Effects of f(O2) and H2O on andesite phase relations between 2 and 4 kbar

Experimental phase equilibria have been investigated on three medium-K sili cic andesite (60-61 wt % SiO2) samples from Mount Pelee at 2-4 kbar, 850-10 40 degrees C, under both vapor-saturated CO2-free and vapor-saturated CO2-b earing 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 inve stigated: 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) ar e the liquidus phases, followed by low-Ca pyroxene (opx); these three phase s 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 foun d 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 + m t on the liquidus above 3.5 kbar. For highly oxidizing conditions, mt is th e 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 systemat ically correlate with changing f(O2) Experimental glasses range from andesi tic through dacitic to rhyolitic, showing systematic compositional variatio ns with pi + opx + mt fractionation (increase of SiO2 and K2O, decrease of Al2O3, CaO, FeOt, and MgO). FeO*/MgO moderately increases with increasing S iO2. For f(O2) conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydro us silicic andesite magmas, and this should stimulate reexamination for the mechanisms of generation of andesites by fractionation from basaltic paren ts.