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
(less than or equal to 1.0 GPa), high temperature (>950 degrees C) and
low to moderate oxygen fugacity (fO(2)). We propose that those
conditions change TiO2 and Al2O3 activity in the melt. Iron oxidation
state in amphiboles is affected by fO(2) or hydrogen fugacity (fH(2)) in
the melt. In contrast to previous suggestions, it is not necessary to
have low water activity (aH(2)O) 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-fH(2)-time paths.
Amphibole dehydrogenation likely occurs at the surface or en route 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
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.
Copyright (C) 1999 Elsevier Science Ltd.