TITANIUM IN QUARTZ: THERMOMETRY, DIFFUSION, AND CATHODOLUMINESCENCE

Despite its abundance in Earth's crust, the mineral quartz – although recognized as an important reactant during metamorphism and as an indicator of silica saturation in magmas – has historically been of little use to petrologists. This status may soon change, however, due to recent advancements in cathodoluminescence (CL) imaging and spectroscopy, and to development of a new Ti-in-quartz (TITANiQ) thermometer that relates T (in kelvin) of crystallization to the Ti content of quartz (in ppm) according to:

log(Ti_quartz) = 5.95 – 4077/T.

Although calibrated for quartz that crystallized in equilibrium with rutile, the thermometer yields a minimum T for rutile-absent assemblages.

When combined with CL imaging, the TITANiQ thermometer can be particularly useful for understanding the thermal histories of igneous and metamorphic rocks, because Ti concentration tends to correlate with CL emission intensity. This is illustrated by CL spectra collected from natural (hydrothermal, metamorphic, and igneous) and Ti-doped synthetic quartz. In all crystals containing at least a few ppm Ti (5 ppm corresponds with crystallization temperatures of roughly 500°C), the visible CL spectrum is overwhelmingly dominated by a 415 nm (blue) emission. Consequently, monochromatic (415 nm) and often panchromatic CL images can be used to map relative quartz crystallization temperatures. In metapelites from Vermont, for example, high-temperature, garnet-rimming quartz appears brighter in CL images than matrix quartz that formed at lower T. In igneous rocks, quartz can reveal information about thermal events in magma chambers: quartz in the Vinalhaven granites (Maine), for example, has bright-CL rims that document a sharp increase in temperature due to injection of hot, mafic melt at depth. With knowledge of Ti diffusivities in quartz (determined experimentally), we can sometimes extract temporal information about thermal events: a step-like Ti concentration profile in quartz from the Bishop tuff (Long Valley caldera, California) indicates that mafic recharge there took place within 100 years of, and hence was likely responsible for, the large-volume eruption at 0.76 Ma.