Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 137/14030. DATA REPORT: GEOCHEMICAL LOGGING RESULTS FROM THE EASTERNEQUATORIAL PACIFIC: HOLE 504B1Elizabeth Lewis Pratson,2 Cristina Broglia,2 Philippe Pezard,3 and Peter K.H. Harvey4ABSTRACTGeochemical well logs were obtained through sediment and basalt at Site 504 of Leg 140. Corrections have been applied tothe logs to account for variations in borehole size, drilling-fluid composition, and drill-pipe attenuation. Concentrations of Th, U,and Gd, as well as oxide weight percentages, have been calculated from the logs and compared with available X-ray-fluorescence(XRF) core measurements. The geochemical processing was performed in the open hole sections of Hole 504B only. The compar-ison of XRF core data to log data is good.INTRODUCTIONThe primary objective of Leg 140 was to deepen the previouslyexisting Hole 504B into the sheeted dike complex (Fig. 1) (ShipboardScientific Party, 1992). The hole was logged with the geochemicallogging tool string (GLT), which measures the major elements of aformation at intervals of 0.1524 m. The GLT measurements providecontinuous, in-situ chemical measurements of the rock and are notaffected by incomplete core recovery or core expansion, a commonproblem in core-based studies.This report describes the basic principles of the GLT and outlinesthe post-cruise processing techniques. It then briefly compares geo-chemical log values with lithologic core descriptions and availablecore measurements.GEOCHEMICAL TOOL STRINGThe geochemical logging tool string (GLT) consists of four toolcomponents: the natural gamma-ray tool (NGT), the compensatedneutron tool (CNT), the aluminum activation clay tool (AACT), andthe gamma-ray spectrometry tool (GST). (GLT, NGT, CNT, AACT,and GST are trademarks of Schlumberger; Fig. 2.) The combination ofthe tool components uses three separate modes of gamma-ray spectros-copy for a comprehensive elemental analysis of the formation. TheNGT, located at the top of the tool string, measures the naturally occur-ring radionuclides, thorium (Th), uranium (U), and potassium (K),before the formation is irradiated by the nuclear sources contained inthe tools below. The CNT, located below the NGT, carries a low-energycalifornium-252 (252Cf) neutron source to activate the Al atoms in theformation. The AACT, a modified NGT, is located below the 252Cfsource, measuring the activated gamma rays in the formation. Bycombining the AACT measurement with the previous NGT measure-ment, the background radiation is eliminated, and a reading of forma-tion Al is obtained (Scott and Smith, 1973). The GST, at the base of thestring, carries a pulsed neutron generator to induce prompt-capturegamma-ray reactions in the borehole and formation and a Nal(Tl)scintillation detector to measure the energy spectrum of gamma raysgenerated by the neutron capture reactions. Because each of the ele-ments in the formation is characterized by a unique spectral signature,1 Erzinger, J., Becker, K., Dick, H.J.B., and Stokking, L.B. (Eds.), 1995. Proc. ODP,Sci. Results, 137/140: College Station, TX (Ocean Drilling Program).2 Borehole Research Group, Lamont-Doherty Earth Observatory of Columbia Uni-versity, Palisades, NY 10964, U.S.A.3 Institut Méditerranéen de Technologic Technopole de Chateau-Gombert, 13451Marseille Cedex 13, France.4 Borehole Research, Department of Geology, University of Leicester, Leicester LEI7RH, United Kingdom.it is possible to derive the contribution (or yield) of each of the majorelements silicon (Si), iron (Fe), calcium (Ca), titanium (Ti), sulfur (S),gadolinium (Gd), and potassium (K) from the measured spectrum,and then, in turn, to estimate the relative abundance in the formationof each element above by combining with the elemental concentra-tions from the NGT and AACT. The GST also measures the hydrogen(H) and chlorine (Cl) in the borehole and formation, but these ele-ments are not used for determining rock geochemistry.The only major rock-forming elements not measured by the GSTare magnesium (Mg) and sodium (Na); the neutron-capture cross sec-tions of these elements are too small relative to their typical abundanceto be detected by the GST. A rough estimate of Mg + Na can be madeby using the photoelectric factor (PEF), measured by the lithodensitytool. This measured PEF is compared with a calculated PEF (a sum-mation of the PEF from all of the measured elements). The separationbetween the measured and calculated PEF is, in theory, attributable toany element left over in the formation (i.e., Mg and Na). Furtherexplanation of this technique is found in Hertzog et al. (1989). The Mgcalculation was not attempted for this leg because including it in thenormalization with the other elements induces noise into all otherelements, which proves to be erroneous in Ocean Drilling Program(ODP) wells (Pratson et al., in press). MgO + Na2O values from coredata are included in the normalization step of the processing. This isexplained further in Step 5 of the following data-reduction section.DATA REDUCTIONThe well-log data from the Schlumberger tools are transmitteddigitally up a wire line and are recorded and processed on the JOIDESResolution in the Schlumberger Cyber Service Unit (CSU). The re-sults from the CSU are made available as "field logs" for initial,shipboard interpretation. Subsequent reprocessing is necessary tocorrect the data for the effects of fluids added to the well, loggingspeed, and drill-pipe interference. Processing of the spectrometry datais required to transform the relative elemental yields into oxide weightfractions. The processing is performed with a set of log-interpretationprograms written by Schlumberger that were modified to account forthe lithologies and hole conditions encountered in ODP holes. Thesteps are summarized as follows:/. Reconstruction of Relative Elemental Yields from RecordedSpectral DataThis first processing step compares the measured spectra from thegamma-ray spectrometry tool with a series of "standard" spectra todetermine the relative contribution (or yield) of each element. Each"standard" approximates the spectrum of each element and is com-339DATA REPORT •10° N7 5°- 0°90° W- 5°SFigure 1. Location of DSDP/ODP Sites 501, 504, and 505 south of the Costa Rica Rift in the eastern equatorial Pacific Ocean, from Hobart et al. (1985). E.R.Ecuador Rift.bined at each depth