Tetrahedral al in amphibole: a potential thermometer for some mafic rocks
It has long been know that the Al(IV) content in amphibole of mafic metamorphic rocks is a function of temperature and pressure as well as of bulk composition. The detailed mechanism of Al substitution must be complex combinations of numerous exchange and net-transfer reactions. However, somewhat to our surprise, the authors have found that the net effect is relatively simple, mainly a function of temperature (with a pressure coefficient of 15-25/sup 0/C/kbar). Thus Al(IV) in Ca-amphibole can serve as a useful geothermometer, provided that it coexists with a suitable buffering assemblage: intermediate plagioclase (An/sub 40-65/), a Mg-Fe silicate, and possibly Fe-Ti oxide. The fugacity of oxygen must be at or below that of the FMQ buffer. Regression of the experimental data of Helz and Spear may be used as a preliminary calibration of the thermometer. All data used for the regression follow the above constraints and range from 600/sup 0/ to 870/sup 0/C and 1 to 5 kbar. Three runs that appear incompletely equilibrated were omitted from the regression. Runs that produced garnet were inconsistent with the others and were also omitted. Application of this thermometer to data on various amphibolite and granulite facies terranes has met with moderate success: amphibole temperatures are usually within +/-25/sup 0/C of those indicated by other thermometers. They have not tested it for greenschist facies rocks. Further experimental calibration of this thermometer is now in progress, with special efforts made to bracket the contents of Al(IV).
- Research Organization:
- State Univ. of New York, Stony Brook (USA)
- OSTI ID:
- 6326945
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
58 GEOSCIENCES
AMPHIBOLE
STRUCTURAL CHEMICAL ANALYSIS
AMPHIBOLITES
GEOTHERMOMETERS
PETROGENESIS
GRANULITES
CHEMICAL COMPOSITION
FEASIBILITY STUDIES
GEOCHEMISTRY
GEOTHERMOMETRY
MOLECULAR STRUCTURE
REGRESSION ANALYSIS
ROCK-FLUID INTERACTIONS
CHEMISTRY
MATHEMATICS
MEASURING INSTRUMENTS
METAMORPHIC ROCKS
MINERALS
ROCKS
STATISTICS
THERMOMETERS
152002* - Geothermal Data & Theory- Properties of Minerals & Rocks
152004 - Geothermal Data & Theory- Isotope & Trace Element Studies
580400 - Geochemistry- (-1989)