Syn- and post-depositional thermal history of ignimbrites: insights from the structural hydroxyl defect content of quartz crystals from the Tecoloquillo ignimbrite, Acoculco Caldera Complex, Mexico

Abstract In this contribution, the structural hydroxyl-defect (OH) content of quartz crystals from the 0.8 ± 0.1 Ma Tecoloquillo Ignimbrite (TI) from the Acoculco Caldera Complex (Puebla, Mexico) was analyzed to examine syn- and post-depositional thermal history of ignimbrites. Pumice-hosted quartz crystals were examined from 5 zones of the TI representing various heights above the basal contact of the ignimbrite. To separate quartz crystal populations, unpolarized micro-FTIR on unoriented crystal fragments was performed to determine the OH-speciation as well as the structural hydroxyl-defect content. Additionally, Raman spectroscopy was used to identify the mineral inclusions of quartz crystals. Two populations of quartz were identified based on the structural hydroxyl-defect concentrations and mineral inclusions: a “hydrous” group with boron-related OH-content (BOH) with evidence of hydrothermal origin (containing abundant rutile needles) and another “anhydrous” group with the absence of any structural hydroxyl-defect content (as well as lack of BOH). The “hydrous” group showed a diffusion profile of BOH with a maximum gradient of 2.4 ± 0.65 wt ppm between the core and the rim of the crystals. The pre-eruptive temperatures of the TI were calculated using the composition of Fe-Ti oxides found in the pumice fragments. Results indicate a 720 ± 20 °C and 790 ± 20 °C temperature for the non-welded main mass (middle level) and 840 ± 30 °C for the welded lower ~ 25 m thick basal level. Based on the pre-eruptive temperatures, the syn- and post-depositional cooling history of the TI was estimated based on thermal modelling. According to the cooling model, the cooling time can reach up to 50 years in the middle level (slow cooling rate: ~3 − 8 °C/s or ~ 1 °C/yr). In spite of such high and long-lasting temperature, the thermal stability of BOH defects (under 600 °C) compared to the common AlOH defects in quartz crystals implies that the ignimbrite emplacement temperature was high enough to cause the total AlOH content to be lost from the quartz crystals, but not as high as the OH-content associated with boron to be diffusively lost. Thus, the structural hydroxyl-defect content of quartz represents a function of the thermal history of the deposit they are embedded in and that of the structural hydroxyl-defect speciation. This is the first study showing the significantly higher thermal stability of BOH structural hydroxyl-defects over AlOH structural hydroxyl-defects within quartz crystals originated from silicic ignimbrites.