H2O‐δD‐FeIII relations of dehydrogenation and dehydration processes in magmatic amphiboles

Stable isotope compositions of a suite of magmatic amphiboles from alkaline basalts and andesitic rocks were examined to constrain the effects of degassing processes on the hydrogen isotope compositions. The Fe³⁺ (as Fe³⁺/Fe_total) and H₂O contents, as well as the H isotope compositions of the amphiboles, differ markedly (27–58%, 0.5–2.2 wt%, ?107 to ?15‰, respectively) but indicate systematic variations. The observed trends can be explained either as dehydrogenation or dehydration processes, both of which are coupled to oxidation processes, the latter most probably related to O^2? substitution within amphiboles. The dehydrogenation‐dehydration models can be used to assess the primary compositions of the magmas. As an important example, δD values of amphiboles of Martian meteorites are discussed in a similar context. Copyright © 2006 John Wiley & Sons, Ltd.