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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