We present major- and trace-element data on the lower sheeted dike complex (SDC) drilled during ODP Leg 148 in Hole 504B. Similar to previously studied rocks from the upper sections of Hole 504B, the rocks recovered during Leg 148 are mildly to moderately evolved mid-ocean ridge basalts (MORB) ranging in their MgO concentrations from 7.8 to 10.0 wt%. The rocks exhibit the same sort of depletion as the lavas of Hole 504B, characteristically by very low incompatible element abundances (e.g., TiO2 = 0.54–1.03 wt%, Zr = 29–53 ppm, and Ce = 2.2–5.5 ppm). Chondrite-normalized La/Sm and Nb/Zr rations are markedly lower than average N-MORB, suggestion a highly depleted mantle source. Fractional crystallizations of plagioclase, olivine, ± clinopyroxene can account for most of the chemical variability, whereby the degree of fractionation is less than 50%. The chemical effects of alteration are more difficult to distinguish because they can sometimes have the same consequences as magmatic differentiation. Formation of chlorite, amphibole, albite-olgoclase, and titanite replacing groundmass and primary minerals are typical for all samples. The results of this type of alteration are slight increases of MgO and H2O and highly depleted in Cu (Zn) and S. This make the patches in the lower SDC possible sources for base-metal deposits in the oceanic crust. The patches are also significantly depleted in REE, Zr, Nb, Ti, and exhibit positive Eu anomalies. Theoretical considerations and mass-balance calculations indicate that this behavior is probably not only the consequence of intensive alteration but requires primary chemical differences between the patches and the neighboring diabases. It remains unclear how far such differences could be related to magmatic differentiation processes within the dikes. The puzzling question of the nature of these alteration patches deserves further attention.
Date of initial receipt: 22 August 1994
Date of acceptance: 15 February 1995
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