No description available.

Understanding the generation and evolution of cratonic lithospheric keels requires detailed knowledge of their age, timescale of formation, and the relationship between the crust and mantle part of the lithosphere. The Re-Os isotope system has been instrumental in providing an improved understanding of the timing of formation of cratonic lithospheric keels because of the large fractionation of Re from Os during mantle melting and the relative immunity of Os during mantle metasomatism. This study uses combined full PGE (Platinum Group Elements; Os, Ir, Ru, Pt, Pd) and Re-Os isotope analyses to place better constraints on the significance of Re-Os model ages in sub-continental lithospheric mantle suites from northern Canada and Lesotho, southern Africa. When combined with major-element studies, the coherence of major-elements with PGE fractionation trends, (Pd/Ir)(_n9), provide a firm basis for evaluating the significance of T(_RD) and T(_MA) model ages obtained from Re-Os isotope systematics. The fractionation of PGEs track melt depletion and melt enrichment end-member processes, and allow us to discern samples which have experienced Pd and Re enrichment at varying stages. The Re-Os isotope and PGE analyses of peridotite xenoliths from the Jericho kimberlite clearly indicate the presence of Archean mantle beneath the Slave craton (T(_RD) range from 3.1 to 2.6 Ga), extending down into the high-temperature garnet facies. Archean ages are also evident in peridotite xenoliths from Somerset Island in the Churchill Province (oldest T(_RD) ca. 2.8 Ga). This is the first indication of Archean sub continental lithosphere beneath this region that is characterised by early to mid Proterozoic crustal ages. Some peridotites from the Jericho and Somerset Island peridotite suites have noticeably disturbed Re-Os isotope and PGE systematics and produce anomalously young model ages for cratonic lithospheric mantle overall. Many of the young model ages observed at Jericho and Somerset Island are coincident with major tectonomagmatic events manifested in the overlying crust and may represent new lithospheric mantle added at this time. Depleted peridotite xenoliths from Lesotho (Kaapvaal craton) have very tightly clustered T(_RD) model ages (2.8 ± 0.1 Ga, n = 26). The model age of the lithospheric mantle beneath this region is significantly younger than the age of the oldest overlying crust on the Kaapvaal craton, but correlates well with the end of major crustal differentiation and stabilisation of the Kaapvaal craton. This indicates that large fractionation of lithospheric mantle beneath the Kaapvaal craton could have formed in the late-Archean.

Intermediate composition arc volcanoes present some of the most serious natural hazards because of their tendency towards violent, explosive eruptions. Very little is known about the long term processes which produce such magmas and the timescales over which they operate. U-series systematics have considerable potential for helping to understand these timescales, although very few studies to date have examined individual volcanoes in detail. A detailed series of ²³⁰Th-²³⁸U and ²²⁶Ra-²³⁰Th analyses are presented in conjunction with major and trace elements and Sr, Nd and Pb isotopes for lavas sampled from coherent stratigraphic sequences on Mount Ruapehu. This andesitic volcano lies at the southern end of the Taupo Volcanic Zone (TVZ) in the centre of North Island, New Zealand. Volcanic activity at Ruapehu is related to the subduction of the Pacifc Plate beneath the Indian Plate along the Hikurangi Trough. Major element trends are dominated by shallow level crystallisation and assimilation processes. Modelling of crystallisation trends suggests that the magmas were hydrous and may have elevated fO₂. Trace element patterns show high LILE/HFSE ratios characteristic of arc lavas. AFC models of isotope data suggest that 50% crystallisation and 10-15% assimilation can explain the observed trends. The assimilant is likely to have been dominated by partial melts of local Torlesse basement. Major element extrapolations suggest that the mantle wedge beneath the TVZ was relatively fertile. Modelling of the trace element budgets in relatively primitive lavas suggests that a three component model is required for magma generation. Partial melts of sediments were added to the mantle wedge at a relatively shallow level, and the modified mantle was then fluxed by fluids derived from subducted oceanic crust at greater depth. 10-90% of many LILE and other elements are derived from fluids and sediments rather than the wedge. U-Th analyses show 1-14% U excess and (²³⁰Th/²³²Th) ratios in the range 0.75-0.85. Data for the coherent stratigraphic sequences fall on linear trends on an equiline diagram. Crystallisation and alteration are ruled out as causes of the observed U/Th variation and although the lavas have been affected by assimilation, links between U/Th and indices of assimilation are difficult to demonstrate. Crustal melts with similar U/Th ratios to the parental basalts and relatively low U and Th abundances are the most likely candidate for the assimilant. It is argued that the U-Th systematics reflect addition of hydrous fluids to the mantle wedge. Whole rock pseudo-isochron ages are derived for the different stratigraphic sequences, and compared with data on eruption ages to estimate combined transit and residence times (CTRTs). Rocks from older sequences dated at -80 ka and -130 ka have very short CïRTs << 40 ka, whereas younger sequences have longer CTRTs on the order of 40-80 ka. It is inferred that the magmas of the younger sequences spent a longer period of time ponded at the base of the crust and underwent limited chemical modification. A detailed model of the behaviour of U-Th-Ra systematics during crustal magmatic processes is also presented. This allows crystallisation, assimilation and replenishment processes to be combined with a time factor to predict magma evolution curves. Whilst detailed systematic studies of individual volcanic centres are required to test the model, the lack of age constraints and complex sub-volcanic history of Ruapehu magmas do not permit the detailed application of this model here.

Organic geochemical investigations of terrestrial geothermal vents have provided a veritable treasure chest of unusual results. In particular, recent investigations of microbial lipids preserved in mineral and organo-sedimentary depositions have been shown to be useful in reconstructing vent populations, recording changes in physico-chemical conditions over time and inferring the presence of uncharacterised microbiota. Studies centred on geothermal environments provide a rich source of information that is directly applicable to a range of scientific disciplines including those concerned with the early evolution of life on Earth and other planetary bodies. The organic geochemistry of a particular group of bacterial lipid biomarkers, bacteriohopanepolyols, and their degraded counterparts, geohopanoids, contained within sinters from silica-depositing geothermal vents from New Zealand and Chile provides the basis for this study. This has been complimented with an investigation of BHP distributions that derive from mat-forming microbial consortia that colonise the outflow channels of geothermal vents from California, Nevada and New Zealand. Examination of BHP and hopanoid distributions preserved in siliceous sinters using Atmospheric Pressure Chemical Ionisation-High Performance Liquid Chromatography-Multistage Mass Spectrometry (APCI-HPLC-MSn) and Gas Chromatography-Mass Spectrometry (GC-MS) shows that polyfunctionalised composite BHPs deposited in sinters are well preserved. BHPs and geohopanoids have been analysed from five geothermal locations within the Taupo Volcanic Zone (TVZ). Composite-BHPs, such as bacteriohopanetetrol cyclitol ether and bacteriohopanepentol cyclitol ether, are commonly observed as the most abundant compounds in sinters from Champagne Pool (CP), Loop Road (LR) and Rotokawa (RK). A diverse array of additional BHPs is detected in sinter samples collected from these sites, including a number of novel BHPs. A suite of BHPs have been tentatively-assigned as ‘oxo-BHPs’, i.e. with a ketone group, most likely at C-32 or C-31 of the side chain. Guanidine- substituted bacteriohopanepentol cyclitol ether was also identified and may indicate the presence of methylotrophic bacteria at the sites. Sinters collected from Opaheke Pool (OP) contain BHPs that indicate the presence of methanotrophic bacteria and cyanobacteria. Likewise, 2-methyl BHPs indicative of cyanobacteria were identified in sinters collected from Orakei Korako (OK), including a vent of 97.8oC, potentially indicating another source of 2-methyl BHPs at this site. Sinters from the El-Tatio Geyser Field (ETGF) show a predominance of cyanobacterial signatures that likely derive from endolithic cyanobacterial colonies. Concentrations and the structural diversity of BHPs are greater in sinters from the TVZ when compared to those from ETGF. A survey of the BHP distributions of mat-forming microbial consortia from geothermal locations in Nevada, California and New Zealand highlighted the presence of six currently uncharacterised composite BHP structures with the same novel terminal group at C-35 that appear to comprise a diagnostic BHP signature for mat-forming microbial consortia. Hopanoic acids are the most abundant geohopanoids identified in older sinters and C32 hopanoic acid is the predominant diagenetic product. Geohopanols are more abundant in sinters deposited under acidic conditions, such as those collected from CP, LR and RK and C31 homohopanol is the most abundant geohopanol identified in sinters from these sites. The observed difference is thought to derive from reduction of hopanoic acids to hopanols. The diagenetic end products from each location appear to be different to any previously studied environmental setting indicating an unusual degradation pathway affecting BHP preservation in silica sinters. This indicates that depositional setting not biological input is the key factor determining geohopanoid distributions in these environmental samples. The results of this study complement previous investigations concerning the influence of environmental conditions upon BHP and hopanoid distributions and further elaborate upon the ecology of known BHP-producing bacteria contributing to the sedimentary record.

Deltaic basins of Tertiary age constitute a significant percentage of the sedimentary environments of the world's known hydrocarbon reserves and with more prospects of huge discoveries, renewed exploration activities are on-going in the progressively deeper waters of the continental shelf of deltaic basins (e.g. Gulf of Mexico, Niger Delta and Beaufort-Mackenzie Delta). Hydrocarbon exploration success depends on the knowledge of the petroleum systems (source rock, migration pathways, reservoirs, traps and seal) contributing to an oil accumulation. Although oil continues to be found in Tertiary delta reservoirs, multidimensional interpretation of literature data on crude oil geochemistry from these deltaic basins reveals a paradox in the geochemical characteristics of some of the reservoired oils with respect to the alleged Tertiary delta source rocks. Consequently there is no consensus on the origin of most of the deltaic petroleum accumulations. A poor understanding of the petroleum system (in particular the source rocks) escalates the risk of exploration as the presence, let alone the composition and phases, of petroleum could prove difficult to predict in undrilled prospects. Fairly representative crude oil samples taken from accumulations in Tertiary reservoirs of the Assam Delta (India), Beaufort-Mackenzie Delta (Canada), Gulf of Mexico (USA), Niger Delta (Nigeria) and the Kutei Basin (Indonesia) have been characterised for the purpose of predicting their source rock organic facies (organic matter type, depositional environment, age and thermal maturity). Biomarker and stable carbon isotopes analyses have been conducted on 120 samples from these basins in order to better understand the petroleum systems producing them. All the oils contain the angiosperm higher plant biomarker oleanane, in addition to other biomarkers like bicadinane and lupane which are not ubiquitous, thus providing evidence of higher plant inputs. Novel terpenoid biomarkers were discovered in many of the oils, whose relative abundances correlate strongly with those of conventional terrigenous biomarkers. Oils from Assam and the Kutei Basin are compositionally similar with pronounced terrigenous biomarker and stable carbon isotope characteristics that suggest expulsion from coaly to delta top shales deposited under highly oxygenated environments. Conversely, oils from the Gulf of Mexico show mixed marine-terrigenous and high marine algae source input biomarker and stable carbon isotopes signatures that suggest limited higher plant contribution to their organIc matter. Source rock deposition was under sub-oxic conditions (pristane/phytane) below a stratified water column (presence of gammacerane). On the basis of biomarker and stable carbon isotope data, oil accumulations from the Beaufort-Mackenzie and the Niger Deltas show greater diversity, grouping as dominantly terrigenous and dominantly marine algae sourced end-members, reflecting expulsion from source rocks deposited under oxic non-stratified and sub-oxic stratified water column conditions, respectively. These clear variations in the geochemistry of the oil accumulations can be attributed to oil sourcing from two discrete units: 1) Oils expelled from within the lean but thick source rock volume of deltas (intra-delta) which are characterised by abundant terrigenous biomarkers and stable carbon isotope signatures; 2) Oils expelled from source rocks rich in marine algal kerogen and envisaged as being laid down prior to the delta prograde, hence now buried below the Tertiary delta (sub-delta). The evidence suggests that sub-delta source rocks charge oil accumulations in the Gulf of Mexico, Beaufort-Mackenzie Delta and the deepwater Niger Delta. Bitumen extracts of core samples of the organic-rich late Cretaceous Araromi shale, from the Gbekebo well drilled in the Dahomey Basin on the western margin of the Niger Delta, are comparable both molecularly and isotopically to the deepwater Niger Delta oil set. This provides confirmatory evidence that similar sub-delta source facies may charge the deepwater Niger Delta accumulations.

In marine hydrothermal sediments metal-rich, reduced fluids and oxidised seawater form steep geochemical gradients supporting abundant, but as yet poorly defined, chemosynthetic life and biogeochemical cycling. This project elucidates the functional diversity of chemosynthetic micro-organisms, and metabolic processes on the geochemistry of ferruginous (-50% Fe), arsenic-rich (-400 ppm As) hydrothermal sediment at Santorini, Greece (20-40°C, pH 6.0-6.3, Eh 0 to 155 mV). Culture and molecular-phylogenetic techniques revealed abundant chemosynthetic prokaryotes capable of transforming a range of chemical species common to marine and hydrothermal sediments via: anaerobic Fe(III), sulfate, nitrate and As(V) reduction, and nitrate-dependant Fe(lI) oxidation; micro-aerobic Fe(lI) monosulfide (FeS) oxidation; and aerobic As(lII) and free-sulfide oxidation. Geochemical analyses showed that oxidised species [Fe(III), As(V), sulfate and presumably Mn(IV)] dominated in the suboxic surface sediment layer (0-5 cm deep), but that reduced species [namely Fe(II), As(III), Mn(II)] increased in the lower suboxic-anoxic transition zone (5-20 em depth). From a biological perspective sulfate is the energetically least favourable of the oxidised species to be reduced, and sulfate concentrations were consistent throughout the portion of the sediment column analysed (0-30 em depth). The presence of a black precipitate (probably FeS) at the lower limit of the transition zone, however, did suggest at least a small amount of sulfate reduction was occurring. In contrast, no nitrate was detected indicating rapid reduction of this . energetically favourable compound at· the water-sediment interface. Microbiological and geochemical data, in combination, indicated that redox cycling of Fe, Mn and As were most likely key biogenic processes in biogeochemically stratifying the sediment, whereas S-cycling bacteria were comparatively minor contributors. Other principal findings included the cultivation of a novel neutrophilic Fe(II)-oxidiser related to marine microaerophilic Fe(II)oxidising bacterium Mariprofundus ferroxydans. This bacterium dominated the Fe(lIl) (oxy-)hydroxide-rich surface sediment, and most probably plays a significant role in catalysing suboxic Fe(lI) oxidation. Fe(II)-oxidising prokaryotes comprise a functional group not well understood in marine settings. Additionally, bacteria belonging to a ubiquitous, heterotrophic marine genus, Marinobacter, were shown to oxidise Fe(II). Of these, Marinobacter santoriniensis, was isolated and characterised, and also shown to both oxidise As(llI) and respire As(V), providing a rare example of a marine prokaryote capable of these functions. Futher to this, incubation experiments, conducted to establish the effect of microbial respiration on arsenic mobilisation, demonstrated that microbial Fe(lll) reduction largely controlled shifts in arsenic partitioning in the sediment. Specifically, results support other studies (mainly concerned with the toxic effects of arsenic) showing arsenic sequestration by Fe(II)-bearing minerals. This is contrary to the more common supposition of arsenic release to porewaters upon reductive dissolution of Fe(lIl) minerals. Overall, these results highlight the importance of microbial enzymatic processes in sediment geochemistry.

This thesis describes methodologies to detemiine organic and inorganic phosphorus in water and discusses biological, physical and chemical controls on the transformations of organic and inorganic phosphorus in laboratory microcosms of estuarine particle-water systems. Phosphorus is an important nutrient and the accurate determination of phosphorus species in natural waters is essential for understanding the biogeochemical cycling of the element. Chapter one reviews phosphorus impacts on water quality and relevant UK legislation, the sources, speclation and fractionation of phosphorus, the sediment/water phosphorus cycle and analytical methods for the determination of dissolved Inorganic phosphorus (DIP) and organic phosphorus (DOP) species In natural waters. Chapter two introduces the technique of segmented flow analysis and describes methods for the determination of inorganic phosphorus and total dissolved phosphorus after autoclaving. Segmented flow analysis is suitable for determining inorganic P and organic P species in freshwaters in the range of 1 - 300 pg P L&quot;\ Calibrations were linear (R^ &gt; 0.997) over the range 0.5 - 100 pg P L'^ with good precision (RSDs &lt; 3 %). The LCD was 0.4 pg P L'^ for DIP and 3 pg P L'^ for DOP. The recoveries of 7 model organic phosphorus compounds (adenosine-5-triphosphate di-sodium salt (ATP), phytic acid (PTA), sodium tripolyphosphate (STP), methyltriphenylphosphonium bromide (MPT), p-nitrophenyl phosphate magnesium salt (p-NPP), p-D-glucose-6-phosphate monosodium salt (G-6-P) and cocartjoxylase (COCA)) were all &gt; 90 %. Chapter three reports on bacterial uptake studies of phosphorus species in a plug-flow microcosm. Experiments using inert Duran and Siran beads as supports for bacterial bioflim development showed similar P uptake behaviour. Glucose-6-phosphate was almost fully utilised and converted to DIP after 3 days suggesting that labile organic phosphorus species could be a bioavailable source of P for bacteria. Phytic acid (a refractory P compound) released significant concentrations of DIP to the water column due to physical displacement. Chapter four describes phosphonjs transformations in a sediment-water flume due to turbulent resuspension events. The released DIP concentration had a strong linear relationship with shear stress in pure water and artiflcial seawater. However, no signtflcant relationship between DOP release and shear stress was observed. The sediment type and the water matrix significantly affected the magnitude of DIP and DOP release to the water column. Chapter flve extends the investigation of P exchange to constant resuspension events. The water matrix had a significant effect on the rate of DIP and DOP release from sediment and the sedimentAvaler equilibration time. Typically 6 - 12 % of total phosphorus in the sediment was released as DIP and DOP into the water column. When orthophosphate and phytic acid were added to both UHP and river water. SPM was able to buiffer theses additions. However, the SPM buffering capacity was much lower for G-6-P. The buffering capacity of the SPM for phytic acid was significant and permanent in UHP and biotic river water, whereas phytic acid was cycled in the abiotic experiments.

No description available.

For all three ophiolites, variations in chromitite major and trace element geochemistry occur on several different scales. At the smallest level, single thin sections of chromite may sometimes display large variations in composition. At the next level, intrapod chemical variation can be extensive, though predominantly displaying an inverse correlation between Cr# and Mg#. Finally, interpod variation, and its relationship to stratigraphic height, changes a great deal between ophiolites, sometimes providing an apparently continuous change with height (e.g. Al 'Ays) and in other places (e.g. Shetland) varying discontinuously with height. These differences can provide important clues to the process controlling chromite composition. They suggest that multiple controls may sometimes be involved including, the number of phase crystallizing, post-solidus re-equilibration, fractionation, melt-rock reaction and volatile unmixing.

This thesis covers two separate but complimentary themes. Firstly, work on the spinel composition of the ultramafic rocks of has provided evidence that they formed as Alaskan-type intrusions. This has implications both for prospectivity and for the understanding of neoproterozoic plate movements. Additionally, the study of the rock and soil geochemistry has been used to identify exploration targets and evaluate the prospectivity of the major complexes. It is proposed here that the ultramafic complexes Tulu Dimtu, Kingy, Daleti, Ankori and Yubdo, in the Western Ethiopian Shield are Alaskan type intrusions. Alaskan-type intrusions are concentrically zoned ultramafic intrusions thought to be the feeder pipes of volcanoes. They have dunite at the core and grade outwards to clinopyroxenite and sometimes hornblendite. These intrusions typically occur in continental arc settings such as Alaska, British Colombia and the Urals. This compares with ophiolite complexes which are thought to be obducted oceanic upper mantle and crust. The two types of complex may be difficult to distinguish because if ophiolites are dismembered and deformed fragments, they may resemble Alaskan-type intrusions. This thesis documents several features of these Ethiopian complexes that are typical of Alaskan type intrusions. Mapping of the Yubdo complex has shown a circular out crop pattern with concentric zones of clinopyroxenite at the edge to dunite at the centre. New mapping of the Tulu Dimtu area has shown a similar zoned circular body. Both the Yubdo and Tulu Dimtu complexes show concentric zones of Cr values, where the greatest values occur at the edge and the lowest in the centre. These mafic and ultramafic intrusions are very altered by surface weathering but contain chrome- spinels which have a geochemistry which is unlike ophiolites and similar to Alaskan-type intrusions. The analysis of these spinels demonstrates how the compositions of spinel Fe2+ and Cr values may be used to distinguish between an Alaskan type or ophiolite complexes. In Alaskan-type intrusions, spinels with Fe2+ values greater than 0.85 frequently have Cr values which are lower than 0.5. Such low Cr values in spinels with high Fe2+ values are rare in ophiolite complexes. The variations in Ni, Cu, Cr and Al contents of highly altered ultramafics and fresher ultramafics have been used to investigate the magmatic and post-magmatic ore forming processes that have influenced the Pt and Pd content of the complexes. It is expected that medium to low temperature hydrothermal activity may have had a significant impact on the geochemistry of even the fresher rocks. The discovery of high Pd values and low Pt in lithologies such as talc-schists and quartzite support the idea that Pd is more mobile than Pt in medium to low temperature conditions. Furthermore, in the Tulu Dimtu Main Intrusion, the altered rocks indicate that Pd has been removed from the magmatic sites of concentration. In a few places it is possible to see through the extensive alteration and potential magmatic processes can be considered. Within the Tulu Dimtu Main Intrusion and the Main Yubdo Intrusion, the most primitive rocks occur at the flanks. Furthermore, it is indicated that sulphide segregation may have occurred in the Tulu Dimtu Main Intrusion and Daleti Ultramafic. In the Daleti Ultramafic, regardless of the presence of sulphides, the Pt and Pd values remain low - it is therefore unlikely that the complex hosts economic grades. However, at Tulu Dimtu and Yubdo, it is recommended that any future exploration be targeted at the flanks of the complexes, where magmatic and post-magmatic processes may potentially co-incide to elevate the grade. Much work has been published to document the nature of platinum-group minerals (PGM) in the alluvial and eluvial placers around the Yubdo area and some PGM have been discovered in the serpentinsed dunites of the main Yubdo intrusion. In this thesis further discoveries of PGM have been made in the serpentinised dunites and chromites from the Yubdo area. In the course of this work new base maps covering the ultramafic complexes and the surrounding basement have been produced. These have revealed many smaller ultramafic bodies which are referred to here as the Lensoid Ultramafics. Both spinel and whole rock geochemistry supports the hypothesis that these are slivers of rock "sheared-off" the outside of larger complexes. The conclusion that these complexes have an Alaskan-type origin has consequences for the understanding of plate movements in this part of the Neoproterozoic Western Ethiopian Shield. Additionally, the identification of exploration targets will help focus efforts to uncover any potential economic mineralisation.