Effects of f O2 and H 2 O on andesite phase relations between 2 and 4 kbar

Abstract

Experimental phase equilibria have been investigated on three medium‐K silicic andesite (60–61 wt % SiO 2 ) samples from Mount Pelée at 2–4 kbar, 850–1040°C, under both vapor‐saturated CO 2 ‐free and vapor‐saturated CO 2 ‐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 )‐H 2 O range (875–950°C and 5–7 wt % H 2 O in melt). Amphibole (am) is stable under near vapor‐saturated CO 2 ‐free conditions at 876°C. At 900°C, ilmenite (ilm) is found only in experiments less than or equal to NNO. Upon increasing pressure ( P ) under vapor‐saturated CO 2 ‐free conditions, pl + 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 pl and opx, except in the most H 2 O‐rich part of the diagram at 930°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 pl + opx + mt fractionation (increase of SiO 2 and K 2 O, decrease of Al 2 O 3 , CaO, FeO t , and MgO). FeO * /MgO moderately increases with increasing SiO 2 . 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.