The Shiant Isles Main Sill, of Tertiary age, is a classic example of a composite, differentiated alkaline basic sill. The first unit to be intruded was a 2 m thick olivine teschenite which was emplaced with phenocrysts of olivine (rag > 83) [rag = Mg#] and, perhaps, plagioclase. This was intruded by a 24 m thick picrite sill consisting of a mush of melt and suspended olivine phenocrysts (rag > 83) with a D-shaped modal profile. The 140 m thick picrodolerite-crinanite unit was formed by a magma carrying ~ 10% olivine (rag > 83) as the main phenocryst phase, together with some calcic plagioclase phenocrysts, being emplaced into the top of the picrite unit before the host rock was completely solidified. The olivine phenocrysts settled towards the bottom to form the picrodolerites. In-situ differentiation processes occurred under conditions of almost perfect fractional crystallization, during which very strongly zoned ophitic crystals of olivine (fayalitic rims) and clinopyroxene (hedenbergitic rims), and zoned laths of plagioclase (anorthoclase rims), formed. The last unit consists of ~ 2 m of granular olivine picrodolerite which was intruded into the upper crinanites, again before the host rock was fully solid.The mineral zoning patterns are interpreted using published cation diffusion coefficient data, and used to show that the picrite unit might have cooled to the blocking temperatures for Mg and Fe diffusion in < 5 years, and that even the relatively thick crinanite unit cooled very fast, so preserving the zoned Fe-Mg olivine and pyroxene compositions. The compositions of coexisting ilmenites and spinels define a redox trend which initially lies close to fayalite-magnetite-quartz buffer conditions, but later becomes more reducing and approaches magnetite-wustite buffer conditions. The final stages of development occurred during sub-solidus deuteric processes and involved formation of analcime and zeolites, as well as localized sulphide mineralization.
2 3In recent years, geological disposal of radioactive waste has focused on placement of high-and intermediate-levelwastes in mined underground caverns at depths of 500-800 m. Notwithstanding the billions of dollars spent to date on this approach, the difficulty of finding suitable sites and demonstrating to the public and regulators that a robust safety case can be developed has frustrated attempts to implement disposal programmes in several countries, and no disposal facility for spent nuclear fuel exists anywhere. The concept of deep borehole disposal was first considered in the 1950s, but was rejected as it was believed to be beyond existing drilling capabilities. Improvements in drilling and associated technologies and advances in sealing methods have prompted a re-examination of this option for the disposal of high-level radioactive wastes, including spent fuel and plutonium. Since the 1950s, studies of deep boreholes have involved minimal investment. However, deep borehole disposal offers a potentially safer, more secure, cost-effective and environmentally sound solution for the long-term management of high-level radioactive waste than mined repositories. Potentially it could accommodate most of the world's spent fuel inventory. This paper discusses the concept, the status of existing supporting equipment and technologies and the challenges that remain.
SUMMARY. The liquidus temperature 0198 ~ and equilibrium phase relations of a sample of Columbia River basalt from the Picture Gorge section have been determined at I atmosphere by heating in a controlled atmosphere. When this basalt is cooled from above its liquidus temperature the liquidus phase (plagioclase) may fail to crystallize depending on the degree of undercooling and the duration of the experiment. A field in temperature-time space in which plagioclase fails to crystallize on cooling is separated from another in which plagioclase always crystallizes by a third in which the nucleation of plagioclase is unpredictable in terms of temperature and time. The extent to which this basaltic liquid can be supercooled without the crystallization of plagioclase is independent of the time it is held above the liquidus or the temperature in excess of the liquidus to which it is heated.The exceptionally long times required to ensure the nucleation of plagioclase at or near the liquidus temperature suggest that many so-called 'equilibrium' phase relations determined from experiments of a few hours' duration could be in serious error if the 'equilibration' involves a nucleation process.It is demonstrated that, over a range of cooling rates, the temperature at which plagioclase begins to crystallize on cooling varies markedly and the temperature and times required for both possible and certain nucleation of plagioclase are calculated for a range of constant cooling rates. The range of cooling rates over which the nucleation temperature of plagioclase varies is likely to occur in nature only in certain lava flows and small minor intrusions. In such cases this could lead to changes in the order in which the minerals appear on cooling and other petrologically significant effects.MANY silicate liquids can be cooled below their equilibrium liquidus temperature without crystallization of the liquidus phase. This tendency to supercool is generally recognized (Schairer, ~959) and necessitates some caution in interpreting crystalliquid relations in experimental charges that have been above the intended equilibration temperature at any time before or during the experiments.Whether or not a silicate liquid can be supercooled below its liquidus temperature seems to depend largely on the composition of the liquid and the nature of the liquidus phase. For example, in systems where forsteritic olivine is the liquidus phase there are no recorded instances of supercooling: on the contrary, olivine appears to crystallize very readily as soon as its saturation level is reached, as evidenced by the difficulties encountered in quenching Mg-rich liquids (Yoder, I952; Kushiro and Yoder, ~969; Green, I973). On the other hand, there are several examples of silicate melts failing to crystallize pyroxene or feldspar below the temperature at which the solid phase is in equilibrium with the liquid. In studying the melting relations of some calc-alkaline volcanic rocks Brown and Schairer (1968) completely melted their samples at I3oo ~ in order to ...
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