Journal of the Virtual Explorer

The samples from Naxos with gaseous fluid inclusions have a much higher strength than those with aqueous inclusions. The two samples with gaseous inclusions (experiments 32 and 34) reached a differential stress of ∼1300 MPa, but did not reach a steady state flow stress (Fig. 1). The samples with aqueous inclusions, deformed with flow stresses of 320 (10^-5 s^-1) and 450-650 MPa (10^-5 s^-1; both Victoria and Scandinavia samples, experiments 25, 27, 30, and 33; Fig. 1). There is scatter in sample strength for different starting materials as well as for the same material. This variability in strengths may be related to the c-axis orientation in the sample, and to inhomogeneous distribution of fluid inclusions. However, the difference in strength between samples with different fluid inclusions composition is too large to be due to one of these factors and we therefore interpret it related to fluid composition.

Figure 1. Stress-strain curves of the deformation experiments

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Curves labelled 32 and 34 refer to gas-rich Naxos quartz, 25, 27 and 30 to Victoria samples, and 33 to Scandinavia material, both rich in aqueous inclusions. Details on the experiments are listed in Table 1.

All samples with aqueous inclusions have similar microstructures after deformation and annealing. They contain recrystallized grains showing straight extinction, which is in contrast to the original material showing undulose extinction (Fig. 2a and c). The sample that was annealed for 2 hours (experiment 25, Table 1) has 1 vol.% recrystallized grains; the samples that were annealed for longer times have more recrystallized grains, indicating that most recrystallization occurred during the annealing stage of the experiment. Recrystallized grains are elongated perpendicular to the shortening direction and their size varies somewhat with annealing/recrystallization time, but reaches > 100 μm in all samples. The recrystallized grains are normally free of internal fluid inclusions (Fig. 2b and f), but occasionally contain some large, gaseous ones. However, in some cases many fluid inclusions exist along boundaries of the recrystallized grains (Fig. 2f).

Samples with gaseous inclusions have different microstructures compared with those containing aqueous inclusions. Recrystallized grains also are elongated perpendicular to the shortening direction, but the grains are smaller in size. In these samples fluid inclusions are found inside the recrystallized grains, though somewhat less than in the surrounding quartz, and also on grain boundaries.

Several different features are observed in SEM on the grain boundaries in the H₂O-rich samples (Figs. 3a-c): Grain boundaries in between unrecrystallized grains are irregular with many pits of fluid inclusions (Fig. 3a). In contrast, the grain boundaries of the recrystallized grains are often smooth or exhibit channel-like features (Fig. 3b and c). Grain boundaries of recrystallized grains have rough interfaces in some cases, but are less distinct than the unrecrystallized areas (Fig. 3d-f). Triple junctions are often open (Fig. 3d-f). On the edges of the triple junction channels irregular channels are found in some cases (Fig. 3f).

Figure 2. Optical micrographs

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Images a,c, and e had crossed polarisers while b,d, and e had plane polarised illumination.
      
a and b) Sample 27, illustrating elongated recrystallized grains with straight extinction and udulatory extinction
     in surrounding material. Fluid inclusions are concentrated on grain boundaries. Shortening direction is
     horizontal (width of photos is 140 μm).
     
c and d) Sample32, overview of recrystallized grains in matrix with undulatory extinction.  Shortening direction is
     horizontal (width of photos is 5.5 mm).
     
e and f) Sample 33, recrystallized grains contain fewer fluid inclusions as the original material, there are fewer inclusions
     on grain boundary than in sample 27 (a-b). Shortening direction is horizontal (width of photos is 140 μm).

Figure 3. Scanning electron micrographs of sample 33

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The polycrystals break along grain boundaries:
a) Starting material containing many fluid inclusions in the center and inclusion-free recrystallized grains surrounding it; 
b) Recrystallized grains showing channel-shaped features on their grain boundaries; 
c) View is similar to b) but lower magnification; 
d) Smooth grain boundaries and open triple junctions; 
e) Lens-shaped pores on grain boundaries; 
f) Open triple junctions and irregular features possibly representing healing of the boundary.

The size and distribution of fluid inclusions changed during the experiments as listed in Table 2 and described below:

The sample of Victoria quartz annealed under stress for 2 hours after deformation (experiment 25). It has large and small fluid inclusions along grain boundaries of recrystallized grains. In non-recrystallized regions the distribution of inclusions has not changed. In general, fluid inclusions decreased in size (see Table 2), and increased in number, suggesting the original inclusions have split up and re-annealed. In some cases this process is illustrated by elongated inclusions that narrow down towards the centre (Fig. 2f). Inclusions are often elongated parallel to σ₁. The sample that was annealed for 32 hours at 900°C (experiment 30) has ∼15% recystallized grains and the distribution of fluid inclusions has not changed in the unrecrystallized areas. In the regions with recrystallized grains the fluid inclusions occur preferentially along grain boundaries and have decreased in size compared to the starting material. In the sample with the most recrystallized grains (30%) that was annealed the longest (experiment 27, 107 h at 800 °C) fluid inclusions appear in clusters and bands sub-parallel to σ₁. Inclusions occur on grain boundaries of recrystallized grains and are elongated. Outspread depressions along grain surfaces are interpreted representing fluid films which connect the inclusions (Fig. 3f).

At conclusion of the experiment the Scandinavia quartz sample, experiment 33, has approximately 30% recrystallized grains, similar to experiment 27, and shows some other features similar to that sample. Fluid inclusions that occur on the grain boundaries of recrystallized grains are large and in some cases connected by fluid films. This feature is also inferred based on the SEM observations on this sample (Fig. 3). In this sample the fluid inclusions have also decreased in size and increased in number. The recrystallized grains contain very few inclusions. The large inclusions are concentrated in the pressure shadow regions of the recrystallized grains.

In the gas-rich Naxos samples, experiments 32 and 34, there are around 10% recrystallized grains. Large, elongated fluid inclusions are found along grain boundaries of recrystallized grains, and some inside the grains. Small inclusions occur in recrystallized and unrecrystallized regions (more in unrecrystallized), and are aligned in bands.