High-Ti, bright-CL rims in volcanic quartz: a result of very rapid growth

Abstract

Volcanic quartz commonly contains Ti-enriched zones and CO2-enriched melt inclusions, and crystallization temperatures and pressures derived from Ti-in-quartz geothermobarometry and H2O–CO2 glass geobarometry applied to these compositions are typically high. Consequently, these features are generally interpreted to represent high temperatures and/or pressures. Yet, growth rate estimates from some high-Ti/bright-CL quartz rims suggest they grew at rates orders of magnitude faster than did cores and interiors of the crystals. This observation is notable in light of studies that suggest that fast crystal growth rates can produce a boundary layer in the melt surrounding a growing crystal that is enriched in components that diffuse comparatively slowly in the melt. In these circumstances, the composition of zones or melt inclusions formed from such a boundary layer melt will not accurately represent that of the far-field melt, and temperatures and pressures estimated from these compositions will be anomalous. We use a numerical model based on the coupled growth–diffusion equation of Lasaga (1982) to assess the effect of growth rate on the production of high-Ti/bright-CL zones and high-CO2 melt inclusions in quartz in rhyolitic melts. Simulations span a wide range of growth rates (10−7 to 10−13 m/s) and timescales (1 minute–1 year), and results suggest that quartz growth at 10−10 m/s or faster can produce a boundary layer enriched in these components. This suggests that appropriate application of Ti-in-quartz and H2O–CO2 glass geothermobarometry is contingent upon the verification that the compositions used are not those of boundary layer melts. Applying our model to the Bishop Tuff, which contains quartz displaying high-Ti/bright-CL rims and high-CO2 rim-hosted melt inclusions, we find that growth rates of 10−7 to 10−9 m/s can produce the observed enrichments in these components over the timescales estimated for the growth of the rims (days–weeks); these growth rates are consistent with those estimated independently for these rims. Thus, it is not necessary to invoke changes in temperature and/or pressure to explain these features. More broadly, the results suggest that the high-Ti/bright-CL rims commonly observed in volcanic quartz may be a record of fast growth, potentially associated with the growth of groundmass crystal populations in response to eruptive decompression.