Analysis of H in Natural and Experimental Nominally Anhydrous Minerals: A Comparison of Elastic Recoil Detection Analysis, Infrared and SIMS Measurements.

Abstract:

Experiments to determine the H storage capacity of nominally anhydrous minerals (NAMs) can be used to place strong constraints on the degree of hydration required to initiate volatile fluxed melting in the mantle. Moreover, accurate measurements of H in NAMs from mantle xenoliths may be used to constrain the rheological properties of the source region. However, accurate measurements of H concentrations using infrared and ion beam techniques require mineral specific calibrations. Standardless measurements of H using elastic recoil detection analysis (ERDA) in some cases have provided valuable verification of calibrations for other techniques, but so far have had limited application to experimental samples owing to sample size and analytical background. Results of recent measurements of H in garnet, pyroxene and feldspar will be used to illustrate the potential of the technique. Some of the results we will present include the following:

1) Forsterites grown at 2 and 3 GPa in the MgO-Al₂O₃-SiO₂-H₂O system exhibit low frequency absorption bands in the OH stretching region between 3300 and 3400 cm^-1, but very limited absorption at higher frequencies where the strongest absorption bands are usually seen for Al-free olivine. Analyses of H in these samples by ERDA will be used to develop a tentative site-specific molar absorption coefficient for the low frequency OH absorption band.

2) ERDA measurements of H in silicate glass standards verify previous determinations by Karl Fischer titration and FTIR spectroscopy, validating the ERDA technique. Samples of these rhyolitic glasses are available on request for use as analytical standards, and come with a compendium of analyses that have been used to characterize major and minor elemental composition and H content.

3) ERDA measurements of three homogeneous grossular garnets from East Africa show an excellent correlation (r² = 1) with absorbance in the O-H stretching region and indicate an integral molar absorption coefficient significantly higher than that determined by Rossman and Aines (1991) based on reduction of a global data set on grossulars and hydrogrossulars with 0.2-13 wt% H₂O. These new results can be reconciled well with a model for matrix effects in SIMS analysis of H in NAMs presented elsewhere at this meeting (Mosenfelder and Rossman).