The origin of Ni and Mn variations in Hawaiian and MORB olivines and associated basalts

Abstract

The origins of differences in the Mn and Ni contents of olivine phenocrysts from ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are disputed. One prevalent hypothesis is mixing between peridotite melts and melts of an olivine-free pyroxenite that forms by reaction between large extents of melting of recycled ocean crust and peridotite. A second is peridotite melting at various pressures without contributions from recycled materials, and a third is mixing of recycled crust into peridotite sources. All models purport to account for olivine data, but an additional requirement is that the models account for corresponding liquid compositions. Here we demonstrate that none of the proposed models alone are able to account for both olivine and liquid data. The pyroxenite model predicts variations in liquid Mn to account for high Fe/Mn ratios, in contrast to the data where the high ratios result primarily from Fe variations. Using an updated model of Ni partitioning, combined with quantitative modeling of polybaric melting, confirms the important role of pressure on olivine compositions, and in addition shows that extent of melting and mantle fertility are also important. Most MORB are consistent with ∼10% -15% melting of peridotite at lower pressures, but low Ni olivines found on ridges adjacent to hot spots require a depleted mantle source. High Ni olivines can be produced by low degree melting of peridotite at high pressure, but such models do not produce the high Ni liquids that are also observed, nor the trace element abundances such as high TiO2. Mixing basalt and peridotite sources is not successful because it fails to account for the low Na/Ti ratios of OIB. A successful model for OIB involves adding ∼3–5% of a low degree eclogite melt with high trace element abundances and low Na/Ti to mantle peridotite, and then melting this source to small extents at high pressure. The need for a low degree melt of eclogite also rules out models that call upon high degrees of melting of eclogite that react with peridotite to produce OIB sources, and shows that this melting does not occur during upwelling beneath hot spots, where high degree melts of eclogite would occur. A firm conclusion of this work is that olivine compositions cannot be used to calculate the amount of recycled crust in mantle sources. Further work to explore variations within OIB and MORB will benefit from the realization that olivine compositions alone cannot be interpreted reliably without quantitative polybaric melting models and attention to co-existing liquids and trace element abundances.