Re-equilibration of quartz inclusions in garnet

<p>Inclusion–host elastic thermobarometers are widely used to determine the pressure and temperature (<span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span>) histories of metamorphic rocks. Complex metamorphic <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> paths can affect the pressures that develop in host–inclusion systems. There are limited experimental studies that investigate how changing <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> 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 <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> 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<span class="inline-formula">⁻¹</span> Raman bands of quartz inclusions to determine the inclusion pressures (<span class="inline-formula"><i>P</i>_inc</span>) and entrapment pressures (<span class="inline-formula"><i>P</i>_trap</span>) at the experimental temperature. The multi-stage experiments show that elasticity primarily controlled changes to <span class="inline-formula"><i>P</i>_inc</span> values that occur from <span class="inline-formula"><i>P</i>_trap</span> through quenching to room conditions and that <span class="inline-formula"><i>P</i>_inc</span> values measured at room conditions along with elastic modelling can be used to accurately calculate <span class="inline-formula"><i>P</i>_trap</span>. Quartz <span class="inline-formula"><i>P</i>_inc</span> values in two-stage experiments re-equilibrated to give <span class="inline-formula"><i>P</i>_inc</span> values between <span class="inline-formula"><i>P</i>₁</span> and <span class="inline-formula"><i>P</i>₂</span>. The three-stage isothermal experiments show that the observed changes to inclusion pressures are reversible along different <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> paths to restore the re-equilibrated <span class="inline-formula"><i>P</i>_inc</span> values back to their original entrapment isomeke at <span class="inline-formula"><i>P</i>_trap</span>. For rocks that underwent protracted metamorphism along complicated <span class="inline-formula"><i>P</i></span>–<span class="inline-formula"><i>T</i></span> paths, the re-equilibration experiments and viscoelastic calculations show that QuiG may underestimate maximum <span class="inline-formula"><i>P</i>_trap</span> conditions.</p>