An Effect of Brittle Deformation on Metamorphic Reactions as Seen in the Albite to Jadeite + Coesite Transition

We present an experimental study in which brittle faults in albitite provide localization sites for jadeite and coesite nucleation once the rock is in the conditions of the high pressure-phase stability field. Cores of natural albitite (grain size of ~120 x 40 x 40 μm, vacuum dried at 110°C for 12 hrs, sealed in Pt cans) were used in three types of experiments. In the first type, a sample was pressurized to 2.6 GPa at room temp. and developed faults 30° to the length of the sample, corresponding to the maximum compressive stress direction during pressurization. In the second type, samples were pressurized to 3.5 GPa, then annealed at 800°C for 3 hrs, and developed zones of new phases along the pre-existing (and still visible) faults. In the third type, a sample was pressurized to 1.3 GPa, faulting occurred, then held at 800°C for 24 hrs in order to anneal the brittle structures in the albite field. Temp. was then lowered to 500°C and pressurization continued to 2.8 GPa. Once at 2.8 GPa, the temperature was raised to 800°C, bringing the metastable albite into a temp. range which promoted the kinetics of the transformation. The specimen was annealed for 1.75 hrs prior to deformation at a strain rate of 10-4s#^{- 1} for .75 hrs resulting in a total natural strain of 0.45. While under load, the experiment was quenched to room temperature in less than one minute. In the second type of samples, the new phases are heterogeneously distributed. Within the fault zones, 40 to 60% of the material is new phases: jadeite + coesite, whereas outside the fault zones, less than 10% of the material is jadeite + coesite. In the third type of sample, only 1% of the material is new phases: some in the healed fault and some outside of it. Transmission electron microscopy of the faulted-only sample (the first type) reveals the extremely fine grain size (~40 nm dia.) associated with the fault. This microstructural data combined with the differences in amount of new phases in the second and third types of samples, indicate that the presence of the faults, and the reduced grain size associated with them, promoted nucleation of the denser phases.