Chinese scientists have revealed the mechanism of submarine volcanic eruptions in the Okinawa Trough

The Okinawa Trough is a nascent post-arc basin located at the outer edge of the continental shelf in the East China Sea, with submarine volcanoes widely distributed, but there is a lack of understanding of the continuous active time of the overlying magma reservoir of the Okinawa Trough submarine volcano and its eruption driving mechanism.

The reporter learned from the Institute of Oceanography of the Chinese Academy of Sciences on June 6 that through the analysis of orthopneziite and magnetite extracted by the research ship “Science” from the Angyonite and its iron-magnesia magma envelope on the slope of the Dragon 1 volcano in the southern Okinawa Trough, Zeng Zhigang’s team, a researcher at the Key Laboratory of Marine Geology and Environment of the Chinese Academy of Sciences, revealed the mechanism of submarine volcanic eruptions in the Okinawa Trough, and the research results were published in the international geoscience journal Lithosphere.

Geological characteristics of the Fire Dragon 1 submarine volcanic area in the southern Okinawa Trough Photo courtesy of the Institute of Oceanography

Zeng Zhigang’s team conducted detailed mineralogical and micro-element content analysis of orthorhombic and magnetite in the collected Angysitee and its ferromagnesia magma inclusion, and the results showed that orthorhombic pyroxene in Angyneite showed obvious compositional rings, compared with the edge, the core part had high manganese oxide content, significant enrichment of transition metal elements, and the core part was balanced with basalt magma, while the edge part was balanced with rhyolite melt.

This indicates that the edge of orthorhombic pyroxene crystallized in a shallow acid magma reservoir, and the core part is a “recirculating crystal” from a deep mafic magma reservoir, which is entrained by rising magma into a shallow magma reservoir.

Further studies showed that orthorhombic pyroxene diffusion chronology showed that the orthorhombic pyroxene core experienced a long retention time after entering the shallow magma reservoir, about 600 years. It is also indicated that shallow magma reservoirs have a long continuous active time. 

The team found that the diffusion rate of Ni elements in magnetite is faster, however, there is a significant difference in the content of Ni elements in magnetite in the inclusion body and the host Angyneite, which reflects that the formation of inclusions in shallow acid magma reservoirs by the mafic magma must occur in a very short time – it must be less than 24 hours, thereby suppressing the diffusion equilibrium of Ni elements in magnetite. 

The iron-magnesia inclusion is relatively rich in pores relative to the host Angynesite, and a large number of magnetite-bubble structures are distributed around the stomata, which indicates that the volatile saturated mafic magma is injected into the shallow acid magma reservoir. The injection of high-temperature, volatile-rich mafic magma can reduce the density and viscosity of acidic magma on the one hand, and on the other hand, cause the pressure in the shallow magma reservoir to increase, resulting in the melt in the grain gap being compacted and extruded, and then forming poor speckled crystalline andranite with rhyolite composition. Thus, the last injection of volatile mafic magma within 24 hours prior to the eruption triggered the eruption of a shallow acid magma reservoir.

Model diagram of deep magmatic processes and their timescales in the southern Okinawa Trough of the Fire Dragon 1 submarine volcano Courtesy of the Institute of Oceanography

The first/corresponding author of the paper is Dr. Chen Zuxing, Institute of Oceanology, Chinese Academy of Sciences, and Zeng Zhigang is the co-corresponding author. The research was jointly supported by the National Natural Science Foundation of China, the Strategic Leading Science and Technology Project of the Chinese Academy of Sciences, and the Natural Science Foundation of Shandong Province. (Source: China Science News, Liao Yang, Wang Min)

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