NEW SIMS REFERENCE MATERIALS FOR MEASURING WATER IN UPPER MANTLE MINERALS

Trace amounts of water in the nominally anhydrous minerals of the upper mantle can dramatically affect their thermodynamic and rheological properties. Secondary ion mass spectrometry (SIMS) has become a mainstream technique for quantifying small amounts of water in minerals, but depends on reference materials with known concentrations of water. The current standards in use for mantle minerals are very well-characterized (Hauri et al., 2002; Koga et al., 2003; Aubaud et al., 2007; Mosenfelder and Rossman, 2013a,b), but a lack of extra material has limited the spread of this technique to other laboratories.

We present new SIMS measurements on natural mantle xenolith grains that are suitable to be used as calibration reference materials. They have homogeneous water contents, defined as a standard deviation of <10% for analyses across multiple grains. Reference materials for H₂O in orthopyroxene and clinopyroxene cover ranges from 52 to 328 ppm and from 9 to 559 ppm respectively, covering most of the observed range of mantle water contents. The sample concentrations are evenly distributed over these ranges. The orthopyroxene reference materials can also be used to measure water in olivine based on previous observations that these two minerals have the same calibration slope. These new reference materials are calibrated based on the pyroxene standards currently in use at the Department of Terrestrial Magnetism, Carnegie Institution of Washington (Koga et al., 2003; Aubaud et al., 2007).

Sample materials are available by request to us for material from our collection and to the Department of Mineral Sciences, Smithsonian Institution, for material in their collection. We have also created a calibration mount of orthopyroxene and clinopyroxene grains analyzed in this study, along with a set of basaltic glasses and a Suprasil 3002 glass blank. This calibration mount is available for use in cross-calibrating new grains of these reference materials against the specific grains measured in this study. New grains from these samples should plot within the currently measured water concentration ranges of these samples. However, since intergranular variations are always possible, we suggest that future grains from these samples be calibrated against the material measured in this study.