Investigating pressure effects of Ti and Zr partitioning into zircon, quartz, and rutile at crustal temperatures

Abstract

The dependency of Ti partitioning between quartz and zircon on the activity of TiO₂ and Zr partitioning between zircon and rutile on the activity of ZrO₂ suggest that an intercalibration among the three minerals (i.e., concentration information from all three phases in the same experiment) could reduce propagated errors when using multiple systems simultaneously. Experiments were undertaken to assess pressure effects in Ti and Zr partitioning in the zircon-quartz-rutile system and intercalibration of the three phases at low concentrations (down to ∼20 ppm). Analysis of small crystals (down to ∼8 μm) was possible due to the high spatial resolution of the CAMECA ims1290 ion microprobe. Regressions for pressure-temperature-phase domains for experiments between 800 and 1000 °C and 10 and 15 kbar array about the initial calibration data providing confidence in their merit despite recent criticisms. Ti and Zr partitioning into quartz, rutile, and zircon can be quantified as:$log\left(\mathrm{Ti}-\mathrm{in}-\mathrm{quartz}\right)+log{a}_{\mathrm{Si}{O}_2}^{\alpha -\mathrm{quartz}}=6.71\left(\pm 0.11\right)-\frac{383\left(\pm 10\right)}{T\left(K\right)}-0.122\left(\pm 0.07\right)P-0.00197\left(\pm 0.00024\right)P^2$$log\left(\mathrm{Zr}-\mathrm{in}-\mathrm{rutile}\right)+log{a}_{S{\mathrm{iO}}_2}^{\alpha -\mathrm{quartz}}=7.39\left(\pm 0.21\right)-\frac{4262\left(\pm 220\right)}{T\left(K\right)}-0.021\left(\pm 0.0051\right)P$

and$log\left(\mathrm{Ti}-\mathrm{in}-\mathrm{zircon}\right)+\log a_{S{\mathrm{iO}}_2}^{\alpha -\mathrm{quartz}}=\frac{-4147\left(\pm 555\right)}{T\left(K\right)}-5.30\left(\pm 0.41\right)$

where T is temperature (Kelvin), P is pressure (kbar), concentrations have units of ppm, and $a_{\mathrm{Si}{O}_2}^{\alpha -\mathrm{quartz}}$ is the activity of SiO₂ referenced to α-quartz. Importantly, if α-quartz is present, $a_{S{\mathrm{iO}}_2}^{\alpha -\mathrm{quartz}}$is 1 and $log\left(a_{S{\mathrm{iO}}_2}^{\alpha -\mathrm{quartz}}\right)$ is zero.