Re-equilibration of quartz inclusions in garnet

Abstract

Inclusion–host elastic thermobarometers are widely used to determine the pressure and temperature (P–T) histories of metamorphic rocks. Complex metamorphic P–T paths can affect the pressures that develop in host–inclusion systems. There are limited experimental studies that investigate how changing P–T conditions may re-equilibrate or “reset” residual pressures of inclusions. To evaluate re-equilibration of the quartz-in-garnet (QuiG) elastic thermobarometer, we performed single-, two-, and three-stage isothermal experiments. In the first stage of the experiments, oxide starting materials hydrothermally crystallised to grow garnet crystals with quartz inclusions between 700 and 800 °C and 1.0 and 3.2 GPa with constant P–T conditions for 48 h. In the second and third stage of the experiments, we isothermally changed pressure by 1.0 to 1.2 GPa for durations up to 38 d. We used Raman spectroscopy to measure strain-induced changes to the 128, 207, and 465 cm−1 Raman bands of quartz inclusions to determine the inclusion pressures (Pinc) and entrapment pressures (Ptrap) at the experimental temperature. The multi-stage experiments show that elasticity primarily controlled changes to Pinc values that occur from Ptrap through quenching to room conditions and that Pinc values measured at room conditions along with elastic modelling can be used to accurately calculate Ptrap. Quartz Pinc values in two-stage experiments re-equilibrated to give Pinc values between P1 and P2. The three-stage isothermal experiments show that the observed changes to inclusion pressures are reversible along different P–T paths to restore the re-equilibrated Pinc values back to their original entrapment isomeke at Ptrap. For rocks that underwent protracted metamorphism along complicated P–T paths, the re-equilibration experiments and viscoelastic calculations show that QuiG may underestimate maximum Ptrap conditions.