Influence of subduction components on magma composition in back‐arc basins: a comparison between the Mariana and Okinawa troughs

The Okinawa Trough is an initial marginal back‐arc basin. In this paper, we present a comparative study of pyroxenes in white and black pumice from the Okinawa Trough to explore the magmatic evolution of the region and the characteristics of the magmatic system. The results show two species of pyroxenes in the white pumice (clinopyroxene and orthopyroxene) but only one species in the black pumice (orthopyroxene). Many isomorphic replacements among the cations occurred during clinopyroxene crystallization. Clinopyroxene‐melt thermobarometry yielded a temperature of 847–988°C and a pressure of 2.73–7.42 kbar, which vary considerably and correspond to a maximum depth of approximately 24 km. All of the orthopyroxenes in both the white and black pumice crystallized at similar pressures ranging from 0.67 to 4.59 kbar, which correspond to a maximum depth of approximately 15 km. The clinopyroxenes crystallized mainly in the lower crust, whereas the orthopyroxenes formed at the boundary depth between the upper crust and the lower crust. The similarities in the pyroxene chemical compositions and the whole‐rock characteristics between the two types of pumice indicate that both types of pumice are linked to the same magmatic system at depth. The pyroxenes crystallized from genetically related melts within a magmatic system that had undergone various degrees of differentiation. The physicochemical characteristics of the pyroxenes show that the magma chamber module changed during the crystallization process from clinopyroxenes to orthopyroxenes. This change most likely resulted from local characteristics of the near‐surface structure. These observations support the model of a ‘two‐layered magma chamber’ in the northern Okinawa Trough. Copyright © 2016 John Wiley & Sons, Ltd.

Section: Discussionsupporting

confidence: 55%

A complex magmatic system beneath the middle and northern Okinawa Trough: evidence from pyroxene characteristics

Liao

Huang

2016

“…Research results indicate that subduction components play a key role in island-arc and back-arc magmatism and the crustmantle material recycling process (Pearce et al, 2005). In the Okinawa Trough, which is affected by Philippine Sea Plate subduction since it is a young back-arc basin, the injection of subduction components is always a research focus for geological scholars (Shinjo et al, 1999;Yan and Shi, 2014;Yu et al, 2016;Zhao et al, 2016). As pumice is loose and porous, either in the magma eruption from the seafloor and subsequent condensation and rock-forming process or in the seawater immersion process after pumice formation, modification of seawater and the injection of sediments should have occurred.…”

Section: The Influence Of the Subduction Compositionmentioning

confidence: 99%

Sr–Nd–Pb isotopic geochemistry of phenocrysts in pumice from the central Okinawa Trough

Guo

Zhai

et al. 2016

Pumice is the most widely distributed magmatic rock in the Okinawa Trough. Numerous scholars have conducted extensive research on the mineralogy, petrochemistry and whole‐rock isotopic characteristics of pumice. However, even with extensive research, some information about pumice is still unknown, including the initial provenance of magma, influences from subducting plates and the possibility of crustal material (including sediments) mixing in during pumice formation. As pumice is loose and porous, seawater re‐modification and injection of sediments may occur during the condensation and rock‐forming process after magma erupts from the seafloor, or in the seawater immersion process after pumice formation. In order to get a better understanding of the isotopic characteristics of magmatism phase, phenocrysts were first separated from the pumice and divided into both light and heavy minerals to analyse their Sr–Nd–Pb isotopic composition. This information was combined with existing isotopic data of volcanic rocks from the Okinawa Trough and its adjacent areas to assess the magma source composition, influence from subduction components and mantle properties in the magmatic source. The research results indicated light minerals had an average 87Sr/86Sr ratio of 0.70454, while heavy minerals averaged 0.70546. The magma, formed pumice, was a product of the crystal fractionation evolution of basic magma, with the additional injection of crustal material. The magma source in the central trough is a mixture of depleted MORB mantle (DMM) and enriched mantle (EMI‐like and EMII‐like) end members, where the EMI‐like end member is the residual subcontinental lithosphere or lower crustal material and the EMII‐like end member is subducted oceanic sediments. The Pb isotopic composition indicated that the magma source was affected by subduction components (sediments) and that pumice magma was mixed with more sediment than basalt magma material in the same area. The mantle of the magma source in the Okinawa Trough showed signatures of the Indian Ocean‐type mantle. Copyright © 2016 John Wiley & Sons, Ltd.

“…These basalts and rhyolites are believed to be cognate, as they have similar Sr–Nd isotopic compositions (Guo, Zhai, Yu, Cai, & Zhang, ; P. Huang, Li, & Jiang, ; Shinjo & Kato, ), which could reflect the isotopic compositions of the mantle sources in the OT. These mantle sources record Indian Ocean mid‐ocean ridge basalt (MORB) mantle or DUPAL‐like signatures (Hoang & Uto, ; Yan & Shi, ; Zeng et al, ) and are influenced by subduction components from the Philippine Sea Plate (Guo, Zhai, Yu, Wang, et al, ; Pi, You, & Wang, ; Zhao, Luo, Lai, Tian, & Xu, ).…”

Section: Introductionmentioning

confidence: 99%

High‐potassium volcanic rocks from the Okinawa Trough: Implications for a cryptic potassium‐rich and DUPAL‐like source

Zhang

Zeng

et al. 2017

Major element, trace element, and Sr–Nd–Pb isotopic compositions of high‐potassium calc‐alkaline rhyolites (the T6 rhyolites) collected from the western slope of the middle Okinawa Trough (MOT) are reported in this article. These high‐potassium rhyolites have similar major and trace elemental compositions as Type 2 rhyolites from the MOT, except that the former contain higher concentrations of K2O, Th, U, and Rb and lower concentrations of Sr and Eu. Experimental data indicate that the T6 rhyolites cannot be sourced from low‐ to mid‐potassium MOT rocks by either fractional crystallization or partial melting processes but are instead derived from a cryptic potassium‐rich source beneath the Okinawa Trough (OT). Considering that high‐potassium volcanic rocks are distributed throughout the East China Sea Shelf, the Ryukyu Arc, and Kyushu Island, the high‐potassium OT rocks could represent an important connection between the OT and its adjacent regions. Compared to volcanic rocks from the MOT axial zone, the T6 rhyolites have more depleted Sr–Nd isotopic compositions (87Sr/86Sr = 0.7035–0.7036, 143Nd/144Nd = 0.51291–0.51297) but much more enriched compositions of 207Pb (207Pb/204Pb = 15.60–15.63) and 208Pb (208Pb/204Pb = 38.45–38.57), thus recording a significant DUPAL‐like signature. These isotopic anomalies cannot be explained by the incorporation of subducting sediments or crustal contamination but instead imply the presence of a DUPAL‐like mantle source beneath the OT. The decrease of the DUPAL‐like anomaly from the T6 rhyolites to the MOT axial zone volcanic rocks is consistent with the injection of an normal mid‐ocean ridge basalt (N‐MORB) mantle source, such as asthenospheric material, into pre‐existing DUPAL‐like mantle during back‐arc extension.