High-resolution geochemical variations at Sites 723, 728, and 731: a comparison of X-ray fluorescence and geochemical logs

Geochemical logging is a routine part of the Ocean Drilling Program, yet the reliability of ODP geochemical logs has rarely been evaluated quantitatively. On ODP Leg 117, geochemical logs were obtained at Sites 723, 728, and 731. We report here an evaluation of ODP geochemical log quality based on high-resolution sampling and X-ray fluorescence measurement of 398 core samples from the three sites. At these sites we lacked the complete suite of high-quality logs needed for accurate log-based estimation of elemental percentages; only calcium and silicon logs had magnitudes similar to those from XRF. However, relative variations of log-based elemental abundances could be determined. Our comparisons of the XRF analyses with the character of variations in geochemical logs indicates that the reliability of ODP geochemical logs varies substantially, within short intervals and particularly between sites. In general, the geochemical logs are capable of detecting changes in formation geochemistry that are larger than the following thresholds: 2% for Ca, 2%-6% for Si, 0.5%-l% for K, 0.1% for Ti, 0.5% for Fe, and 0.4% for Al. All sulfur variations observed in the XRF data, as well as many of the iron variations, were below the resolving power of the geochemical logging tools. These precisions are generally similar to those determined at the Conoco test well by Chapman et al. (1987), in spite of the very different ODP logging conditions.