Hydrogen in nominally anhydrous crustal minerals

Abstract

Systematic infrared and nuclear magnetic resonance investigations of common crustal minerals were undertaken to better understand the geologic significance of minor components of structural hydrous species within these nominally anhydrous minerals. The absolute hydrogen concentration in three alkali feldspars and eight plagioclase samples was measured with 1H nuclear magnetic resonance spectroscopy. The mid-infrared integral absorption coefficient was determined to be 15.3 ? 0.7 ppm-1?cm-2, allowing quantitative analysis of OH and H2O in feldspars with infrared spectroscopy. A survey of hydrous species in igneous feldspars found that feldspars contain structural OH (0-512 ppm H2O), H2O (0-1350 ppm H2O), and NH4+ (0-1500 ppm NH4+) groups as well as fluid inclusions and alteration products. Composition and crystal structure influence the type of hydrous species that can be incorporated into feldspars, but the concentration and speciation of structural hydrogen is at least partially determined by the geologic environment. The diffusivity of H in OH-bearing plagioclase was determined at 800-1000?C (D0=5.7?2.5x10-4 m2/sec and Q=224?33 kJ/mol). A millimeter-sized volcanic feldspar phenocryst would be expected to lose a significant proportion of its OH concentration on the timescale of a typical eruption (hours to weeks). The structures and compositions of low albite and ussingite, Na2AlSi3O8(OH), are similar. The strong hydrogen bonding in ussingite is found to be fundamentally different from the hydrogen bonding environment of OH in feldspars. Comparison of the infrared spectra of structural isomorphs reedmergnerite, NaBSi3O8, and low albite suggest that OH is incorporated in both structures through protonation of the most underbonded oxygen site. The concentration of structural OH in diopside was determined for four granulite facies siliceous marble samples from the Adirondacks, New York. Diopside OH concentration increases monotonically with increasing estimated water fugacity for each outcrop. Hydrogen concentration is correlated to Ti concentration in zoned grossular skarn garnets from Birch Creek, CA. Decrease of Ti and H from garnet cores to rims may be related to the solubility of Ti in the skarn-forming fluid. Skarn garnets from an Adirondacks, NY, wollastonite ore deposit exhibit a large range of OH concentrations broadly related to rock type that are due to recrystallization and partial dehydration.