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
>
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