ETD Collection

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Author: search.filters.author.Coney, Louise

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    Mineralogy and geochemistry of the impact breccias of the Bosumtwi impact structure, Ghana : genesis and secondary proccesses
    (2009-05-28T10:56:53Z) Coney, Louise
    Abstract The 10.5 km diameter and 1.07 Ma old Bosumtwi structure in Ghana is a complex impact structure that displays a variety of preserved impactites from both within and outside of the crater and that is associated with the Ivory Coast tektite strewn field. Consequently, this crater structure provides a rare opportunity to examine a wide range of impact-related deposits, including within-crater deposits and proximal as well as distal ejecta. Access to the within-crater deposits was provided by two drill cores obtained by a 2004 drilling project of the International Continental Scientific Drilling Program (ICDP). The focus of the present study has been to determine the petrographic and geochemical characteristics of the different impactites, inside and outside of the crater and how these relate to the regional target rocks / drilled crater basement with the aim of understanding how the impactites formed during the cratering process. Two hard rock cores were recovered from the ICDP drilling project: one through the thickest section of the crater fill (LB-07A) outside of the central uplift and one into the flank of the central uplift (LB-08A). The former has been the focus of this study. No coherent melt sheet was intersected and the shock state of the impactites is lower than expectations from pre-drilling numerical modeling (2.6 – 3.9 km3 expected; <0.2 km3 melt encountered). Suevites from south and north of the crater have also been examined in a similar manner to those from within the crater. The LB-07A core has been subdivided on the basis of different impactite types into: An upper impactite section (333.28 - 415.67 m depth), which consists of seemingly alternating polymict lithic and suevitic breccias; this sequence is underlain by a lower impactite section (415.67 - 470.55 m depth) consisting of monomict breccias (after metagreywacke, phyllite or shale), which, in turn, is underlain by basement metasediments (470.55 - 545.08 m depth). The basement metasediments are dominated by shale over metagreywacke. Both the lower impactites and basement metasediments contain cm-scale thin dykes (at 430.13 m, 445.22 m, 483.00 m and 513.90 m depths) with suevitic fill. The matrices of all the suevites are clastic and contain discrete and tiny (50 – 100 μm) melt fragments. A 30 cm thick granophyric-textured lithology, interpreted to represent a hydrothermally altered granitioid intrusion, occurs at 487.12 m (in the basement metasediments). The subdivision of the core has provided a useful correlative tool for geophysical and geochemical observations. The target rock variety comprises metasediments (mostly metagreywacke, shale and phyllite) that form the bulk (ca. 95%) of the material from which the impactites were derived, and granite that forms a minor component (< 0.5 %). The metagreywacke and shale (together with phyllite) form near equal contributions (44 and 47 % respectively) to the target stratigraphy. Additionally carbonate, in the form of calcite bands, contributes to the target rock composition (ca. 2 %). Other contributors include schists and quartz derived from argillitic or greywacke metasedimentary precursors (6.5 % overall). The impactites within the crater are chemically quite homogeneous. Platinum group element (PGE) and osmium isotope analysis failed to detect an unequivocal contribution from the meteoritic projectile in the within-crater breccias. It is concluded that the meteoritic signature is masked by the high abundances of siderophile and platinum group metals in the target lithologies (associated with the regional, preimpact gold mineralization). The degree of shock of clasts in the impactites of the within-crater deposits is lower than expected from pre-drilling numerical modeling (in that no melt layer is in evidence). A maximum of 3 PDF sets in quartz clasts are noted. On average, only 5.9 % of all quartz grains display 1 or more PDF sets in the upper impactites, with this number decreasing to 4.4 % in the lower impactites, and 1.8 % in the basement metasediments. No PDFs in feldspar have been noted. Diaplectic quartz glass is present in the upper and lower impactites, with a distinct absence of this shock feature in the basement metasediments. Rare diaplectic feldspar glass has been observed. A general decrease in shock pressure, in terms of proportions of melt and shocked quartz, is noted along both cores. Melt particles reach a maximum of 1 cm in size, and a maximum proportion of 36 vol% in a non-representative sample, but are generally less than 5 vol% in abundance. Melt particles of the different core sequences differ in terms of colour and, to a lesser degree, chemical composition. The particles are chemically heterogeneous at the single particle scale. The melt particles consist of mixes of minerals derived from the target lithologies (quartz, feldspars, phyllosilicates). Small areas of suevites have been found to both the north and south of the crater (1.5 km2). The suevites from outside of the crater have a larger proportion of granite - and a distinct absence of carbonate – clasts. Overall, the suevites from the south have clast populations dominated by shale, phyllite and slate, followed by granitoid, metagrewacke and schist; those from the north are dominated by granitoid, schist, and relatively smaller proportions of phyllite, slate and shale. The clast populations are notably different from those of the within-crater suevites that are dominated by metagreywacke and shale, though more similarities can be observed between those from the south and the within-crater suevites. Additionally, the out-ofcrater suevites are distinct from the within-crater suevites in terms of their geochemical signature (which reflects the different clast populations), and – particularly - shock degree of clasts (up to 4 PDF sets in quartz, presence of ballen cristobalite and higher proportions of diaplectic glass in the out-of-crater suevite). Melt particle sizes range up to 40 cm and melt is volumetrically more significant (18 – 37 vol%) than in suevite within the crater. Melt particles outside the crater are more vesiculated. Additionally, some melt particles from the north contain microlites. The particles also display internal heterogeneity (particularly noticeable for those from south of the crater), and those from the north are relatively enriched in Al2O3 and depleted in FeO and MgO, in comparison to those from south of the crater. The particle compositions from south of the crater are more similar to the within-crater melt analyses than to those from north of the crater. The melt compositions indicate that these melt particles also formed from mineral mixes. The impactite distribution (out-of-crater suevites occur to the north and south of the crater, but are absent from the east and west) is consistent with a low-angle oblique bolide impacting from the east. Due to the differing petrographic and shock characteristics of the suevites, it has been concluded that the within-crater and out-ofcrater suevites formed by different mechanisms. A method of formation involving relatively less shocked material and derivation from metagreywacke and shale involves either slumping off the crater walls or lateral movement of melted and displaced target rock within the crater. Limited admixture of fallback material from the ejecta plume is proposed to explain the accretionary lapilli layer found above the impactite sequence. The out-of-crater suevites were formed by fallout from the ejecta plume (thus resulting in higher proportions of shocked material). However, the ejecta plume itself was differentiated laterally, which resulted in the manifestation that the clast populations of the suevites outside of the crater are different to each other. A weak post-impact hydrothermal system affected the crater fill, as testified by calcite veins and pods, quartz and chlorite veins, and sulphide formation in the breccia matrices. This constrains the hydrothermal system to lower greenschist facies conditions, as no minerals from higher metamorphic assemblages have been found. The present study has indicated that the impactites from Lake Bosumtwi are distinct from each other in terms of petrographic and geochemical characteristics. Furthermore it is proposed that these impactites formed by different mechanisms. This study has provided petrographic and geochemical data against which to correlate predrilling expectations.
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    Mineralogical - Geochemical Investigation of two sections across the Permian-Triassic Boundary in the Continental Realm of the Southern Karoo Basin, South Africa
    (2006-11-17T06:21:34Z) Coney, Louise
    The Late Permian (251.0 ± 0.4 Ma) mass extinction is universally acknowledged as the most consequential of the five major Phanerozoic mass extinctions. More than 90% of marine species, ~70% of terrestrial vertebrates, and ~90% of plant life were lost in a very short interval. The nature of the Permian-Triassic (P-Tr) boundary and the cause of the mass extinction associated with it have been the subject of extensive international debate. Possible causes for the P-Tr extinction include asteroid/comet impact, oceanic anoxia, volcanism, methane clathrate dissociation, or combinations of these causes. Geochemical studies of the P-Tr boundary have traditionally been focused on the marine realm, as the boundary in continental sections is typically difficult to pinpoint. One continental setting of the P-Tr boundary that has, however, received much attention is that in the main Karoo Basin, South Africa. The Karoo Basin is a large retro-arc foreland basin which accumulated sediment from the Carboniferous (300 Ma) through to the Early Jurassic (180 Ma) in southwestern Gondwana. Mineralogical and geochemical investigations across two palaeontologically well-constrained continental P-Tr boundary sections at Commando Drift Dam and Wapadsberg in the southern Karoo Basin of South Africa have been undertaken in order to aid in our understanding of this extinction event. The Commando Drift Dam section is also constrained palaeomagnetically. There is a change in paleosol colour across the P-Tr boundary from green-grey to red-brown, which is believed to reflect a change of oxidizing conditions at the P-Tr boundary. Quartz grains were examined for possibly impact-produced microdeformation features, but these were not found. Iridium concentrations are below the detection limit (by instrumental neutron activation analysis) and the sections could not be evaluated as to whether any significant enrichment has taken place at the P-Tr boundary. Major element chemical profiles are dominated by the signatures of carbonate nodular horizons in both sequences. Iron contents (and accompanying siderophile element abundances) increase across the palaeontologically-defined P-Tr boundary, followed by a decrease thereafter. The major element concentrations, together with the effects of weathering, largely control trace element distribution. Carbon isotopic results from the Commando Drift Dam section show a gradual decrease in values before the P-Tr boundary, with a larger negative excursion at the P-Tr boundary. Above the boundary, gradual recovery to initial ratios is observed, followed by another gradual decrease in values to the palaeomagnetically defined boundary. No evidence supporting an extraterrestrial impact extinction mechanism has been found. Rather, the carbon isotope data from this study support two gradual palaeoclimatic changes separated by a sudden change in the carbon isotopic content of the atmosphere. The size and nature of these excursions support the addition of large amounts of anoxic material into the atmosphere. This is proposed to have been caused by the multiple influx of carbon dioxide, methane and other greenhouse gases at various times and by different mechanisms. Such a release of carbon dioxide, methane and other greenhouse gases could have been caused by the coincident volcanic event (the formation of the Siberian Traps) and the episodic release of methane clathrates.