Effects of fO2 and H2O on andesite phase relations between 2 and 4 kbar

Experimental phase equilibria have been investigated on three medium-K silicic andesite (60-61 wt % SiO2) samples from Mount Pelee at 2-4 kbar, 850-1040 degrees C, under both vapor-saturated CO2-free and vapor-saturated CO2-bearing conditions. Most experiments were crystallization experiments using dry glasses prepared from the natural rocks. Both normal- and rapid quench experiments were performed. Two ranges of oxygen fugacity (f(O2)) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pl) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (T)-H2O range (875-950 degrees C and 5-7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-free conditions at 876 degrees C. At 900 degrees C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure (P) under vapor-saturated CO2-free conditions, pi + mt is replaced by am + mt on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is the sole liquidus phase at 2 kbar, followed by pi and opx, except in the most H2O-rich part of the diagram at 930 degrees C, where opx is replaced by Ca-rich pyroxene (cpx) and am. Compositions of Ferromagnesian phases systematically correlate with changing f(O2) Experimental glasses range from andesitic through dacitic to rhyolitic, showing systematic compositional variations with pi + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing SiO2. For f(O2) conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic parents.