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The basalts recovered from the Costa Rica Rift by drilling at Deep Sea Drilling Project Sites 501, 504, and 505 during Legs 68, 69 and 70 of the Glomar Challenger are the most depleted in the most-hygromagmaphile elements (Th, Ta, Nb, and La) of all MORB recovered to date by the Glomar Challenger. The invariant ratios Nb/Ta, Zr/Hf, and Y/Tb show "chondritic values" (expected for Nb/Ta because of the very low concentrations in these elements). Four samples from a single unit are exceptions: they present a flat to slightly enriched, extended Coryell-Masuda plot, and at the same time their La/Ta ratio is 9 (normalized ratio = 1) instead of 19 (normalized ratio = 2), the value for all other samples. Only one of these two values of the La/Ta ratio had been found so far within a single hole, and moreover within large areas of the oceanic crust (several holes or dredges). The present result shows that local heterogeneity of the upper mantle with respect to the La/Ta ratio may exist.

Costa Rica Rift: Variably Depleted Basalts in the Same Hole