Magnetization of oceanic crustal layer 2—results and thoughts after DSDP Leg³⁷

Abstract

The detailed study of 455 basement samples from DSDP Leg 37 reveals magnetic properties, particularly inclinations and intensities, different from those commonly considered representative of Layer 2. Non-dipole inclinations are the most common. The deepest hole (582 m) has a vector average intensity of 24.3 × 10−4 emu cm−3 (24.3 × 10−1 A/m) and an inclination of only −14.5°. Induced magnetization never dominates and is usually much less than remanent magnetization, with Q ratio averaging 35 for basalts and 2.6 for plutonic rocks. Viscous magnetization acquisition constant, S, ranges widely from 0.001 to 1 × 10−4 emu cm−3 (0.001 to 1 × 10−1A/m), but is very rarely sufficient to cause VRM to dominate NRM.The major carrier of NRM is cation-deficient titanomagnetite produced by low-temperature oxidation of stoichiometric titantomagnetite. There is no trend of alteration with depth. All the magnetic properties are controlled by conditions within the individual basalt pillows or more massive units. A high degree of cation deficiency is associated with reduced NRM intensity, initial susceptibility, saturation magnetization, and VRM acquisition and increased MDF, Q ratio, and Curie point. Zones of low cation deficiency are presently found only in parts of massive units. With the exception of rare individual samples pillow sequences are highly oxidized throughout.A discussion is given of the kinds of ocean crust drilling and laboratory experiments required to solve the problems of the magnetic structure of Layer 2 as seen at the Leg 37 sites.

Effect of low-temperature oxidation on the remanence properties of titanomagnetites.

“…The stability decreases in the range of z>0.8. tendency of increasing as z increases. These results are consistent with those of 10HNS4N and ATWATER (1977) and RYALL et al (1977).…”

Rock Magnetic Properties of Igneous Rock Samples—Leg 45

“…Comparison of the ratios of the intensity of remanence for the two sites extent of weathering in the (presumed) permeable pillow basalt units of both sites. This increase in Curie temperature with increasing age is a consequence of progressive low-temperature oxidation of the initial titanomagnetite grains to titanomaghemite, and has been previously observed in submarine pillow basalts from the Mid-Atlantic Ridge (Irving, 1970;Schaeffer and Schwarz, 1970;Johnson and Atwater, 1977;Ryall et al, 1977).…”

Paleomagnetism of Basalts and Interlayered Sediments Drilled during DSDP Leg 49 (N–S Transect of the Northern Mid-Atlantic Ridge)

“…Leg 49 mean intensity from the FAMOUS area is within the range of (3.6 ±2.3) × 10~3 emu/cm 3 reported by Ryall et al (1977), this being the core mean of all basalts from Leg 37, where the deepest penetration was 582 meters. When the NRM intensities of drilled and dredged basalts from the FAMOUS area are plotted against distance from the ridge axis ( fig.…”

“…Basalts from all the holes have weak low-field susceptibility (x); the mean Qn for 44 specimens is 21 (Table 3 and Figure 3). This demonstrates that remanence dominates the induced magnetization in the ocean crust, as it does in basalts from previous DSDP holes in several oceans and in dredged specimens (Lowrie, 1977) and in basalts from the FAMOUS area (Johnson and Atwater, 1977;Ryall et al, 1977). The NRM intensities must therefore contribute to the observed magnetic anomalies.…”

“…Boundaries between them are also lithologic boundaries. The magnetic units at Holes 407 and 408 are comparable with Magnetic Superunits B of Ryall et al (1977) in the deep holes of Leg 37. At Hole 409, similar inclinations of a few consecutive specimens from flows indicate that they were erupted within a period short enough to give an instantaneous sample of the secular variation of the geomagnetic field, but the exact number of magnetic units cannot be determined on paleomagnetic evidence alone.…”