Hydrogen, trace, and ultra-trace element distribution in natural olivines
Résumé
We investigate the coupling between H, minor, trace, and ultra-trace element incorporations in 17 olivines from ten different locations covering various petrological origins: magmatic, hydrothermal and mantle-derived context. Concentrations in major element are determined by micro X-ray fluorescence. Minor, trace, and ultra-trace element are determined by laser ablation inductively coupled plasma mass spectrometry. Hydrogen concentrations are quantified using unpolarized and polarized Fourier transform infrared spectroscopy (FTIR). Forsterite contents (83.2 to 94.1 %) reflect the petrogenetic diversity. Hydrogen concentrations range from 0 to 54 ppm H2O wt. Minor element concentrations (Ni + Mn) range from 3072 to 4333 ppm, and impurities are dominated by Ni, Mn, Ca or B. Total trace element concentrations range from 8.2 to 1473 ppm. Total rare Earth and extended ultra-trace elements concentrations are very low (< 0.5 ppm). Magmatic and hydrothermal olivines show the most and least amount of impurities, respectively, and mantle-derived olivines have concentrations between these two extremes. Combined with minor, trace, and ultra-trace element concentrations, the hydrogen concentrations and FTIR OH bands reflect the point defect diversity imposed by different geological settings. Hydrogen concentrations are inversely correlated with divalent impurities, indicating their competition for vacancies. However, a broad positive correlation is also found between OH bands at 3575 and 3525 cm-1 and Ti, confirming the existence of Ti-clinohumitelike point defect in mantle olivines. Nonetheless, Ti does not exclusively control hydrogen incorporation in olivine due to the coexistence with other mechanisms, and its effect appears diluted. Our results confirm that hydrogen behaves as a peculiar incompatible element, and furthermore as an opportunistic impurity in olivine.
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