On pseudoternary diagrams, locations of experimentally produced melt compositions that coexist with plagioclase and anhydrous mafic minerals vary as functions of pressure (a geobarometer) and water content (a geohygrometer). Projection of rock compositions onto diagrams allows constraints to be placed on magma chamber depths and meltwater content. Application of the experimentally calibrated geobarometer and gehygrometer to Aleutian and Mariana rock suites suggests Aleutian magma chambers to be at depths of 7 to 17 km (pressures from 2 to 5 kbar) and meltwater content of andesites near 2 wt%. Mariana magma chambers are at shallow depths (pressure less than 2 kbar), and their water content is less than 2 wt%, probably near 1 wt%. Magma chamber depths can be interpreted to be controlled by density differences between crystallizing magmas and crust, which in turn may be controlled by crustal thermal structure. Differences in water content are correlated with amounts of sediment incorporated into basaltic magmas formed near the subducting oceanic slab.