A hydrous melting and fractionation model for mid‐ocean ridge basalts: Application to the Mid‐Atlantic Ridge near the Azores

Abstract

The major element, trace element, and isotopic composition of mid‐ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H 2 O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge‐hotspot interaction therefore requires a comprehensive model that accounts for the effect of H 2 O in the source on melting behavior and for the effect of H 2 O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO 2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ∼700 ppm H 2 O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H 2 O‐rich melting regimes. An on‐axis thermal anomaly of ∼35°C in potential temperature explains the full crustal thickness increase of ∼4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO 2, trace element and isotopic composition of the H 2 O‐rich component in a way that self‐consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.