) Province. In South Africa, the Caledon River Catchment in the FS Province has been identified as one of the regions where soil erosion has been prevalent for decades. Evidence across many parts of the catchment show a widespread of soil erosion and the contaminant flux associated with sediment into river systems and reservoirs; including the Welbedatcht dam and Carthcart-drift dam in Ladybrand. It is of these issues that the current work aimed at enhancing the understanding of sediment sources and soil erosion dynamics at the Caledon River Basin. The objectives of the study were to locate sources of suspended sediments and to assess and quantify the contribution of Land Use Cover (LUC) classes to water erosion and sediment yield at a sub – catchment level of the Caledon River Basin. To achieve the objectives set out for this research, a study was conducted at a sub - catchment level of the Caledon River Basin in the FS Province, South Africa. The sediment–fingerprinting approach and the Revised Universal Soil Loss Equation (RUSLE) model were used in the study under Geographic Information System (GIS) settings. A qualitative and quantitative interpretation of the geochemical data were used to evaluate the potential for distinguishing catchment sediment sources. The application of multivariate sediment mixing models incorporating Monte Carlo simulations was undertaken to investigate recent variations in sediment sources. Lastly, to document the impact of LUC change on soil erosion; data from soil profile database, Landsat 8 OLI–TIRS and climate (i.e. rainfall) were used to assess and map the spatial and temporal pattern changes of soil erosion at a sub – catchment level as related to LUC changes. In this study, the sub–catchment was classified into 6 LUC classes. Thereafter soil erosion was quantified for three consecutive years namely; 2015, 2016 and 2018 using the soil erosion factors as GIS–layers. The investigation of sediment source types and spatial provenance in the catchment showed that the grassland areas have consistently been the main sediment source (83 percent) throughout the study period. Findings further showed that there was an increase in contributions from cultivation and abandoned cultivated fields. Sediment contribution from surface sources was dominant (54 percent) and thereafter, subsurface sediment input increased (62 percent). This trend is indicative of increased severity of gully erosion in the area and thus is consistent with other studies. To comprehend the influence of LUC class modification dynamics on soil erosion, water erosion in particular at the sub-catchment commencing from 2015 to 2018 (4 years), multi-temporal Landsat 8 information jointly with the RUSLE model were used. A post-classification, LUC class alteration comparison revealed that water bodies, shrubs and forested region and grassland declined by 0.27 percent, 15.60 percent, and 37.60 percent, respectively. On the other hand, regions under Bad lands, and bare-soil and built-up regions including agricultural region expanded by 2.22 percent, 5.78 percent, and 45.67 percent respectively, between 2015 to 2018 study period. The average yearly soil loss decreased at the sub-catchment and was 10.23,5.71 and 5.82 t ⋅ ha -1 ⋅ yr-1 for 2015, 2016 and 2018 respectively. Although soil loss lessened for the duration of the perceived period, a closer scrutiny revealed that there were nonetheless seeming signs of persistent escalation in soil loss risk. These signs were mostly shown in the elevated parts of the sub-catchment as shown by the red regions on the soil loss map. Additional examination of soil loss findings by LUC classes categories further indicated that most LUC classes categories, including Bare-soil and built-up area, agricultural-land, grassland, and region under shrubs and forests, showed increased soil loss levels during the 4 years’ study period at the sub-catchment. The information on the comparative vividness of diverse sediment sources given by the study must be observed as a noteworthy development towards an understanding of the sediment source dynamics in agricultural river based catchments; more so of the Caledon River Basin. Further research is recommended for other erosion prone catchments in South Africa to identify additional evidence of the spatial and temporal variations in soil erosion and sediment sources. The results of the study suggest that the procedure of assimilating the GIS and RS with the RUSLE model is not just precise, time-efficient and exact in recognizing soil erosion susceptible regions in geospatial and temporal standings. However, it is a cost-efficient substitute to standard field-founded approaches. , Thesis (MSc) (Soil Science) -- University of Fort Hare, 2021
- Full Text:
- Date Issued: 2021-04
- Authors: Taeni, Thembalethu (https://orcid.org/ 0000-0001-7662-8652)
- Date: 2021-04
- Subjects: Geographic information systems , Soil erosion , River sediments
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10353/20920 , vital:46747
- Description: Soil erosion by water is the major source of soil degradation in the world, and South Africa (SA) is not an exception particularly in the Free State (FS) Province. In South Africa, the Caledon River Catchment in the FS Province has been identified as one of the regions where soil erosion has been prevalent for decades. Evidence across many parts of the catchment show a widespread of soil erosion and the contaminant flux associated with sediment into river systems and reservoirs; including the Welbedatcht dam and Carthcart-drift dam in Ladybrand. It is of these issues that the current work aimed at enhancing the understanding of sediment sources and soil erosion dynamics at the Caledon River Basin. The objectives of the study were to locate sources of suspended sediments and to assess and quantify the contribution of Land Use Cover (LUC) classes to water erosion and sediment yield at a sub – catchment level of the Caledon River Basin. To achieve the objectives set out for this research, a study was conducted at a sub - catchment level of the Caledon River Basin in the FS Province, South Africa. The sediment–fingerprinting approach and the Revised Universal Soil Loss Equation (RUSLE) model were used in the study under Geographic Information System (GIS) settings. A qualitative and quantitative interpretation of the geochemical data were used to evaluate the potential for distinguishing catchment sediment sources. The application of multivariate sediment mixing models incorporating Monte Carlo simulations was undertaken to investigate recent variations in sediment sources. Lastly, to document the impact of LUC change on soil erosion; data from soil profile database, Landsat 8 OLI–TIRS and climate (i.e. rainfall) were used to assess and map the spatial and temporal pattern changes of soil erosion at a sub – catchment level as related to LUC changes. In this study, the sub–catchment was classified into 6 LUC classes. Thereafter soil erosion was quantified for three consecutive years namely; 2015, 2016 and 2018 using the soil erosion factors as GIS–layers. The investigation of sediment source types and spatial provenance in the catchment showed that the grassland areas have consistently been the main sediment source (83 percent) throughout the study period. Findings further showed that there was an increase in contributions from cultivation and abandoned cultivated fields. Sediment contribution from surface sources was dominant (54 percent) and thereafter, subsurface sediment input increased (62 percent). This trend is indicative of increased severity of gully erosion in the area and thus is consistent with other studies. To comprehend the influence of LUC class modification dynamics on soil erosion, water erosion in particular at the sub-catchment commencing from 2015 to 2018 (4 years), multi-temporal Landsat 8 information jointly with the RUSLE model were used. A post-classification, LUC class alteration comparison revealed that water bodies, shrubs and forested region and grassland declined by 0.27 percent, 15.60 percent, and 37.60 percent, respectively. On the other hand, regions under Bad lands, and bare-soil and built-up regions including agricultural region expanded by 2.22 percent, 5.78 percent, and 45.67 percent respectively, between 2015 to 2018 study period. The average yearly soil loss decreased at the sub-catchment and was 10.23,5.71 and 5.82 t ⋅ ha -1 ⋅ yr-1 for 2015, 2016 and 2018 respectively. Although soil loss lessened for the duration of the perceived period, a closer scrutiny revealed that there were nonetheless seeming signs of persistent escalation in soil loss risk. These signs were mostly shown in the elevated parts of the sub-catchment as shown by the red regions on the soil loss map. Additional examination of soil loss findings by LUC classes categories further indicated that most LUC classes categories, including Bare-soil and built-up area, agricultural-land, grassland, and region under shrubs and forests, showed increased soil loss levels during the 4 years’ study period at the sub-catchment. The information on the comparative vividness of diverse sediment sources given by the study must be observed as a noteworthy development towards an understanding of the sediment source dynamics in agricultural river based catchments; more so of the Caledon River Basin. Further research is recommended for other erosion prone catchments in South Africa to identify additional evidence of the spatial and temporal variations in soil erosion and sediment sources. The results of the study suggest that the procedure of assimilating the GIS and RS with the RUSLE model is not just precise, time-efficient and exact in recognizing soil erosion susceptible regions in geospatial and temporal standings. However, it is a cost-efficient substitute to standard field-founded approaches. , Thesis (MSc) (Soil Science) -- University of Fort Hare, 2021
- Full Text:
- Date Issued: 2021-04
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