A precise olivine-augite Mg-Fe-exchange geothermometer |
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Authors: | Robert R Loucks |
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Institution: | (1) Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia, AU |
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Abstract: | Olivine and augite that were experimentally equilibrated in the temperature interval 1175–1080°C at 1 bar in natural basaltic
and andesitic bulk compositions are used to calibrate an Mg-Fe2+ cation-exchange geothermometer. Within its temperature interval of experimental calibration, and over a broad range in olivine
Mg/Fe ratio, the geothermometer has a standard error of ±6°C. In compositionally simpler synthetic systems, the same calibration
retrieves appropriate experimental temperatures up to at least 1250°C. In application to intermediate and felsic volcanic
rocks erupted at ∼1080 –800°C (below the range of experimental calibration), calculated olivine-augite temperatures are in
good agreement with Fe-Ti oxide thermometry in the same samples. These results encourage confidence in the olivine-augite
geothermometer over at least the 800–1250°C interval at low pressures. Sparse experimental data up to 1250°C at higher pressures
for olivine + augite in the assemblage olivine + plagioclase + augite ± pigeonite or orthopyroxene suggest that the low-pressure
calibration recovers experimental temperatures without systematic bias to pressures of 10 kbar. Examples illustrate applications
to determining igneous equilibration temperatures in holocrystalline extrusive and intrusive rocks, and to estimating intratelluric
H2O content dissolved in magmas.
Received: 24 February 1995 / Accepted: 1 March 1996 |
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