Abstract
A number of oceanic island basalts display isotopic characteristics which fall below the so-called Nd-Sr mantle array. The most extreme (lowest Nd) samples from each suite form a linear alignment in Sr-Nd-Pb-space; this low Nd array, termed here the LoNd array, is defined by basalts from Tubuaii, St. Helena, New England Seamounts, Comores, San Felix and Walvis Ridge. Intra-island isotopic variations for these suites are not colinear with the array but initiate on the array and trend towards the “mantle plane” of Zindler et al. (1982) at high angles. This suggests that mixing between the two mantle reservoirs which are endmembers of the LoNd array (Tubuaii-Walvis) precedes the mixing with reservoirs closer to the “mantle plane” that produces the intra-island variations. Full elucidation of such a mantle mixing “chronology” can provide essential constraints on the location of various mantle reservoirs. If further study substantiates the LoNd array, we argue that the endmember reservoirs of this array are most probably sited in the subcontinental lithosphere, and are introduced into the oceanic mantle circulation by delamination of this lithosphere.
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