Geochemistry and Fe/(Ti + Ti), x, Curie temperature, and oxidation parameter z at DSDP Hole 597C

Electron microprobe and thermomagnetic analyses of selected basalt samples from Hole 597C were performed. The main purpose of this work was to investigate and estimate the degree of oxidation of the samples using the ratios of Fe to Ti and the Curie temperatures obtained from thermomagnetic curves. The results show that the magnetic properties of samples from Hole 597C change at a sub-bottom depth of 100 m, and that low-temperature and high-temperature oxidation processes prevailed above and below 100 m, respectively.

Curie temperatures and cell edges of ODP Leg 120 samples (Table 1)

Basalt samples recovered on Ocean Drilling Program Leg 120 from the Kerguelen Plateau were investigated by thermomagnetic analysis, X-ray diffraction, and ore microscopy. The basement samples could be divided into two groups based on Curie temperatures, cell-edge parameters, and optical magnetic mineralogy. Samples from Sites 748 and 750 underwent only low-temperature oxidation and displayed Curie temperatures for the titanomaghemites that ranged from 340° to 395°C. The basalts from Sites 747 and 749 mainly experienced high-temperature oxidation. High-temperature oxidation produced titanium-poor titanomagnetites with ilmenite-exsolution lamellae. Curie temperatures of the deuterically oxidized titanomaghemites varied from 490° to 620°C.

Curie temperatures and hysteresis properties of ODP Leg 183 sites, Kerguelen Plateau and Broken Ridge

In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580°C were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios). The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.

Low-temperature experiments of sediment core GeoB6229-6 at the South American continental margin off the Rio de la Plata estuary

With various low-temperature experiments performed on magnetic mineral extracts of marine sedimentary deposits from the Argentine continental slope near the Rio de la Plata estuary, a so far unreported style of partial magnetic self-reversal has been detected. In these sediments the sulphate-methane transition (SMT) zone is situated at depths between 4 and 8 m, where reductive diagenesis severely alters the magnetic mineral assemblage. Throughout the sediment column magnetite and ilmenite are present together with titanomagnetite and titanohematite of varying compositions. In the SMT zone (titano-)magnetite only occurs as inclusions in a siliceous matrix and as intergrowths with lamellar ilmenite and titanium-rich titanohematite, originating from high temperature deuteric oxidation within the volcanic host rocks. These abundant structures were visualized by scanning electron microscopy and analysed by energy dispersive spectroscopy. Warming of field-cooled and zero-field-cooled low-temperature saturation remanence displays magnetic phase transitions of titanium-rich titanohematite below 50 K and the Verwey transition of magnetite. A prominent irreversible decline characterizes zero-field cooling of room temperature saturation remanence. It typically sets out at ~210 K and is most clearly developed in the lower part of the SMT zone, where low-temperature hysteresis measurements identified ~210 K as the blocking temperature range of a titanohematite phase with a Curie temperature of around 240 K. The mechanism responsible for the marked loss of remanence is, therefore, sought in partial magnetic self-reversal by magnetostatic interaction of (titano-)magnetite and titanohematite. When titanohematite becomes ferrimagnetic upon cooling, its spontaneous magnetic moments order antiparallel to the (titano-)magnetite remanence causing an drastic initial decrease of global magnetization. The loss of remanence during subsequent further cooling appears to result from two combined effects (1) magnetic interaction between the two phases by which the (titano-)magnetite domain structure is substantially modified and (2) low-temperature demagnetization of (titano-)magnetite due to decreasing magnetocrystalline anisotropy. The depletion of titanomagnetite and superior preservation of titanohematite is characteristic for strongly reducing sedimentary environments. Typical residuals of magnetic mineral assemblages derived from basaltic volcanics will be intergrowths of titanohematite lamellae with titanomagnetite relics. Low-temperature remanence cycling is, therefore, proposed as a diagnostic method to magnetically characterize such alteration (palaeo-)environments.

(Table 1) Magnetic properties and Curie temperatures of basalts from DSDP Leg 65 holes

During DSDP Leg 65, we achieved significant basement penetration at three sites (482, 483, and 485) in the mouth of the Gulf of California (Lewis and Robinson, 1983, doi:10.2973/dsdp.proc.65.1983). Since these holes were all drilled into extremely young crust near the crest of the East Pacific Rise, the rocks we recovered present an unusual opportunity to study the magnetic properties of submarine basalts before alteration has become pervasive. To take advantage of this opportunity and to complement the paleomagnetic studies conducted on these basalts by Day (1983, doi:10.2973/dsdp.proc.65.138.1983), we have studied, in the same samples, the following properties: saturation magnetization (Js); intensity and stability of isothermal remanent magnetization (IRM); hysteresis parameters, such as the coercive force (Hc), the remanent coercive force (HRC), and the ratio of saturation remanence (JRS) to saturation magnetization; susceptibility (x); and Curie temperature (Tc). In this chapter we will discuss the results of these studies in conjunction with the opaque mineralogy of the samples.

Magnetic properties and chemical composition at DSDP Hole 77-538A

Magnetic properties of doleritic and some metamorphic basement rocks underlying Catoche Knoll are studied. Doleritic rocks show a high saturation magnetic moment (2-5 emu/g) compared to metamorphic rocks (0.1-1 emu/g). Magnetic minerals of rocks from this hole show a high stability when heated in vacuo up to 600°C at a fixed rate of heating. Curie temperatures are distributed close to 550°C. These properties differ markedly from those of common submarine basalts observed before. X-ray microprobe analysis techniques were used to determine internal structures of ferromagnetic minerals; in most of ferromagnetic minerals there exist two different types of magnetic phases (i.e., products of high-temperature and low-temperature oxidations). Interpretations on the coexisting, seemingly contradictory, phases can be made based upon present analyses.

Magnetic properties of Cenozoic MORB and Cretaceous MORB from DSDP and ODP holes

The fact that the natural remanent magnetization (NRM) intensity of mid-oceanic-ridge basalt (MORB) samples shows systematic variations as a function of age has long been recognized: maximum as well as average intensities are generally high for very young samples, falling off rather rapidly to less than half the recent values in samples between 10 and 30 Ma, whereupon they slowly rise in the early Tertiary and Cretaceous to values that approach those of the very young samples. NRM intensities measured in this study follow the same trends as those observed in previous publications. In this study, we take a statistical approach and examine whether this pattern can be explained by variations in one or more of all previously proposed mechanisms: chemical composition of the magnetic minerals, abundance of these magnetization carriers, vectorial superposition of parallel or antiparallel components of magnetization, magnetic grain or domain size patterns, low-temperature oxidation to titanomaghemite, or geomagnetic field behavior. We find that the samples do not show any compositional, petrological, rock-magnetic, or paleomagnetic patterns that can explain the trends. Geomagnetic field intensity is the only effect that cannot be directly tested on the same samples, but it shows a similar pattern as our measured NRM intensities. We therefore conclude that the geomagnetic field strength was, on-average, significantly greater during the Cretaceous than during the Oligocene and Miocene.

(Table 2) Geochemistry of ore minerals and magnetization properties at the DSDP Leg 73 Holes

The samples investigated in this study come from DSDP Leg 73 Drill Holes 519A, 522B, and 524, all of which are in the South Atlantic. A general petrographic description of the basalts is given by Carman et al. (1984; doi:10.2973/dsdp.proc.73.120.1984).