On the composition of ocean island basalts (OIB): The effects of lithospheric thickness variation and mantle metasomatism

We have examined island-averaged geochemical data for 115 volcanic islands with known eruption ages and ages of the underlain lithosphere from the Pacific, Atlantic and Indian Oceans. These age data allow calculation of the lithosphere thickness at the time of volcanism. After correcting the basalts (including alkalic types) (< 53% SiO2) for fractionation effect to Mg# = 0.72, we found that the island-averaged Si72 and Al72 decrease whereas Fe72, Mg72, Ti72 and P72 increase with increasing lithosphere thickness. The island-averaged [La/Sm]CN and [Sm/Yb]CN ratios also increase with increasing lithosphere thickness. These statistically significant trends are most consistent with the interpretation that the mean extent of melting decreases whereas the mean pressure of melting increases with increasing lithosphere thickness. This is physically consistent with the active role the lithosphere plays in limiting the final depth of intra-oceanic mantle melting. That is, beneath a thin lithosphere, a parcel of mantle rises to a shallow level, and thus melts more by decompression with the aggregated melt having the property of high extent and low pressure of melting. By contrast, a parcel of mantle beneath a thick lithosphere has restricted amount of upwelling, and thus melts less by decompression with the aggregated melt having the property of low extent and high pressure of melting. This demonstrates that oceanic lithosphere thickness variation exerts the first-order control on the geochemistry of ocean island basalts (OIB). Variation in initial depth of melting as a result of fertile mantle compositional variation and mantle potential temperature variation can influence OIB compositions, but these two variables must have secondary effects because they do not overshadow the effect of lithosphere thickness variation that is prominent on a global scale. The mantle potential temperature variation beneath ocean islands cannot be constrained with the existing data. Fertile mantle source heterogeneity is required to explain the large OIB compositional variation on a given island, between islands and between island groups. The OIB mantle source heterogeneity must have multiple origins, but an incipient melt in the seismic low-velocity zone and its metasomatic lithologies in the lithosphere are best candidates that contribute to the incompatible element enriched OIB geochemistry on two different time scales: (1) melt–lithosphere interaction during OIB magmatism, and (2) recycled metasomatized lithosphere in the OIB source regions.