Different magma sources and evolutions of white and black pumice from the middle Okinawa Trough: Evidence from major, trace elements and Sr–Nd–Pb isotopes

Abstract

The Okinawa Trough (OT) is a back-arc, initial marginal sea basin that is located behind the Ryukyu arc–trench system. Pumice, including both white and black pumice, is the most widely distributed magmatic rock in the OT. Here, the major and trace element concentrations and Sr–Nd–Pb isotopic compositions of the white and black pumice from the middle OT (MOT) are presented and combined with previously published elemental data for volcanic rocks from the MOT. Their different magma sources and magmatic evolutions are discussed, and their different magma chamber structures are provided in this paper. These results demonstrate that the white pumice has two magma sources: One was produced by the crystallization-based differentiation of basaltic magma, and the other was produced by the assimilation-fractional crystallization (AFC; with less than 5% contamination) of basaltic magma. The black pumice also includes two magma sources: One is similar to that of the first white pumice, whereas the other was produced by basaltic magma that experienced more than 20% upper crustal contamination. There are two different structure magma chambers for white and black pumice in the MOT crust: The deeper one generated no contaminated pumice, whereas the shallow one generated contaminated black pumice. The contamination of the white pumice (<5%) may have occurred during the eruption of magma on the seafloor, rather than in the magma chamber. Different magma sources and evolutions of white and black pumice imply the complexity of magmatism in the Okinawa Trough.