Browsing by Author "Howarth, Geoffrey H"

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    A multi-isotopic geochemical investigation of the Lower Zone, Bushveld Complex, South Africa: implications for a crustal component for parental magmas
    (2020) Edwards, Hunter R; Howarth, Geoffrey H
    The current study focuses on the Lower Zone of the Bushveld Complex, South Africa using multiple geochemical and isotopic systems to determine the origin of crustal signatures, i.e. crustal assimilation or recycled crust in the mantle source, present throughout the Rustenburg Layered Suite (RLS) such as elevated  18O values. These geochemical and isotopic systems include major elements, trace elements, highly siderophile elements, oxygen isotopes, 87Sr/86Sr, 143Nd/144Nd, and Os-Os isotopes. Samples come from the Nooitgedacht Borehole 2 (NG2) at Union Section of the western limb of the Bushveld Complex, which sampled the Lower Zone. The 87Sr/86Sri (0.7043 – 0.7086) and Ndi values (-7.40 - -4.97) calculated in this study are in agreement with published data for the Bushveld. The majority of NG2 samples contain  18O greater than mantle peridotite (5.50‰) and MORB (5.70‰), in which NG2  18O ranges from 5.60 up to 8.00‰ for olivine, orthopyroxene, and clinopyroxene separates. These high  18O values suggests the Bushveld magmas sourced a crustal reservoir, either through crust assimilation or recycled crustal materials in the mantle source. This is the first study utilizing the Re-Os isotope system for the Lower Zone. The Osi values for the NG2 suite range from -4.37 to +35.9, which overlap with published data for the Critical Zone and the Platreef, the only previous Re-Os studies on the Bushveld. However, there are no previously reported negative Osi values for the Bushveld. The range in Osi values for the NG2 samples suggest mixing of at least two geochemical reservoirs. In addition to Lower Zone NG2 samples, sample NG2-773.65 is a chilled margin sample at the base of the NG2 borehole that contains high  18O ( 18O = 9.42 – 9.78‰) and radiogenic Osi (Osi = +62.5), in which the crust and/or recycled crust in the mantle source caused these high values. Sample NG2-734.64 contains the second lowest  18O ( 18O = 5.67‰) and most unradiogenic Osi (Osi = -4.37) values for the NG2 suite, evidence for a harzburgitic SCLM (H-SCLM) mantle source component. A lack of correlations for Osi values with  18O and 87Sr/86Sri values are indices for crustal assimilation processes. This lack in correlation may suggest a crustal component in the mantle is more likely than assimilation of the crust during ascent of the magmas toward the surface. The geochemical data presented in this study suggest the Lower Zone parent magma had a H-SCLM mantle source component in addition to the assimilation of the crust and/or the eclogitic SCLM (E-SCLM).
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    Petrology of the micaceous Koidu and Tongo - Tonguma kimberlites, Man Craton, Sierra Leone
    (2022) Viljoen, Anton; Howarth, Geoffrey H
    Sierra Leone contains two Jurassic-aged diamondiferous volcanic clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes at Koidu and NESW trending dikes at both Koidu and Tongo. Petrographically, the diamondiferous rocks from both clusters have features of both micaceous kimberlites and unevolved Kaapvaal lamproite, making classification ambiguous. To successfully classify and constrain the petrogenesis of these rocks, a combination of detailed petrography, and phlogopite, spinel and olivine chemistry is used in this study. The Koidu rocks are predominantly macrocrystic in texture, whereas the Tongo rocks can broadly be separated into macrocrystic and aphanitic varieties. Olivine is present as macrocrysts and microcrysts, while phlogopite is occasionally present as macrocrysts and abundantly present as groundmass microcrysts (33 vol.% average). Other groundmass minerals include spinel, perovskite, apatite, and calcite set in a base of serpentine possibly containing secondary carbonate. These characteristic features can be present in both micaceous kimberlites and unevolved Kaapvaal lamproites, making classification based on petrography ambiguous. Phlogopite and spinel have similar compositional trends for both Koidu and Tongo, comparable to trends observed for archetypal kimberlites rather than lamproites. This in combination with previously studied trace element geochemistry and isotopic compositions from rocks within the Tongo cluster, indicate characteristics of an archetypal kimberlite. The overall phlogopite abundances further indicate that these rocks are micaceous kimberlites. Olivine core compositions are divided into Mg-rich (Fo >89) and Fe-rich (Fo < 89) endmembers, with Koidu and Tongo cores being dominantly Mg-rich with fewer Fe-rich cores. The overall olivine core compositions range between Fo 84.7 to 94.5 for Koidu and between 83.2 to 94.6 for Tongo. The Mg-rich olivine cores often contain intensely resorbed margins and no groundmass inclusions, consistent with a xenocrystic origin, whereas the Fe-rich population have resorbed margins to a lesser extent and are interpreted to be derived from a Cr-poor megacryst suite. Olivine rim compositions have Fo contents which are relatively homogenous, with compositional overlaps observed between Tongo and Koidu. Olivine rims often contain mineral inclusions of groundmass spinels and are interpreted as having crystallized from a magma. Previous models have suggested that olivine chemistry and groundmass mineralogy in worldwide diamondiferous rocks can be used to constrain their origin and evolution. However, the Koidu-Tongo rocks are the first example where multiple kimberlite clusters do not align with these models. The Koidu-Tongo rocks are highly micaceous and contain Mg-rich olivine core-rim compositions and correspond with Kaapvaal lamproites and so an alternative model has been suggested where the rocks are likely derived from the asthenosphere, like a typical kimberlite, but assimilated phlogopite-rich material in the SCLM.