Quantifying alteration in the ocean crust through the use of wireling logs.

Alteration has long been known as a characteristic of ocean crust, the degree of alteration apparently increasing with age. Sampling of alteration is often biased by the lack of recovered core, and consequently quantitative estimates of the extent of alteration are difficult to obtain. The products of alteration change the petrophysical character of fresh basalt in such a way that they can be detected by some downhole measurements. In this contribution we describe a novel method for quantifying the degree of alteration in an oceanic section. The method is based on increases in the neutron absorption cross-section, as measured by a number of logging tools, as alteration develops. This increase is mainly due to the addition of trace elements, particularly boron, during the alteration process. The technique is applied to two sections in ODP Hole 801, (Leg 185, Mariana-Izu-Bonin Convergent Margin).