Magnesium and boron isotope evidence for the generation of arc magma through serpentinite mélange melting

Subduction zones serve as a critical link between Earth’s surface and deep layers, playing a vital role in the global cycling of materials. Arc volcanism are widely observed in oceanic subduction zones, but their formation processes and mechanisms remain a topic of significant debate. Previous studies commonly suggest that these rocks are formed by mantle melting triggered by slab-derived fluids. However, this model struggles to explain the correlated isotopic variations in elements such as Sr and Nd observed in these rocks. Some researchers have proposed a sediment-dominated mélange melting model, yet this approach also falls short due to the inherent geochemical characteristics of sediments and their derived fluids, which cannot account for the heavy Mg isotopic signatures of arc volcanic rocks.

“How to find the appropriate method to decode the formation of arc magma? This is one important problem in solid Earth science.” Said Yi-Xiang Chen, the leading author of the paper, who is a professor from the University of Science and Technology of China (USTC) located in Hefei, eastern China.

To tackle this problem, the research team led by Professor Yi-Xiang Chen, focused on volcanic rocks and forearc serpentinites from the South Sandwich Island arc. This arc, located in the South Atlantic between South America and Antarctica, is a globally significant and geologically simple system. Utilizing a novel tracer approach combining magnesium (Mg) and boron (B) isotopes, the team demonstrated the advantages of these isotopes in tracing arc magma formation. Their findings unveiled a new mechanism: the formation of arc volcanic rocks through the melting of serpentinite-dominated mélanges.

The South Sandwich Island arc is an ideal natural laboratory for such studies due to its young age, tectonic simplicity, and minimal continental influence. The study led by Chen's team discovered that volcanic rocks and forearc serpentinites in this region exhibit high δ²⁶Mg and δ¹¹B values. Considering the Mg and B contents and isotopic compositions of mantle and slab-derived fluids, these features cannot be explained by traditional models of slab-fluid metasomatism. Mass balance calculations reveal that less than 3% of fluid mass is sufficient to explain the B isotopic composition of these rocks. However, accounting for their heavy Mg isotopic signatures would require over 60% of fluid addition, a scenario that contradicts established geochemical observations (Figure 1).

“It is unlikely for adding fluid with a mass fraction of 60% into the mantle.” Chen explains. This suggests that some processes other than fluid metasomatism must have played an important role in the formation of island arc. “What about the partial melting of serpentinite-dominated mélanges? The heavy Mg isotopic signatures can be easily explained by incorporation of the serpentinite component.” Chen thought.

Building on their previous research on serpentinites, the team proposed a model involving the diapiric rise and partial melting of serpentinite-dominated mélanges in the shallow mantle wedge. This model effectively explains the coupled heavy Mg-B isotopic signatures of arc volcanic rocks (Figure 2). The mélange is predominantly composed of serpentinites with heavy Mg isotopic compositions, along with minor contributions from sediment or altered oceanic crust (Figure 2). Geochemical simulations further indicate that this model not only aligns with the trace elemental and Sr-Nd isotopic characteristics of SSI magmas but also accounts for their systematically heavy Mg-B isotopic compositions (Figure 1).

“Our result demonstrates that the combined use of Mg-B isotopes not only effectively identifies recycled components in the mantle source of island arcs but also provides new insights into the mechanisms of subduction material recycling.” Chen said.

Although joint studies of Mg-B isotopes in island arc magmatic rocks are still limited, recent data suggests that volcanic rocks from other island arc systems, such as the Lesser Antilles and Mariana arcs, also exhibit heavy Mg-B isotopic compositions. Chen believes that these results indicate that serpentinite-dominated mélange diapiric melting may play a broader role in the formation of arc volcanic rocks globally, although this idea warrants further investigation. If this process indeed significantly contributes to arc volcanic rock formation worldwide, it necessitates a thorough re-examination of volatile cycling in subduction zones and the mechanism of crust-mantle interactions.

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See the article:

Magnesium and boron isotope evidence for the generation of arc magma through serpentinite mélange melting

https://doi.org/10.1093/nsr/nwae363

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