Rates of retrograde metamorphism and their implications for crustal rheology

摘要

The rate of reaction of a natural hornblende garnet granulite with water under a range of mid- to lower crustalnconditions has been investigated experimentally. In runs of between 7 and 84 days small but measurablenamounts of water were consumed, and sheet silicates (300u0002C, 300 MPa and 400u0002C, 400 MPa) and ⁄ or secondarynactinolite (400u0002C, 400 MPa and 500u0002C, 500 MPa) were observed to have grown. When normalized to the sur-nface area of the starting materials, hydration rates were in the range of 2–5 · 10)8ngm)2nsec)1n. These reactionnrates imply that a film of water that infiltrated a planar crack with a half width of 100 lm would be completelynconsumed within c. 100 years. These results imply that where water infiltrates the crust along faults or underlyingnshear zones in response to a deformation, it will remain as a free phase for only a finite period of time, which innsome cases will be less than the repeat time for major earthquakes in the fault system. Thus, the rheology of faultnzones and shear zones is likely to be cyclical, with the zone becoming stronger with time as water is consumed,nand then weakened by infiltration of water after each rupture.