A study of rotational remanent magnetization

A magnetic study was carried out on lacustrine sediments from the Zoigê basin, Tibetan Plateau, in order to obtain a better understanding of palaeoclimatic changes there. Gyromagnetic remanence (GRM) acquisition is unexpectedly observed during static three‐axis alternating field (AF) demagnetization in about 20 per cent of a large number of samples. X‐ray diffraction (XRD) analysis on a magnetic extract clearly shows that greigite is the dominant magnetic mineral carrier. Scanning electron microscopy (SEM) reveals that the greigite particles are in the grain size range of 200–300 nm, possibly in the single‐domain state. Greigite clumps of about 3 μm size are sealed by silicates. Fitting of XRD peaks yields a crystalline coherence length of about 15 nm, indicating that the particles seen in the SEM are polycrystalline. GRM intensities of most samples are of the same order as the NRM, while others show much stronger GRM although their magnetic properties are similar. Variation of the demagnetization sequence confirms that GRM is mainly produced perpendicular to the AF direction. The anisotropy direction can be derived from GRM, but more systematic studies are needed for detailed conclusions. An attempt to correct for GRM failed due to high GRM intensities and because smaller GRM acquisition was also found along the demagnetization axis. Behaviours of acquisition and AF demagnetization of GRM are comparable with those of NRM, ARM, IRM, indicating fine grain sizes of remanence carriers.

mentioning

confidence: 99%

Gyromagnetic remanence acquired by greigite (Fe3S4) during static three-axis alternating field demagnetization

Appel

Hoffmann

et al. 1998

“…Later experiments confirmed these initial findings and examined the importance of the relative orientation and frequencies of the specimen rotation and alternating field (Brock & Iles 1974;Edwards 1980aEdwards , 1984Stephenson 1985). Further work has compared the characteristics of RRM with other modes of remanence (Edwards 1980b(Edwards , 1984Stephenson 1976). It is believed that RRM may explain the erratic changes in remanence that are sometimes produced by alternating field demagnetizers which tumble the specimen.…”

Section: Introductionmentioning

confidence: 93%

Some observations of the gyroremanent magnetization acquired by rocks in a rotating magnetic field

Noël

1988

A rotating magnetic field of approximately constant magnitude was used to induce a gyroremanent magnetization (GRM) in natural and synthetic rock specimens. Negligible GRM was produced in fields below 10mT but in higher fields growth was rapid. GRM was induced either parallel or antiparallel to the field rotation vector in different specimens. The results are'interpreted by extending a model for rotational remanent magnetization to include the interaction between an anisotropic rotating field and rock magnetic fabric.

“…There have been several other works broadly similar to that described in this article. Stephenson (1976) progressively demagnetized an ARM, and an RRM, each acquired in an A F of 60 mT, in A F s of strengths up to 40 mT and concluded that there is a fundamental difference between ARM and RRM demagnetization curves. In separate studies Stephenson (1985;1983, respectively) has given theoretical accounts of the acquisition of PRARM, and the acquisition and subsequent static demagnetization of ARM.…”

Section: Introductionmentioning

confidence: 99%

Static alternating field demagnetizations of anhysteretic and rotational remanent magnetizations in rocks and synthesized samples

Edwards

Desta

1989

S U M M A R Y Expressions for intensities of rotational anhysteretic remanent magnetization (RARM) and partial RARM (PRARM) as functions of demagnetizing alternating magnetic field (AF) strength are derived on the basis of a dilute isotropic assembly of identical single-domain particles. These are extended in the case of PRARM to simulate an actual sample by including the effects of several assemblies of particles with discrete anisotropy fields within the limited coercivity range of PRARM, and tested on a variety of samples including rocks and synthesized specimens using demagnetizing AF strengths, B, up to three times the inducing AF strength, Bi. Unexplained discrepancies between expected and observed results appear when B = 2Bi, where theoretically, demagnetization should be completed. Demagnetization curves of partial rotational remanent magnetizations (PRRM) induced in a manner analogous t o PRARM are experimentally identical to those of PRARM. One of several possible reasons which might account for the observed discrepancies is that particles other than single domain (e.g. pseudo-single domain and/or multidomain) might be contributing to PRARM and PRRM. A tentative conclusion that the effective gyrofield is independent of particle orientation supports a similar conclusion reached by Stephenson & Potter (1987).