New IR spectroscopic data for determination of water abundances in hydrous pantelleritic glasses

To aid current work on the genesis of pantelleritic magmas, and the desire to use IR spectroscopy to measure water contents in natural (e.g., melt inclusions) and experimental glasses of pantelleritic composition, we have determined molar absorptivities for near-infrared (NIR) absorption bands related to molecular water (5200 cm-1) and OH groups (4500 cm-1) in synthetic hydrous pantelleritic glasses, with compositions similar to natural pantellerites from the Eburru complex of the Kenya Rift Valley. The experiments were conducted at P = 30 to 150 MPa and T = 850-900 °C using a synthetic pantelleritic starting composition with (wt%) SiO2 = 76.60, Al2O3 = 8.48, FeO∗ = 5.48, K2O = 3.68, Na2O = 4.72, and with molar ratio (Na+K)/Al = 1.38. The experiments were H2O undersaturated (~1.1 to 6.5 wt% H2O), and the run products were analyzed by Karl-Fischer Titration (KFT) for total dissolved H2O abundance. Different combinations of baseline types (GG or TT) and intensity measurements (peak height and peak area) were applied to measure both hydroxyl group (OH) and molecular water (H2O) in the experimental samples. For instance, evaluating the peak heights and using the TT baseline e4500 results to be equal to 0.98(4) (L mol-1cm-1) and e5200 to 1.92(2) (L mol-1 cm-1); these values differ by ~20 to 50% from published values for metaluminous rhyolitic compositions.