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The Aluminium-in-Hornblende barometer of Johnson & Rutherford (1989)

This is a (very) simple Al-in-Hornblende barometer. According to Johnson & Rutherford (1989), metamorphic pressures can be estimated by means of the aluminum content in hornblendes. For this purpose, the total content of aluminum (Al^IV and Al^VI) in hornblende, based on a structural formula with 23 oxygen atoms, is used. The formula proposed by Johnson & Rutherford is:

P (±0.5 kbar) = -3.46 (±0.24) + 4.23 (±0.13) ∙ Al^tot

The calibration was performed experimentally by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780 °C.

References

Johnson, M.C. & Rutherford, M.J., 1989: Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks. – Geology (1989) 17 (9): 837–841.
See this publication on GeoScienceWorld (GSW), a nonprofit collaborative and comprehensive resource for research and communications in the Earth Sciences.

The barometer

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