Correlations of Mid-Ocean Ridge Basalt chemistry with the geoid

The major-element composition of Mid-Ocean Ridge basalts (MORB) shows geochemical variations controlled by the temperature, pressure and source composition produced by convection in the mantle beneath the spreading centers. The long-wavelength signal of the geoid reflects deep mantle processes. In this paper, we characterize the correlations between the major-element chemistry of MORB and the geoid anomalies at different wavelengths to determine the extent to which the major-element composition of the MORB reflects deep mantle circulation. The chemical effects of the low-pressure fractionation have been corrected using a new method based on the FeOt/MgO ratio. The fractionation trends modeled by a linear trend in the geochemical parameter space normalized by the MgO denominator converge to a common point with ∼8% MgO. The separation vector d between the fractionation trends and this common point defines a new parameter whose sodium component shows a negative correlation significant at the 99% level with the axial depth for both the Mid-Atlantic Ridge (MAR) and the East Pacific Ridge (EPR) and with the geoid only for the MAR. The latter correlation suggests that the major-element composition of the MORB is clearly related to deep mantle convection by material and thermal coupling between the lower and upper mantle. The lack of correlation at long or medium wavelengths between the mantle enrichment inferred from the K2O/TiO2 ratio and the geoid indicates that either source composition does not influence MORB chemistry or that hotspot material is injected along relatively narrow conduits with little interaction with the surrounding upper mantle.