The interpretation of thermal neutron properties in ocean floor volcanics

Using near-complete chemical analyses, computed thermal absorption cross-section values (sigma) are calculated for ocean-floor basalts from ODP Holes 896a and 1179D. Comparison with nuclear measurement of sigma, demonstrates that this computational method is valid and clarifies the interpretation of sigma in ocean-floor basalts. Boron, lithium and the rare-earth elements are important controls on the sigma-values, and, of these, the distribution of the rare-earth elements is controlled by primary magmatic processes, whereas the distribution of boron and lithium is strongly influenced by secondary low-temperature alteration processes. Consequently, computed sigma-values can be used to discriminate between various basalt types and to identify areas of secondary alteration. Following calibration of the 'fresh basalt' signature, it is possible to interpret log derived neutron absorption measurements of a sample (Sigma-values), which, when integrated with other log responses, allow the distribution of alteration within a drilled section to be mapped. Examples from ODP Hole 801C demonstrate the potential of this technique.