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Magmas undergo an intense CO2 degassing process, as discovered by a recent study.


Fig. 1 Volatile compositions of matrix glass and metal-containing melt inclusions in the southern Okinawa Basin. Credit: IOCAS

The Okinawa Basin is a newly formed back-arc basin located at the outer edge of the continental shelf of the East China Sea. Submarine hydrothermal activities are widely developed in the Okinawa Basin. A typical feature of the basin is that hydrothermal fluids are very rich in carbon monoxide2.

An increasing number of studies indicate that carbon dioxide2 In hydrothermal fluids may come directly from the discharge of molten gases. However, the CO2 The discharge in the Okinawa basin of magma is poorly understood, and it is not clear whether the magma can release enough carbon dioxide2 in the hydrothermal system.

Recently, Dr Zhang Yuxiang of Professor Zeng Zhigang’s team of the Institute of Oceanography of the Chinese Academy of Sciences (IOCAS), in collaboration with Professor Glenn Gaitani of the Woods Hole Oceanographic Institution, selected volcanic rock samples from areas near the hydrothermal fields in the Basin. Southern Okinawa by analysis of volatile compositions (CO2h2O, S, F, Cl) in metal melt inclusions and matrix glass using an ion probe, and discuss the degassing process.

The study has been published in Lithos On March 23rd.

The researchers found that carbon dioxide2 The contents in the matrix glass and some solubilization impurities were very low (two contents positively correlated with water contents, and CO2 The contents in the matrix glass were even below the automated detection limit (2) which came from basement rock leaching. On the other hand, it indicated that the magma was exposed to carbon dioxide intensely.2 unloading process.

The study reveals an intense degassing of carbon dioxide in the magma

fig … 2 pieces of h2O/Ce versus a B (δ11B). Credit: IOCAS

They estimated that at this level of carbon dioxide2 unloading, 1 km3 of magma can support rising carbon dioxide2 Hydrothermal vents have flowed for more than 500 years. Therefore, from the perspective of magma degassing, this study demonstrates that magma is an important potential source of CO22 hydrothermal system.

The researchers also found that magma in the Okinawa Basin also underwent a certain degree of degassing of S, but the halogens (F and Cl) did not change significantly during magma degassing.

“These understandings can provide research support for the investigation of the physical link between the magmatic system and the hydrothermal system at the subduction zone,” said Dr Chang, first author of the study.

In addition, a melt inclusion analysis was combined with a plagioclase densimeter to estimate the magmatic water content and its variation from the recent arc front to the posterior arc spreading centers. Together with data from the Mariana and Tonga subduction zones and mid-ocean basalts, there is a good positive correlation between H2O/Ce and the boron isotope (δ11b) for igneous rocks in subduction zones.

The isotope of boron is an effective indicator for submerged serpentine, which is very rich in water and is an important source of fluids in the subduction zone. This correlation also indicates that fluids derived from submerged serpentines have an important contribution to the water budget of arc magmas.

more information:
Yuxiang Zhang et al, Pre-eruptive water content and volatile discharge processes in southern Okinawa through magmas: implications for subduction zone water recycling and magmatic contributions to hydrothermal systems, Lithos (2023). DOI: 10.1016/j.lithos.2023.107145

Provided by the Chinese Academy of Sciences

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