Oxy-substitution and dehydrogenation in mantle-derived amphibole megacrysts

Results from major element and hydrogen micro-analyses of titanium-rich man tle-derived amphiboles from the SW USA are combined with previous experimen tal studies. We show that the distinctive chemistry of mantle-derived amphi boles, especially relatively high Ti, variable ferric/ferrous iron, and hyd rogen contents, result from both initial crystallization conditions and deh ydrogenation. On the basis of previous experimental work, it is concluded that the Ti-ric h nature of mantle-derived amphibole megacrysts is a result of crystallizat ion from mafic-ultramafic melts at low to moderate pressure (less than or e qual to 1.0 GPa), high temperature (>950 degrees C) and low to moderate oxy gen fugacity (fO(2)). We propose that those conditions change TiO2 and Al2O 3 activity in the melt. Iron oxidation state in amphiboles is affected by f O(2) or hydrogen fugacity (fH(2)) in the melt. In contrast to previous sugg estions, it is not necessary to have low water activity (aH(2)O) to crystal lize 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. Amp hiboles from lava flows and scoria cones have low to variable H contents (O H < 1.4 atomic formula units) and individual megacrysts from a single local ity commonly have different H contents. Amphibole H contents and Fe3+/Fe2are a function of both initial crystallization conditions and dehydrogenati on, with variations occurring due to different pressure-temperature-fH(2)-t ime paths. Amphibole dehydrogenation likely occurs at the surface or en rou te to the surface where fH(2) 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 crystallograp hic 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 rap idly 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. Copyright (C) 1999 Elsevier Sc ience Ltd.