Probing Ti in Quartz: Application of the TITANiQ Thermometer to the Bishop Tuff

The composition of igneous quartz is rarely listed in tables of mineral chemistry, and is seldom used in attempts to understand the thermal and chemical evolution of silicic magma systems. Because it contains trace levels of Ti, however, quartz may soon play a more prominent role. According to the recently launched TITANiQ (Titanium-in-Quartz) thermometer of Wark and Watson (2004 Goldschmidt conference), Ti levels can be translated into T of crystallization (Ti decreases exponentially with inverse T) if Ti activity is known. Furthermore, Ti levels correspond with intensity of cathodoluminescence (CL) emissions, which document zones of crystal growth and dissolution. This leads to the possibility of characterizing the thermal history of individual quartz crystals by analyzing Ti in different CL zones. We tested application of the TITANiQ by analyzing Ti (by electron probe) in several Bishop Tuff quartz phenocrysts studied by Peppard et al. (Am. Min., v. 86; 1034-1052; 2001). We focused on late-erupted samples, in which quartz reveals a dark CL core surrounded by a bright CL rim zone, with each zone exhibiting smaller-scale variations in CL intensity. Based on differences in melt inclusion compositions, Peppard et al. suggested that the rim zones reflected higher-T growth. This is consistent with our analyses: in dark cores, Ti oscillates between 35 and 50 ppm with most values in the 40-45 ppm range, whereas rim Ti contents are higher (70-100 ppm, with most 80-85 ppm). These numbers are consistent with ion probe data of A. Davis, reported in Peppard et al. Using the TITANiQ thermometer (assuming TiO₂ activity of 0.6, which is considered reasonable for rhyolite melt), the median rim content of 82.5 ppm Ti corresponds with a T of 790° C, which is near the upper T limit estimated for Bishop magma by two-oxide thermometry (if TiO₂ activity were lower by 10%, T would increase by ~10° C). TITANiQ predicts a total T range of 705-815° C for these phenocrysts, with most core growth taking place between 720 and 730° C and most rim growth between 785 and 795° C. These values are consistent with earlier suggestions of higher-T rim growth. Additionally, the range of calculated temperatures closely matches the range estimated by others for the entire Bishop Tuff.

Accepting equilibrium crystallization and uniformity of Ti activity, Ti-rich rim zones apparently document increasing T of crystallization, which could reflect crystals sinking into hotter regions of the magma system, or underplating by newly injected, hot mafic melt.