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The DSDP/ODP Hole 504B,drilled in the 5.9 Ma southern flank of the Costa Rica Rift,represents the deepest section through modern ocean floor basaltic basement.The hole penetrates a 570 m thick volcanic zone,a 210 m thick transition zone of volcanic rocks and dykes,and 1 056 m of dykes.A representative selection of these basalt types has been investigated with respect to Nd and Pb isotopes.The ε 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 shows significant variation,with 206Pb/ 204Pb varying from 17.90 to 18.82.The isotopic data show that a small volume of enriched mantle existed in the source.The large ranges in isotopic composition in a single drill hole demonstrate the importance of small-scale mantle heterogeneities in the petrogenesis of MORB.Fractional melting and extraction of small magma batches by channelled flow,and small,short-lived crustal magma reservoirs,with limited potential for mixing of the mantle derived magmas,are favored by these isotopic data.
The DSDP / ODP Hole 504B, drilled in the 5.9 Ma southern flank of the Costa Rica Rift, represents the deepest section through modern ocean floor basaltic basement. Hole penetration a 570 m thick volcanic zone, a 210 m thick transition zone of volcanic rocks and dykes, and 1 056 m of dykes. A representative selection of these basalt types has been investigated with respect to Nd and Pb isotopes. The ε 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 shows significant variation, with 206Pb / 204Pb varying from 17.90 to 18.82. The isotopic data show that a small volume of enriched mantle existed in the source. in isotopic composition in a single drill hole demonstrate the importance of small-scale mantle heterogeneities in the petrogenesis of MORB. Fractional melting and extraction of small magma batches by channelled flow, and small, short -lived crustal magma reservoirs, with limited potential for mixing of the mantle derived magmas, are favored by these isotopic data.