The age and distribution of mantle heterogeneity along the Mid-Atlantic Ridge (31-41 degrees N)

New trace element and isotopic data for basalts from the mid-Atlantic ridge between 31 and 41 degrees N allow a better description of the geochemical gradient south of the Azores triple junction, and the systematics of mantle source heterogeneity, There is a long wavelength enrichment in incompatibl e trace elements and isotopes associated with the Azores hot spot that exte nds from the Kurchatov fracture zone near 41 degrees N to the Hayes fractur e zone near 33 degrees N. Superimposed on this gradient are local spikes of enrichment, the most prominent being the anomaly near the Oceanographer Fr acture Zone (NOFZ). The Oceanographer anomaly spike is reflected modestly i n the morphology of the ridge axis, but is not obviously related to a plume . The isotopic data alone are consistent with involvement of subcontinental material, but the samples do not contain the negative Nb-Ta anomalies whic h are usually associated with the presence of continental material in the m antle source. Away from the prominent enrichment spikes associated with the Azores and Oceanographer fracture zone, there are systematic relationships in this region between parent/daughter element ratios and isotope ratios. The Ph, Sr and Nd isotope systems all give apparent ages in the range 100 M a to 300 Ma, with the age increasing with likely parent/daughter fractionat ion during melting (U/Pb < Rb/Sr < Sm/Nd age). Monte Carlo simulations of a n enrichment event in a depleted heterogeneous mantle at 250 Ma produce res ults that correspond well with the observations for all three isotopic syst ems. Since this age also corresponds to the pre-opening of the North Atlant ic, it raises the possibility that some of the heterogeneity in this region is associated with shallow level mantle heterogeneity resulting from the r ifting of Gondwanaland rather than from interaction with mantle plumes. The data may also reflect a mean mixing time for the heterogeneities in the up per mantle source. Sr isotope systematics reveal correlations in a Sr-87/Sr -86 versus Rb-87/Sr-86 plot, which are geographically controlled. Data poin ts from 10-24 degrees N samples and data points from 31-38 degrees N sample s (excluding NOFZ samples) plot on two offset trends of similar slope. Irre spective of the origin of the isotopic variations, these data require end m ember depleted mantle with distinct isotopic characteristics. Depleted sour ces with low Rb-87/Sr-86 (0.005-0.04) and low (La/Sm)(N) (<0.5), have Sr-87 /Sr-86 values that vary between 0.70215 and 0.7029, Therefore the depleted mantle source of N-MORB is not a homogeneous reservoir, but shows isotopic variations almost as large as the differences between generic depleted mant le (0.7025) and the enriched Atlantic plumes. Creation of a very heterogene ous depleted mantle in terms of isotopic composition needs to be included a s a constraint on models of mantle mixing and convection. (C) 1999 Elsevier Science B.V. All rights reserved.