Dehydration and partial melting of tremolitic amphibole coexisting with zoisite, quartz, anorthite, diopside, and water in the system H2O-CaO-MgO-Al2O3-SiO2 |
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| Authors: | Diane M Quirion David M Jenkins |
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| Institution: | (1) Department of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, New York, 13902-6000 USA, US |
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| Abstract: | The greenschist to amphibolite transition as modeled by the reaction zoisite+tremolite + quartz= anorthite+diopside+water
has been experimentally investigated in the chemical system H2O−CaO− MgO−Al2O3−SiO2 over the range of 0.4–0.8 GPa. This reaction is observed to lie within the stability fields of anorthite + water and of zoisite
+ quartz, in accord with phase equilibrium principles, and its position is in excellent agreement with the boundary calculated
from current internally-consistent data bases. The small dP/dT slope of 0.00216 GPa/K (21.6 bars/K) observed for this reaction supports the pressure-dependency of this transition in this
chemical system. Experimental reversals of the Al content in tremolitic amphibole coexisting with zoisite, diopside, quartz,
and water were obtained at 600, 650, and 700°C and indicated Al total cations (atoms per formula unit, apfu) of only up to 0.5±0.08 at the highest temperature. Thermodynamic
analysis of these and previous compositional reversal data for tremolitic amphibole indicated that, of the activity/composition
relationships considered, a two-site-coupled cation substitution model yielded the best fit to the data and a S
0 (1 bar, 298 K) of 575.4±1.6 J/K · mol for magnesio-hornblende. The calculated isopleths of constant Al content in the amphibole
are relatively temperature sensitive with Al content increasing with increasing temperature and pressure. Finally, several
experiments in the range of 1.0–1.3 GPa were conducted to define the onset of melting, and thus the upper-thermal limit, for
this mineral assemblage, which must involve an invariant point located at approximately 1.05 GPa and 770°C.
Received: 24 January 1997 / Accepted: 2 October 1997 |
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