Garnet-Hornblende Geothermometer: A Comparative Study
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Garnet-Hornblende Geothermometer: A Comparative Study Harel Thomas* and Haritabh Rana Department of Applied Geology, School of Engineering & Technology, Dr Harisingh Gour Vishwavidyalaya, Sagar - 470 003, India *E-mail: [email protected]
ABSTRACT The abundance of garnet - hornblende pair in a wide range of rocks primarily from upper green schist to granulite facies has made it one of the most widely used pairs for estimation of temperature at which once rocks equilibrated. Several models of garnet- hornblende thermometer have been formulated by a number of researchers in last four decades. In this contribution, four thermometers: Graham & Powell (1984); Perchuk et al., (1985); Powell, (1985) and Ravna, (2000) formulated since 1984 have been applied. 79 sample data (32 granulites facies, 37 amphibolite facies, 1 greenschist facies, 6 UHP/eclogite facies, 1 UHT and 2 migmatites) from the global literature were collected and processed through the “Gt-Hbl.EXE” software. Based on the present study, the best among all the four models have been identified where the regression bearing on inverse of temperature verses LnKD of each model as well as relation of distribution coefficient on composition were compared. It is concluded that Perchuk et al., (1985) is the most valid and reliable thermometers for a wide range of rock types such as greenschist to eclogite facies. INTRODUCTION A valid geothermobarometry serves as the fundamental tool for inferring crustal evolution by decoding the metamorphic condition. In last few decades, several geothermobarometers have been proposed for determining the metamorphic conditions of rocks covering a large span of pressure from greenschist to eclogite facies. In the study of metamorphic condition, the validity of a thermometer is called upon. Several thermobarometric studies have been undertaken in the past few years, which led to the development of a range of thermometers and barometers. Ravna (2000) investigated the relationship of equilibrium between garnet and hornblende based on experimental data compiled from various experiments (Sen and Dunn, 1994; Patino Douce and Beard, 1995; Carroll and Wyllie ,1989; Springer and Seck , 1997; Skjerlie and Johnston , 1996; Ernst and Liu, 1998) as well as natural data set given by Graham and Powell (1984). Thomas H. and Rana H., (2019) reviewed twenty one garnetbiotite models proposed since Ferry and Spear (1978) till Kaneko and Miyano (2004) on granulites investigating the compositional dependence on distribution coefficient and regression of inverse of temperature for each model. Thomas H. et al., (2018) have done a comparative study on fourteen garnet - orthopyroxene thermometer proposed till date to find the best model. Thomas H. et al., (2020) (In press) did a comparative study on eleven garnet - cordierite geothermometers finding out the highest regression of models showing best fit values to find the suitable model. The garnet - hornblende pair has a history of four decades which is confirmed by several experiments such as experimenta
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