DSDP/ODP Hole 504B, located in the Eastern Equatorial Pacific in 5.9 Ma old crust, penetrates both basaltic pillows and dykes down to a depth of 1562.1 m below the sea floor. Core recovery is poor, averaging 230ut falling to a 12% average in the deepest sections. Geochemical logs give a continuous measure of elemental (Si, Ca, Fe, S, Ti, Gd, H, Cl, K, Th, U) abundances. Correlation between laboratory (XRF) geochemical data and log data is difficult because of the lack of accurate depth information for the cores. Pattern recognition techniques are of little help for locating sample depths in the uniform basaltic lithologies. Non-hierarchical clustering techniques prove the existence of relationships between core descriptions and gross geochemical variations; major lithostratigraphic zones are readily identified from the log-derived data.

Attempts to interpret the geochemistry of the oceanic crust are constrained by the representative nature, quality, and amount of material recovered from an individual site. The most effective sampling method has been that of continuous coring while drilling, although the amount of material recovered from a single hole can vary with depth from 0 to 100%, and is often, though not necessarily, low. Observations suggest there is inevitably a biasing towards the recovery of unaltered material. DSDP/ODP Hole 504B has been successfully drilled in a series of stages and a 1500 m section of the oceanic crust has been sampled. The core recovery in Hole 504B is, however, generally poor; consequently the study of both igneous and alteration processes at this site is limited. An alternative to this scenario is offered by down-hole geochemical (nuclear) measurements. The nuclear responses provide a limited but near-continuous geochemical dataset within this hole. The derived chemo-stratigraphy correlates well with that based upon conventional geophysical measurements. Alteration has played an important role in producing the observed chemical signatures and may partly explain the poor core recovery within the hole. Downhole nuclear measurements in the ocean crust provide a novel geochemical dataset in the study of both primary and secondary processes. This information must be integrated with lithological, petrological and geophysical data to produce the most complete appraisal yet of the oceanic crust.

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