Bibliography: pages 225-253. / The 3540 million year old komatiitic and tholeiitic lavas in the Onverwacht Group (Barberton greenstone belt) crop out in the rugged terrain of the eastern Transvaal lowveld. The results of an investigation into the geochemistry of the lavas, mainly from the three lower Formations of the Onverwacht Group - the Lower Ultramafic Unit (LUU) - are reported. While the lavas generally show excellent textural preservation, their primary phases have usually been reconstituted to greenschist facies mineral assemblages. Although original phenocryst phases are often pseudomorphed, they can still be identified from occasionally preserved relict grains and from the secondary mineral assemblages. In this way all the major phases that occur in the various lava types could be identified. However, before the geochemical data could be used to examine the effects of partial melting and/or crystal fractionation processes in the development of the magma compositions, it was necessary to investigate which elements had been redistributed by later metamorphic and other alteration events that have occurred in the history of the lavas.

Bibliography: p. 238-268. / Bulk rock compositional variations in lavas from four localities in the Atlantic Ocean are evaluated quantitatively in terms of fractional crystallisation and partial melting models. Samples studied are from the Mid-Atlantic Ridge at 36°49 'N (FAMOUS), from the Islas Orcadas fracture zone on the Southwest Indian Ocean Ridge, from the Conrad fracture zone on the America-Antarctica Ridge and from Bouvet Island, situated at the southernmost tip of the Mid- Atlantic Ridge and in the vicinity of both the fracture zones mentioned above. Bulk rock major and trace element variations in selected basalts from the FAMOUS area, in conjunction with a detailed study of the chemistry of phenocryst minerals and associated melt inclusions are used to place constraints on the genetic relationships between the various lava types. The distribution of NiO in olivine and Cr-spinel phenocrysts distinguishes the picritic basalts, plagioclase phyric basalts and plagioclase-pyroxene basalts from the olivine basalts. The NiO content of these phenocrysts in the former three basalt types is low, or a given Mg Mg+Fe²⁺ atomic ratio of the mineral, relative to the phenocrysts in the olivine basalts. Consideration of the Zr/Nb ratio of the lavas similarly distinguishes the olivine basalts from the plagioclase phyric and plagioclase- pyroxene basalts but, in addition, distinguishes the picritic basalts from the former three basalt types.

The objects of the work presented here were: to develop an accurate method for the determination of gallium on a routine basis in rocks and meteorites at concentrations down to or less than 1 ppm Ga with a precision (c.o.v.) at the 10 ppm level and above of 1-2% (1s); to apply this technique to the determination of Ga in rocks and meteorites; by critically examining the high quality data thus obtained, to compare the distribution of gallium in various meteorite types, as well as in igneous, sedimentary and metamorphic rocks, seeking to define, where possible, the fine structure of its distribution and its relationship with aluminium during geological processes; and to evaluate the possibility of using variations in the gallium contents and Ga/Al ratios of rocks both to assist in the interpretation of the geochemical history of differentiated suites and to define regional differences, as has been done with other trace elements. Bibliography: pages 219-247.

Bibliography: pages 174-194. / The xenolith suite from the Bultfontein Mine, Kimberley, South Africa, comprises lherzolites, harzburgites, and wehrlites with and without garnet; garnet olivine websterites and polymict peridotites. Many of the peridotites contain phlogopite and/or potassic-richterite which are interpreted as metasomatic minerals together with ilmenite, rutile, sulphides, and rare zircon. The variations in texture, mineral chemistry, equilibration conditions and bulk chemistry are described. No diamond-bearing assemblages have been found. The textures vary from coarse to porphyroclastic and to fluidal mosaic and laminated and disrupted. The deformed xenoliths have textures which may vary over distances of centimetres in individual specimens. The range of textures is greater than that reported for the xenoliths from Thaba Putsoa and Matsoku kimberlite pipes of northern Lesotho. There is no clear correlation between texture, mineral chemistry, or bulk chemistry in the Bultfontein xenoliths. Temperatures and pressures of equilibration have been calculated using three different geothermometers and three geobarometers, and the uncertainties involved in their application have been discussed. Some of the deformed xenoliths appear to have equilibrated at marginally higher temperatures and pressures than the undeformed xenoliths. The four-phase garnet lherzolites do not define a "perturbed geotherm" similar to that found in several other kimberlite localities in southern Africa and elsewhere.

Includes bibliographical references (leaves 82-89). / The Central Benguela inner shelf is characterised by hypoxic to anoxic sub-thermocline water conditions on a semi-permanent basis. Historical understanding of the incidence of hypoxia is that the inner shelf area off Namibia is one of the main areas of formation of low oxygen water (LOW) in the Benguela upwelling system. Local biogeochemical remineralization of organic matter mostly related to the primary production in surface waters (Chapman and Shannon, 1985) is thought to drive seasonal variability in sediment oxygen demand. New understanding of the system suggests that shelf hypoxia is driven by a remotely forced equatorial hypoxic boundary condition thought to trigger anaerobic remineralization and increased sediment fluxes of reduced metabolites (Monteiro et al., in press). The study focused on seasonal variability of sediment oxygen demand on the Namibian inner shelf and its relation to particulate organic carbon and reduced metabolite fluxes.

Includes bibliography. / The Eshowe melilitites intruded marginal cratonic crust at approximately 80my. Their intrusion followed after a long period of extensive rift tectonism associated with the breakup of Gondwanaland. The intrusives represent the final phase of alkaline and basaltic magmatism that commenced at about 200my. This magmatism was probably related to mantle processes responsible for the continental fragmentation and was controlled by a fluctuating mantle thermal regime. The Eshowe melilitites intrude an area of attenuated crust in an essentially rift valley setting. Petrological, and chemical evidence suggest that the Emtilombo melilitite does not represent crystallisation of a primary magma. The magma was generated in the asthenosphere but reacted with incompatible element and probably CO₂ and H₂O enriched lithosphere and perhaps crustal sources on its way to the surface. The Emtilombo melilitite contains microxenoliths of mantle derived spinel peridotite and of a suite of dunitic rocks that are believed to be high pressure cumulates of earlier alkaline magmas. These latter trapped melts would have introduced metasomatising agents into the lithosphere. The dunitic melts are believed to represent earlier intrusions related to the Eshowe alkaline volcanism. The chemistry of the olivine phenocrysts and microphenocrysts and complex zonation patterns on olivine xenocrysts (macrocryst and some complex phenocrysts) suggest the Emtilombo magma formed by mixing of several batches of melt. The 'parental magmas' to the Eshowe occurrences are therefore considered to consist of a mixture of asthenospheric and lithospheric components and a variety of different melts. The 'parental magma' to the Emtilombo dyke was an incompatible element enriched ultramafic melt that contained microxenoliths of spinel lherzolite and the dunitic suite of rocks. Changing oxygen fugacities are believed to be controlled by a loss of volatiles at relatively shallow depths in the mantle or lower crust. These changes are reflected by the spinel chemical trends and perovskite crystallisation. The change occurred after complex zonation patterns had developed on the olivines. The microphenocryst olivines are believed to be the only population of grains that crystallised from the Emtilombo 'parental magma'. Mode of emplacement of the melilitites is probably influenced by the nature and volume of magma in the intrusion, its volatile content, and to some extent the nature of the country rock. The Eshowe melilitites show a wide variety of intrusive modes and also demonstrate how late stage processes, possibly during or post consolidation, influence the geochemistry of the rock type.

Six different types of authigenic carbonate were identified associated with the no.2 coal seam in the Witbank basin. These are: l) cell-filling carbonate, 2) early formed spberulites, 3) massive carbonates, 4) cleat-filling carbonate, 5) fracture-filling carbonates and 6) carbonate cement of associated sands. The textural relationships, depth of burial and chemical evolution of these carbonates were studied with the aid of X-ray diffraction, total organic carbon analysis, vitrinite reflectance, reflected and transmitted light microscopy, scanning electron microscopy and electron microprobe analysis. Comparison with the studies of Matsumoto and Iijima (1981), Curtis and Coleman (1986) and other literature showed the Witbank carbonates to have followed a fairly complex path in terms of chemical evolution. The chemistry of the earliest cell-filling carbonates is indicative of an early brackish, alkaline environment with high sulphate levels, with occasional local oxidising conditions prevailing during precipitation. Spherulite chemistry is largely representative of the continuation of alkaline conditions with high sulphate supply. Those forming closer to palaeohighs are sideritic and possibly reflect the input of fresher run-off water, allowing more acidic conditions locally. Compositional zonation of some examples indicates initial acidic conditions and later alkaline conditions. Massive carbonates are considered to have formed, at least in some cases, from amalgamations of these earlier spherulites. They are high-Ca carbonates, siderite or dolomite. A number of the high-Ca massive carbonates were found to be aragonite. These are considered to have precipitated from water with high ionic strength, in the presence of humic acids. The chemistries of these massive carbonates are representative of the continuation of relatively, alkaline conditions, or, in some cases, represent a strong brackish or marine imprint over the earlier fresh water chemistries. Chemical trends and isotope data suggest continued formation with burial. Later formed carbonates filling cleats and fractures show more diverse compositions and are interpreted as the results of mixing of pore waters at depth, primary silicate dissolution and later percolation of groundwaters after uplift. The overall evolution of the authigenic carbonates in the Witbank no.2 seam leads to the interpretation that the early swamp was only locally "fresh", and provides strong evidence for a marine influence at both early and late stages of deposition and diagenesis.

The determination of 14 trace elements, namely As, Ag, Bi, Cd, Cu, Ga, Ge, Hg, In, Pb, Sb, Sn, Tl and Zn, in silicate rocks using d.c. arc optical emission spectrography (O.E.S.) and X-ray fluorescence spectroscopy (X.R,F.) was investigated. X.R.F, was shown to be capable of determining Cu, Ga, Pb and Zn in normal silicate rocks and Sn, As and Ge in samples enriched in these latter three elements. Improvement of the sensitivity of the trace elements selected employing d.c. arc excitation in artificial atmospheres was examined in detail. A Margoshes and Scribner (M & S) design gas jet was constructed, and proved to have numerous advantages over those gas jets which incorporate a glass canopy. Excitation in argon, argon-oxygen and nitrogen resulted in many advantages. The most prominent were, remarkable enhancement of the element Zn, and selective distillation of the volatile elements when the d.c. arc was surrounded by argon. The drastic reduction 1.n electrode temperature associated with excitation in argon resulted in an unacceptable decrease in the volatilization rates of the more abundant volatile elements. Several successful methods of overcoming these reduced volatilization rates were found. Increased amperage coupled with a special electrode design was one while the use of a small carrier electrode was another. It was also found that the carrier design electrode was most suitable for use with argon and nitrogen atmospheres as its use promoted selective distillation of the volatile elements. The lack of improvement in volatilization characteristics through the use of additives was attributed to the dominating influence of argon on electrode temperature. The principal advantage of excitation in argonoxygen was the suppression of CN emission essential for ultimate sensitivity of the element Tl. While excitation in nitrogen improved the volatilization characteristics of the more abundant elements, severe CN emission and increased spectral background limited the use of nitrogen to those elements with spectral lines not affected by CN emission.

Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusually scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho, and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration and one appears to link together the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in TiO₂ to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenoliths, or megacrysts. A small number of concentrate grains have compositions matched by the diamond inclusions. Attempts to apply geothermobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which are ascribed to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromite.

The 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.