30. DATA REPORT: GEOCHEMICAL LOGGING RESULTS FROM THE EASTERN EQUATORIAL PACIFIC: HOLE 504B1

Geochemical well logs were obtained through sediment and basalt at Site 504 of Leg 140. Corrections have been applied to the 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. it is possible to derive the contribution (or yield) of each of the major elements 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 formation of 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 GST are magnesium (Mg) and sodium (Na); the neutron-capture cross sec- tions of these elements are too small relative to their typical abundance to be detected by the GST. A rough estimate of Mg + Na can be made by using the photoelectric factor (PEF), measured by the lithodensity tool. This measured PEF is compared with a calculated PEF (a sum- mation of the PEF from all of the measured elements). The separation between the measured and calculated PEF is, in theory, attributable to any element left over in the formation (i.e., Mg and Na). Further explanation of this technique is found in Hertzog et al. (1989). The Mg calculation was not attempted for this leg because including it in the normalization with the other elements induces noise into all other elements, which proves to be erroneous in Ocean Drilling Program (ODP) wells (Pratson et al., in press). MgO + Na2O values from core data are included in the normalization step of the processing. This is explained further in Step 5 of the following data-reduction section.