The origin of the geochemical zonation with temperature in Bishop high-SiO2 rhyolite: evidence of rapid times scales between melt segregation and eruption?

High-silica rhyolites are the most evolved magmas on Earth and constitute some of the largest explosive eruptions. An example is the Bishop Tuff, which displays two distinct types of geochemical zonation: one related to magma mixing (Late-type) and the other unrelated to magma mixing and best displayed in the high-SiO₂ rhyolite portion of the Bishop Tuff (Early- and Transitional-type). The results of high-resolution Fe-Ti two-oxide thermometry and two-feldspar thermometry on all pumice clasts give strongly correlated temperatures (R²=0.94; Jolles and Lange, 2019). The focus here is on the origin of the strong variations in several trace element concentrations as a function of temperature (700-750°C) in Bishop high-SiO₂ rhyolite, where some (Rb, Cs, Y, Hf, U, Th, Tb, Yb, Lu) decrease with T, while others (La, Ce, Nd, Ba, Sr) increase with T. In most cases, remarkably strong linear correlations with T are observed (e.g., R² ≤ 0.9). These distinctive geochemical gradients can be explained by extraction of interstitial melt from a leucogranite crystalline mush that contains the same nine phenocryst phases as found in the high-SiO₂ Bishop rhyolite, where T is linearly correlated with melt fraction and/or source depletion. The slopes of element concentration with temperature can be used to determine their relative compatibility/incompatibility in the interstitial melt relative to the bulk crystalline mush. Using updated crystal-melt partition coefficients, the observed geochemical gradients (including increasing Fe³⁺/Fe^T) with temperature are readily explained by extraction of interstitial melt at different melt fractions (and/or source depletion) from a leucogranite (qtz+Kspar+plag) that contained ~3% biotite, ~0.9% titanomagnetite, ~0.4% ilmenite, ~0.08% allanite, ~0.04% apatite and ~0.015% zircon. If this explanation of the geochemical gradients with temperature in the Bishop high-SiO₂ rhyolite is correct, a new question emerges: given the relatively short time scales for titanomagnetite-ilmenite re-equilibration in hydrous rhyolite melt (~2 weeks), how is it that conditions of melt segregation remain recorded in the Fe-Ti oxide compositions in erupted pumices? One possibility is that the time scale between melt segregation, crystal growth, and eruption is extremely rapid (<1 month).