A K-feldspar-liquid hygrometer specific to alkaline differentiated magmas

- A K-feldspar-liquid hygrometer has been calibrated for alkaline differentiated magmas.
- The error of estimate is minimized through a test for equilibrium based on Or-Ab exchange.
- Compositions from Phlegrean Fields were used to test the reliability of the new hygrometer.

We present a K-feldspar-liquid hygrometer specific to alkaline differentiated magmas that is calibrated through the regression analysis of sanidine and anorthoclase crystals coexisting with trachyte and phonolite melts. Partial-regression leverage plots were used to determine the minimum number of regression parameters that closely describe the variance of the dataset. The derived model was tested using K-feldspar-liquid pairs not included into the calibration dataset in order to address issues of systematic errors. When K-feldspar and plagioclase crystals coprecipitate from the same alkaline liquid under identical P-T-X-fO2-H2O conditions, the ability prediction of the new hygrometer is comparable to that of previous plagioclase-liquid models. To minimize the error of H2O estimate caused by the inadvertent use of disequilibrium data in natural samples, we have also calibrated a new test for equilibrium based on Or-Ab exchange between K-feldspar and coexisting melt. As an immediate application for both equilibrium and hygrometer models, we used as input data K-feldspar-liquid pairs from alkaline explosive eruptions at the Phlegrean Fields. The estimates of H2O dissolved in natural trachyte and phonolite magmas closely match those determined by melt inclusion analysis and H2O solubility modeling. This leads to the conclusion that our new models can significantly contribute to a better quantitative characterization of the H2O content in differentiated alkaline magmas feeding large-volume explosive eruptions.