Downhole Magnetic Measurements in Oceanic Crustal Hole 395A on the Mid-Atlantic Ridge

Abstract

Magnetic measurements of the vertical field component and of magnetic susceptibility were made within the ocean crust in Deep Sea Drilling Project Hole 395A on the Mid-Atlantic Ridge during Leg 78B. The variations in the vertical anomalous component of the field confirm the reversals of magnetization in the section described by Johnson (1979a). The reversals are shown to be very sharp, and are concluded to represent breaks in volcanism within which field reversals occurred. The continuous log of the vertical component of the field gives a far more complete and detailed view of the magnetic stratigraphy than was obtained from the minicores alone. The susceptibility log shows two positive anomalies in the upper part of the section that probably relate to the presence of serpentinite in the walls of the hole. At one of these horizons some serpentinite was recovered in the core but not at the other. The log thus provided a valuable and sensitive adjunct to the recovered samples.

Site 395

“…The results of these measurements are characterized by high resolution and accuracy and by a high signal-to-noise ratio. The result of the measurements by the self-contained three-component magnetometer (Plate 2, in back pocket) agrees well with the result of Ponomarev and Nechoroshkov (1984) on Leg 78B.…”

“…The most conspicuous aspect of this log is the uniformity from a depth of 200 mbsf to the bottom. The susceptibility log measured on Leg 78B (Ponomarev and Nechoroshkov, 1984) shows a similar behavior, except for the anomalous bulge at 430 mbsf. Such uniformity over such great depth is not normal; most susceptibility logs measured in basalts show much more variation and structure.…”

“…The next lower maxima can be attributed to massive basalts (Hole 395A, Cores 14 and 15, DSDP Leg 45) having the larger grain sizes (20-100 j«m) of the titanomagnetites (Johnson, 1978a). As indicated by the Russian scientists on Leg 78B (Ponomarev and Nechoroshkov, 1984), further serpentinites between 175 and 200 mbsf could also occur because of the similarity of the maxima in the susceptibility log. The peak in the depth of 345 mbsf is caused by a carbonate-cemented breccia zone (Hole 395A, Core 32, DSDP Leg 45) having low-titanium magnetite grains that contain ilmenite lamellae (Johnson, 1978b).…”

“…netic study of recovered rock samples from these holes and can provide data about the precise stratigraphic structure within the holes. In Hole 395A, the vertical component of the magnetic field was measured during Leg 78B (Ponomarev and Nechoroshkov, 1984). However, measurements of the three components of the magnetic field would provide more information about the nature of the young oceanic crust surrounding the hole.…”

Magnetization of the Oceanic Crust Inferred from Magnetic Logging in Hole 395A

“…In Hole 395A, the vertical component of the magnetic field was measured during Leg 76B (Ponomarev and Nechoroshkov, 1984). However, measurement of all three components of the magnetic field would provide more information about the nature of the young oceanic cruet surrounding the hole.…”