WATER IN PLAGIOCLASE FELDSPAR IN BASALTIC TO RHYOLITIC ROCKS

Synchrotron-generated infrared radiation has been used to identify and quantify water in plagioclase feldspar crystals from a variety of volcanic settings, and to examine the budget of water during crystallization of magmas of contrasting composition on the basis of water concentration in successively crystallizing populations of feldspar crystals. In addition to collection of spectra through Fourier transform infrared (FTIR) spectroscopy, the brightness of synchrotron-generated radiation permits the collection of maps of overall water concentration, as well as concentration of water species, including molecular water and OH-. FTIR spectra have been collected from samples of Columbia River basalt (USA), Deccan traps basalt (India), and Purico dacite and andesite (Chile). In many samples, water concentrations are lower in phenocrysts that are larger and appear to have formed earlier in a crystallization sequence than in subsequent smaller blocky plagioclase crystals and latest-formed laths, and water concentrations are generally lower in the cores of crystals than near their edges. However, some samples from each of the complexes studied have plagioclase crystals that show complex zoning in water concentration that seems to mimic variations in Ca/Na. These variations suggest that zoning of water molecules and/or OH complexes across plagioclase crystals, like Ca/Na zoning, can be a long-lived compositional feature of the crystal. Because all of the plagioclase crystals are surrounded by relatively water-rich groundmass (to 3 wt.%), some water is likely to have diffused into the crystals, possibly accounting for elevated water concentrations in plagioclase rinds. However, the extent of diffusion of water into phenocrysts can be characterized by mapping water concentrations across crystal/groundmass boundaries. This study confirms both the complexity of partitioning of water species during crystallization of melts, and the status of feldspar as a reservoir of significant water in the Earth’s crust.