PYROXENE AND PLAGIOCLASE COMPOSITIONS AS AN INDICATOR OF MAGMATIC WATER CONCENTRATIONS: TWO HYGROMETERS

It is well known that magmatic water plays an important role in controlling the composition of plagioclase phenocrysts in arc magmas, with higher water concentrations leading to more calcic plagioclase. This has led to the development of a new thermodynamic model for the plagioclase hygrometer (Lange and Frey, 2008), which is based on the exchange reaction of the anorthite and albite components between plagioclase and magmatic liquid. We have applied this plagioclase hygrometer to numerous crystal-poor (0-10 vol%), lavas in western Mexico that contain euhedral plagioclase crystals that are individually homogenous in composition, but collectively span a remarkably wide (e.g., An80-An40) compositional range. Using the plagioclase hygrometer, we have shown that this compositional range can be explained by a strong variation in dissolved magmatic water concentration as expected during degassing upon ascent through the upper crust (Frey and Lange, 2008). Thus, variable plagioclase compositions in arc lavas cannot be immediately inferred to imply magma mixing, but may merely reflect H2O degassing during crystallization. In these same crystal-poor lavas, sparse orthopyroxene and clinopyroxene phenocrysts also have euhedral rims and are individually homogenous. However, they also collectively span a compositional range in terms of their Mg#, leading to a variation in apparent Mg-Fe KD values of 0.25-0.45 between pyroxene and liquid. Here, we argue that this variation reflects the differential effect of dissolved H2O concentration (most likely the hydroxyl component) on the relative activity of the MgO vs. FeO liquid components, which is supported by phase equilibrium experiments in the literature where water and oxygen fugacity are both carefully controlled (e.g., Moore and Carmichael, 1998). It appears that water concentrations favor more iron-rich pyroxenes, and thus degassing may explain observations of reverse zoning in pyroxenes in arc lavas. We will show that a thermodynamic model can be constructed for a pyroxene hygrometer based on the exchange reaction between enstatite and ferrosilite components between pyroxene and liquid, which is similar in form to that developed for the plagioclase hygrometer.