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

Abstract

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 Fe 8; the 230Th excess is positively correlated with Fe 8 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 Fe 8 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 Fe 8, Ti 8 and Ca 8/Al 8 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 Fe 8, Ti 8 and Ca 8/Al 8) 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.