Constraints on the composition of the Aleutian arc lower crust from V _P/V_S

Determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust. We present new constraints on the composition of high P-wave velocity (V_P = 7.3-7.6 km/s) lower crust of the Aleutian arc from best-fitting average lower crustal V_P/V_S ratio using sparse converted S-waves from an along-arc refraction profile. We find a low V_P/V_S of ∼1.7-1.75. Using petrologic modeling, we show that no single composition is likely to explain the combination of high V_P and low V_P/V_S. Our preferred explanation is a combination of clinopyroxenite (∼50-70%) and alpha-quartz bearing gabbros (∼30-50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that ∼30-40% of the full Aleutian crust comprises ultramafic cumulates. These results also suggest that small amounts of quartz can exert a strong influence on V_P/V_S in arc crust.