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

Abstract

A nuclear microbeam technique called elastic recoil detection analysis (ERDA) or forward recoil spectroscopy which is capable of yielding bulk H in silicates at ppm sensitivities is described. This technique is nondestructive and uses a 4He+ beam which may be routinely focused to dimensions less than 5 × 5 μm. 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 the range 0–2 wt% H2O using a set of mineral standards of known H2O concentration with sensitivities of 0.04 wt% H2O achieved. There is a good correlation between H2O contents derived by spectra simulations and concentrations derived by empirical calibration, although the former yield data 10–20% lower when compared to known values. The equilibration of olivine with a potassic silicate melt at high pressures (1.5 to 10 GPa) in experiments shows more H is accommodated in the mineral with increasing pressure. The olivine-melt KH2O (expressing H as wt% H2O concentration in mineral/concentration in melt) at 1.5GPa (1400°C) was ca. 0.04 ± 0.015. At 5.8–6 GPa (1740°C), olivine-melt KH2O increased to 0.13 ± 0.03. A single experiment at 10 GPa (ca. 2000°C) yielded a minimum KH2O of 0.12. The amount of H which minerals accommodate is also highly correlated with bulk system composition (which controlled melt composition). In alkali-absent bulk systems, the KH2O for olivine equilibrated with a MgSi melt was 0.30 at 1 GPa (1400°C), an order of magnitude increase over the alkali-bearing system at this pressure. This reflects the reduced facility of a wholly MgSi melt to accommodate H2O relative to an alkali-bearing melt. The increase in KH2O with pressure for olivine-melt, combined with data for KH2O of natural olivine (and orthopyroxene) in basaltic 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 melt extraction.