Journal of the Virtual Explorer

Three types of quartz single crystals with different fluid inclusions compositions were used in this study, originating from Victoria (Australia), Naxos (Greece, Schuiling and Kreulen, 1979), and Scandinavia (Table 1).

The fluid inclusions that occur in these samples can roughly be divided into three categories: large, medium, and small inclusions. Detailed descriptions of the quartz and the inclusions are given below. The fluid composition in our starting materials was determined using a heating stage in optical microscopy. We did not characterise the water related point-defect structure of the samples by FTIR (Keppler and Rauch, 2000). The large quantity of bulk water in fluid inclusions makes FTIR characterisation of point defects very difficult. Therefore we cannot determine a possible interaction between properties of the samples and their point defect structure, but only with the bulk fluid composition.

The Victoria samples are milky vein quartz with straight extinction, with approximately 4 vol.% fluid inclusions. The larger fluid inclusions occurring in clusters are 20-40 μm in size and irregular in shape. These inclusions contain 0-20 vol.% gas and 80-100 vol.% liquid. The medium size inclusions (5-10 μm) are homogeneously distributed throughout the grains and have a smooth shape. They contain 0-20 vol.% gas and 50-100 vol.% liquid, and 0-30 vol.% solid (in the form of cubic crystals). The small inclusions are found in trails and have a spherical shape. These inclusions are 1-5 μm in size and contain 0-70 vol.% gas and 30-100 vol.% liquid (Table 2).

The material from Naxos has faint undulose extinction and contains ∼1-2 vol.% fluid inclusions which are inhomogeneously distributed in clusters and along healed cracks. The large, 5-20 μm fluid inclusions contain 20-100 vol.% gas and 0-80 vol.% liquid. These inclusions are irregular and prolate in shape. Small inclusions (<5 μm) contain only gas (Table 2).

The quartz from Scandinavia is clear to milky, contains 4-5 vol.% fluid inclusions, and has blocky undulose extinction. Large inclusions (10-35 μm) are irregular in shape and cluster along healed cracks. These inclusions contain 0-60 vol.% gas, but mostly in the range of 10 vol.%. Medium-size inclusions are 1-5 μm in size and spherical to round in shape. They contain 10 vol.% gas and 90 vol.% liquid, and infrequently they also contain cubic crystals, inferred to be NaCl. The small, spherical inclusions of 0.5-3 μm contain liquid.

Cylindrical samples (length ∼12 mm, diameter 5.9 mm) were cored at an angle of 56° - 86° with respect to the c-axis (see Table 1). Samples were bench-dried and then weld-sealed in thick-walled gold or platinum capsules (Table 1). The samples were deformed in a Tullis-modified Griggs-rig (Tullis and Tullis, 1986) at the Utrecht University, The Netherlands, using an all-salt assembly (for details see den Brok, 1992). For this type of assembly no pressure correction is required (Johannes, 1978).

Stresses were applied at temperatures between 710 and 820°C, 1200 MPa confining pressure and strain rates ranging from 0.9 to 11 x 10^-6 s^-1 (see Table 1). In order to avoid decrepitation of the fluid inclusions as much as possible during heating and pressurisation of the samples, the pressure and temperature were raised along the isochore of water density (1 gcm^-3, Fisher, 1976). Samples were then axially shortened to bulk strains ranging from 6.5 - 38.4 %. After the deformation the samples were held at high temperature and pressure for various time periods (2 – 280 h) allowing annealing and static recrystallization of the samples. Annealing temperatures were similar to or higher than temperatures during deformation (Table 1). At the conclusion of deformation, the σ₁–piston was not reversed, so that the samples annealed under stress. At the conclusion of each experiment the sample was cooled rapidly in order to avoid changes in microstructure. We prepared standard thin-sections (30 μm) parallel to the shortening direction. Stress-strain curves were calculated from the force-displacement record assuming homogeneous shortening of the sample and assuming the same linear increase in friction before and after the top piston reaches the sample (den Brok, 1992).

Slabs of material 1 mm thick were cut parallel to the shortening direction and prepared for Scanning Electron microscopy (SEM, glued to SEM stubs and carbon and gold coated). The slabs were carefully broken along planes either parallel to σ₁ or perpendicular to σ₁. The polycrystals break along the grain boundaries of recrystallized grains and this method thus allows observation of the grain boundary morphology (see also Urai 1983, Olgaard and Fitz Gerald, 1993; Mancktelow et al. , 1998).