In a groundbreaking study published in "National Science Review," researchers led by Professor Yi-Xiang Chen from the University of Science and Technology of China (USTC) have unveiled significant insights into the formation processes of arc magma through an innovative approach combining magnesium (Mg) and boron (B) isotopes. The study focuses on volcanic rocks and forearc serpentinites found in the South Sandwich Island arc, positioned in the South Atlantic Ocean, a region characterized by its geological simplicity and youth.
Subduction zones are pivotal in connecting Earth’s surface to its deep interior, playing a crucial role in the global recycling of materials. Arc volcanism, commonly observed in oceanic subduction zones, has long intrigued scientists due to the complex processes involved in the genesis of these rocks. Traditional models predominantly attribute the formation of arc volcanism to melting mechanisms driven by fluids released from subducting slabs. However, these models often fail to account for the observable isotopic variations in strontium (Sr) and neodymium (Nd) within the volcanic products, leading to persistent debates within the geoscience community.
Professor Chen articulated the challenges faced by researchers in decoding the intricacies of arc magma formation, stating, “How to find the appropriate method to decode the formation of arc magma? This is one important problem in solid Earth science.” This vivid assertion underscores the ongoing quest to unravel the mechanisms underlying subduction-related volcanic activity.
In their study, Chen’s team employed a unique tracer methodology that harnessed the power of Mg and B isotopes. Through this approach, the research team was able to demonstrate the utility of these isotopes in tracing the formation of arc magmas, leading to the proposal of a novel melting mechanism driven by the partial melting of serpentinite-dominated mélanges. These mélange formations arise from the interaction of serpentinite and other materials at subduction zones, a process previously overlooked in arc magmatism studies.
One of the critical findings from this study is the revelation that volcanic rocks and forearc serpentinites from the South Sandwich Island arc exhibit elevated values of δ^26Mg and δ^11B. The presence of these isotopic signatures calls into question the validity of established models concerning slab-derived fluid metasomatism. Notably, the research determined that a minimal fluid mass, less than 3%, is theoretically sufficient to explain the boron isotopic composition exhibited by the arc volcanic rocks. However, attempting to reconcile the observed heavy Mg isotopic signatures requires a fluid contribution exceeding 60%, a claim that contradicts accepted geochemical observations.
Professor Chen elaborated on the improbability of such significant fluid fluxes, asserting, “It is unlikely for adding fluid with a mass fraction of 60% into the mantle.” This observation suggests that alternative mechanisms are at play in the formation of island arcs, urging researchers to reconsider the role of fluid metasomatism in these geological processes. Chen hypothesizes that the partial melting of serpentinite-dominated mélanges could account for the heavy Mg isotopic signatures detected in the volcanic rocks.
To further substantiate their claims, the research team proposed a model involving the diapiric ascent and subsequent partial melting of serpentinite-dominated mélanges located within the shallow mantle wedge. This innovative model provides a comprehensive explanation for the coupled heavy Mg and B isotopic signatures manifesting in the arc volcanic rocks. The composition of the mélange is intrinsically comprised of serpentinites exhibiting heavy Mg isotopic values, complemented by lesser quantities of sediments or altered oceanic crust.
The implications of this research are profound, as the geochemical simulations conducted confirm that the proposed model aligns with the trace elemental and isotopic characteristics of the magmas derived from the South Sandwich Island arc. In doing so, it also accounts for the systematic heavy Mg-B isotopic compositions observed in the region, which were previously challenging to explain through existing paradigms.
Professor Chen emphasized the broader implications of their findings, stating, “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.” This statement reflects the potential for this study to redefine existing paradigms in subduction zone dynamics and arc volcanism.
While the study notably sheds light on the South Sandwich Island arc, its findings resonate across other volcanic systems. Recent data indicates that volcanic rocks from different island arcs, including the Lesser Antilles and Mariana regions, exhibit similarly heavy Mg-B isotopic signatures. According to Chen, this consistency suggests that serpentinite-dominated mélange diapiric melting could serve as a prevalent mechanism in the formation of arc volcanic rocks on a global scale, warranting further scientific inquiry.
In conclusion, this enlightening study opens the door to a potential paradigm shift in understanding arc volcanism. If the proposed mechanisms of serpentinite mélange melting gain traction, it will necessitate a critical re-evaluation of the dynamics governing volatile cycles in subduction zones and the intricate interactions between the crust and mantle. Such insights could pave the way for future research endeavors aimed at unraveling the complexities of our planet’s geological processes.
Subject of Research: Magma Generation in Arc Settings
Article Title: Magnesium and boron isotope evidence for the generation of arc magma through serpentinite mélange melting
News Publication Date: October 2023
Web References: National Science Review
References: National Science Review, DOI: 10.1093/nsr/nwae363
Image Credits: ©Science China Press
Keywords: subduction zones, arc volcanism, magnesium isotopes, boron isotopes, serpentinite mélange, volcanic rocks, geochemistry, Earth’s crust, mantle processes, arc magma formation.
