Fe³⁺/H⁺ and D/H in kaersutites—Misleading indicators of mantle source fugacities

Hornblende megacrysts from basaltic flows in several locations have been studied by means of the electron microprobe, Mössbauer spectroscopy, uranium extraction techniques, and mass spectrometry to define the interrelations between the concentrations of Fe³⁺ and H⁺, and the hydrogen isotope partitioning (D/H) in mantle amphiboles. Results indicate a nearly perfect inverse relation between Fe³⁺ content and H⁺ content; no relation is observed between D/H partitioning and wt% H₂O. A review of diffusion data for amphiboles suggests that the observed increase in Fe³⁺ content with decreasing H⁺ content may represent partial dehydration of the amphiboles during ascent. Thus, Fe³⁺ and H⁺ contents measured on amphiboles at Earth's surface may not represent mantle conditions. Instead, high Fe³⁺ contents may be indicative of high H⁺ contents in the mantle source region rather than high mantle oxygen fugacities.