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Sr and Nd isotopic composition of 23 basalts from Sites 556-559 and 561-564. are reported. The Sr/Sr ratios in fresh glasses and leached whole rocks range from 0.7025 to 0.7034 and are negatively correlated with the initial Nd/ Nd compositions, which range from 0.51315 to 0.51289. The Sr and Nd isotopic compositions (in glasses or leached samples) lie within the fields of mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) from the Azores on the Nd-Sr mantle array/fan plot. In general, there is a correlation between the trace element characteristics and the Nd/Nd composition (i.e., samples with Hf/Ta > 7 and (Ce/Sm)N < 1 [normal-MORB] have initial Nd/Nd > 0.51307, whereas samples with Hf/Ta < 7 and (Ce/Sm)N > 1 (enriched-MORB) have initial Nd/Nd compositions < 0.51300). A significant deviation from this general rule is found in Hole 558, where the N-MORB can have, within experimental limits, identical isotopic compositions to those found in associated E-MORB. The plume-depleted asthenosphere mixing hypothesis of Schilling (1975), White and Schilling (1978) and Schilling et al. (1977) provides a framework within which the present data can be evaluated. Given the distribution and possible origins of the chemical and isotopic heterogeneity observed in Leg 82 basalts, and some other basalts in the area, it would appear that the Schilling et al. model is not entirely satisfactory. In particular, it can be shown that trace element data may incorrectly estimate the plume component and more localized mantle heterogeneity (both chemical and isotopic) may be important.

Isotopic and Trace Element Composition of Basalts from Sites 556-559 and 561-564: Constraints on Some Processes Affecting Their Composition