The Significance of Fluids for Strain Partitioning
Bruce Yardley
School of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
Metamorphic and igneous rocks that have cooled below their original temperature of formation
generally form a closed system to fluid species, and the assemblage of coexisting volatile-poor and
volatile-rich phases (including those formed during incipient retrogression) buffers the partial
pressures of volatile species to very low values. This is inconsistent with the presence of free fluid.
Intracrystalline deformation of many minerals may be facilitated by the presence of traces of water in
defects within grains, irrespective of the amount and pressure of fluid in the grain boundary network,
but this is not true of all deformation mechanisms. Mechanisms that involve dissolution and
precipitation require the presence of a water phase, and cannot operate in cooled crystalline rock
unless fluid is added. As a result, the addition of water to cooled crystalline rocks has a significant
weakening effect, permitting the operation of deformation mechanisms that require a component of
solution mass transfer at lower stresses than those required to deform nominally dry rock. The
presence of aqueous fluid in cooled rocks is however only transient, as it is consumed by fast
retrograde reactions until the host is fully hydrated at the ambient P-T conditions.
Cooled rocks, typical of the middle and lower crust, are a potent sink for fluids from the surface or
deep sources, but often preserve the minerals, textures and structures present at the peak of
metamorphism with little or no modification. This is in marked contrast to the extensive pervasive
deformation that can accompany all grades of wet progressive regional metamorphism. Where later,
lower-T, deformation has occurred, it is localised in zones and frequently associated with hydration.
In the Ox Mountains, Ireland, amphibolite facies staurolite-kyanite-garnet-biotite schists with minor
retrograde chlorite have been converted to lower greenschist facies assemblages within a broad shear
zone, They are dominated by muscovite and chlorite, with sparse garnet relicts, and shimmer
aggregate pseudomorphs after the highly aluminous phases may contain minor chloritoid. At the P-T
conditions of this deformation, water activity in the largely unaltered rocks around the shear zone was
approximately 2 orders of magnitude lower than for fluid saturation, which was transiently attained in
the shear zone. Thus the addition of water to the shear zone rocks was coupled to a reduction in
strength that allowed them to deform while the surrounding dry rocks were too strong to yield in the
regional stress field. Shear zones in the crust are normally sites of marked retrograde hydration, and
their formation can be directly linked to the large contrast in strength between dry and wet rocks.