Penetration of deformation-driven meteoric water into ductile rocks: Isotopic and model observations from the Southern Alps, New Zealand

A stable isotope laser fluorination probe was used to examine oxygen isotope zoning within foliation-parallel quartz veins of two Alpine Fault mylonite samples and two Alpine Schist samples, all of amphibolite facies. The technique used has a spatial resolution of c. 1 mm, and we found variation of delta(18)O values within individual samples and between schist and mylonites. delta(18)O values from individual mylonite quartz veins showed a slight variation of up to 1 parts per thousand. By contrast, there was up to 2.4 parts per thousand variation within veins from outside the fault zone. Equilibrium fluid delta(18)O values were calculated for aqueous fluids from which the quartz precipitated. The calculated values range from 5 to 13 parts per thousand. They suggest the involvement of a near-surface fluid just beneath the brittle-ductile transition which is depleted in O-18 relative to the fluid calculated to be in equilibrium with the local metamorphic assemblage. The geochemical evidence of penetration of fluids depleted in O-18 into ductile rocks constrains models of the mechanics of fluid flow. Ductile rocks are impermeable to fluids driven solely by thermal and topographic gradients. Simple numerical experiments illustrate the need for coupling of deformation and fluid flow to allow access of meteoric water into ductile material.