An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

We report new experimental data on the compo- sition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total alu- minium content (Altot) of the synthetic calcic amphiboles varies systematically with pressure (P), although the rela- tionship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experi- mental studies, which suggests of a general relationship between Altot and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An15–80) + biotite + quartz + alkali feldspar + ilmenite/ titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagio- clase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Altot (in atoms per formula unit, apfu) according to the expression:P (kbar) = 0.5 + 0.331(8) × Altot + 0.995(4) × (Altot)2This expression recovers equilibration pressures of our cal- ibrant dataset, comprising both new and published experi- mental and natural data, to within ±16 % relative uncer- tainty. An uncertainty of 10 % relative for a typical Altot value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.