TY - JOUR

T1 - Effects of fo2 and H2O on andésite phase relations between 2 and 4 kbar

AU - Martel, Caroline

AU - Pichavant, Michel

AU - Holtz, Francois

AU - Scaillet, Bruno

AU - Bourdier, Jean Louis

N1 - Copyright: Copyright 2018 Elsevier B.V., All rights reserved.

PY - 1999/12/10

Y1 - 1999/12/10

N2 - Experimental phase equilibria have been investigated on three medium-K silicic andésite (60-61 W10 % SiO2) samples from Mount Pelée at 2-4 kbar, 850-1040°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 (fo2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pi) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (7>H2O range (875-950°C and 5-7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-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 vaporsaturated 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°C, where opx is replaced by Carich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changing f02 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 A12O3, CaO, FeO', and MgO). FeO/MgO moderately increases with increasing SiO2. For fo2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andésite magmas, and this should stimulate reexamination for the mechanisms of generation of andésites by fractionation from basaltic parents.

AB - Experimental phase equilibria have been investigated on three medium-K silicic andésite (60-61 W10 % SiO2) samples from Mount Pelée at 2-4 kbar, 850-1040°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 (fo2) were investigated: NNO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderately oxidizing conditions, plagioclase (pi) and magnetite (mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature (7>H2O range (875-950°C and 5-7 wt % H2O in melt). Amphibole (am) is stable under near vapor-saturated CO2-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 vaporsaturated 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°C, where opx is replaced by Carich pyroxene (cpx) and am. Compositions of ferromagnesian phases systematically correlate with changing f02 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 A12O3, CaO, FeO', and MgO). FeO/MgO moderately increases with increasing SiO2. For fo2 conditions typical of calk-alkaline magmatism (approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andésite magmas, and this should stimulate reexamination for the mechanisms of generation of andésites by fractionation from basaltic parents.

UR - http://www.scopus.com/inward/record.url?scp=0033545023&partnerID=8YFLogxK

M3 - Article

AN - SCOPUS:0033545023

VL - 104

SP - 29453

EP - 29470

JO - Journal of Geophysical Research: Solid Earth

JF - Journal of Geophysical Research: Solid Earth

SN - 2169-9313

IS - B12

ER -