THE EFFECT OF PRE-EXISTING STRUCTURES ON PHASE TRANSFORMATIONS AND SUBSEQUENT STRAIN LOCALIZATION AS SHOWN IN THE ALBITE TO JADEITE + COESITE TRANSITION

Metamorphic phase transformations can result in mechanically weaker phases than the "parent" phase or phases resulting in strain localization. In this paper we present an experimental study in which pre-existing brittle faults in albitite provide localization sites for jadeite and coesite nucleation once the rock is subjected to the conditions of the high pressure-phase stability field. During subsequent deformation of the partially transformed aggregate, strain is localized along the zones of preferential transformation. Cores of natural albitite (grain size of ~120 x 40 x 40 mm, vacuum dried at 110°C for 12 hrs, sealed in Pt cans) were used in three types of experiments. In the first type, samples were only pressurized to 2.6 GPa at room temperature in a piston-cylinder deformation apparatus and developed faults 30° to the length of the sample, which corresponds to the maximum compressive stress direction during pressurization. In the second type, samples were pressurized to 3.5 GPa, then hydrostatically annealed at 800°C for 3 hrs, and developed zones of new phases along the pre-existing (and still visible) brittle faults. In the third type, samples were pressurized to 3.5 GPa, and held at 800°C for 1 to 3 hrs. Then the temperature was ramped at 4°C/min to the run T (900 to 1170°C), and held for 0.5 hrs before the samples were shortened at ~10^-4s^-1. Specimens were quenched under load to 400°C in <30 s. In these samples no faults can be seen but there are bands of crystals of jadeite and SiO₂ oriented at 30 to 45° to the maximum compressive stress direction. In the higher strain samples of this type, foliations, and grain-size reduction have developed in these transformed zones, whereas outside of these zones strain is not localized. Thus, the presence of the faults promoted nucleation of the denser phases, which subsequently localized deformation. The mechanism by which the jadeite and coesite accommodated deformation within the shear zones is still under investigation.