Plagioclase stability at elevated temperatures and water pressures
The breakdown of plagioclase to zoisite, kyanite, quartz, and a more sodic plagioclase in the system NaAlSi/sub 3/O/sub 8/-CaAl/sub 2/Si/sub 2/O/sub 8/-H/sub 2/O has been investigated with reversed reactions at 8, 9, and 10 kbar. At these pressures all plagioclase compositions from approx.An/sub 40/ to An/sub 100/ break down along the same P-T curve as pure anorthite, which can be described by the equation P = -4590 + 20.4T(P = bars, T = /sup 0/C). The isobaric curves in the T-X section are thus represented by a horizontal line (constant temperature) between approx.An/sub 40/ and An/sub 100/. At compositions more sodic than approx.An/sub 40/ the curves dip rather sharply to lower temperatures. In the regions beneath the curves, the composition of sodic plagioclase along the down-dipping limb is in equilibrium with Zo + Ky + Qz. The remarkable behavior in the An/sub 40/-An/sub 100/ region suggests that under the hydrothermal conditions of this study, regions of ordered anorthite develop in the plagioclase and subsequently react with water as would independent crystals of An. Partial melting of pure An in the presence of water begins at 10.2 kbar and 725/sup 0/C. At 10.8 kbar a region of partial melt extends at nearly the same temperature from An/sub 30/ to An/sub 100/. (JMT)
- Research Organization:
- Univ. of Chicago, IL
- OSTI ID:
- 6641982
- Journal Information:
- Am. Mineral.; (United States), Vol. 67:7-8
- Country of Publication:
- United States
- Language:
- English
Similar Records
Anatexis of garnet amphibolites from a subduction zone metamorphic terrane
Pressure-induced amorphization in plagioclase feldspars: A time-resolved powder diffraction study during rapid compression
Related Subjects
15 GEOTHERMAL ENERGY
FELDSPARS
PHASE STUDIES
ANORTHITE
DECOMPOSITION
MINERALOGY
PHASE DIAGRAMS
PRESSURE DEPENDENCE
QUARTZ
TEMPERATURE DEPENDENCE
CHALCOGENIDES
CHEMICAL REACTIONS
DIAGRAMS
MINERALS
OXIDE MINERALS
OXIDES
OXYGEN COMPOUNDS
SILICATE MINERALS
SILICON COMPOUNDS
SILICON OXIDES
580300* - Mineralogy
Petrology
& Rock Mechanics- (-1989)
152003 - Geothermal Data & Theory- Rock-Water-Gas Interactions