This section outlines the experimental designs used during this project, and the problems associated with each. As will become clear in chapters 3 and 4, even the most successful method in this study (cementing crystal seeds to the crucible floor using alumina cement) has problems which have not yet been overcome.
Technique a) The Pt wire loop technique
A Pt8()Rh20 wire loop is used as a sample holder (Weill and McKay 1975, Lofgren et al 1974). The sample consists of a pressed pellet (figure 2.01a) of powdered glass and chips of the mineral to be overgrown. The pellet is attached to the wire by heating the wire with an electrical current which melts the outside of the pellet (figure 2.01b). The loop is then slung from a rod and inserted in the furnace. This technique was used in two series of preliminary experiments checking the phase relations of both melts used in this study. The chips of crystal sink to the bottom of the bead if they are denser than the melt. In theory they should remain here during the experiment, but there is a possibility that they may move in the bead of liquid, especially if buoyant, escaping air bubbles attach to them. This risk is minimal as most air is expelled from the pellets when they are pressed during preparation, however this technique provides no guarantee of fixing crystal seeds during experiments.
The main problem with the technique, however, involves the effect of differential surface tension at the points where crystal seeds come into contact with the meniscus of the bead. Variations in surface tension at these points may override the buoyancy forces in a compositional boundary layer and cause the melt at the crystal-liquid interface to be dragged away from the crystal onto the meniscus (Donaldson, 1993). For these reasons this technique has not been used for all experiments in this study. The wire-loop technique does, however, provide a ready method of testing phase relations in order to identify the most suitable temperature conditions for further experiments.
Pressed powder pellets (3 mm in diameter), mounted on 2.5 mm diameter loops of Pt8()Rb20 wire (0.2 mm thick) (see figure 2.01), were fused to form spherical beads of melt in a quenching furnace, held above the liquidus of the system (1300°C for the cobalt basalt and 1270°C for the natural basalt) for 10 minutes, and then cooled rapidly to a subliquidus temperature to allow crystal overgrowths to form on the seeds. The beads were quenched in air, and vertically-aligned, polished thin sections were made for optical and electron-probe micro-analysis.
FIGURE 2.01. Sketches outlining the wire-loop technique.
b) After attaching the wire to a vertical ceramic rod, the pellet is fused in the furnace to form a bead of melt which is held on the wire by surface tension.
zx
c) Vertically-Aligned thin sections through beads show how crystals have settled at the bottom of the charge. The indentations on the meniscus are the points at which the bead was attached to the wire.
FIGURE 2.02. Phase diagram for a typical mineral showing solid solution (eg. olivine). There is a bigger compositional difference (y»x) between the original melt composition and melt at the crystal-liquid interface at lower temperatures.
FIGURE 2.03. Summary of the change in olivine morphology with varying cooling rates and degrees of supercooling (from Donaldson, 1974).
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SO 3 00 t'tro1 0 zo 30
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1+0 0 0 w A ja./ 1 wFIGURE 2.04. Cross section of an alumina crucible in which an olivine seed is suspended by platinum wire.
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This technique provides an estimate of the rate of crystal growth at a given temperature, and indicates the morphology of the crystals that will grow at that temperature. If the temperature is too low then crystals will nucleate throughout the melt and interfere with analysis of the resultant glass because compositional gradients are established adjacent to all crystals, not just in the glass adjacent to the central crystal seed. This is especially relevant in the cobalt-basalt, as olivine nucleates readily in this melt when it is supercooled.
Figure 2.02 illustrates the importance of finding the optimum temperature conditions for overgrowing crystal seeds in these experiments. If an experimental run is made at temperature Ti, using an initial melt composition of COrig/ the composition of melt in equilibrium with the growing crystal at the crystal-liquid interface is given by Ci. At a higher degree of supercooling in the same liquid composition, (i.e. at T2), the melt at the crystal-liquid interface has the composition C2. At T2 the interface melt has a larger compositional difference with Corig than at Ti (denoted by the length of X and Y in figure 2.02)- If convection does not remove it, the boundary layer at T2 will have a larger compositional contrast with COrig- However, at greater supercoolings crystal nucleation rate is higher, and more elongate and skeletal crystal morphologies grow (Donaldson, 1976, and Lofgren 1974). This will produce a complex interface along which boundary layer movement will be potentially hard to trace. At smaller
supercoolings (eg Ti in figure 2.02), a thinner boundary layer will be produced due to the slower crystal growth rates, but the crystal overgrowth will have a simple
morphology and so examination of the boundary layer and crystal morphology is not complicated by a complex crystal morphology (see figure 2.03) whereas at lower temperature it might be.
If convection does not interfere with boundary layer development then the width of boundary layers will be controlled by chemical diffusion rates. At large supercoolings, where interface melt has a bigger compositional difference with the original melt composition, diffusion rates will be relatively slow. In this case diffusion rates will not be able to smooth out the compositional gradients formed by crystal growth, and so the large compositional difference is spread over a wide boundary layer. At small supercoolings diffusion rates are faster and so boundary layers will not develop to the same thickness as they do at larger supercoolings. In such cases small compositional variations will exist across thin boundary layers. Therefore a problem exists in
choosing the degree of supercooling at which boundary layer development should be studied. At small supercoolings small compositional differences will develop in thin boundary layers, if convection does not interfere with their development, while at larger supercoolings a bigger, and possibly more easily detectable, boundary layer will form with a larger compositional difference. From this it may seem that larger
aforementioned problems of crystal morphology come into play more at larger supercoolings. In this study runs are made at small degrees of supercooling.
In examining the glasses produced on quenching crystal-growth experiments, using either optical or electron microscopy, it should be easier to detect larger compositional differences in melts. This suggests that a larger degree of supercooling should be used. However the problems of extra crystal nucleation and increasingly complex crystal- liquid interface morphologies then need to be considered. An intermediate
temperature (1230°C for the synthetic basalt, and 1250°C for the basalt) would therefore seem to be the most suitable condition for developing and observing compositional boundary layers in these experiments.
In summary, experiments using the wire-loop technique serve a dual purpose:- 1) They allow phase relations of a magma system to be checked.
2) They allow crystal growth rates and crystal morphologies at different temperatures to be gauged.
From this information a suitable temperature can be selected for experiments, one at which boundary layer thickness and compositional differences across it should be sufficient to be detected, and at which nucleation of extra crystals in the charges will not interfere unduly with glass analysis.
Technique b) Crystals suspended by Pt wire in alumina crucibles
This technique has been used successfully in high-temperature, short-duration, crystal- dissolution experiments in silicate melts (Donaldson 1993). It involves suspending a drilled crystal (olivine in the cobalt-basalt) in an alumina crucible, by threading the two together with 0.05mm diameter Pt-Rh wire through drilled holes in the crucible walls (see figure 2.04). Holes were drilled in the crucible walls with a 0.8mm diameter, diamond tipped dental drill bit. The wire is looped around the crystal once, to keep the seed in a fixed position, and the wire-ends are tied together on the outside wall of the crucible. The holes in the sides of the crucible are plugged using alumina cement, and a 2-3 mm thick layer of this cement is applied to the outside of the charge to prevent leakage of melt through the crucible walls. After adding the alumina cement, the charges were allowed to dry at 100°C for several hours.
The crucibles were filled with chips of cobalt-basalt glass, held at I300°C until the glass had fused, removed from the furnace and refilled if the melt level fell to anywhere near the upper surface of the crystal (to avoid any surface-tension driven convection). The
glass was then remeHeJ. and held above the for 10 minutes before being cooled
to the appropriate subliquidus temperature. At the end of the experimental runs the charges were quenched in air (to avoid the shattering that occurs in water-quenched
experiments). Polished thin sections, cut vertically through crystal seeds, were made for optical and electron microscope analysis.
This technique was successful for short-duration runs (<5 hours), but after longer periods of time the pores in the alumina cement became saturated with melt, and then the melt leaked onto the outside of the crucible, forcing runs to be aborted. Long duration runs (which are necessary to produce any noticeable differentiation in the charges) are therefore not possible using this technique. After several failures due to melt leakage this method was abandoned. Another problem of the technique involves contamination of the melt by a certain amount of dissolution of alumina cement, this is discussed further in chapters 3 and 4.
Technique c) Crystals suspended in clay crucibles
With the aid of the School of Materials at Leeds University clay crucibles were made in which crystals could be suspended internally, without the problem of weaknesses caused by holes in the crucible walls. These crucibles were made using clay slip provided by the technique described in appendix A. This technique involves attaching small handles of clay to the inner walls of a clay crucible before the crucible is fired in a furnace. The clay handles are pierced vertically with a needle to form holes two-thirds of the way down the walls of the crucible. Olivine seeds can be hung in the crucible by threading them with Pt wire, passing this through the holes in the handles, and tying the wire at its ends (see appendix B for details).
Several experiments using the cobalt-basalt were run in these crucibles. However problems arose due to dissolution of the crucibles in this melt. This type of crucible is usually used for glass and ceramic experiments at 1000°C, with only minimal
contamination of melts being reported. It appears that a combination of higher
temperature in these experiments, and the corrosive properties of basaltic melts causes increased dissolution of the crucible walls. Making the walls thicker only prolongs the life of the crucible before leakage occurs from the charge, while the melt becomes more contaminated with dissolved clay.
However the dissolution does cause convection in the crucibles by producing a less dense melt at the walls and on the floor (see plate 3.04 in chapter 3). Dissolution of the crucible takes place at all liquid-clay interfaces, but is fastest at the meniscus-wall contact where flux-line attack causes accelerated wall thinning. In some of the runs using these crucibles dissolution causes clay handles to detach from crucible walls so that the crystal seed does not retain its fixed position during the run. This is discussed further in chapter 3.