Geochemical features of trace and rare earth elements of pumice in middle Okinawa Trough and its indication of magmatic process

Abstract

Pumice, the most widely distributed volcanic rock in Okinawa Trough, is loose and porous. Since its formation, it has definitely suffered from the denudation of the sea to different degrees. In order to truly reveal the geochemical features of pumice, we choose the method of mineral separation. Firstly, the phenocryst is separated from glass. Then the phenocryst is divided into light and heavy mineral compositions. By ICP-MS (inductively coupled plasma mass spectrometry) analytical technology, the contents of trace and rare earth elements in the whole pumice, the glass and the heavy and light mineral compositions are determined respectively. By researching the elemental geochemical features, the magma dynamic processes are found. It shows that the initial magma for the pumice in Okinawa Trough came from the depleted mantle, from which the N-MORB (normal type of mid-ocean ridge basalt) is formed, homologous with the local basalts. But they are formed in different periods of magma crystal fractionation. Featured with sufficient crystal fractionation for pumice, it is found that the earlier crystallizing minerals are olivine, plagioclase and pyroxene. The pumice magma, formed from the depleted mantle, was mixed with additional subduction-related materials (components), and contaminated with the mass from upper crust when it rose up into the crust. As the Okinawa Trough is a back-arc basin in its early back-arc spreading stage, its magmatism has a series of its own unique characteristics, different from not only the mid-ocean ridge expansion, but also the mature back-arc basin.