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Quartz vein 18O/16O ratios across a 500 km transect through the Lachlan fold belt of southeastern Australia are remarkably uniform (±1–1.5 permil) at both local (centimeter to meter) and regional (over 104 km2) scales. They define isotopic zones that correlate with the tectonic divisions of the Cambrian through Devonian (dominantly Ordovician) quartz‐rich turbidites determined by regional mapping. From west to east these divisions are (1) Stawell, δ18O=14.7±1.1 (47 samples); (2) Bendigo‐Ballarat, δ18O=17.5±1.3 (204 samples); (3) Melbourne, δ18O=19.0±1.6 (80 samples); (4) Tabberabbera, δ18O=16.3±1.9 (12 samples); and (5) Omeo, δ18O=14.4±1.0 (26 samples). Isotope profiles across zone boundaries, and intrazone fault zones particularly within the Bendigo‐Ballarat zone, show steps indicating abrupt changes across faults with little or no evidence of fluid mixing within the fault zones. δ18O values of veins are insensitive to relative age, type of vein, and immediate host rock lithologies. The δ18O values of coexisting vein and host rocks show nonequilibrium relationships which can be explained in terms of rock buffering under conditions of low fluid/rock ratios (water/rock ≪ 1). Limited D/H determinations on fluid inclusions fall mainly in the range −70 and −100, with one value as low as −140. These low deuterium values when considered in the context of paleolatitude may have been inherited from deutrium‐depleted detrital minerals and do not necessarily require the direct penetration of meteoric fluids to midcrustal depths. The work suggests that vein formation is possible in regions where the integrated water/rock (w/r) ratio is very low (w/r ≪ 1) as long as a pervasive fluid phase is present. This fluid appears to achieve a steady state isotopic composition on scales of hundreds of meters, and once a quasi steady state has been reached, the isotopic compositions of the resultant quartz veins are rather insensitive to the diachronous and nonisothermal conditions under which vein growth occurs. Advective cycling of the fluid on this scale is by episodic dilatancy “pumping” in fracture networks associated with localized faulting.

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Pressure-solution and associated crystallization are subclasses of a diffusive mass transport process which involves diffusion in grain boundary and pore solutions. The manner in which they give rise to permanent deformation is examined in three steps: (a)A simplified reversible non-hydrostatic crystal-solution thermodynamic criterion (first order, 2-components) based on Gibbs provides a manageable basis for determining the direction in which the process will run (regions of dissolution or growth) in stressed porous, non-porous, closed and open systems. (b) Considerations of irreversible diffusion and deformation indicate certain restrictions on the displacements accompanying permanent growth or dissolution and hence on the form of the solution-transfer strain rate tensor. (c)The way in which the process develops, and its rate, are governed by kinetic factors, especially diffusion kinetics. As well as having an exponential dependence on stress, the displacement rate is influenced by absolute temperature, grain boundary diffusivity, initial solubility and geometric scale.

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