Ocean Drilling Program Hole 896A was drilled 1 km southeast of the ODP deep-basement Hole 504B, 200 km south of the Costa Rica Rift. The magnetic properties (intensity of remanence, bulk susceptibility, inclination of stable remanence, and median destructive field) of 166 minicores from Hole 896A are presented. The basement in Hole 896A can be divided into three sections on the basis of its magnetic properties. The upper part, to 330 m below seafloor (mbsf), has relatively high intensity of remanence (J0) and low susceptibility values. The middle section, between 330 and 360 mbsf, has high (J0) and intermediate susceptibility values, and the lower section, below 360 mbsf, has low (J0) and high susceptibility values. The magnetic mineralogy was investigated by monitoring the effect of heating on susceptibility. These results suggest that titanomaghemites occur throughout the hole and that titanomagnetites occur mainly below 330 mbsf. Some samples have evidence for minor amounts of hematite. The main cause of variation in the susceptibility and the median destructive field can be related to grain size. Grain size also controls J0, although the degree of maghematization and the remanence mechanism of the magnetization also contribute. The magnetic properties of the upper part of Hole 896A, above the massive flow Unit 24, correlate with the upper part of Hole 504B above Unit 2D, at depths of 3796 and 3773 m below sea level, respectively. We suggest that the upper section corresponds to a late volcanic pile extruded onto a ponded flow that masks the earlier basement topography. This flow has probably acted as a major barrier to hydrothermal circulation. These results have significant implications for the interpretation of magnetic field data from ridges. The uppermost layer of late extrusive rocks has a high magnetization and a highly variable thickness, and may be an important contributor to variations in the magnetic signal at ridges.
Date of initial receipt: 15 August 1994
Date of acceptance: 24 February 1995
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