(Table 1) Lead isotopic composition and concentration at DSDP Leg 82 Holes

The aim of DSDP Leg 82 was to decipher the temporal and spatial evolution of Azores Plume. The Pb-isotopic results of this leg are rather complex, and can be summarized as follows: 1. At a given site (561, 558), variations of Pb isotopic compositions are generally accompanied by major changes in trace-element ratios, indicating significant heterogeneity of the source region. There is a correlation between Pb isotopes and trace elements. 2. In contrast, if all the data (i.e., all studied sites) of Leg 82 are considered together, no correlation can be discerned between Pb isotopes and trace elements. Site 556, especially, shows abnormal behavior. 3. Leg 82 samples not only cover the entire range of Pb isotopic composition previously established for the Atlantic Ocean, but extend this field to more radiogenic values. 4. The data are compatible with the hot-spot model proposed by Schilling (1975), if one considers that the Azores Plume itself is isotopically heterogeneous, and that it has been progressively contaminated to various degrees by upper mantle material.