The diffusion behavior of hydrogen in plagioclase feldspar at 800-1000C:

Implications for OH re-equilibration in volcanic phenocrysts


Elizabeth A. Johnson
Department of Geology and Environmental Science
James Madison University, 395 S. High St., MSC 6903, Harrisonburg, VA 22807

George R. Rossman
Division of Geological and Planetary Sciences, California Institute of Technology, MS 170-25, Pasadena, CA 91125


ABSTRACT

Understanding the diffusive behavior of hydrogen in plagioclase is necessary in order to use structural hydroxyl (OH) concentrations preserved in volcanic phenocrysts to constrain the magmatic water content prior to eruption.   In this study,  diffusion coefficients for a natural OH-bearing plagioclase feldspar (Ab66An30Or3 ) were determined from a series of integrated loss heating experiments performed at 800-1000C and 1 atm under air, nitrogen gas, and a CO2-H2 mixture at the FMQ oxygen buffer.  Using a one-dimensional diffusive loss model for an infinite slab, the diffusion behavior for hydrogen in plagioclase is described by the diffusion parameters log D0 = -1.620.31 and EA=26677 kJ/mol for all experiments, and log D0 = -0.970.35 and EA=27890 kJ/mol for experiments conducted under nitrogen gas.  Nearly complete (83-97%) loss of OH from the andesine was achieved in 900C and 1000C heating series, except for the 900C FMQ buffer experiment in which only 64% of the total OH was lost at long heating times. The diffusion rates of hydrogen in the Cima plagioclase at 800-1000C are similar to interpolated diffusion rates for sodium diffusion in An30 feldspar, implying that Na+ and H+ both diffuse via Frenkel defects involving A-sites and interstitial ions.  The D values for hydrogen in plagioclase are lower than most reported diffusion data for hydrogen in nominally anhydrous minerals, and are most similar to D reported for pure forsterite, unaffected by iron redox reactions.  Based on the hydrogen diffusion parameters in this study, a 1 mm plagioclase phenocryst experiencing dehydration under lowered water activity during ascent and eruption at 800C retains 50% of its initial OH concentration after 34 days. At 900C and 1000C, a 1 mm phenocryst retains 50% of its initial OH concentration after only 1.3 days and 0.25 day, respectively.  OH concentrations in plagioclase are therefore most indicative of magmatic water contents during the latest stages of ascent and eruption.