delta (super 37) Cl systematics of a backarc spreading system; the Lau Basin

Determinations of delta (super 37) Cl 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 delta (super 37) Cl to near zero values. In contrast, low-Cl glasses from the Mangatolu triple junction have distinctly negative delta (super 37) Cl, compatible with recent estimates for delta (super 37) Cl of the depleted mantle source. Along the Valu Fa Ridge, subduction input manifests as delta (super 37) Cl 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.