A new study has shed light on the mechanisms of material transfer beneath the South Sandwich Islands arc, focusing on how the processes of mélange dehydration and melting influence the geochemical signatures of arc lavas. Researchers suggest this unique setting could provide answers to longstanding questions about the transfer of materials from subducted slabs to volcanic arcs.
Subduction zones, which serve as portals through which Earth's surface materials are transported to its deep interior, are known for their significant role in shaping both geological and climatic processes. The South Sandwich Islands arc, located northeast of the Antarctic Peninsula, has been identified as particularly conducive to studying these processes due to its unique geological characteristics.
The research elucidates the significant differences between two competing models: one positing fluid-driven metasomatic processes at the slab-mantle interface, and the other focusing on the involvement of mélanges—composite materials of subducted sediments and altered oceanic crust. By analyzing barium (Ba) isotope variations alongside strontium (Sr) ratios, the study offers insights on the origins of the materials contributing to arc magma formation.
The findings reveal substantial barium isotope variations among the lavas from the South Sandwich Islands, which are accompanied by invariant strontium ratios, indicating the significance of mélange mobilization beneath the arc. Notably, northern arc lavas exhibited greater variations than anticipated from the direct influences of subducted materials, pointing to barium isotope fractionation processes during transport.
Marine barites, which constitute the primary barium-bearing sediments involved, display lighter barium isotopic signatures, likely due to fractionation occurring during precipitation from surface seawater. This study establishes how the composition of these sediments, along with their interactions with underlying mélange rock, governs the Ba signatures observed in the lavas.
Importantly, sediment contributions to the Ba budgets of these arc lavas are amplified due to their high concentrations relative to altered oceanic crust, which suggests they are the dominant source of barium. The research indicates systematic variations among isotope compositions, likely influenced by differing melting processes and the nature of sedimentary influx beneath the northern and southern segments of the arc.
According to the authors, the results support the notion of mélange dehydration and melting as key processes affecting the isotopic and trace element compositions of arc lavas, with significant ramifications for our knowledge of subduction processes. Future research directions may include more elaborate and broader assessments across different subduction zones to refine our models of material transfer and the geological impacts of arcs.