Post-extrusive hydrothermal alteration within basaltic lavas has been examined with special reference to the Lower Carboniferous Clyde Plateau Basalts of the Midland Valley of Scotland. Within this province several steep-sided, narrowly constrained zones of intense water-rock interaction have been located and examined. It is thought that these zones, or metadomains, developed where hydrothermal fluids exploited permeability contrasts within the volcanic pile and as such they represent foci for geothermal fluids which advected through the lavas. In the field metadomains are often manifest as areas of intense alteration and hydrofracting of the host basalts. Characteristic alteration mineralogies are often hydrous and typically include evolved phyllosilicate assemblages, prehnite, analcime and calcite. Petrography reveals that prehnite and analcime are always paragentetically earlier than calcite across the whole of the lavas. Also, metadomain development was accompanied by significant elemental mobility within the host basalts; Si, Ti, Fe, Mn, Mg, Ca, Na and K all exhibit varying degrees of mobility, whilst Sr, Rb, Cu and Zn display extreme mobility. Furthermore, metadomain basalts have higher volatile contents than there unaltered counterparts. Fluids present during metadomain development typically were Ca2+, Na+, Cl- dominated brines which exhibited both a wide range in salinity (approximately 0 to 25 equiv. wt. % NaCI) and temperatures (approximately 50°C to 300°C). As the development of metadomains proceeded it appears that the fluid present was a hybrid formed from the interaction and mixing of a low temperature, low salinity fluid which was ultimately of surface origin and a higher temperature brine derived from the underlying sediments. The presence of minor amounts of hydrocarbon material within the fluids testified to the involvement of organic rich sequences within the underlying Inverclyde Group sediments. These fluids also record a number of physical processes which are observed within currently active geothermal systems such as mixing and boiling indicative of near surface conditions. Such processes appeared to be largely responsible for controlling the gas content of the fluids. Combined fluid inclusion and stable isotopic analysis of alteration phases (specifically analcime and calcite) has been used to determine fluid origins and evolution during metadomain development. Fluids present during the precipitation of analcime appears to have originated as a surface derived fluid which interacted (isotopically and chemically) with reactive, carbonate rich sediments, possibly the underlying Inverclyde Group. Fluids present during calcite precipitation can be separated into two distinct groups; a low to intermediate (c. 90 to 200°C) surface derived fluid and a higher temperature fluid which also appears to have interacted with reactive sediments. Carbon isotopic results confirm that organic derived material interacted with these fluids. Uniformity in carbon isotopic compositions suggests that the carbon reservoir was active prior to calcite precipitation. As geothermal activity waned calcite precipitation still continued whilst fluid temperatures decreased due to convective cooling of the lava pile. During this stage it appears that fluids were dominantly of surficial origin.

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Lherzolite nodules in basaltic rocks from three localities in British Columbia include rocks of mantle origin and crystal cumulates. Partial chemical analyses show that the compositional ranges of the minerals are narrow for both major and minor elements and fall within the ranges reported for lherzolite nodules elsewhere. Each suite is characterised by a definite range of concentrations of some elements. Olivine in nodules from Castle Rock and Jacques Lake show fabrics resulting from deformation in the solid state prior to their incorporation into their host rocks but those from Nicola Lake are undeformed. The distribution of iron and magnesium between coexisting phases is examined using an ideal ionic solution model. Differences in the distribution coefficients between the suites are probably due to different temperature and pressure conditions at the source of the nodules. The distribution of iron and magnesium between coexisting spinel and olivine gives nominal temperatures of formation of 838°C for Nicola Lake nodules, 1085°C for Jacques Lake nodules and >l600°C for Castle Rock nodules. Differences among the suites in the distribution, of Ni, Co, Mn and Zn between coexisting silicates are independent of variations in composition and are apparently due to different conditions of formation. The Castle Rock and Jacques Lake lherzolites are residual fragments of the upper mantle left after extraction of an under-saturated basaltic liquid from parental mantle rock. The source of the Castle Rock nodules probably lies at greater depth than that of the Jacques Lake nodules. The Nicola Lake nodules are crystal cumulates and formed at an early stage of basalt genesis within the upper mantle or lower crust. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Seventy-one samples of subalkaline and alkaline basalts from British Columbia and the adjacent Pacific seafloor were analyzed for 33 major, trace and rare earth elements using X-ray flourescence (XRF) and instrumental neutron activation analysis (INAA). These basalts are all less than 22 Ma in age and come from various magmatic belts, each with a distinct, well-known, tectonic setting; (1) Convergent margin (Garibaldi and Pemberton Belts), (2) Back-arc (Chilcotin Basalts), (3) Hotspot (Anahim Volcanic Belt), (4) Incipient rift (Stikine Volcanic Belt), (5) Arc-trench gap (Alert Bay Volcanic Belt) and (6) Ocean floor (Offshore basalts of the Juan de Fuca-Explorer Ridge Systems). Element abundances and ratios were plotted on eighteen diagrams that have been proposed to discriminate between tectonic settings on the basis of magma chemistry. Although eruption through continental crust has modified the abundances of Ba, Th, U, K and Sr, in most cases this did not affect the ability of the diagrams to distinguish tectonic setting. On most diagrams basalts from back-arc, hotspot, incipient rift and arc-trench gap settings plotted in the within plate basalt (WPB) field, but distinction between these different WPB settings could not be made. Two samples from the Masset Formation on the Queen Charlotte Islands, included with the Anahim Belt hotspot suite, were consistently classified as convergent margin. Samples from the ocean floor plotted in the N-MORB or E-MORB fields. Three convergent margin samples from the Pemberton Belt always plotted in the convergent margin field, but on most diagrams all eight samples from the Garibaldi Belt plotted in the WPB field because of their depletion in LIL elements. La is the only rare earth element obtained by INAA that is essential for identifying the tectonic environment of magma genesis. The ratio La/Nb, is an effective separator of within plate basalts (WPB), including E-MORB, (La/Nb less than 1.2) from convergent margin basalts (La/Nb greater than 2.0). N-MORB lie between the ratios 1.2 and 2.0. Th, Ta and Hf also obtained by INAA, are important discriminant elements. However, Nb and Zr, obtained by XRF analysis convey much of the same information. The ratio Nb/16 as an estimate of Ta and Zr/39 as an estimate for Hf produced acceptable results on diagrams that originally incorporated Ta and Hf. Effective discrimination can therefore usually be achieved using XRF elements alone. Convergent margin, within plate and ocean floor tectonic settings were best distinguished on Th-Hf/3-Ta, Ti-Zr-Y and Ti-Zr-Sr, Ti/Y vs. Nb/Y, Th/Yb vs. Ta/Yb, (Ba/La)CH vs. (La/Sm)CH and V vs. Ti/1000. Slightly less effective plots were MnO-Ti0₂-P₂0₅, La vs. Th, La vs. Nb and K₂0/Yb vs. Ta/Yb. On the other hand Ti0₂-K₂0-P₂0₅, MgO-Fe0*-Al₂0₃ and La vs. Ba provided little information concerning the tectonic setting of individual samples. Ti/Cr vs. Ni, Sm/Ce vs. Sr/Ce, Cr vs. Ce/Sr and Cr vs. Y diagrams were useful for distinguishing unfractionated convergent margin basalts from MORB plus WPB. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

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In an attempt to understand the phase equilibria and petrogenesis of MORB anorthitic plagioclase, Cr-spinel commonly hosted within anorthitic plagioclase has been investigated petrographically and compositionally. Based on spinel-anorthite relationships from three samples of plagioclase ultra-phyric basalt (PUB; Southeast Indian Ridge, Axial Seamount and West Valley Segment, Juan de Fuca Ridge) our work finds that Cr-spinel hosted within anorthitic megacrysts consistently exhibit rounded, partially dissolved morphologies. In addition, spinel included in anorthitic plagioclase are often accompanied by melt in a composite 2 phase inclusion. Cr-spinel compositionally exhibits collinear negative correlations in Mg# (Mg/Mg+Fe; 0.6-0.73) and Cr# (Cr/Cr+Al; 0.2-0.6), and positive correlations of Cr# and Fe�����# (Fe�����/ Fe�����+Cr+Al; <0.1) with TiO��� wt% (0.3-0.8). Additionally, all spinel appear to exhibit mantle affinity (Fe�����#<0.1; Barnes and Roedder, 2001). Based on historical interpretations of Cr-spinel (Dick and Bullen, 1984), we conclude that Cr-spinel hosted within MORB anorthitic plagioclase preserve melt-mantle reaction signatures. Such reactions, potentially forming dunite, result when ascending low-a[subscript silica] primitive melts interact and consume upper mantle silicates (i.e. clinopyroxene), and include Cr,Al-rich spinel. Thus, both Al and Ca are released into the derivative melt stabilizing anorthitic plagioclase. Given that olivine has never been found in contact with plagioclase >An������, we propose that anorthitic plagioclase precipitates from the derivative liquid prior to olivine. Recently, studies have used the trace element signatures of MORB anorthitic plagioclase as probes of early differentiation processes beneath MOR (Adams et al., 2011; Weinsteiger et al., in review). However, these studies have outlined the need to decipher the geochemical signals of individual anorthitic plagioclase so that population trends may be interpreted. In response, this thesis also reports detailed trace element profiles of individual anorthitic plagioclase crystals and population trends from two samples of PUB (Southeast Indian Ridge [SEIR] sample, Axial Seamount sample). Profiles can be categorized as dominantly stochastic since correlations between trace elements and trace and major elements are largely not found; potentially precluding a role for diffusive re-equilibration. We propose that trace element heterogeneities found within individual crystals reflect the degree of trace element variability present within upper mantle and lower crust conduits. However, by observing population data specific processes may be seen. Similarities in Axial Seamount plagioclase trace elements suggest a relatively uniform source that was evolving largely as a result of plagioclase only fractionation. In contrast, the array of trace element concentrations of SEIR plagioclase positively correlates with major element variations. This suggests a complex process of melt aggregation of increasing percent melts concurrently with anorthitic plagioclase precipitation. Further process related signatures can be derived by viewing plots combining trace and major elements from both samples. In this format, each samples data suggest that similar large scale processes occur in conduits within the upper mantle where these anorthite populations' form. Although plagioclase only fractionation appears to frame the trend, the correlation is diffuse and potentially reflects additional magmatic processes (i.e. AFC, % melt, and melt aggregation). Calculated equilibrium liquids based on Axial Seamount and SEIR plagioclase are considerably depleted relative to their host glass and natural glasses documented to occur near the sampled site. The differentiation processes linking these melt compositions is currently unknown. / Graduation date: 2013

Thesis (Ph.D.) -- Memorial University of Newfoundland. / Typescript. Bibliography : leaves 344-373. Also available online.

Thesis (Ph. D.)--Oregon State University, 2006. / Printout. Includes bibliographical references (leaves 224-238). Also available on the World Wide Web.

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