Paleomagnetic and rock magnetic properties of ODP Leg 173 sites, west Iberia continental margin

We present detailed paleomagnetic and rock magnetic results of rock samples recovered during Leg 173. The Leg 173 cores display a multicomponent magnetization nature. Variations in magnetic properties correlate with changes in lithology that result from differences in the abundance and size of magnetic minerals. The combined investigation suggests that the magnetic properties of the "fresher" peridotite samples from Site 1070 are controlled mainly by titanomagnetite, with a strong Verwey transition in the vicinity of 110 K, and with field- and frequency-dependent susceptibility curves that resemble those of titanomagnetites. These results are in excellent agreement with thermomagnetic characteristics where titanomagnetites with Curie temperature ~580°C were identified from the "fresher" peridotites. In contrast to the magnetic properties observed from the "fresher" peridotites, the low-temperature curves for the "altered" peridotites did not show any Verwey transition. Thermomagnetic analysis using the high-temperature vibrating sample magnetometer also failed to show evidence for titanomagnetites. The remanent magnetization is carried by a thermally unstable mineral that breaks down at ~420°C, probably maghemite. The field- and frequency-dependent relationships are also directly opposite to those in the reversal zone, with no signs of titanomagnetite characteristics. Altogether, these rock magnetic data seem to be sensitive indicators of alteration and support the contention that maghemite is responsible for the magnetic signatures displayed in the altered peridotites of the upper section. The magnetic minerals of the basement rocks from Sites 1068, 1069, and 1070 are of variable particle size but fall within the pseudo-single-domain size range (0.2-14 µm). The average natural remanent magnetization (NRM) intensity of recovered serpenitinized peridotite is typically on the order of 20 mA/m for samples from Site 1068, but ~120 mA/m for samples from Site 1070. The much stronger magnetization intensity of Site 1070 is apparently in excellent agreement with the observed magnetic anomaly high. Nearly half of the NRM intensity remained after 400°C demagnetization, suggesting that the remanence can contribute significantly to the marine magnetic anomaly.

Calcium carbonate content and anisotropy of magnetic susceptibility of ODP Leg 172 sites

Sediments from Sites 1057 and 1061 of Ocean Drilling Program Leg 172 on the Blake Outer Ridge exhibit nearly isotropic magnetic susceptibility. Resolving the degree of anisotropy of magnetic susceptibility proved difficult in many samples because of the generally weak magnetic susceptibility of the sediments relative to the noise level of the susceptibility meters used. Lineation varies from 1.0 to 1.013 and foliation varies from 1.0 to 1.08 in the samples that pass rejection criteria. In general the foliation is better resolved than the lineation, particularly at Site 1061, where the foliation exhibits long-term trends that mimic the mean susceptibility. The changes in the foliation at this site are likely the result of changes in the magnetic mineralogy of the sediment. The poorly developed or absent magnetic fabric in the sediments overall can be attributed to high carbonate concentrations and to a circulation regime that was diffuse or with currents too weak to effectively align magnetic particles.

Rock magnetic properties of ODP Leg 134 sites

During Leg 134, the influence of ridge collision and subduction on the structural evolution of island arcs was investigated by drilling at a series of sites in the collision zone between the d'Entrecasteaux Zone (DEZ) and the central New Hebrides Island Arc. The DEZ is an arcuate Eocene-Oligocene submarine volcanic chain that extends from the northern New Caledonia Ridge to the New Hebrides Trench. High magnetic susceptibilities and intensities of magnetic remanence were measured in volcanic silts, sands, siltstones, and sandstones from collision zone sites. This chapter presents the preliminary results of studies of magnetic mineralogy, magnetic properties, and magnetic fabric of sediments and rocks from Sites 827 through 830 in the collision zone. The dominant carrier of remanence in the highly magnetic sediments and sedimentary rocks in the DEZ is low-titanium titanomagnetite of variable particle size. Changes in rock magnetic properties reflect variations in the abundance and size of titanomagnetite particles, which result from differences in volcanogenic contribution and the presence or absence of graded beds. Although the anisotropy of magnetic susceptibility results are difficult to interpret in terms of regional stresses because the cores were azimuthally unoriented, the shapes of the susceptibility ellipsoids provide information about deformation style. The magnetic fabric of most samples is oblate, dominated by foliation, as is the structural fabric. The variability of degree of anisotropy (P) and a factor that measures the shape of the ellipsoid (q) reflect the patchy nature of deformation, at a micrometer scale, that is elucidated by scanning electron microscope analysis. The nature of this patchiness implies that deformation in the shear zones is accomplished primarily by motion along bedding planes, whereas the material within the beds themselves remains relatively undeformed.