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Pumpellyite is certainly one of the
most critical index minerals in sub-greenschist rocks, and also the
most complex in terms of its compositional variations (i.e., ferric
iron, ferrous iron, aluminum, and magnesium substitutions) and
stoichiometry (i.e., variable anion content dependent on
ferrous/ferric ratio). As noted by Beiersdorfer and Day (in press),
one major shortcoming of chemographic techniques for investigating
equilibria in pumpellyite-bearing metabasites is the limited amount
of data on the ferrous/ferric ratio in pumpellyite. For their
projections, Beiersdorfer and Day (in press) estimate this ratio by
employing standard assumptions on the distribution of iron between
octahedral sites in pumpellyite. Although various
colorimetric/gravimetric and spectroscopic techniques are available
for the direct determination of ferrous/ferric ratios, it is
generally impractical, if not nearly impossible, to separate
fine-grained pumpellyite crystals out of low grade metabasites.
Aguirre et al. (in press) examined vein-filling pumpellyite in dolerites
from northern Spain and found good agreement between ferrous/ferric
ratios determined directly by wet chemistry and those based on
assumptions such as described by Beiersdorfer and Day.
Aside from the layer silicates and pumpellyite, research
activity on the other mineralogic constituents of low grade
metabasites was much less intense. Enami et al. (1992) examined the
amphiboles in metabasites from the Salton Sea geothermal field. As
might be anticipated, amphiboles which equilibrated with highly
saline brines are enriched in Cl, but the Cl-content is strongly
dependent on both the ferrous iron and edentite content of the
amphibole. Liou (1993) and Arnason et al. (1993) reviewed the
stability of natural epidote. Although epidote is arguably the most
common calc-silicate mineral in many low grade metabasites (and
thus so suitable as a projection point in chemographic techniques),
it is now known to occur in certain granites as a magmatic phase as
well as in coesite-bearing eclogites stable at mantle depths.
Next: Submarine Geodynamic Settings
Up: Mineralogic studies
Previous: Chlorite and chlorite/smectite.
U.S. National Report to IUGG, 1991-1994
Rev. Geophys. Vol. 33
Suppl., © 1995 American Geophysical Union