Experimental studies of the P–T–H2O near-liquidus phase relations of basaltic andesite from North Sister Volcano, High Oregon Cascades: constraints on lower-crustal mineral assemblages

Abstract

We have mapped the mineralogy onto the H2O-undersaturated liquidus surface of basaltic andesite from North Sister Volcano to constrain the crystalline assemblage with which, and P–T–H2O conditions at which, the melt last equilibrated before erupting. Combining our high pressure experimental results with examples of tectonically exposed lower arc crust, geophysical constraints, trace element geochemistry, and melt inclusion volatile contents, we conclude that an anhydrous, augite-rich gabbro at ∼12 kbar and ∼1,175°C is the most probable lithology with which North Sister basaltic andesite with ∼3.5 wt% H2O last equilibrated before erupting. We speculate that reaction between this gabbro and primitive mantle-derived precursor melts buffered the compositions of magmas erupted from this volcano resulting in their remarkably limited compositional range.