CORRELATIONS OF MIDOCEAN RIDGE BASALT CHEMISTRY WITH THE GEOID

The major-element composition of Mid-Ocean Ridge basalts (MORE) shows geochemical variations controlled by the temperature, pressure and sou rce composition produced by convection in the mantle beneath the sprea ding centers. The long-wavelength signal of the geoid reflects deep ma ntle processes. In this paper, we characterize the correlations betwee n the major-element chemistry of MORE and the geoid anomalies at diffe rent wavelengths to determine the extent to which the major-element co mposition of the MORE reflects deep mantle circulation. The chemical e ffects of the low-pressure fractionation have been corrected using a n ew method based on the FeO1/MgO ratio. The fractionation trends modele d by a linear trend in the geochemical parameter space normalized by t he MgO denominator converge to a common point with similar to 8% MgO. The separation vector d between the fractionation trends and this comm on point defines a new parameter whose sodium component shows a negati ve correlation significant at the 99% level with the axial depth for b oth the Mid-Atlantic Ridge (MAR) and the East Pacific Ridge (EPR) and with the geoid only for the MAR. The latter correlation that the major -element composition of the MORE is clearly related to deep mantle con vection by material and thermal coupling between the lower and upper m antle, The lack of correlation at long or medium wavelengths between t he mantle enrichment inferred from the K2O/TiO2 ratio and the geoid in dicates that either source composition does not influence MORE chemist ry or that hotspot material is injected along relatively narrow condui ts with little interaction with the surrounding upper mantle, (C) 1997 Elsevier Science B.V.