DENSITY CHANGES OF FLUID INCLUSIONS IN HIGH-PRESSURE LOW-TEMPERATURE METAMORPHIC ROCKS FROM CRETE - A THERMOBAROMETRIC APPROACH BASED ON THE CREEP STRENGTH OF THE HOST MINERALS

The densities of fluid inclusions in quartz from high-pressure, low-te mperature metamorphic rocks are generally inconsistent with the P-T co nditions derived from solid-phase equilibria. Therefore, some process of change must regularly have occurred during exhumation. However, evi dence of decrepitation is frequently lacking. Consideration of the rat es of change of P and T in metamorphic rocks shows that very low strai n rates are sufficient for volume adaptation. Dislocation creep appear s to be a feasible deformation mechanism, allowing for continuous ball ooning driven by a small differential pressure during decompression do wn to temperatures of about 300 degrees C in quartz. Decrepitation req uires a higher differential pressure. In natural rocks, it is therefor e restricted to relatively low temperatures. This concept was tested o n inclusions in the Phyllite-Quartzite Unit of Crete. In these rocks, cooling to below ca. 300 degrees C at confining pressures of 3 to 4 kb ar is indicated, corresponding to depths of 10 to 13 km. It is anticip ated that in metamorphic rocks the isochores of all fluid inclusions i n quartz that have been formed at higher temperatures and remain unaff ected by decrepitation, should intersect close to the 300 degrees C is otherm, thereby defining a point on the P-T paths rarely accessible by other methods. Extension of this concept to other minerals with known mechanical properties is straightforward, and can under favourable ci rcumstances be used to fix several points on P-T paths. (C) 1997 Elsev ier Science B.V.