Craig and all,
Consider using the plagioclase model used by Berman and Aranovich (1996,
CMP). However, can one be sure that garnet equilibrated with a
plagioclase-pyroxene symplectite? If the symplectite is replacing garnet,
it may not have equilibrated with it because the equilibrium is easily
overstepped. I think that this is generally the case unless neoformed and
not relict garnet is seen in the symplectite. For GADS one also needs to
be certain about the presence of quartz at the time the symplectite formed,
or to regard the results as an upper baric limit. Many symplectites form
from garnet or omphacite without equilibrating with quartz. Several phases
in symplectites often indicate a low activity of SiO2, e.g., spinel,
tschermakitic pyroxene, aluminous amphibole, or even glass with a
relatively low SiO2 content (as found in some symplectites in
garnetiferous xenoliths).
Eric
I have recently started a PhD looking at the structural and
>metamorphic history of the Glenelg Inlier in NW Scotland.
>As a recent graduate I am having a little difficulty in establishing
>which plagioclase activity model to apply to the GADS geobarometer.
>I am trying to apply GADS to a Diopside-Oligoclase symplectite in
>equilibrium with garnet within a retrogressed eclogite (i.e. early
>high-P Granulite overprint).
>Please could someone aid me with an appropriate expression or, even
>better, an idea of where I might get a fully worked example of
>activity models.
>
>Regards,
>
>Craig Storey
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