P. Upton et al., PENETRATION OF DEFORMATION-DRIVEN METEORIC WATER INTO DUCTILE ROCKS -ISOTOPIC AND MODEL OBSERVATIONS FROM THE SOUTHERN ALPS, NEW-ZEALAND, New Zealand Journal of Geology and Geophysics, 38(4), 1995, pp. 535-543
A stable isotope laser fluorination probe was used to examine oxygen i
sotope zoning within foliation-parallel quartz veins of two Alpine Fau
lt 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 be
tween 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 su
ggest the involvement of a near-surface fluid just beneath the brittle
-ductile transition which is depleted in O-18 relative to the fluid ca
lculated to be in equilibrium with the local metamorphic assemblage. T
he geochemical evidence of penetration of fluids depleted in O-18 into
ductile rocks constrains models of the mechanics of fluid flow. Ducti
le rocks are impermeable to fluids driven solely by thermal and topogr
aphic gradients. Simple numerical experiments illustrate the need for
coupling of deformation and fluid flow to allow access of meteoric wat
er into ductile material.