P-T-H2O PHASE RELATIONS OF AN ALEUTIAN HIGH-MAGNESIA BASALT: RESULTS FROM THERMODYNAMIC MODELING AND HYDROUS EXPERIMENTS

The central Aleutians have erupted compositionally heterogeneous lavas ranging from fractionated high-alumina basalts to rarer, more primitive high-MgO basalts (HMB).  HMBs erupted in arc systems represent near-primary magmas that may be unmodified products of mantle melting beneath the arc.  Anhydrous experiments on natural HMBs have constrained the P-T conditions at which the melts last equilibrated with a dry mantle peridotite assemblage.  However, the presence of a hydrous fluid in the mantle wedge causes melting to occur at lower pressures and higher temperatures than under dry conditions.  

Previous anhydrous experimental studies on a natural HMB (ID-16) show that it is multiply saturated with five phases (olivine, spinel, clinopyroxene, orthopyroxene, and plagioclase) at 12 kbar and ~1300˚C.  However, is unlikely that ID-16 equilibrated under anhydrous conditions at this pressure and temperature.  It is our goal to infer the P-T-H₂O conditions (if any) at which ID-16 is in equilibrium with a hydrated mantle peridotite using thermodynamic modeling and hydrous, piston-cylinder experiments.   

We have modeled the near-liquidus phase relations for ID-16 using MELTS and pMELTS under equilibrium, water-undersaturated conditions.  The liquidus phases under dry conditions are orthopyroxene (<9 kbar), olivine (9-12 kbar), clinopyroxene (12-20 kbar), and garnet (>20 kbar).  At high pressures, garnet is the liquidus phase (0-13 wt% H₂O) shifting to biotite at >13 wt% H₂O.  At moderate pressures (10-20 kbar), clinopyroxene is the liquidus phase, shifting to olivine at higher water contents.  According to the phase relations predicted by MELTS and pMELTS, ID-16 is not multiply saturated with a mantle assemblage under any P-T-H₂O conditions.   

A second modeling technique developed by Wood (2004) tracks the migration of the multiple saturation point of fertile peridotite in P-T space under variable water conditions.  This model predicts that ID-16 is in equilibrium with fertile mantle at the following conditions: 12.3 kbar, 1304˚C, 0 wt% H₂O; 14.1 kbar, 1200˚C, 3 wt% H₂O; 15.3 kbar, 1186˚C, 5 wt% H₂O; and 18.3 kbar, 1171˚C, 10 wt% H₂O.  These results are confirmed by available anhydrous data.  It is our intention to test this model further with hydrous, piston-cylinder experiments at 3, 5, and 10 wt% H₂O.