We report new experimental data on the composition 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 aluminium content (Al^tot) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^tot 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 (An_15–80) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase 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 Al^tot (in atoms per formula unit, apfu) according to the expression: $${\textit{P }}\left( {\text{kbar}} \right) = 0.5 + 0.331\left( 8 \right) \times {\text{Al}}^{\text{tot}} + 0.995\left( 4 \right) \times \left( {{\text{Al}}^{\text{tot}} } \right)^{2}$$ This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^tot 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.

We present new experiments, combined with a re-evaluation of published data, to characterize the topology of the silicate–carbonate two-liquid solvus in the five-component system SiO₂–Na₂O–Al₂O₃–CaO–CO₂ (SNAC + CO₂). Conjugate liquid compositions have been determined for a wide range of pressures (0·1–2·5 GPa) and temperatures (1225–1700°C) as well as variable degrees of CO₂ saturation. The expansion of the two-liquid field with increasing pressure and/or decreasing temperature, and the contraction of the two-liquid field for conditions where P_CO2 < P_total is accurately presented for the first time. The shape of the two-liquid solvus suggests that alkali-rich carbonatites can have a range of SiO₂ + Al₂O₃ contents down to very low values (<1 wt %), but that low-alkali or alkali-free immiscible carbonatites will always have SiO₂ + Al₂O₃ contents greater than 10–15 wt %. The most commonly observed carbonatite rock compositions observed at the Earth’s surface all tend towards low contents of alkalis SiO₂ and Al₂O₃ and would have fractionated silicate phases from the carbonatite parental melts, possibly associated with alkali loss to coexisting fluids. Our results also show that carbonate liquid exsolution can occur from a CO₂-undersaturated (P_CO2 < P_tot) silicate melt. Although the expanded high-pressure miscibility gap appears favourable for producing natural silicate melt compositions, a low-pressure (<1·0 GPa) magma chamber in the crust or perhaps in the shallow mantle below a rift provides the most likely environment for immiscibility to arise owing to the lower CO₂ demand of the silicate magma. Unusual textures in some experiments, suggestive of a deformable liquid state for the CaCO₃ phase, are conclusively shown to be characteristic of a non-quenchable, high-temperature polymorph of solid calcite. Similar calcite globules with this rounded appearance, which are also observed in some nephelinite lavas and mantle xenoliths, must be solid calcite and not immiscible liquids. This is consistent with the high SiO₂ + Al₂O₃ requirement of low-alkali or alkali-free immiscible carbonate liquids.