δ³⁷Cl systematics of a backarc spreading system: The Lau Basin

Abstract

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.