A TEM study of disequilibrium plagioclase breakdown at high pressure: the role of infiltrating fluid

Abstract

High-pressure metamorphism (∼600° C, ∼20 kbar) of the Allalin Gabbro (Western Alps) resulted in the breakdown of plagioclase (∼An₆₃) to fine-grained zoisite, jadeite, kyanite and quartz. In rare cases this reaction failed to reach completion. The resulting textures of partial reaction have been studied by transmission and analytical electron microscopy. In localised regions of a plagioclase crystal where the extent of reaction is <10%, only zoisite developed and the orientation relationship

$$\left( {100} \right)_{{\text{Zo}}} //\left( {101} \right)_{{\text{P1}}} $$

(1)

and

$$\left( {012} \right)_{{\text{Zo}}} //\left( {010} \right)_{{\text{P1}}} $$

(1)

is frequently present. In regions where 10–50% of plagioclase has transformed, the reaction plagioclase+H₂O→zoisite+kyanite+quartz +(NaSiCa₋₁Al₋₁)_pl has occurred. The systematic orientation relationship between plagioclase and zoisite is absent at ≥50% transformation. Complete breakdown of plagioclase occurred in localized micron-scale domains by the reaction plagioclase+H₂O→zoisite+jadeite+kyanite+quartz and the reaction products are variably orientated with respect to each other. Incomplete reaction, together with the concentration of reaction products around cracks in original plagioclase grains, suggests that extent of reaction was controlled primarily by the availability of H₂O. The textural observations are interpreted in terms of two possible disequilibrium reaction models. (1) Plagioclase persists metastably with its original igneous composition to a pressure > 17 kbar at 600° C. Reaction to the equilibrium assemblage then develops adjacent to cracks in response to the presence of aqueous fluid. At intracrystalline sites, only partial reaction occurs because Jadeite, and sometimes kyanite and quartz, fail to nucleate for kinetic reasons. (2) Localized regions of a plagioclase crystal partially equilibrate at several stages during the increase of pressure (∼9–17 kbar at 600° C), possibly due to discrete episodes of fluid infiltration. In both these models, the extent of reaction may be limited by NaSi-CaAl interdiffusion in plagioclase.