Fine-scale isotopic heterogeneity in the sub-Atlantic mantle

The isotopic composition of Sr and Nd in oceanic basalts reflects the character and chemical evolution of their source regions with respect to Rb/Sr and Sm/Nd. Combined studies of the Sr, Nd and Pb isotopic systems in mid-oceanic ridge basalts (MORB)^1,2 have confirmed previously recognized heterogeneities on a relatively large scale for the oceanic mantle. These heterogeneities have been ascribed to both long- and short-lived mantle processes and to mixing between enriched and depleted mantle sources (for example, see refs 1-4). However, data for the mid-Atlantic ridge (MAR) remote from known hot spots are available for only 14 samples of which only 3 are basaltic glass, the most 'reliable' sample type. Thus, attempts to explain MORB mantle heterogeneities are founded on limited data and remain largely speculative. The extent of variation in isotopic composition of 'normal MORB' over a limited area of the MAR is critical to any isotopic model for the oceanic mantle. Using high resolution seafloor mapping and sampling techniques, we can evaluate isotopic composition on a fine scale both within a ridge segment and across the fracture zones terminating that segment. We report here studies of a series of basaltic glass samples from the MAR intersection with the Kane Fracture Zone located at 22°-24° N (Fig. 1) which have implications for the dynamics of mantle melt segregation and the role of a major transform in mantle convection.