Formation of island arc dacite magmas by extreme crystal fractionation: An example from Brothers Seamount, Kermadec island arc (SW Pacific)

Dacitic to rhyolitic glasses from Brothers Seamount in the southern Kermadec island arc lie on very tight major element trends and have formed by fractional crystallization from a basaltic magma rather than by partial melting of amphibolitic lower crustal rocks. The radiogenic isotope and highly incompatible element ratios of the dacites resemble the composition of basalts from other volcanoes in the same segment of the island arc which probably represent analogues to the basaltic parental magma of the Brothers Seamount lavas. Glass compositions show that the magmas did not degas extensively but retained much of their volatile inventory with very high Cl (4000 to 7000 ppm) but low water contents (∼2 wt.%). The Cl / K of 0.25 is constant in the Brothers Seamount lavas and probably reflects the composition of a sedimentary slab component in the subarc mantle below the volcano. The Brothers Seamount Cl / K reflects a sedimentary fluid rather than a melt and is lower than the Cl / K observed in Valu Fa Ridge lavas showing an influence of subducted altered basaltic crust, probably due to the higher K contents of sediments than in basalts.