Constraints on the composition of the Aleutian arc lower crust from VP/VS

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 similar to 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 (similar to 50-70%) and alpha-quartz bearing gabbros (similar to 30-50%). This is consistent with Aleutian xenoliths and lower crustal rocks in obducted arcs, and implies that similar to 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.