AN EXPERIMENTAL REEXAMINATION ON BA ZONING IN SANIDINE: WHAT MAGMATIC PROCESSES IS IT ACTUALLY RECORDING?

Barium zoning in sanidine is widely documented in igneous rocks and has been used to interpret timescales and mechanisms of magma formation, evolution, and storage. Although these studies have important implications for the timing and nature of volcanic eruptions, and even the evolution of the continental crust, there is a surprising lack of consensus on the processes that control Ba distribution in sanidine. Additionally, reported partition coefficients for Ba between sanidine and rhyolite liquid (as well as other trace elements such as Mg, Sr, Pb, etc.) are uncertain – showing a wide range of values (K_D = 1–45), and with little agreement on what controls partitioning behavior (i.e., temperature, pressure, orthoclase content, bulk composition, etc.). Here we report Laser Ablation ICP-MS analyses of experimentally derived sanidine and rhyolite glass to assess how different variables affect partitioning. Two sets of cold-seal pressure vessel experiments were analyzed. First, new experiments were run using a starting mixture of high-silica rhyolite of Lava Creek Tuff, Yellowstone, run at 740–800ºC and 25–100 MPa with a controlled oxygen fugacity of NNO+1 and water-saturated conditions. The second set of experiments analyzed were those conducted by Befus & Gardner (2014) using high-silica rhyolite of Solfatara Lava flow, Yellowstone as the starting mixture, run at 700–800ºC and 25–150 MPa, buffered at either NNO+1 or QFM, and at water-saturated conditions. When focusing on sanidine-glass pairs in experiments with the least amount of Ba variability in the glass as best candidates for equilibrium, we find a range of Ba partition coefficients of 15–60 and in a few cases up to 200, which is higher than values reported in the literature. Orthoclase content, temperature, and pressure have little to no apparent control on Ba partitioning. However, Ba content in glass appears to have a negative relationship with Ba partitioning. Further experiments and analysis are planned to confirm these observations and explore the role of kinetics on Ba distribution in sanidine from experiments and nature.