Helium isotopes in volcanic rocks from the Okinawa Trough—impact of volatile recycling and crustal contamination

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

“…Therefore, several investigations of the OT have been conducted (e.g., Lee, Shor Jr, Bibee, Lu, & Hilde, 1980;Letouzey & Kimura, 1986;Sibuet et al, 1998). However, although considerable geophysical and geological data have been generated, many issues remain regarding types of mantle sources and the process of magma evolution (e.g., Huang, Li, & Jiang, 2006;Ishizuka, Kawanobe, & Sakai, 1990;Shinjo, 1999;Yu, Zhai, Guo, Zhou, & Zong, 2016;Zeng et al, 2010). As a potential reflection of deep mantle activity, volcanic rocks can provide evidence of magma evolution and geodynamic mechanisms in the OT.…”

Section: Funding Informationmentioning

confidence: 99%

Geochemistry of silicate melt inclusions in middle and southern Okinawa Trough rocks: Implications for petrogenesis and variable subducted sediment component injection

Zeng

Yang

et al. 2018

Self Cite

Silicate melt inclusions (SMIs) in olivine, clinopyroxene, and orthopyroxene in basalt and trachyandesite of the middle Okinawa Trough (MOT) and in basaltic andesites of the southern Okinawa Trough (SOT) were investigated in detail to understand the evolution of magma and geochemical characteristics of the basaltic magma source. Both the SMIs and host bulk rocks are more heavily enriched with Pb and large ion lithophile elements than with high field strength elements and rare earth elements. The major element compositions of SMIs in the MOT trachyandesite show a scattered variation trend of medium‐K to shoshonitic series and suggest that a K‐rich magma source may exist in the Okinawa Trough. Two groups with distinct element distribution patterns observed in SMIs of the MOT trachyandesite may represent magmas with different evolutionary histories. Compared with fractional crystallization and partial melting processes, magma mixing may serve as a reasonable process for the production of trachyandesite in the MOT. Fractional crystallization has played an important role in basaltic magma evolution in the MOT and SOT. Simulations conducted using MELTS show that the fractional crystallization evolution trends observed correspond well to the presence of a magma chamber at 300 MPa with an H2O content level of 0.2 wt% in the MOT and to the presence of a magma chamber at 300 MPa with an H2O content level of 0.4 wt% in the SOT. Ratios of incompatible trace elements (e.g., Th/Ce, Th/Sm, Ba/Th, and Th/Nb); positive Pb anomalies; negative Nb, Ta, and Ti anomalies; and the existence of organic hydrocarbons (CnHm) in the SMIs can be attributed to the involvement of subducted sediment from the subducting slab. Modelling results suggest that more primitive magma represented by SMIs from the MOT has formed through the melting of less than 6% of a metasomatized depleted mantle with contributions of 1.2% sediment melt and 0.8% sediment fluid. In contrast, more primitive magma from the SOT formed through the melting of 4–10% of the depleted mantle with contributions of 3% sediment melt and 1% sediment fluid. The different features of the SMIs together with different tectonic settings of the MOT and SOT have formed in response to the westward subduction of the Philippine Sea Plate.

“…The continental lithosphere had been broken at the axial part of the central and southern OT, and the new oceanic crust had been developing at the central spreading centre since 2 Ma (Fig. Yu et al (2016) analysed the helium isotope characteristics of volcanic rocks from the OT and proposed that the contamination of basaltic magma from the southern OT by continental crust was negligible and the magma from the central OT was contaminated by crustal gases. The NNE-trending faults in the central and southern OT were switched into transform faults, and those of the northern OT kept their dextral strike-slip motion and made the axial grabens continue to rift under extension.…”

Section: Tectonic Modelmentioning

confidence: 99%

“…The high heat flow in the northern OT may be related to the well-developed continental rifting; the extremely high heat flow in the central OT may be related to initial seafloor spreading on account of the crustal disequilibrium; the relatively low heat flow in the southern OT may be related to further seafloor spreading, as the crustal development is trending to a state of isostasy. Yu et al (2016) analysed the helium isotope characteristics of volcanic rocks from the OT and proposed that the contamination of basaltic magma from the southern OT by continental crust was negligible and the magma from the central OT was contaminated by crustal gases. This also indicates that the southern OT spread faster than the central OT.…”

Section: Tectonic Modelmentioning

confidence: 99%

The geological nature and geodynamics of the Okinawa Trough, Western Pacific

Liu

Suo

et al. 2016

There is a continuing hot debate on whether the geodynamics in the West Pacific indicate either a marginal basin or back‐arc basin, since the nature, evolution and formation mechanism are not well known. Research on the Okinawa Trough (OT), which is an active back‐arc basin, has important scientific significance for discussing the opening and development of a marginal basin or back‐arc basin. This paper synthesizes data on the crustal thickness, heat flow, hydrothermal activity and fault patterns to determine the geological nature of the OT. By comparing the thicknesses of initial oceanic crusts of global passive continental margins and marginal basins in the West Pacific, the central and southern OT are interpreted to be at the early stage of seafloor spreading, generating new oceanic crust at the spreading centre, while the northern OT is at the mature stage of continental rifting, having the character of transitional crust. The NE‐ and NNE‐trending faults are dominant in the OT. The NE‐trending strike‐slip faults control the stratigraphic succession and structural framework of the OT. The NNE‐trending strike‐slip faults triggered the back‐arc rifting or extension and evolved into transform faults with the emergence of oceanic crust. The Cenozoic tectonic jumping in the East Asia Continental Margin, including the OT, indicates that the eastward movement of the mantle flow under the Eurasian Plate should be the main dynamic source. Because the Philippine Sea Plate is obliquely and passively subducted, the evolution of the OT can be divided into three stages. The first stage is the initial rifting, triggered by the back‐arc normal faulting. The second stage is the passive spreading, triggered by the strike‐slip pull‐apart process or transtensional faulting of NNE‐trending faults. The third stage is the initial divergence of the relic and Ryukyu island arcs, triggered by new‐born oceanic crustal spreading. Copyright © 2016 John Wiley & Sons, Ltd.