by Michael Athans. / AFOSR Grant AF-AFOSR-72-2273.

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The mantle is the largest reservoir of many of the Earth’s volatile species. Detailed isotopic studies of noble gases within the mantle volatiles have demonstrated that they are of a primordial origin, which have been trapped in the mantle since the Earth’s accretion. This original volatile signature has continually evolved over time, due to the production of in situ radiogenic isotopes and through the recycling of surface volatiles back into the mantle (Lupton and Craig, 1975; Holland and Ballentine, 2006). The study of noble gases within magmatic samples has enabled the composition and structure of the mantle to be determined and has distinguished the multiple volatile reservoirs (primordial, crustal, marine etc.) that have contributed to the mantle composition sampled. Together with the halogens (Cl, Br and I) they represent key tracers of volatile transport processes in the Earth. Therefore a combined analytical approach including the halogens and noble gases is not only be able to track the influx of surface volatile into the mantle, but also provide a greater understanding to the fundamental controls of transport, storage and partitioning of volatiles within the mantle. A combined noble gas and halogen study was undertaken on three different geological samples sets to determine how surface volatiles interact with the mantle on a variety of different scales. Firstly continental xenoliths from the Western Antarctic Rift were analysed to establish the role of subduction volatiles in the creation of the rifts volcanic products. The xenoliths have 3He/4He ratios of 7.5RA indicating that the rift is dominated by the rising asthenospheric mantle. However the Br/Cl and I/Cl ratio and heavy noble gases within the xenoliths indicate that marine derived volatiles have been incorporated into the mantle beneath the rift and may have provided and fundamental control on the formation of the rift itself. Secondly the role of surface contamination on mantle samples has been evaluated. A transect along a MOR pillow basalt has been analysed for its halogen concentrations in conjunction with the previously determined noble gases. The outer sections are enriched in Cl relative to Br and I due to the assimilation of a high salinity brine during eruption. In contrast the crystalline interior of the pillow has MORB like Br/Cl and I/Cl ratios but elevated 132Xe/36Ar ratios indicative of the incorporation of pelagic sediments. This small scale analytical approach has shown that submarine pillow basalts are prone to contamination from the surrounding marine environment and provides a method for the identification and quantification of marine contamination. Finally the halogens within olivine phenocrysts from three Emperor Seamounts have been analysed to determine the distribution of the halogens within the lower mantle. The I/Cl ratio of the samples evolves from a MORB-like ratio in the oldest seamount to elevated values similar to sedimentary pore fluids and chondrites in the younger seamounts. This indicates that the Hawaiian mantle plume contains isolated pockets of subducted or primordial material which have been isolated from whole mantle mixing and have therefore retained a halogen signature distinct from the average mantle values.

Iceland straddles the Mid-Atlantic Ridge and overlies a mantle hotspot. This tectonic setting produces voluminous tholeiitic magmas. Volcanism in Iceland is focussed along three neovolcanic spreading ridges. During the Holocene, the Eastern Volcanic Zone (EVZ) in southeast Iceland has been the most volcanically active and has been the site of several large (>6 km3) eruptions, including the only floodbasalt type eruption in recorded history, the 1783-84 Laki eruption. Three eruptions of large volume have been sampled for this study: the 1783-84 Laki eruption (15.1 km3); the 3,000-4,000 yBP Thjórsárdalur eruption (probably >4 km3); and the ~8,600 yBP Thjórsá eruption (>21 km3). The products of these eruptions have been analysed using a range of analytical techniques, with the specific aim of investigating crystallisation, degassing and mixing processes in the magma reservoirs that feed large eruptions. The Laki eruption has been the particular focus of this study. Samples from different parts of the lava flow show fine-scale variations in trace element concentrations and ratios. This compositional variation is not fully explained by fractional crystallisation processes, but is strongly controlled by crystal accumulation as whole-rock incompatible trace element concentrations show a linear, negative correlation with the mass fraction of crystals in the sample. Simple crystal accumulation models, however, fail to explain the compositional variation, and one explanation is that the homogeneous Laki melt mixed with varying proportions of a crystal mush that contained its own liquid. The results of thermobarometry calculations indicate that the erupted Laki liquid was in equilibrium with olivine, plagioclase and augite at 1-3 kb. Most of the crystals carried by the flow are too primitive to have crystallised from the erupted liquid and barometry calculations indicate that clinopyroxene crystallised at 3-7 kb. The majority of the large crystals hosted in the Laki basalt samples are therefore antecrysts that grew within the same magma plumbing system as the Laki carrier melt but are not in direct chemical equilibrium with it. This finding is verified by the fact that olivine crystals that are too magnesian to be in chemical equilibrium with the Laki whole-rock composition contain melt inclusions with average La/Yb values that are the same within error as the whole-rock values. The wide range of La/Yb values in melt inclusions hosted in the most magnesian (Fo86) olivine crystals in comparison to the least magnesian (Fo<74) indicates the initial variability of the Laki magma prior to concurrent crystallisation and extensive mixing, which acted to homogenise the carrier melt composition. The preservation of a wide range of La/Yb within the melt inclusions in comparison to the whole-rock composition, and a range of La/Yb values in different inclusions from the same crystal, indicates short timescales between melt inclusion entrapment and quenching during eruption. Melt inclusion studies also reveal the dissolved volatile content of the Laki magma at the onset of olivine crystallisation, although the majority of H2O concentrations have almost certainly been reset by low pressure diffusive exchange with the host crystal or surrounding magma. Comparison of the behaviour of volatiles with that of incompatible elements in the melt inclusions indicates that CO2 was degassing during olivine crystallisation, but S, F and Cl were not. New estimates of total volatile loading to the atmosphere during the eruption based on melt inclusion volatile concentrations show SO2 and HCl loading comparable to previous estimates, but higher HF loading. Mass balance calculations show that the observed H2O and CO2 concentrations of melt inclusions hosted in olivines in chemical equilibrium with the Laki whole-rock composition are ~50% and ~93% lower respectively than would be expected if no pre-eruptive degassing of the magma reservoir had occurred, meaning that pre-eruptive degassing of H2O and CO2 from the magma must have been significant. Lava flows from Thjórsá are more compositionally variable than those from Laki, and have different average major and trace element compositions. Compositional variation within the Thjórsá whole-rock composition is explained by a combination of source variation and fractional crystallisation, and, unlike Laki, is not strongly controlled by crystal accumulation.

Critical metals are of growing economic importance for the low carbon sector but are susceptible to resource restrictions and have no viable substitutes in their applications. In this study, 134 samples of the Cornubian Batholith, SW England, with associated early Permian mafic and ultramafic rocks were sampled and analysed by ICP-MS and XRF for their major, trace and critical metal (Li, Be, Ga, Ge, Nb, Ta, In, Sb, W and Bi) abundance. The mineral chemistry of feldspars, micas, tourmaline, topaz and cordierite was determined for 8 samples by EPMA and LA-ICP-MS. The Cornubian Batholith is a peraluminous, composite pluton intruded into Devonian and Carboniferous metasedimentary and volcanic rocks. Geochemical fractionation trends recorded by whole rock geochemistry and mineral chemistry permit trace element modelling of two distinct fractional crystallisation series, biotite-muscovite (>282 Ma) and biotite-tourmaline (<282 Ma). The biotite-muscovite granites formed through muscovite and minor biotite dehydration melting of a metagreywacke source at moderate temperatures and pressures. Fractionation of an assemblage dominated by feldspars and biotite, enriched muscovite granites in Li (average 340 ppm), Be (13 ppm), Nb (16 ppm), Ta (3.7 ppm), In (77 ppb), Sn (17 ppm), W (12 ppm) and Bi (2.6 ppm) and are spatially associated with greisen style Sn-W mineralisation. Muscovite is the major host of In, Sn and W, and as muscovite is late-stage / subsolidus this implies these metals are highly incompatible in magmatic minerals and likely to partition into fluids exsolving from evolved muscovite granites. The biotite-tourmaline granites formed through higher-T melting than the first suite due to underplating of the region by mantle-derived melts during tectonic extension. Fractionation of feldspars, biotite and cordierite enriched Li (average 525 ppm), Ga (28 ppm), In (122 ppb), Sn (14 ppm), Nb (30 ppm), Ta (5.5 ppm), W (7.1 ppm) and Bi (2.7 ppm) in the tourmaline granites with retention of Be in the biotite granite due to partitioning of Be into cordierite. Distribution of Nb and Ta is controlled by accessory phases such as columbite within the evolved tourmaline granites, promoting disseminated Nb and Ta mineralisation. Lithium, In, Sn and W are hosted in biotite group micas which may prove favourable for breakdown on ingress of hydrothermal fluids and partitioning of the critical metals into mineralising fluids emanating from evolved tourmaline granites. Topaz granites are analogues of Rare Metal Granite described in France and Germany. They contain albite, polylithionite and topaz as major minerals and show differing trends on major and trace element plots relative to the other two granite series. These granites are enriched in Li (average 1363 ppm), Ga (38 ppm), Sn (21 ppm), W (24 ppm), Nb (52 ppm) and Ta (15 ppm) and formed through partial melting of a biotite-rich residue left after melting that formed early biotite granites.

The Columbus Basin, offshore Trinidad, is a mature gas producing basin with a number of major fields now in decline. Focus for infield exploration and production is shifting, with thin bedded sandstones as a secondary pay target. This basin is exceptional as age equivalent analogues to the subsurface reservoirs are exposed along the south east coast of Trinidad at Mayaro Bay (16 – 25 m sections). This research utilizes these outcrops and integrates findings with subsurface core data to present an improved understanding of thin bedded sandstones in three significant areas: 1) depositional setting on a wave dominated delta through description and interpretation of their large scale architecture and facies associations, 2) reservoir quality and connectivity of the facies and microfacies that comprise these heterolithic units through petrography and pore system characterization and 3) pore scale reservoir quality and connectivity through micro CT imaging and 3D modelling of their pore system morphology. Detailed sedimentological analysis has revealed that thin beds are highly interbedded units with thicknesses of 0.1 – 10 cm and have a lenticular geometry. Their lateral extent, controlled by their exposure, varies from 3 to 10s m in some areas. Field sampling and microfacies analysis, revealed five distinct lithofacies types and five microfacies types that make up two principal facies associations (FA): (FA1) axial distal delta front facies and (FA2) lateral distal delta front facies. The reservoir quality poroperm data achieved for the thin sandstones of these two facies associations are consistent with routine core analysis data from basin core and industry assigned values for conventional thicker bedded sandstones, inferring their secondary reservoir potential. Utilizing new techniques such as X-Ray tomography, a high resolution 3D model of the thin sandstone pore systems has been created for qualitative and quantitative reservoir characterization, especially vertical and lateral connectivity within the thin bedded units. This detailed dataset of 3D pore dimensions that can be used as conditioning data for other reservoir models. The observations and conclusions of this research give an insight into the depositional architecture and thin bedded sandstones on a distal delta front and their associated reservoir properties and connectivity mechanisms that facilitate an effective reservoir. These findings may inform and guide future exploration and appraisal, development and production and well completion and configuration programmes for thin bedded reservoirs as explained by the implications and recommendations at the end of this thesis.

The Western Gneiss Region (WGR) in Norway is home to some of the world’s most spectacular exposures of high pressure (HP) and ultrahigh pressure (UHP) eclogites. Despite extensive petrological studies into their pressure, temperature and time (PTt) histories, relatively few have reported on their trace element compositions. Such data can be used to supplement our understanding of the provenance and history of Norwegian eclogites, as well as to further our understanding of trace element fluxes during HP to UHP metamorphism in subduction zone settings. In order to address this shortfall in data availability, the first step was to investigate and apply the best dissolution techniques for preparing eclogite samples for chemical analysis. Eclogites commonly contain up to a few weight percent rutile (TiO2), which is known to be an important host for a variety of major and trace elements (e.g. Ti, Nb and Ta). However, typical rock digestion procedures are incapable of dissolving rutile, and thus may lead to inaccurate measurements. It was found that that total dissolution of rutile can be achieved by dissolving samples in sealed pressure vessels at increased pressures and temperatures, ultimately leading to greatly increased data accuracy for analyses of any rutile-bearing lithology. The solutions were analysed by standard ICP-MS techniques and the results compared to analyses of powders by XRF spectrometry. Our high-accuracy and high-precision data were subjected to immobile trace element discriminant analyses that suggested eclogites belonged to three broad geochemical groups: eclogites with mid ocean ridge Basalt (MORB)-like composition; eclogites with arc-like composition; and eclogites with geochemical signatures significantly perturbed by metamorphism. The geochemistry of eclogites in the first two groups are shown to likely reflect protolith composition, and as such we used model protolith compositions to calculate estimated element mobilities (EMMs) for those elements considered relatively mobile during metamorphism. It was not possible to determine protoliths for eclogites in the third category using trace elements alone. Finally, the trace element geochemistry of a large number of separated eclogite-hosted rutiles was studied. The data collected were used to demonstrate that rutile contains significant amounts of the whole-rock’s high field strength element (HFSE) budget, and may exert significant control on the HFSE composition of passing hydrothermal fluids. Furthermore, Zr-in-rutile thermometry (ZRT) was applied to separated rutiles. This temperature information was used to better our understanding of the thermal history of the WGR, as well as to create a map of eclogite temperatures in the Nordfjord-Statlandet area. This high-resolution thermal map of arguably the most important area of the WGR, supports current interpretations that during the Caledonian Orogeny the leading edge of the Baltica plate was consumed in a northwest to north-northwest-dipping subduction zone under Laurentia. Furthermore, isotherms on this map mimic several major fold hinges in the region rather well, thus providing support to the hypothesis that such structures were most likely formed during the collapse of the Scandinavian Caledonian Orogen after the peak metamorphism of most eclogites.

Volcanic rocks which outcrop over 40 sq. km in eastern Jersey and which have a maximum thickness of 25 km have been mapped in detail. Basaltic and andesitic lavas, tuffs and agglomerates, which have been affected by keratophyric alteration, are overlain by numerous ignimbrite cooling units, the youngest of which are generally aphyric, the oldest being porphyritic. Local rhyolites, tuf'fs and andesites occur among the igniinbrites. The volcanics are essentially conformable upon Brioverian sediments and are regarded as being late Precambrian in age. Cadomian E-W and N-B fold phases, with associated faulting, are largely responsible for the present outcrop pattern. The petrology and geochemistry of these volcanics have been studied. Together with the regional distribution of similar late Precambrian volcanism they indicate that the caic-alkaline suite was generated soon after the initiation of the global tectonic movements which eventually produced the Caledonides. The basalts and andesites are thought to result from the partial melting of a descending lithospheric plate within the mantle, the magmas thus produced being modified en route to the surface by fractionation and crustal contamination under a variable The rhyolitic maginas were produced by crustal anatexis and collected in high-level magma chambers prior to their eruption. Post.-orogenic Cadomian plutonics which invade the volcanics are also caic-alkaline but are chemically distinct from the extrusives. The petrology and geochemistry of a dyke swarm which invades the volcanics and just postdates emplacement of the plutonics is also discussed. This appears genetically related to the plutonic complexes and was generated close to the base of the crust.

The aim of this research was to provide a basis for quantifying the ability of red sandstone to oxidise dissolved organic carbon (DOC), and determine if results of experiments on synthetic minerals can be applied to geological materials. Ascorbic acid (H\(_2\)A) was used as a probe. Preliminary experiments revealed that it can reductively dissolve sandstone hematite and Mn oxide as research on synthetic minerals had previously shown. Sorption of H\(_2\)A to sandstone was similar to that seen for synthetic hematite (a two-slope linear isotherm). Anoxic batch experiments were undertaken under biotic and abiotic conditions. Release of Fe and Mn was found to be dependent on the concentration of H\(_2\)A and pH. Decrease in H\(_2\)A concentration exceeded increase in Fe and Mn concentrations corrected for sorption, and this was modelled by sorption of both ascorbate and dehydroascorbic acid. The rate of H\(_2\)A oxidation was higher in biotic experiments than in abiotic experiments, probably due to the differences in pH rather than bacterial presence. The rate of ascorbic acid oxidation by natural oxides was higher than found by previous researchers for synthetic hematite. This result was not expected, and only in part can be ascribed to the effect of Mn oxides. However, the mechanism of reductive dissolution determined by the synthetic mineral studies appear relevant also to the sandstone system. Many questions remain but the study shows that the sandstone provides a significant natural oxidative attenuation capacity and that the results of experimentation on synthetic minerals can only be applied qualitatively.

No description available.