Lay summary
Stable isotope analyses of natural substances and measurements of the isotope fractionations between pairs of cogenetic substances have played a major role in understanding a wide range of important geological and cosmological processes. For hydrogen isotope fractionations, there is a lack of consensus between different experimental calibrations. While some of the differences between calibrations have been shown to be related to pressure effects, others are not readily explained. One of the major unknowns is the compositional control, particularly the Fe-content and its valency on H-isotope fractionations between minerals and fluids. This research is aimed at determining the importance of compositional variations in minerals to H-isotope fractionations in natural mineral-melt-fluid systems of well-studied but chemically variable alkaline intrusive rocks. Resolution of this unkown factor will then also provide information on the origin of highly reducing conditions that are indicated by mineralogic and fluid inclusion studies for a number of undersaturated alkaline and peralkaline granitic systems. The H-isotope compositions of amphibole from alkaline complexes not only have an extremely wide range, but also have extremely low D-content in peralkaline to agpaitic rocks. We have measured amphiboles from complexes of Ilímaussaq (Greenland) and Khibina (Russia) with D values as low as -202 and -178‰, respectively; Tugtutoq Complex (Greenland) has values for amphiboles around 137‰ and the Canadian Mount Saint Hilaire intrusives about -152‰. All of these, however, also have values more typical of mantle-derived magmatic rocks (about -90‰). In contrast, amphiboles from the Okenyenya and Messum alkaline complexes (NW Namibia) only have values of between -70 and -89‰, supporting a normal mantle origin. Because the extremely low values occur in different complexes, a common perhaps magmatic control on the compositions is indicated. The present data of mineralogical and geochemical compositions from a number of complexes clearly indicates that the extremely low D-contents are only observed for complexes that experienced conditions of extreme alkalinity.
Fe-content and valency state do not seem to play an important role. The O-isotope compositions are also homogeneous for all complexes and well in line with a mantle-derived magmatic origin. Analyses of profiles for amphiboles exhibit no obvious zonation in H-isotope composition, suggesting that late secondary process, such as a diffusive exchange with meteoric water but also with a reducing fluid, had no effect.
Collectively, the results imply a complex fractionation process in the presence of highly alkaline fluids, a model that is to be tested further through experiments in progress.