Calibración

2.3.1 Pressure Generation Within the Earth.

At any point within the Earth, pressure is generated primarily by the action of gravitational field on the overlying matter: at the Earth's surface, atmospheric pressure is due to the gravitational attraction between the atmosphere and Earth's centre of gravity. Difference in pressure between a point at radius r from the centre of gravity and a point, dr closer to the centre of gravity, can be expressed as dP = dr.g.p, where g is the gravitational field strength ^g = and p is the density of the material between r and dr. From this, it can be seen that:

1) Pressure within the earth is a positive function of depth,

2) Rate of change of pressure is dependent on gravitational field strength, and density of overlying rocks. As the centre of gravity is approached, the gravitational field strength diminishes due to the gravitational attraction of the overlying mass. At the same time, however, increasing pressure increases the density of the surrounding matter and the two effects tend to cancel each other within the depth range under study here.

3) Total lithospheric pressure at a given point can be expressed as: r

Pressure within the deep Earth is generally considered to be hydrostatic, that is the confining pressure acting on a point area is of equal magnitude in all directions, as it would be in a fluid pressure medium. If volumes larger than points are considered, there is a non-hydrostatic component due to dr.g.p across the depth of the region, but this effect is negligible. Within the lower crust and upper mantle, the assumption of hydrostasicity is reasonable because, even where significant shear is taking place, the magnitude of hydrostatic pressure will be far greater than any non-hydrostatic loading and, except in rare cases, phase relations will not be significantly altered by the non­

significantly altered by shear within the Earth resulting in large textural variations between hydrostatic and non-hydrostatic growth environments.

2.3.2 Solid-State Pressure Media: Advantages and Disadvantages.

In order to replicate the conditions within the Earth it is necessary to experimentally generate high hydrostatic pressures, with minimal shear component and simultaneous high temperatures within the sample. The simplest way to generate hydrostatic pressure within a sample is the use of a liquid or gaseous pressure transm itting medium (PTM). Although this solves the problem of pressure hydrostacisity and fluid media devices are common for low pressure experiments up to about 0.5 GPa, at higher pressures, other potentially dangerous difficulties are encountered. The first of these is that of pressure confinement; since fluid media have the property of flowing, sealing of joints, particularly around moving parts such as the pump components, is a very real problem. Secondly, the possibility of the PTM undergoing phase change during the experiment, which would cause a highly discontinuous pressure generation history. Careful planning can negate this problem and even sometimes make use of phase changes during pressurisation, for instance diamond anvil cells have been loaded under a confining pressure in order to liquefy inert gasses as the PTM. In general, however, phase changes encountered during pressurisation can cause problems with pressure measurement and generation and can obscure the physical meaning of results. The pressure range of a PTM is, therefore, usually confined to pressures below where phase changes would be encountered. Although solid media also potentially undergo phase changes with pressure, because they are already condensed phases, the associated volume changes are far smaller than those in liquids or gasses and solid PTM materials can be selected in which the pressures of phase transformation are much higher. Finally, due to the high compressibilities of liquid and, particularly gaseous PTM the energy required to generate high pressures is significantly higher than for solid PTM. The danger with this is in the case of the pressure vessel rupturing, there is a large amount of kinetic

energy stored in a compressible PTM available for explosive depressurisation. Because of these reasons, solid media devices have been developed for experiments at high pressures and are now capable of reaching up to 50 GPa. It should be noted that the majority of diamond anvil cell experiments ensure hydrostatic conditions using liquid pressure transmitting media and in this case the above problems are largely avoided by the reduction of sample volume. However, diamond anvil cell experiments are not ideal for the study of multi-component phase equilibria and many problems exist in the generation of high sample temperatures.

2.3.3 Experiments up to 2 GPa: Piston-CyUnder Apparatus.

The piston-cylinder apparatus generates pressure in a solid PTM through the uniaxial compression of a cylindrical pressure cavity by a single tungsten carbide (WC) plunger (figure 2.4). The upper limit on pressure generation is imposed by the breaking strength of the pressure vessel, either the plunger or the WC cylinder, or else by extrusion of the PTM. A cross section of the PTM cell is shown in figure 2.5 and it can be seen that the only possible area for extrusion is through the thermocouple inlet. The danger of thermocouple extrusion, and the shear strength of the PTM put an effective lower limit on pressure generation in the piston-cylinder of about 0.3 GPa.

Boyd-England (1960) type piston-cylinder vessels of 3/4 and 1/2 inch internal diameter were used for experiments up to 3 GPa. These have a steel prestressed jacket around the WC inner cylinder and are additionally end-loaded to increase the rupture strength of the inner cylinder. The upper working limit of this design of piston cylinder is 5 GPa, when pistons break regularly. If pressure is confined to 3 GPa, the lifetime of pistons is extended to 50 or more experiments, depending on the quality of WC used.

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ram iston

Figure 2.4 Schematic o f the Boyd-England type piston-cylinder. Brickwork represents WC components,

close hatching hardened steel and open hatching soft steel. Unlabelled arrows represent cooling water

inlets.

The PTM assem bly was essentially identical to the talc-boron nitride cell used by H uang and W yllie (1975), except the talc com ponents were pre-dried at 800 °C for 1 hour before use. The small shrinkage was insufficient to have a detrim ental effect on cell assem bly or pressure generation and the resultant PTM was essentially anhydrous. P ressure w ithin the 1/2 inch assem bly w as calib rated using the calcite-arag o n ite equilibrium curve o f Carlson (1980).

Starting m aterial for the pressure calibration was a finely ground m ixture o f 50 % natural calcite and 50 % natural aragonite. Run products w ere exam ined by pow der XRD, described below, to determ ine direction o f reaction. A change in relative peak intensity o f m ore than 15 % averaged over four diagnostic peaks for each phase was used as criteria for reaction. This allowed bracketing o f equilibrium to d: 0.05 G P a and the necessary correction to pressure for internal friction was found to be -12.5 % o f nom inal pressure. See Appendix 1A for details o f pressure calibration.

T/c tube _Pyrophyllite

]WC piston

Figure 2.5 Enlargement from figure 2.4; the talc-boron nitride cell used for 1/2 inch piston-cylinder

experiments. The sample cavity, containing the capsule and MgO powder is unlabelled.

Tem perature in the resisiively heated cylindrical graphite furnace was controlled by a E u ro th e rm 815 tem p eratu re c o n tro lle r w ith a feedback loop. T he control th erm o co u p le w as also the sam ple th erm o co u p le, w hich for e x p e rim e n ts below 1200 °C w as ch ro m el-alu m el (K -type) and for ex p erim en ts above 1200 °C w as P t-13% R h/P t-0% R h (R -type); therm ocouples w ere electric-arc w elded in air. The estim ated differen ce betw een sam ple and therm ocouple tem perature w as 25 °C at 1500 °C , but w as alw ays higher in the sam ple, because the sam ple w as situated at the ce n tre o f the furnace. No c o rrectio n has been ap p lied for this e rro r in stated tem perature and no correction has been applied for pressure effect on therm ocouple e.m .f. in any experim ents.

Samples were encapsulated in arc-welded Au-capsules for experiments up to 1000 °C and Pt-capsules at higher temperatures. The capsules were welded at one end, cleaned for several days in acetone and dried at 110 °C for at least 3 hours before packing. The sample was lightly tamped down into a capsule using a WC drill bit and then the open end of the capsule was crimped shut. The packed and crimped, but not yet sealed capsules were stored at 115 °C for K2C0 3-free samples and 300 °C for those containing K2CO3 before being welded. In order to minimise the possibility of contamination of the sample due to diffusion of hydrogen through the capsule walls, the capsule was packed in MgO powder inside the furnace. This MgO reacts with free water at experimental conditions to produce brucite:

MgO + H2O = Mg(OH)2 ( 1 )

There was empirical evidence of the occurrence of reaction (1) by the fact that the MgO powder had produced a reasonably sintered white or pale grey mass around the capsule at temperatures as low as 700 °C. Despite these precautions, it is unlikely that experiments were entirely anhydrous, especially in K2C0 3-rich compositions.

Pressure was generated 'piston-out' in that the cell was pressurised to just below experimental pressure and then heat was applied. The thermal expansion due to heating was sufficient to bring the cell up to experimental pressure. Duration of experiments was limited to 15 minutes for experiments above 1000 °C, in order to reduce the likelihood of contamination by hydrogen, but below 1000 °C, experiments lasted from 30 minutes to over 1 hour. There was no evidence from repeat near- liquidus experiments varying from 30 minutes to 3 hours that the shorter experiments had not attained equilibrium. Experiments were terminated by cutting electrical power to the furnace, which produced quench rates in excess of 300 °C/min to below 200 °C. Pressure was subsequently slowly released over about 30 minutes to 4 hours for 0.5 GPa to 3 GPa experiments respectively.

2.3.4 Experiments Over 2 GPa: Multi-Anvil-Press Experiments.

The common factor in all multi-anvil press (MAP) designs is the triaxial compression of a solid, geometrical PTM. This serves to minimise the non-hydrostatic component of pressure and, within the sample region, produces an essentially hydrostatic environment. With few exceptions (most notably, split-sphere type; Kawai & Endo, 1970), pressure is achieved through uniaxial compression of the anvils, with force being transmitted from uniaxial mode to triaxial mode across inclined surfaces. Because pressure is generated within the PTM triaxially, the surface area of the anvils in contact with the PTM must be smaller than the face of the PTM in contact with the anvil (plate 2.1.a). The result of this gap between anvils is a large negative pressure gradient between the centre and edges of each face of the PTM, resulting in the commonest form of pressure failure; catastrophic extrusion of the PTM through this low pressure region, known as 'blow-out' (plate 2.1.c).

The geometry of the PTM is configured to minimise the risk of blow-out as seen in plate 2.1.b. A ceramic gasket is extended into the gap between the anvils which, under compression, pinch onto the gasket, thus providing support to the pressurised PTM. This gasket region has the additional role of providing support to the anvils and this, combined with the shaping of the anvils themselves to provide maximum support to their contact faces allows the regular attainment of 5 times higher pressures than that attainable in piston-cylinder devices using the same materials, in other words up to 25 GPa.

Two types of MAP were used in the present study. The first a Walker-type 6 - 8 double stage MAP was used at University College London for phase equilibria experiments. Secondly, for the in situ determination of physical properties of carbonate melts, the cubic dia-type MAP, MAX 80, was used at the National Laboratory for High Energy Physics, Japan in conjunction with the high intensity X-rays provided by the

a) riic Walker-type niulli-aiivil press.

Ih c W alker-type M AP was modi lied from the 6-8 type doiihle-slage presses com m on in Japan with the intent o f providing a MAP unit sufficiently com pact and self- contained to be used with already existent piston-cylinder load-fra m es (W alker c( al ,

1990). Eight cubic anvils, with corners truncated to produce an octahedral pressure cavity, are com pressed by six sliding wedges contacting the anvils at 45 ‘' to the axis of

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