Serpentinized oceanic peridotites collected during Ocean Drilling Program Leg 147 at Site 895 in the Hess Deep were studied magnetically to further our understanding of the magnetization of the oceanic lithosphere. The majority of the samples are either dunites and harzburgites, with varying degrees of serpentinization. Rock magnetic studies suggest that the dominant magnetic mineral is relatively pure pseudo-single-domain magnetite. Optical observations reveal that the magnetite has a secondary origin related to the serpentinization process. The dunites and the harzburgites have median natural remanent magnetization values of 3.2 and 1.2 A/m and magnetic susceptibility values of 0.053 and 0.023 (SI volume units), respectively. The NRM is the dominant component of magnetization and has a magnitude similar to values observed for oceanic basalts of Layer 2a. Similarly serpentinized peridotites residing within the oceanic lithosphere are likely contributors to magnetic anomalies. Thermal and alternating-field demagnetizations usually yield a stable remanent magnetization direction that often has an inclination much greater than expected for the time-averaged geomagnetic field (>5°) at Site 895. This disparity suggests that the samples acquired their magnetization and were subsequently reoriented. Hole 895E samples have relatively coherent inclinations (30°) that become shallower in the bottom 30 m of the hole. Inclinations in Hole 895D vary from +60° to -60°, with no systematic trends evident with depth in the hole, suggesting differential vertical rotation of blocks on a scale of a few meters. Experimental evidence suggests that room-temperature magnetic viscosity does not significantly effect the magnetization intensity or direction of these samples.

Date of initial receipt: 2 August 1994
Date of acceptance: 11 January 1995

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