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

Abstract:

New trace element and isotopic data for basalts from the mid-Atlantic ridge between 31 and 41°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 incompatible trace elements and isotopes associated with the Azores hot spot that extends from the Kurchatov fracture zone near 41°N to the Hayes fracture zone near 33°N. Superimposed on this gradient are local spikes of enrichment, the most prominent being the anomaly near the Oceanographer Fracture Zone (NOFZ). The Oceanographer anomaly spike is reflected modestly in 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 which are usually associated with the presence of continental material in the mantle 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 Pb, Sr and Nd isotope systems all give apparent ages in the range 100 Ma to 300 Ma, with the age increasing with likely parent/daughter fractionation during melting (U/Pb < Rb/Sr < Sm/Nd age). Monte Carlo simulations of an enrichment event in a depleted heterogeneous mantle at 250 Ma produce results that correspond well with the observations for all three isotopic systems. Since this age also corresponds to the pre-opening of the North Atlantic, it raises the possibility that some of the heterogeneity in this region is associated with shallow level mantle heterogeneity resulting from the rifting of Gondwanaland rather than from interaction with mantle plumes. The data may also reflect a mean mixing time for the heterogeneities in the upper mantle source. Sr isotope systematics reveal correlations in a 87Sr/86Sr versus 87Rb/86Sr plot, which are geographically controlled. Data points from 10–24°N samples and data points from 31–38°N samples (excluding NOFZ samples) plot on two offset trends of similar slope. Irrespective of the origin of the isotopic variations, these data require end member depleted mantle with distinct isotopic characteristics. Depleted sources with low 87Rb/86Sr (0.005–0.04) and low (La/Sm)N (<0.5), have 87Sr/86Sr 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 mantle (0.7025) and the enriched Atlantic plumes. Creation of a very heterogeneous depleted mantle in terms of isotopic composition needs to be included as a constraint on models of mantle mixing and convection.

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