TY - JOUR

T1 - Melting of plagioclase in granite and related systems

T2 - Composition of coexisting phases and kinetic observations

AU - Johannes, Wilhelm

AU - Holtz, François

N1 - Copyright: Copyright 2020 Elsevier B.V., All rights reserved.

PY - 1992/1/1

Y1 - 1992/1/1

N2 - Melting experiments with plagioclases were performed in the systems Ab-An, Ab-An-H2O, Qz-Ab-An-H2O, and Qz-Or-Ab-An-H 2O-CO2. The experimental products were analysed by electron microprobe, and the kinetics of the reactions were studied qualitatively. Melting of the plagioclase in the system Ab-An (P = 1 atm, T = 1420°C) is very fast in the first minutes but becomes slower with increasing run duration and is incomplete even after 1000 hours. The Ab/An fractionation between melt and residual plagioclase is similar to that described by Bowen (1913). Melting kinetics of plagioclase in the system A b - A n - H2O (PH2O = 5 kbar, T = 1000°C) is controlled by the diffusion of water into the plagioclase structure. Melting is especially fast parallel to the a-axis. The experimental products show separation of melts and crystals. In the tonalité system Qz-Ab-An-H2O, equilibrium melting could be observed down to 830°C (PH2O = 2 kbar) but not a lower temperatures. The kinetics of the reaction is enhanced by deficiency or excess of alumina in the aluminosilicate melt surrounding the plagioclase crystals. The fractionation of Ab and An between melt and plagioclase crystals is more pronounced in the presence of quartz than in the A b - A n - H2O system. The ratio An/An + Ab is approximately 0-35 in the melt and 0-85 in the coexisting plagioclase (T = 880°C). In the haplogranodiorite system Qz-Or-Ab-An-H2O-CO2, melting reactions were performed at P = 0-5 kbar, T = 880°C, and aH2O of approximately 0-5. It is assumed that near equilibrium compositions of melt and coexisting residual plagioclase could be obtained in long duration runs (run time = 60 days). The distribution of Ab and An between melt and minerals is similar to that observed in the tonalité system. The partial melt coexisting with an An-rich plagioclase and Or-rich K-feldspar is relatively poor in An.

AB - Melting experiments with plagioclases were performed in the systems Ab-An, Ab-An-H2O, Qz-Ab-An-H2O, and Qz-Or-Ab-An-H 2O-CO2. The experimental products were analysed by electron microprobe, and the kinetics of the reactions were studied qualitatively. Melting of the plagioclase in the system Ab-An (P = 1 atm, T = 1420°C) is very fast in the first minutes but becomes slower with increasing run duration and is incomplete even after 1000 hours. The Ab/An fractionation between melt and residual plagioclase is similar to that described by Bowen (1913). Melting kinetics of plagioclase in the system A b - A n - H2O (PH2O = 5 kbar, T = 1000°C) is controlled by the diffusion of water into the plagioclase structure. Melting is especially fast parallel to the a-axis. The experimental products show separation of melts and crystals. In the tonalité system Qz-Ab-An-H2O, equilibrium melting could be observed down to 830°C (PH2O = 2 kbar) but not a lower temperatures. The kinetics of the reaction is enhanced by deficiency or excess of alumina in the aluminosilicate melt surrounding the plagioclase crystals. The fractionation of Ab and An between melt and plagioclase crystals is more pronounced in the presence of quartz than in the A b - A n - H2O system. The ratio An/An + Ab is approximately 0-35 in the melt and 0-85 in the coexisting plagioclase (T = 880°C). In the haplogranodiorite system Qz-Or-Ab-An-H2O-CO2, melting reactions were performed at P = 0-5 kbar, T = 880°C, and aH2O of approximately 0-5. It is assumed that near equilibrium compositions of melt and coexisting residual plagioclase could be obtained in long duration runs (run time = 60 days). The distribution of Ab and An between melt and minerals is similar to that observed in the tonalité system. The partial melt coexisting with an An-rich plagioclase and Or-rich K-feldspar is relatively poor in An.

KW - Granite

KW - Melting

KW - Plagioclase

KW - Reaction kinetics

KW - Tonalité

UR - http://www.scopus.com/inward/record.url?scp=84879647042&partnerID=8YFLogxK

U2 - 10.1130/SPE272-p417

DO - 10.1130/SPE272-p417

M3 - Article

AN - SCOPUS:84879647042

VL - 272

SP - 417

EP - 422

JO - Special Paper of the Geological Society of America

JF - Special Paper of the Geological Society of America

SN - 0072-1077

ER -