Crustal Construction Along the Eastern Lau Spreading Center: Mantle Water, Magmatic Differentiation, and a Compositionally Zoned Basin

Abstract:

The Eastern Lau Spreading Center exhibits variations in mantle source composition, melt supply, and crustal characteristics consistent with a decreasing influence of slab volatiles on mantle melting with distance from the active Tofua arc. A series of recent geophysical and geochemical studies show that over the past 4 Myr the spreading center has formed distinct zones of oceanic crust as the ridge migrates away from the arc and its underlying mantle melting region. Recent seismic tomographic images reveal that a fundamentally distinct type of oceanic crust forms along back-arc spreading centers located near (<50 km) the active arc. This crust is anomalously thick (8–9 km) and seismically layered, and by inference chemically differentiated, in a manner not observed in crust produced at mid-ocean ridges. Lava samples from this region are enriched in subduction-related components, including water, and have compositions that extend to very low (~1 wt%) MgO contents, indicating erupted lavas have experienced considerably more crystal fractionation on average than typical mid-ocean ridge basalts. Further from the arc, crust produced at the spreading center abruptly transitions (within ~5 km) to a more normal thickness (~6 km) and seismic structure as compared to oceanic crust formed at intermediate-rate mid-ocean ridges. Here, lavas primarily consist of less evolved basalts (~6-8+ wt% MgO) with relatively low arc-like enrichments. Our work suggests a model for the formation of arc-proximal, chemically-stratified oceanic crust that is counter to other published interpretations of stratified crust detected in other back-arc systems: water from the downgoing slab is entrained in the melting zone beneath the ridges where it enhances melting and crustal production. Thereafter, water in the primary magmas suppresses plagioclase crystallization relative to olivine and clinopyroxene, creating an unusually mafic lower crust; residual low-viscosity melts buoyantly rise upward to form a more felsic, porous upper crust. Testing this model, and advancing our understanding of magmatic processes along water-rich spreading centers, is a primary focus of future research.