CRYSTAL CHEMICAL CONTROLS ON HALOGEN AND HYDROXYL PARTITIONING INTO IGNEOUS AMPHIBOLES

Dr. Frank Hawthorne has been a major figure in the field of amphibole crystal chemistry. 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, O^2-, F, and Cl, amphibole composition provides important information about water and halogen contents, oxidation state, and other features of its parental magma. However, 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—as Frank has pointed out in the past.

Here, we have assembled models and insights from previous work in order to create a methodology that allows the estimation of magmatic H₂O and Cl contents from amphibole analyses. We begin with a deeper investigation of the consequences of different cation normalization schemes for amphibole analyses. We then show how the existing model of Popp and coworkers can be reversed to estimate the OH and ^[O(3)]O^2- contents of amphiboles in synthesis experiments. Using a synthetic dataset collected from the literature (39 analyses), we calibrate preliminary partitioning model equations for the OH/Cl competition on the O(3) site of igneous amphiboles. We then combine the Popp et al. (2006) model with our new model to link the occupancy of the amphibole O(3) site and other crystal chemical parameters to the Cl and H₂O content of melts crystallizing amphibole. The competition between OH and Cl for this site can be used to calculate the melt and amphibole OH/H₂O contents, as well as the speciation of Fe in amphibole, provided that the Cl content of both the amphibole and its coexisting melt is known.

This work shows that the volatile content of magmas can be determined using amphibole chemistry, and also provides information about the crystal chemistry of amphibole. We conclude by providing results from an experimental campaign to gather better data and calibrate a more reliable model for O(3) site partitioning. We believe this work will lead to a far better understanding of magmatic volatile contents and the role that F and Cl play in igneous petrology.