Generation and evolution of magma beneath the East Pacific Rise: Constraints from U-series disequilibrium and plagioclase-hosted melt inclusions

Mid-ocean ridge basalt (MORB) samples from the East Pacific Rise (EPR 12°50'N) were analyzed for U-series isotopes and compositions of plagioclase-hosted melt inclusions. The ²²⁶Ra and ²³⁰Th excesses are negatively correlated; the ²²⁶Ra excess is positively correlated with Mg# and Sm/Nd, and is negatively correlated with La/Sm and Fe₈; the ²³⁰Th excess is positively correlated with Fe₈ and La/Sm and is negatively correlated with Mg# and Sm/Nd. Interpretation of these correlations is critical for understanding the magmatic process. There are two models (the dynamic model and the "two-porosity" model) for interpreting these correlations, however, some crucial parameters used in these models are not ascertained. We propose instead a model to explain the U-series isotopic compositions based on the control of melt density variation. For melting either peridotite or the "marble-cake" mantle, the FeO_t content, ²³⁰Th excess and La/Sm ratio increases and Sm/Nd decreases with increasing pressure. A deep melt will evolve to a higher density and lower Mg# than a shallow melt, the former corresponds to a long residence time, which lowers the ²²⁶Ra excess significantly. This model is supported by the existence of low ²²⁶Ra excesses and high ²³⁰Th excesses in MORBs having a high Fe₈ content and high density. The positive correlation of ²²⁶Ra excess and magma liquidus temperature implies that the shallow melt is cooled less than the deep melt due to its low density and short residence time. The correlations among Fe₈, Ti₈ and Ca₈/Al₈ in plagioclase-hosted melt inclusions further prove that MORBs are formed from melts having a negative correlation in melting depths and degrees. The negative correlation of ²²⁶Ra excess vs. chemical diversity index (standard deviation of Fe₈, Ti₈ and Ca₈/Al₈) of the melt inclusions is in accordance with the influence of a density-controlled magma residence time. We conclude that the magma density variation exerts significant control on residence time and U-series isotopic compositions.