Viscous Remanent Research Articles

Single domain grain distributions II. The distribution of single domain iron grains in Apollo 11 lunar dust

A quantitative analysis of some of the magnetic properties of an Apollo 11 lunar dust sample is given. The measurements involved are the acquisition of isothermal and viscous remanence, the thermal demagnetization of isothermal and thermoremanence, and the temperature and frequency dependence of the initial susceptibility. It is shown that the experimental results are in good agreement with those predicted from single domain theory and can be explained on the basis of an assembly of single domain iron grains distributed such that the number of grains within a given volume range is inversely proportional to the square of the volume.

Natural Remanent Magnetization and Viscous Magnetization of Apollo 11 Lunar Materials

The natural remanent magnetization (NRM) of three Apollo 11 lunar materials, i. e. a crystalline rock and two kinds of microbreccia, is examined in detail with special attention to the possible effect of viscous magnetization acquirable in the terrestrial magnetic field.The intensity of stable NRM (In), the critical AC-demagnetization field (H*) which represents the upper limit for keeping approximate invariance of NRM direction, and the effective demagnetization field (H0) which reduces the intesity of NRM to (1/e) of the initial value are determined as follows; (a) In=8.9×10-6emu/gm, H*=30Oe, H0=25Oe for a crystalline rock (NASA No. 10024.22): (b) In=1.50×10-5emu/gm, H*=40Oe, H0=35Oe for a weakly impacted microbreccia (No. 10021.32): (c) In=1.50×10-4emu/gm H*>500Oe, H0≅1400Oe for a strongly impacted microbreccia (No. 10085·16).The intense and extremely stable NRM of the strongly impacted lunar microbreccia may suggest that a strong impact of lunar materials can cause an acquisition of a particularly stable remanent magnetization even in a very weak magnetic field.A certain portion of metallic irons in the lunar materials are very fine particles of several hundreds of Angstrom in mean diameter. These particles behave almost superpara-magnetically. Saturated viscous remanent magnetization acquired in the geomagnetic field being denoted by ΔIv, the ratio of 44 to the stable remanence In is determined to be 0.015, 8.4 and 0.23 respectively in cases of (a), (b) and (c).The larger value of the ratio ΔIv/In indicates that the larger portion of fine particles of metallic irons. The relaxation time of the viscous magnetization at 20°C ranges from 2×10 to 7×104sec., which implies that the grain size of the find iron particles is 170-180A in diameter.

Paleomagnetism of Franciscan ultramafic rocks from Red Mountain, California

Results from the study of the magnetism in serpentinized ultramafic rocks of the Red Mountain intrusion southeast of San Francisco, California, suggest that these rocks possess a chemical remanent magnetization acquired during serpentinization. The stability of magnetization decreases with serpentinization, owing to the growth of larger magnetic grains. Thus, in highly serpentinized peridotites (serpentinites), most of the CRM is destroyed and the NRM becomes mainly viscous remanent magnetization, rendering these rocks unsuitable for paleomagnetic work. On the other hand, in partially serpentinized peridotites, pyroxenites, and dunites, the CRM is highly stable and paleornagnetically reliable. A study of the stable directions of magnetization indicates that the Red Mountain ultramafic body was emplaced and serpentinized prior to the folding of the Franciscan formation. The stability of magnetization and removal of viscous components were verified by the results of a storage test and alternating-field demagnetization experiments, by the divergence of the mean directions of magnetization from that of the present field, and, most important, by the convergence of the directions after tilt correction. Paleomagnetic pole positions calculated from the mean directions suggest that the magnetic pole rapidly migrated southward into the Atlantic Ocean during late Mesozoic, possibly more than once, as proposed previously by Gromme and Gluskoter.

Magnetic Properties and Petrology of Rocks near the Crest of the Mid-Atlantic Ridge

MAGNETIC and petrological studies have been made on nine dredge samples taken in the course of detailed geological and geophysical investigations of a small area near the crest of the mid-Atlantic Ridge, 22° 30′ N., 45° 30′ W. (refs. 1 and 2). Samples were taken in 1964–65 on cruises Chain-44 and Washington 1965–1 of the Woods Hole Oceanographic Institution and the Scripps Institution of Oceanography. The magnetic properties of the samples were examined in order to help in the interpretation of observed magnetic anomalies in the area. The remanent magnetizations of twenty cores (2.5 cm by about 3 cm) taken from the nine samples were measured on an astatic magnetometer at the Scripps Institution. Susceptibilities were measured on a susceptibility bridge. The samples were not demagnetized of any isothermal or viscous remanent magnetization components which may have been present. Because all dredge samples were unoriented, the cores were also unoriented. Care was taken to avoid coring outer visibly weathered zones in some samples.

A paleomagnetic investigation of some of the Precambrian igneous rocks of southeast Missouri

Measurements of the magnetic susceptibility and the intensity and direction of remanent magnetization were made on 325 2.5-cm cylindrical cores prepared from 189 samples of the felsites, granites, and diabases exposed in the Ironton, Fredericktown, and Coldwater quadrangles, Missouri. These measurements show, first, that the directions of remanent magnetization, although quite variable, are significantly different from the direction of the present geomagnetic field in southeast Missouri and, second, that partial demagnetization in an ac field of 300 to 700 oe greatly improves the initial scatter of the remanent magnetization vector by diminishing the anomalous components of viscous and isothermal remanent magnetization shown by many samples. The paleomagnetic data from five rock units indicate a Precambrian geomagnetic pole in the vicinity of 150°W and 5°N.