Do along-strike variations in volatile cycling modulate the evolution of back-arc spreading?

Back-arc spreading, tied to the evolution of progressive subduction, records the spatial expression of three-dimensional subduction dynamics. Mantle flow exerts a dominant control on its development; however, the underlying mechanisms driving spatial variability in back-arc spreading extent remain unclear. The opening of the Japan Sea provides an example of evolved back-arc spreading that is driven by mantle flow. We present geochemical data of back-arc basin basalts (BABBs) from Ocean Drilling Program Sites 794 and 797 in the Japan Sea. These sites are located on two profiles characterized by contrasting back-arc spreading extents. Variations in Nb/Zr and 143Nd/144Nd in samples from the two sites reveal contrasts in spreading extent modulated by mantle flow. In addition, Mg isotopic variations reveal the lateral heterogeneity in volatile cycling, and are observed concurrently with along-strike variations in back-arc extension. BABBs from the northern site in limited-spreading profile exhibit extremely high δ26Mg values (−0.30‰ to +0.34‰), suggesting contributions of water-dominated fluids derived from serpentinite. In contrast, samples from the central site in greater-spreading profile show relatively low δ26Mg values (−0.57‰ to +0.10‰), primarily reflecting the involvement of deep subducted carbonates. The marked differences in how water and carbon modulate mantle viscosity as well as weaken the overlying mantle wedge, thereby regulating mantle rheology, are inferred to have the potential to contribute to the striking contrast in the extent of back-arc spreading. Accordingly, distinct volatile cycling processes, yet often overlooked, need to be considered in interpreting at least some of the curved back-arc extension observed in evolved subduction zones.