Petrogenesis of Oceanic Crust at Back-Arc Spreading Centers: Modeling the Effects of Slab-Derived Water on Crustal Accretion in the Lau Basin

Abstract:

Oceanic crust formed along spreading centers in the Lau back-arc basin exhibits a dramatic change in crustal structure and composition with proximity to the nearby Tofua Arc. Results from seismic studies indicate that crust formed near the arc is abnormally thick (~8-9 km) and compositionally stratified, with a thick low-velocity upper crust and an abnormally high-velocity (7.2-7.4+ km/s) lower crust. Lava samples from this area show arc-like compositional enrichments and tend to be more evolved than typical mid-ocean ridge basalts (MgO contents as low as ~1 wt%). We propose that slab-derived water entrained in the near-arc ridge system not only enhances mantle melting, as commonly proposed to explain high crustal production in back-arc environments, but also affects magmatic differentiation and crustal accretion processes. Phase equilibria modeling of the observed compositional trends suggests that the high water contents found in near-arc parental melts can lead to crystallization of an unusually mafic cumulate layer by suppressing plagioclase crystallization relative to olivine and clinopyroxene. Model runs with ~1-2 wt% H₂O in the parental melts successfully reproduce the geochemical trends of the erupted lavas while crystallizing a cumulate assemblage with calculated seismic velocities consistent with those observed in the near-arc lower crust. The resulting residual melts are also water-rich, which lowers their density and aids in the extraction and eventual eruption of unusually evolved magmas. We present preliminary results of this petrological model for the formation of back-arc oceanic crust, which successfully predicts the unusual crustal stratification observed in the near-arc regions of the Lau basin, and helps explain the highly fractionated andesites and dacites that erupt there. We also comment on alternative proposed hypotheses for back-arc crustal accretion and discuss some of the challenges facing them in this particular region.