Eric and all
There has been very little response to my question. I interprete this silence
that nobody has indeed observed PROGRADE plagioclase formation in
olivine-rocks from spinel or chlorite + amphibole or pyroxene. This seems
also to be the case for olivine in plagioclase-rich rocks. The pair olivine +
plagioclase appears to be exclusively of igneous origin. This is, in fact,
all I wanted to know.
Thank you
kurt
Eric J Essene wrote:
> Kurt and all,
> These reactions are well known of course. There are reports of
> plagioclase peridotites both as xenoliths and in exposed rocks. However,
> the stability field of the spinel side of the reaction is markedly
> expanded when chromite rather than spinel is part of the assemblage, and
> in that case the spinel peridotite assemblage is markedly expanded to 0-40
> kbar. The thermobarometric significance of the symplectites has to be
> treated with care.
> eric
>
> On Fri, 26 Oct 2001, kurt bucher wrote:
>
> > Dear all,
> >
> > The three reactions below limit the occurrence of plagioclase in olivin-
> > (+Opx) bearing high-grade rocks (such as meta-harzburgite):
> >
> > En + Di + Spl = 2 Fo + An
> > 4 Spl +2 Tr = 6 Fo + En + 4 An + 2 H2O
> > 4 Chl + 2 Tr = 10 Fo + 5 En + 4 An + 18 H2O
> >
> > Evidence of these reactions in olivine- (and Opx-) bearing rocks is
> > given by the widespread occurrence of Spl + Cpx and Spl + Cam ± Chl
> > symplectites and rinds around decomposing plagioclase. I am not aware of
> > reports of prograde plagioclase formation by the three reactions. Do I
> > have overlooked some important observations?
> >
> > Kurt Bucher
> >
--
Kurt Bucher (Prof.Dr.)
Institute of Mineralogy-Petrology-Geochemistry
University of Freiburg
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D-79104 Freiburg Germany
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