Systematic Decrease of Hydroxyl Defect Concentration in Quartz Phenocryst Fragments Within Ignimbrites: Implications for Post-Depositional Diffusional Loss of Hydrogen

Ignimbrite-hosted quartz phenocryst fragments contain much lower hydroxyl defect than quartz in igneous rocks. Pre-eruptive and post-depositional loss of hydrogen were hypothesized as main processes for lowering the ‘original’ concentrations of hydroxyl defects. Our goal was to examine the hydroxyl defect concentration of quartz phenocryst fragments from various vertical positions above the base of pyroclastic density current – hereafter PDC – deposits. This is to monitor the vertical variations of hydroxyl defect concentrations to have an insight into potential post-depositional hydrogen loss and cooling history of PDC deposits. Ignimbrite-hosted quartz phenocryst fragments were examined from two different, thick ignimbrites in the Bukk Foreland Volcanic Area (North Hungary). Unpolarized micro-FTIR measurements on 23-35 unoriented crystal fragments from each sample were performed representing 4 different vertical positions of each site. Our results imply, that hydroxyl defect concentrations show a pronounced decrease upwards from the base of the deposits . The initial ~12 ppm hydroxyl defect concentration decreases to <3 ppm within few meters from the base. Thus, post-depositional loss of H is supposed to be the main factor causing the low hydroxyl defect concentration of quartz crystals in ignimbrites. Ignimbrites with different degree of welding are characterized by different hydroxyl defect concentrations of quartz phenocryst fragments at the same height above the base. A novel methodology which statistically evaluate the distribution of unpolarized absorbances for a given sample revealed that there may be either some accidental crystals incorporated during pyroclast transport at the base of these deposits or inhomogenities in hydroxyl defect concentrations due to non-equilibrium processes. Hydroxyl defect concentration in quartz from different vertical positions above the base is a relatively easily measurable proxy, which can provide further contribution to modeling the cooling speed, depositional temperature and original thickness of any quartz-bearing PDC deposits. We propose that ‘original’ hydroxyl defect concentration of quartz phenocrysts may only be preserved in a very thin layer (that cool very fast) at the base of PDC deposits where the cooling was instantaneous and no significant post-depositional H loss occurred. The original hydroxyl defect concentration may represent physical and chemical conditions in the magma chamber just prior to eruption.