Application of titanium-in-quartz thermobarometry to greenschist facies veins and recrystallized quartzites in the Hsüehshan range, Taiwan

Abstract

Abstract. The accuracy, reliability and best practises of Tiin-quartz thermobarometry (TitaniQ) in greenschist facies rocks have not been established. To address these issues, we measured Ti concentrations in rutile-bearing samples of moderately deformed, partially recrystallized quartzite and vein quartz from the Hsüehshan range, Taiwan. The spread of Ti concentrations of recrystallized grains in quartzite correlates with recrystallized grain size. Recrystallized quartz (grain size ∼100-200 µm) that formed during early deformation within the biotite stability field shows a marked increase in intermediate Ti-concentration grains (∼1-10 ppm) relative to detrital porphyroclasts (Ti ∼0.1-200 ppm). Fine recrystallized quartz (∼5 % of the samples by area, grain size ∼10-20 µm) has a further restricted Ti concentration peaking at 0.8-2 ppm. This trend suggests equilibration of Ti in recrystallized quartz with a matrix phase during deformation and cooling. Unlike previously documented examples, Ti concentration in the quartzite is inversely correlated with blue cathodoluminescence. Deformation was associated with a minimum grain boundary diffusivity of Ti on the order of 10 −22 m 2 s −1 . Vein emplacement and quartzite recrystallization are independently shown to have occurred at 250-350 • C and 300-410 • C, respectively, with lithostatic pressure of 3-4 kbar (assuming a geothermal gradient of 25 • km −1 ), and with hydrostatic fluid pressure. Estimates of the accuracy of TitaniQ at these conditions depend on whether lithostatic or fluid pressure is used in the TitaniQ calibration. Using lithostatic pressure and these temperatures, the calibration yields Ti concentrations within error of concentrations measured by SIMS. If fluid pressure is instead used, predicted temperatures are ∼30-40 • C too low. TitaniQ has potential to yield accurate PT information for vein emplacement and dynamic recrystallization of quartz at temperatures as low as ∼250 • C, however clarification of the relevant pressure term and further tests in rutile-present rocks are warranted.

Localisation of ductile and brittle shear zones along the Szentlőrinc-1 well in the Mecsekalja Zone using quartz microstructural and well-log data

“…Thus, to know which of these schemes is true in the case of our study area, information about the metamorphic evolution of the footwall and hanging wall formations is essential. This information can be determined using the chemical compositions of the available quartz grains based on the ''Ti-in-quartz'' thermometer (Wark and Watson 2006;Thomas et al 2010;Huang and Audétat 2012;Kidder et al 2013;Haertel et al 2013). If the metamorphic grades of the footwall and hanging wall formations are nearequivalent, then the coincidence of the brittle and ductile shear zones could be caused by primarily evolved softened regions.…”

Constraints on Alpine Fault (New Zealand) Mylonitization Temperatures and Geothermal Gradient from Ti-in-quartz Thermobarometry

“…We note that both these anomalous results and those of Kidder et al (2013), who found a brighter blue CL signal in recrystallized areas, were observed in quartz with Ti concentrations much lower than measured by Wark and Spear (2005).…”

“…The typical association of recrystallized grains with darker-CL and low Ti (e.g. Kidder et al, 2013) was rarely observed in the mylonites and protomylonites, but is common in the ultramylonites. The sample analyzed using hyperspectral CL has a Solid Earth Discuss., https://doi.org/10.5194/se-2018-12 Manuscript under review for journal Solid Earth Discussion started: 7 March 2018 c Author(s) 2018.…”

“…In both experimental and natural deformation, Ti concentrations in quartz are often reset during recrystallization (e.g. Behr and Platt, 2011;Kidder et al, 2013;Kohn and Northrup, 2009;Nachlas and Hirth, 2015;Nachlas et al, 2014). While quantifying temperatures using Ti-in-quartz thermobarometry is complicated by uncertainty in Ti 10 activity (Bestmann and Pennacchioni, 2015;Grujic et al, 2011;Nevitt et al, 2017) and differing calibrations (Huang and Audétat, 2012;Thomas et al, 2015), we find that variations in Ti-in-quartz concentrations in Alpine Fault rocks span a range large enough that these issues can be largely bypassed using independent constraints on maximum and minimum temperatures associated with quartz recrystallization.…”

“…To account for matrix effects between quartz and NIST glass, a correction factor of 0.67, as determined by Behr et al (2010) and independently tested by Kidder et al (2013), was applied for the Ti data. As a Ti-blank, we used Herkimer 'Diamond', a natural quartz containing 4-5 ppb Ti (Kidder et al, 2013). Analyses of this natural blank suggest an effective detection limit in this study of 78 ± 27 ppb.…”