Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer

Abstract

The Al-in-hornblende barometer, which correlates Al^tot content of magmatic hornblende linearly with crystallization pressure of intrusion (Hammarstrom and Zen 1986), has been calibrated experimentally under water-saturated conditions at pressures of 2.5–13 kbar and temperatures of 700–655°C. Equilibration of the assemblage hornlende-biotite-plagioclase-orthoclasequartz-sphene-Fe-Ti-oxide-melt-vapor from a natural tonalite 15–20° above its wet solidus results in hornblende compositions which can be fit by the equation: P(±0.6 kbar) = −3.01 + 4.76 Al ^tot_hbl r ²=0.99, where Al^tot is the total Al content of hornblende in atoms per formula unit (apfu). Al^tot increase with pressure can be ascribed mainly to a tschermak-exchange ($t\vec k,{\text{ Mg}}_{{\text{ - 1 }}} {\text{Al}}^{{\text{VI}}} {\text{Si}}_{{\text{ - 1}}} {\text{ Al}}^{{\text{IV}}}$) accompanied by minor plagioclase-substitution ($\vec pl,{\text{ Ca}}_{{\text{ - 1 }}} {\text{Na}}^{{\text{M(4)}}} {\text{ Al}}_{{\text{ - 1}}}^{{\text{IV}}} {\text{ Si}}$). This experimental calibration agrees well with empirical field calibrations, wherein pressures are estimated by contact-aureole barometry, confirming that contact-aureole pressures and pressures calculated by the Al-in-hornblende barometer are essentially identical. This calibration is also consistent with the previous experimental calibration by Johnson and Rutherford (1989b) which was accomplished at higher temperatures, stabilizing the required buffer assemblage by use of mixed H₂O-CO₂ fluids. The latter calibration yields higher Al^tot content in hornblendes at corresponding pressures, this can be ascribed to increased edenite-exchange ($\vec ed,{\text{ }}\square _{{\text{ }} - {\text{ }}1}^{ A} {\text{ Na}}^{\text{A}} {\text{Si }}_{ - {\text{ }}1} {\text{Al}}^{{\text{IV}}}$) at elevated temperatures. The comparison of both experimental calibrations shows the important influence of the fluid composition, which affects the solidus temperature, on equilibration of hornblende in the buffering phase assemblage.