On the interpretation of TitaniQ and ZiR thermobarometry in subduction complexes

• Published in: Contributions to mineralogy and petrology 2023, Vol.178 (1), p.8, Article 8
• Main Authors: Spear, Frank S., Wolfe, Oliver M., Cheney, John T.
• Format: Article
• Language: English
• Subjects: Crystals; Earth and Environmental Science; Earth Sciences; Eclogite; Geology; Intersections; Metamorphism; Metamorphism (Geology); Mineral Resources; Mineralogy; Nucleation; Original Paper; Petrology; Quartz; Recrystallization; Rocks, Metamorphic; Rutile; Shear zone; Subduction; Thermometers; Thermometry; Titanium; Titanium dioxide; Zirconium
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-022-01989-4

Titanium and zirconium analyses of quartz and rutile from blueschist samples from the eclogite–blueschist unit (EBU) in northern Sifnos, Greece, have been evaluated to determine the extent to which the intersection of the Ti-in-quartz (TitaniQ) and zirconium-in-rutile (ZiR) thermometers reflect peak metamorphic P–T conditions. Ti concentrations range from 100 to 530 ppb and Zr concentrations range from 14 to 44 ppm; there is no significant zoning observed in any grains and no significant differences in concentrations in samples south and north of the Vroulidia shear zone (VSZ). Lines of equal Keq from the TitaniQ and ZiR thermometers intersect at around 450–500 °C and 2.2–2.4 GPa, which is similar to the peak metamorphic conditions of 530 °C, 1.9 GPa for samples north of the VSZ. However, the intersection is inconsistent with the peak metamorphic conditions of 525 °C, 1.1–1.4 GPa for samples south of the VSZ even when TitaniQ temperatures are corrected for reduced activity of TiO 2 . Rather, TitaniQ temperatures are consistent with Ti values being incorporated into quartz during prograde metamorphism along a subduction geotherm of 10–12 deg/km at around 300 °C. ZiR temperatures are consistent with rutile nucleation and growth at around 450–500 °C along a similar metamorphic geotherm after the rutile-in reaction was overstepped by around 1 kJ/mol-O. The TitaniQ and ZiR thermometers do not, therefore, reflect peak metamorphic P–T conditions in these rocks but rather record recrystallization or growth processes during prograde subduction.