Mössbauer study of Fe3+/Fe2+ ratio in amphiboles to search correlation with hydrogen isotope fractionation

Abstract

There is a general lack of understanding of the hydrogen isotope fractionations between different experimental approaches both at higher and at lower temperatures of exchange. The complexity of bonding related with the hydroxyl ion in most hydrous minerals makes theoretical treatments rather difficult. Though some of the differences between experimental findings have recently been shown to be related to pressure effects that influence the hydrogen isotope fractionation properties of water, some other factors may also have significance. One of the major unknowns is the compositional control, especially the Fe-content and effects of variable Fe3+/Fe2+ ratios on hydrogen isotope fractionations between minerals and fluids (Suzuoki and Epstein, Geochim Cosmochim Acta 40:1229–1240, 1976; Chacko et al. 2001). We have studied a series of amphibole samples by Mossbauer spectroscopy, EPMA and TC-EA-IRMS to examine for possible correlations between D/H fractionation and Fe2+ to Fe3+ ratio/total iron content. Our measurements show that this correlation may exist for one particular origin of samples, but local conditions of mineral formation (most probably chemical composition and reactions accompanying the formation of minerals) may be more important in controlling the hydrogen isotope composition of minerals.