Nd- and Pb-isotopic variations through the upper oceanic crust in DSDP/ODPHole 504B, Costa Rica Rift

The DSDP/ODP Hole 504B, drilled in the 5.9 Ma southern flank of the Costa R ica Rift, represents the deepest section through modern ocean floor basalti c basement. The hole penetrates a 570 m thick volcanic zone, a 210 m thick transition zone of volcanic rocks and dykes, and 1056 m of dykes. A represe ntative selection of these basalt types has been investigated with respect to Nd and Pb isotopes. The epsilon (Nd) Of the basalts varies from 7.62 to 11.16. This range in the Nd-isotope composition represents about 67% of the total range reported for Pacific MORB. The Pb-isotope composition also sho ws significant variation, with (206)pb/Pb-204 varying from 17.90 to 18.82. The isotopic data show that a small volume of enriched mantle existed in th e source. The large ranges in isotopic composition in a single drill hole d emonstrate the importance of small-scale mantle heterogeneities in the petr ogenesis of MORB. Fractional melting and extraction of small magma batches by channelled flow, and small, short-lived crustal magma reservoirs, with l imited potential for mixing of the mantle derived magmas, are favored by th ese isotopic data. (C) 2001 Elsevier Science B.V. All rights reserved.