Stephen C. Elphick

Sequential growth of deformation bands in the laboratory

We investigate the formation and evolution of localised faulting in high porosity sandstone by laboratory triaxial compression of intact 100-mm-diameter core samples. Experiments were carried out dry, at constant confining pressure (34 MPa), constant axial strain rate (5 10 ˇ6 s ˇ1 ) and increasing axial strain (1.5‐11.2%). Tests generated fault zones consisting of sets of distinct pale granulated strands, separated by lenses of apparently undamaged host rock. The sets of strands were sub-parallel to the shear direction but showed complex anastamosing geometry in perpendicular section. The individual strands had reduced grain size, porosity and sorting compared to undeformed rock. A strong correlation was found between the number of strands occurring in a fault zone and the applied axial strain. Mean grain size, however, reached a steady value irrespective of axial strain. This implies that a limited amount of strain is accommodated on each strand with further strain requiring new strands to form. However, no direct evidence for strain hardening was observed in the post-failure macroscopic stress‐strain curves. Our laboratory induced deformation zones strongly resemble the key characteristics of natural deformation bands. We show the first laboratory evidence for the sequential development of increasing numbers of discrete deformation bands with increasing strain. # 1999 Elsevier Science Ltd. All rights reserved.

Experimental determination of cation diffusivities in aluminosilicate garnets I. Experimental methods and interdiffusion data

We have carried out diffusion couple experiments using pairs of single crystals of natural garnet of dissimilar compositions in the range of 30–40 Kbar, 1,300–1,500° C, and measured the induced diffusion profiles by microprobe scanning across the interface. Significant modifications to, and experimentation with, the design of the pressure cell, furnace assembly and sample geometry were needed to obtain measurable volume diffusion at controlled P-T conditions.The diffusion profiles in the pyrope-almandine couples are short enough that retrieval of diffusion data from them must await deconvolution analysis to resolve the effect of spatial averaging of the microprobe beam. However, the profiles in the spessartine-almandine couples are sufficiently long to obviate convolution analysis. They yield interdiffusion coefficients (D) at 40 Kbar of D = 0.82×10−5 exp (−53.6±4.9 Kcal/RT) cm2/s and D=1.2×10−5 exp (−57.1±8.4 Kcal/RT) cm2/s for Fe-rich and Mn-rich compositions, respectively, and an activation volume of ∼4.7 cm3/mole. Preliminary analysis of profiles in a pyrope-almandine couple at ∼40 Kbar, 1,440°C suggests Fe-Mg interdiffusion to be an order of magnitude slower that Fe-Mn interdiffusion, and to increase with Fe/Mg ratio. The interdiffusion data reported here are in sharp disagreement with those of Freer (1979) and Duckworth and Freer (in Freer 1981) on Fe-Mn and Fe-Mg interdiffusion, respectively.

The solubility of water in NaAlSi3O8 melts: a re-examination of Ab−H2O phase relationships and critical behaviour at high pressures

The solubility of water in melts in the NaAlSi3O8−H2O system at high P and T was deduced from the appearance of quenched products and from water concentrations in the quenched glasses measured by ion probe, calibrated by hydrogen manometry. Starting materials were gels with sufficient water added to ensure saturation of the melts under the run conditions. Experiments were carried out for 10–30 h in an internally heated argon pressure vessel (eight at 1400° C and 0.2–0.73 GPa and three at 0.5 GPa and 900–1200° C) and for 1 h in a piston-cylinder apparatus (three at 1200° C, 1–1.3 GPa). No bubbles were observed in the glasses quenched at P<0.5 GPa or from T<1300° C at 0.5 GPa. Bubble concentration in glasses quenched from 1400° C was low at 0.5, moderate at 0.55 GPa and very high at 0.73 GPa and still higher in glasses quenched in the piston cylinder. Water concentration was measured in all glasses, except for the one at 0.55 GPa, for which it was only estimated, and for those at ≽0.73 GPa because bubble concentration was too high. Inferred water solubilities in the melt increase strongly with increasing P at 1400° C (from 6.0 wt% at 0.2 GPa to ∼15 at 0.55 GPa) and also with increasing T at 0.5 GPa (from 9.0 wt% at 900° C to ∼12.9 at 1400° C). The T variation of water solubility is fundamental for understanding the behaviour of melts on quenching. If the solubility decreases with T at constant P (retrograde solubility), bubbles cannot form by exsolution on isobaric quenching, whereas if the solubility is prograde they may do so if the cooling rate is not too fast. It is inferred from observed bubble concentrations and from our and previous solubility data that water solubility is retrograde at low P and prograde at and above ∼0.45 GPa; it probably changes with T from retrograde below to prograde above ∼900° C at 0.5 GPa. Moreover, the solubility is very large at higher pressures (possibly>∼30 wt% at 1.3 GPa and 1200° C) and critical behaviour is approached at ∼1.3 GPa and 1200° C. The critical curve rises to slightly higher P at lower T and intersects the three-phase or melting curve at a critical end point near 670° C and 1.5 GPa, above which albite coexists only with a supercritical fluid.

Experimental determination of cation diffusivities in aluminosilicate garnets - II. Multicomponent simulation and tracer diffusion coefficients

Data from experimentally-induced diffusion profiles at approximately 40 Kbar, 1,300–1,500° C in spessartine-almandine couples and a pyrope-almandine couple at ∼ 40 Kbar, 1,440° C, described in Part I, were used to derive tracer diffusion coefficients (D*) of Fe, Mn and Mg in garnet. The experimental data were fitted by numerical simulations that model multicomponent, compositionally-dependent difussion, including the effects of nonideal thermodynamic mixing. The simulations use the formalism of irreversible thermodynamics and an eigenvector technique of solution. We were able to fit the asymmetrical spessartine-almandine profiles using constant D* and either the Darken/Hartley-Crank or Manning-Lasaga models relating D* and interdiffusion coefficients, and both models yielded DMg*consistent with the direct measurement of DMg*in by Cygan and Lasaga (1985) at lower temperatures (750–900° C). The results (equations 4.1–4.3 and Table 1) indicate that DFe*≅DMg*<DMn*and QFe≅QMg>QMn, where Q is the activation energy. In contrast, the asymmetry of pyrope-almandine profiles is too great to fit with either tracer model assuming constant D* and indicates that DMg*is similar to its value in spessartine-almandine couples but DFe*is an order of magnitude less. The fit also suggests that DCa*< DFe*<DMg*in pyrope-almandine couples. Synthesis of data from the two types of diffusion couples suggests that DMg*is insensitive to compositional changes, whereas DFe*is affected by Mn/Mg and Fe/Mg ratios and probably by other factors. These compositional effects on tracer coefficients are compatible with those documented by Morioka (1983) for cation diffusion in olivine.

An ion microprobe study of anhydrous oxygen diffusion in anorthite: a comparison with hydrothermal data and some geological implications

The diffusion rate of 18O tracer atoms in anorthite (An97Ab03) under anhydrous conditions has been measured using SIMS techniques. The tracer source was 18O2 98.4% gas at 1 bar, in the temperature range 1300° C−850° C. The measured diffusion constants are D0=1−0.6+1×10−9 m2s−1Q=236±8 kJ mol−1 Comparison of these values with published data for 18O diffusion in anorthite under hydrothermal conditions shows that “dry” oxygen diffusivities are orders of magnitude lower than equivalent “wet” values at similar temperatures. The effect of these differences on oxygen isotope equilibration during cooling is discussed.

An experimental study of the diffusion of oxygen in quartz and albite using an overgrowth technique

AbstractDiffusion rates of18O tracer in quartz (∥ c, 1 Kb H2O) and Amelia albite (⊥ 001, 2 Kb H2O) have been measured, using Secondary Ion Mass Spectrometry (SIMS). A new technique involving hydrothermal deposition of labelled materials has removed the possibility of pressure solution-reprecipitation processes adversely affecting the experiments. Reported diffusion constants are:β-quartz (∥ c),

Fault sealing during deformation-band growth in porous sandstone

D_0 = 3.4\left( {\begin{array}{*{20}c} { + 4.8} \\ { - 2.0} \\ \end{array} } \right)x 10^{ - {\text{13}}} {\text{m}}^{\text{2}} {\text{s}}^{ - {\text{1}}}

Influence of confining pressure on the mechanical and structural evolution of laboratory deformation bands

,Q=98±7 KJ mol−1 (600–825° C, 1 Kb); Amelia albite (⊥ 001),