(Table 1) Viscosity coefficients at various temperatures of samples from ODP Hole 176-735B

Ocean Drilling Program (ODP) Hole 735B was drilled to a depth of 1.5 km in a tectonic window of gabbroic lower oceanic crust created at the Southwest Indian Ridge. The gabbros have a very stable natural remanent magnetization (NRM) of reversed polarity with most unblocking temperatures slightly below the Curie temperature of magnetite. The NRM includes a drilling-induced overprint but its intensity decays strongly towards the interior of the drill core. The demagnetization data yield no or only a very small secondary magnetization component acquired during the present Brunhes chron or an earlier normal chron, suggesting cooling through most of the blocking temperature range during chron C5r and a strong resistance against the acquisition of thermoviscous magnetization. A novel furnace has been designed to measure magnetizations and their time dependences at high temperatures (up to 580 deg C) inside a commercial SQUID magnetometer. Magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550 deg C to determine the time and temperature stability of remanent magnetization. Viscosities are generally small and increase little with temperature below the main blocking temperature, where the increase becomes almost an order of magnitude. Extrapolations to geological times infer viscous acquisitions that would be 5-25% of a thermoremanence in 100 kyr and at temperatures of 200-500 deg C. At ocean bottom temperature the predicted magnetization of one sample acquired in the present Brunhes chron should be 10% of the NRM. However, this is not recognized during NRM demagnetization and partial thermoremanent magnetization (pTRM) acquisitions at 250 deg C are also much smaller than predicted. It thus appears that the NRMs are generally magnetically harder than magnetizations acquired after heating to 570 deg C in the laboratory. Susceptibility changes during heating are small (<5%) indicating a seemingly stable magneto-mineralogy, but conspicuous minima occur after heating to 520 deg C. Also, quasi paleointensity experiments reveal characteristic patterns in the NRM/pTRM ratios and also large increases in pTRM capacity after heating to 570 deg C. Moreover, anhysteretic remanent magnetization acquisition in the low field range (<=10 mT) is strongly enhanced after heating by factors up to three. The alteration of the magneto-mineralogy is interpreted to result from the annealing of defects in magnetite that originate from tectonically induced strain. The oceanic gabbros of Hole 735B are thus ideal source layer material for marine magnetic anomalies, and secondary thermoviscous acquisition, as a possible cause for anomalous skewness, is essentially absent.

Magnetic properties of light and dark sediment layers from the Japan Sea

Rock magnetic/paleoclimatic/diagenetic relationships of sediments spanning the last 0.78 Ma have been investigated using samples collected from light and dark layers recovered at ODP Sites 794 (Yamato Basin) and 795 (Japan Basin). Rock-magnetic parameters (K, Kfd, ARM, SIRM, S-ratio) are shown to reflect diagenetic processes and climate-related variations in the concentration, mineralogy and grain-size of the magnetic minerals contained within the sediments. The magnetic mineralogy is dominated by ferrimagnetic (magnetite-type) minerals with a small contribution made by hematite and iron sulphides such as pyrrhotite and/or greigite. Magnetic mineral concentration and grain size vary between light and dark layers with the former characterized by a higher magnetic content and a finer magnetic grain size. Magnetite dissolution, related to sulfate reduction due to bacterial degradation of organic matter, is the process responsible for the magnetic characteristics observed in the dark layers, testifying to the reducing conditions in the basin. Variations in the rock magnetic properties of the sediments are strongly correlated with global oxygen isotope fluctuations, with glacial stages characterized by a lower magnetic mineral content and a coarser magnetic grain size relative to interglacial stages. Major downcore changes in the magnetic properties observed at Site 794 can be related to changes in the oceanographic conditions of the basin associated with the flow of the warm Tsushima Current into the Japan Sea at about 0.35-0.40 Ma ago.

Rock magnetic properties of ODP Hole 109-670A

The magnetic properties of 11 samples from Site 670 of Leg 109, 3 harzburgites and 8 highly serpentinized peridotites, have been studied. Reflected light microscopy and Curie temperatures confirm that magnetite is the dominant magnetic mineral in all samples. However, both rock types show different magnetic behavior. Susceptibility, saturation magnetization, and NRM are higher for the serpentinites, because of the higher magnetite content. The hysteresis parameters indicate magnetite particles with pseudosingle domain structure for both rock types. For the remarkable anisotropy of the magnetic susceptibility no definite explanation could be found, because of the complex texture of the samples. In both rock types the presence of maghemite, a product of low temperature oxidation of magnetite, has been indicated by reflected light microscopy and by thermomagnetic analysis. As the maghemite converts to hematite at temperatures above 350°C, the temperature during the serpentinization was below this value assuming that the maghemitization took place at the same time.

(Table 1) Summary of horizon, lithology, and paleomagnetic of ODP Hole 126-791B basaltic mousse samples

A paleomagnetic study was made on the highly vesiculated basaltic tuff breccia (the basaltic mousse) drilled by Ocean Drilling Program Leg 126 from the Izu-Bonin backarc, Sumisu Rift, to estimate the mode of its emplacement. Thirty-four 10-cm**3 minicore samples were collected from almost all the horizons of the basaltic mousse. Stepwise thermal and alternating-field demagnetization experiments show that the natural remanent magnetizations of many samples are mainly composed of a single stable component. Although remanence inclinations are not expected to be disturbed by rotary drilling, the measured inclinations of remanence show a random directional distribution as a whole. The remanence inclinations, however, show directional consistency on a smaller scale. High-density sampling and measurements from a limited interval of drilled cores, and the measurement of small disks cut from a single minicore sample show that there is directional consistency over several centimeters. Strong and stable remanent magnetization, the existence of remanence direction consistency, and the fresh lithology of the samples suggest the thermal origin of remanence. Combining the paleomagnetic results with other geological, petrographical, and paleontological characters, the Hole 791B basaltic mousse can be interpreted as a subaqueous explosion breccia produced by deep-sea pyroclastic fountaining.

Magnetic parameters of rocks from the Red Sea rift zone

Results of detailed geomagnetic and geomorphological studies carried out by R/V Akvanavt together with data obtained by a side-scanning sonar and high-frequency profiles from a towed Zvuk-4 vehicle plus results of visual observations of deep-sea manned Pisces submersible have shown that the spreading axis is divided into segments, whose strike (330°) differs from the overall strike (310°) of the axial magnetic anomaly. In the study area segments are about 1 km long and transform displacements are 0.5 km. Calculations on a model have shown that spreading is asymmetric: during the Brunhes epoch accretion rate of the African Plate was 6 mm/yr and that of the Arabian Plate 7 mm/yr. Earlier it had been 9 and 11 mm/yr, respectively.

Paleomagnetic of ODP Hole 197-1203A

On the basis of studies of Holocene samples,submarine basaltic glass (SBG) is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield Thellier paleointensity data of exceptional quality. To be useful as a recorder of the long-term geomagnetic field, older SBG must retain these optimal properties. Here, we examine this issue through rock magnetic and transmission electron microscope (TEM) analyses of Cretaceous SBG recovered at Ocean Drilling Program Site 1203 (northwestern Pacific Ocean). These SBG samples have very low natural remanent magnetization intensities (NRM <50 nAm**2/g) and TEM analyses indicate a correspondingly low concentration of crystalline inclusions. Thellier experiments on samples with the strongest NRM intensity (>5*10**-11 Am**2) show a rapid acquisition of thermoremanent magnetization (TRM) with respect to NRM demagnetization. Taken at face value,this behavior implies magnetization in a very weak (617 WT) ambient field. But monitoring of magnetic hysteresis properties during the Thellier experiments (on subsamples of the SBG samples used for paleointensity determinations) indicates systematic variations in values over the same temperature range where the rapid TRM acquisition is observed. A similar change in properties during heating is observed on monitor SBG specimens using low-temperature data: with progressive heatings the Verwey transition becomes more distinct. We suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in SBG during the thermal treatments required by the Thellier method,resulting in paleointensity values biased to low values. We further propose that this process is pronounced in Cretaceous and Jurassic SBG (relative to Holocene SBG) because devitrification on geologic time scales (i.e., tens of millions of years) lowers the transition temperature at which the neocrystallization can commence. Magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in SBG during Thellier experiments.

Component NRM Directions for two sediment cores from the Arctic Ocean

Inclination patterns of natural remanent magnetization (NRM) in Quaternary sediment cores from the Arctic Ocean have been widely used for stratigraphic correlation and the construction of age models, however, shallow and negative NRM inclinations in sediments deposited during the Brunhes Chron in the Arctic Ocean appear to have a partly diagenetic origin. Rock magnetic and mineralogical studies demonstrate the presence of titanomagnetite and titanomaghemite. Thermal demagnetization of the NRM indicates that shallow and negative inclination components are largely "unblocked" below ~300 °C, consistent with a titanomaghemite remanence carrier. Following earlier studies on the Mendeleev-Alpha Ridge, shallow and negative NRM inclination intervals in cores from the Lomonosov Ridge and Yermak Plateau are attributed to partial self-reversed chemical remanent magnetization (CRM) carried by titanomaghemite formed during seafloor oxidation of host (detrital) titanomagnetite grains. Distortion of paleomagnetic records due to seafloor maghemitization appears to be especially important in the perennially ice covered western (Mendeleev-Alpha Ridge) and central Arctic Ocean (Lomonosov Ridge) and, to a lesser extent, near the ice edge (Yermak Plateau). On the Yermak Plateau, magnetic grain size parameters mimic the global benthic oxygen isotope record back to at least marine isotope stage 6, implying that magnetic grain size is sensitive to glacial-interglacial changes in bottom-current velocity and/or detrital provenance.

Magnetic properties and incompatible element geochemistry of some igneous rocks at DSDP Leg 64 Holes

I received five unoriented samples of igneous rocks from four Sites of Leg 64 of the Deep Sea Drilling Project (DSDP). I have measured several magnetic properties, alkalis (K, Rb, and Cs), alkaline-earth (Ba and Sr) element concentrations, and 87Sr/86Sr ratios of these samples. This study reports the results.

Rock magentic investigations of ODP Site 162-983

We report natural remanent magnetization (NRM) directions and geomagnetic paleointensity proxies for part of the Matuyama Chron (0.9-2.2 Ma interval) from two sites located on sediment drifts in the Iceland Basin. At Ocean Drilling Program Sites 983 and 984, mean sedimentation rates in the late Matuyama Chron are 15.9 and 11.5 cm/kyr, respectively. For the older part of the record (>1.2 Ma), oxygen isotope data are too sparse to provide the sole basis for age model construction. The resemblance of the volume susceptibility record and a reference d18O record led us to match the two records to derive the age models. This match, based on Site 983/984 susceptibility, is consistent with available Site 983/984 benthic d18O data. Paleointensity proxies were derived from the slope of the NRM versus anhysteretic remanent magnetization plot for alternating field demagnetization in the 30-60 mT peak field range. Paleointensity lows correspond to polarity reversals at the limits of the Jaramillo, Olduvai, Cobb Mountain, and Réunion Subchrons and to seven excursions in NRM component directions. Magnetic excursions (defined here by virtual geomagnetic polar latitudes crossing the virtual geomagnetic equator) are observed at 932, 1048, 1115, 1190-1215 (Cobb Mountain Subchron), 1255, 1472-1480, 1567-1575 (Gilsa Subchron), and 1977 ka. The results indicate that geomagnetic directional excursions, associated with paleointensity minima, are a characteristic of the Matuyama Chron and probably of polarity chrons in general.

Reflection microscopy and geochemistry at DSDP Leg 55 Holes

Magnetic properties of volcanic rocks are controlled mainly by the physical and chemical state of their constituent ferromagnetic minerals. The most important parameters determining magnetic properties are concentration, composition, grain size, and oxidation state. In sea floor basalts, the main ferromagnetic minerals are titanomagnetites which are either unoxidized or, more commonly, have undergone various degrees of posteruptive low-temperature oxidation to become cationdeficient titanomagnetites, or titanomaghemites. The effects of this low-temperature alteration are seen in the increase of Curie temperature and decrease of saturation magnetization and lattice parameter of ferromagnetic minerals (Readman and O'Reilly, 1972). It is now believed that titanomaghemitization of newly formed mid-ocean ridge crust proceeds with a time constant of about 1 m.y., accompanying drastic decrease of the intensity of the natural remanent magnetization (NRM) (Johnson and Atwater, 1977).

Magnetic properties of plutonic rocks at DSDP Leg 82 holes

Ten samples of gabbro and peridotite, with varying degrees of serpentinization, were studied by magnetic techniques and reflected light microscopy. Evidence from these methods suggests that the natural remanent magnetization is primarily of chemical origin. It is generally weak for the gabbros and much stronger for the peridotites. This difference is offset by the fact that the peridotites have generally lower magnetic stability and Koenigsberger ratios. There is a considerable variation in both magnetic parameters and petrology even among closely spaced samples, which suggests that some combination of source heterogeneity and tectonic mixing was involved in the production of these rocks. However, the small number of samples makes this conclusion tentative. There may also have been significant postemplacement alteration involved. All samples show a significant anisotropy of weak field susceptibility that appears to be related to deformation. This anisotropy may be useful in defining petrofabrics.

Paleomagnetic of ODP Leg 133 holes

Paleomagnetic and rock-magnetic analyses from discrete samples of carbonate sites on the Queensland Plateau were used to determine magnetic polarity reversal stratigraphy and the nature of magnetization in these sediments. Magnetic polarity zones were correlated with the geomagnetic polarity time scale in the upper portions of cores at Sites 812 through 814, usually back to a late Pliocene age. Loss of reliable directional data was coincidental with a major decrease in magnetic intensity, below which, no stable polarity zones could be identified. The intensity reduction is either an in-situ alteration of magnetic grains, or an input signal representing progressive increase in the magnetic component of Queensland Plateau sediments. Although not conclusive at this point, the geochemical conditions and differing age of intensity reduction support the former hypothesis. Rock-magnetic analysis of carbonate sediments suggests that ultrafine-grained magnetite or maghemite crystals is an important carrier of remanence and may be biogenic in origin. Application of a recently calibrated anhysteretic remanent magnetization test to assess configuration of single-domain crystal within a natural matrix indicates that cementation (ooze-chalk-limestone) may be important in post-depositional changes affecting magnetostatic grain interaction.