USE OF THE ELASTIC RECOIL DETECTION ANALYSIS (ERDA) MICROBEAM TECHNIQUE FOR THE QUANTITATIVE-DETERMINATION OF HYDROGEN IN MATERIALS AND HYDROGEN PARTITIONING BETWEEN OLIVINE AND MELT AT HIGH-PRESSURES

A nuclear microbeam technique called elastic recoil detection analysis (ERDA) or forward recoil spectroscopy which is capable of yielding bu lk H in silicates at ppm sensitivities is described. This technique is nondestructive and uses a He-4(+) beam which may be routinely focused to dimensions less than 5 x 5 mu m. The technique is suitable for the analysis of both materials of appreciable H content (e.g., amphiboles ) and materials with trace H content. The technique is calibrated in t he range 0-2 wt% H2O using a set of mineral standards of known H2O con centration with sensitivities of 0.04 wt% H2O achieved. There is a goo d correlation between H2O contents derived by spectra simulations and concentrations derived by empirical calibration, although the former y ield data 10-20% lower when compared to known values. The equilibratio n of olivine with a potassic silicate melt at high pressures (1.5 to 1 0 GPa) in experiments shows more H is accommodated in the mineral with increasing pressure. The olivine-melt K-H2O (expressing H as wt% H2O concentration in mineral/concentration in melt) at 1.5GPa (1400 degree s C) was ca. 0.04 +/- 0.015. At 5.8-6 GPa (1740 degrees C), olivine-me lt K-H2O increased to 0.13 +/- 0.03. A single experiment at 10 GPa (ca . 2000 degrees C) yielded a minimum K-H2O Of 0.12. The amount of H whi ch minerals accommodate is also highly correlated with bulk system com position (which controlled melt composition). In alkali-absent bulk sy stems, the K-H2O, for olivine equilibrated with a Mg-Si melt was 0.30 at I GPa (1400 degrees C), an order of magnitude increase over the alk ali-bearing system at this pressure. This reflects the reduced facilit y of a wholly Mg-Si melt to accommodate H2O relative to an alkali-bear ing melt. The increase in K-H2O with pressure for olivine-melt, combin ed with data for K-H2O of natural olivine (and orthopyroxene) in basal tic glass at P < 0.3 GPa (<0.005), implies that a deep residual mantle would be more H-rich than the shallow mantle for the same degree of m elt extraction. Copyright (C) 1997 Elsevier Science Ltd