Quantifying Parental MORB Trace Element Compositions from the Eruptive Products of Realistic Magma Chambers: Parental EPR MORB are Depleted

At fast-spreading mid-ocean ridges the existence of a near steady-state axial magma lens indicates that melt differentiation is an open-system process. Field relations in ophiolites and tectonic windows at fast-spreading ridges, together with some chemical characteristics of mid-ocean ridge basalts (MORB) and oceanic plutonic rocks, indicate that assimilation is a common process in and around the axial magma lens. Magma and mush zone mixing and mingling is indicated by the petrology of MORB and oceanic plutonic rocks; mush disaggregation provides an efficient mechanism for return of interstitial melt to an eruptible reservoir-a form of in situ crystallization. Despite such copious evidence to the contrary, MORB differentiation is generally modeled assuming perfect fractional crystallization (Rayleigh distillation). Here we present a simple open-system model for MORB differentiation that includes assimilation and in situ crystallization that can be used to generate synthetic basalt datasets to compare with natural sample suites. Inversion of the model allows the parental melt compositions to be estimated quantitatively. We use a numerical Bayesian inversion scheme to determine the parental melt compositions for three large (>150 samples in each) normal-MORB suites from the East Pacific Rise. The parental melt compositions determined this way differ significantly from those that would be calculated assuming closed-system fractional crystallization. Parental MORB are more depleted than commonly assumed, suggesting that the upper mantle is more depleted than generally believed and/or that the extent of melting is larger (for example, with melts poorly focused to the ridge axis). The more depleted character of parental than erupted melts has important implications for using basalt trace element systematics in chemical geodynamic models. For example, the Sm/Nd of parental MORB are significantly lower than those of erupted MORB and this needs considering in models of the Nd-isotope evolution of the mantle.