Oxy-substitution and dehydrogenation in mantle-derived amphibole megacrysts

Results from major element and hydrogen micro-analyses of titanium-rich mantle-derived amphiboles from the SW USA are combined with previous experimental studies. We show that the distinctive chemistry of mantle-derived amphiboles, especially relatively high Ti, variable ferric/ferrous iron, and hydrogen contents, result from both initial crystallization conditions and dehydrogenation. On the basis of previous experimental work, it is concluded that the Ti-rich nature of mantle-derived amphibole megacrysts is a result of crystallization from mafic-ultramafic melts at low to moderate pressure (≤1.0 GPa), high temperature (>950°C) and low to moderate oxygen fugacity (fO₂). We propose that those conditions change TiO₂ and Al₂O₃ activity in the melt. Iron oxidation state in amphiboles is affected by fO₂ or hydrogen fugacity (fH₂) in the melt. In contrast to previous suggestions, it is not necessary to have low water activity (aH₂O) to crystallize Ti-rich amphiboles. Mantle-derived amphiboles typically have homogeneous H contents. Megacrysts from maars and dikes have high H contents (OH > 1.1 atomic formula units) and individual crystals from a single locality have similar H contents. Amphiboles from lava flows and scoria cones have low to variable H contents (OH < 1.4 atomic formula units) and individual megacrysts from a single locality commonly have different H contents. Amphibole H contents and Fe³⁺/Fe²⁺ are a function of both initial crystallization conditions and dehydrogenation, with variations occurring due to different pressure-temperature-fH₂-time paths. Amphibole dehydrogenation likely occurs at the surface or en route to the surface where fH₂ is low, cooling is slow, or grain attributes tend to favor rapid H diffusion. We propose a model for calculating Fe³⁺ in Ti-rich kaersutites where Fe³⁺ = 2.47-0.93(OH)-(Ti + Al(vi)). This equation takes into account crystallographic constraints within an amphibole structure. Our findings have implications for determining the primary oxygen fugacity of the mantle on Earth and Mars (using SNC meteorites). Amphiboles from rapidly cooled volcanic rocks have most likely retained their 'primary' OH and Fe³⁺/Fe²⁺ contents and are the best targets for calculating mantle oxygen fugacities and for stable isotopic analyses.