Experimental calibration of the aluminum-in-hornblende geobarometer with application to Long Valley caldera (California) volcanic rocks

Journal Article · Fri Sep 01 04:00:00 EDT 1989 · Geology; (USA)

DOI:https://doi.org/10.1130/0091-7613(1989)017<0837:ECOTAI>2.3.CO;2· OSTI ID:6918495

Johnson, M C; Rutherford, M J ^[1]

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1. Brown Univ., Providence, RI (USA)

A new geobarometer based on the Al content of igneous hornblendes in equilibrium with melt, fluid, biotite, quartz, sanidine, plagioclase, sphene, and magnetite or ilmenite has been calibrated experimentally. The calibration was performed by equilibrating the required phase assemblage over the pressure range 2-8 kbar at 740-780C, and then analyzing euhedral hornblendes in equilibrium with glass (melt). Experiments were performed on natural samples of both volcanic and plutonic rocks. Earlier empirical calibrations of this geobarometer relied on analyzing natural hornblendes from plutons with the required phase assemblage and inferring pressure from nearby metamorphic country rocks. The experimental calibration differs from the empirical calibrations, especially above 5 kbar, and shows that the Al content of hornblendes in equilibrium with the required phase assemblage is greater for a given total pressure than previously thought. The geobarometers uncertainty is dramatically reduced. The derived equation is P ({plus minus}0.5 kbar) = 3 {minus}3.46 ({plus minus}0.24) + 4.23 ({plus minus}0.13) (Al{sup T}). The geobarometer is applied to post-Bishop Tuff volcanic rocks from Long Valley caldera, California, and reveals that most rhyodacites in this complex erupted from depths of about 6 km. These eruptions occurred over 500,000 yr, suggesting that the rhyodacitic magma reservoir beneath Long Valley had reached a steady P (depth)-T state.

• OSTI ID: 6918495
• Journal Information: Geology; (USA), Journal Name: Geology; (USA) Vol. 17:9; ISSN GLGYB; ISSN 0091-7613
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALUMINIUM
AMPHIBOLE
CALDERAS
CALIBRATION
CALIFORNIA
DEPTH
DIMENSIONS
ELEMENTS
EQUATIONS
EQUILIBRIUM
FEDERAL REGION IX
FLUIDS
GEOBAROMETRY
GEOLOGIC HISTORY
HORNBLENDE
IGNEOUS ROCKS
LONG VALLEY
METALS
MINERALS
NORTH AMERICA
PLUTONIC ROCKS
RESERVOIR FLUIDS
RESERVOIR PRESSURE
RESERVOIR TEMPERATURE
ROCKS
SILICATE MINERALS
STEADY-STATE CONDITIONS
TUFF
USA
VOLCANIC ROCKS