Determinations of δ37Cl in glasses and melt inclusions from backarc basin basalts of the Lau Basin (southern Pacific Ocean) constrain the effects of mantle source, subduction, and shallow assimilation. In sections of the Lau Basin not heavily affected by subduction input (central and eastern Lau spreading centers), Cl derived from unfractionated seawater via assimilation in shallow crustal environments is the predominant source of “excess Cl,” and moderates δ37Cl to near zero values. In contrast, low-Cl glasses from the Mangatolu triple junction have distinctly negative δ37Cl, compatible with recent estimates for δ37Cl of the depleted mantle source. Along the Valu Fa Ridge, subduction input manifests as δ37Cl that is variable but dispersed toward more negative values in both lavas and melt inclusions. These values are compatible with a second source of lighter Cl within a slab-derived flux derived from altered ocean crust, sediments, and/or serpentinized mantle wedge.

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