MELT-FLUID INCLUSIONS AND VOLCANIC PROCESSES Quantitative models linking igneous amphibole composition with magma Cl and OH content

Minerals of the amphibole group are found in igneous rocks on Earth and other rocky bodies. Since the O(3) site of amphibole can contain OH. O2-, F, and Cl, amphibole composition provides important information about water and halogen contents, oxidation state, and other features of its formation and alteration environments. However, the complexity of amphibole crystal chemistry means this information is difficult to extract. Furthermore, it has been regular practice in the era of the electron microprobe to neglect H and Fe3+ analyses for amphibole, critically reducing the amount of information available in amphibole analyses in the literature. We have assembled models and insights from previous work to create a methodology that allows the estimation of magmatic H2O and Cl contents from existing amphibole analyses. Since the methodology requires use of a cation norm, we begin with a deeper investigation of the consequences of different cation normalization schemes for amphibole analyses, and provide grounds for deciding which scheme best fits a given amphibole analysis. We then show how the existing model of Popp and coworkers can be reversed to estimate the OH and [O(3)]O2- contents of amphiboles in synthesis experiments. Using a synthetic data set collected from the literature (39 amphibole analyses), we calibrate a partitioning model for the OH/Cl competition on the O(3) site of igneous amphiboles