Salvation of Primary Remanence From Hydrothermally Altered Oceanic Gabbros in the Oman Ophiolite: A Selective Destructive Demagnetization Approach
Ophiolite massifs provide important insights about magnetic remanence structure of oceanic crust and its modification by water-rock interactions. Widespread hydrothermal alteration and formation of secondary magnetite have been problems for paleomagnetic work on gabbros in the Oman ophiolite. By means of selective removal of silicate minerals, we revealed that large secondary remanence is associated with hydrothermally altered olivine and pyroxenes in gabbro and gabbronorite in the Wadi Rajmi area. Removal of these hydrothermally altered mafic minerals revealed a cryptic remanene component which could not be detected by stepwise demagnetization of bulk rock core samples. After the mechanical removal, samples consist of plagioclase and clinopyroxene and exhibit remanence directions of southeast declination and shallow inclination. This direction is consistent with previously reported paleomagnetic directions at crystallization of the Oman ophiolite. In contrast, bulk rock core samples yielded north declination, resembling the younger remanence directions associated with the obduction of the ophiolite. Microscopic observation revealed that the cryptic remanence is carried by tiny exsolved magnetite in plagioclase and clinopyroxene. On the basis of these paleomagnetic directional analysis and microscopic observations, we argue that the cryptic component is primary remanence direction. Contribution of secondary remanence to bulk natural remanence in our samples was estimated to be as high as 80%, on the basis of comparison between remanence intensities of bulk samples and samples after mechanical removal of hydrothemally altered minerals. Our results suggest that previous paleomagnetic data from whole rock gabbros in the Oman ophiolite as well as in other ophiolites should be taken with care. Moreover, this study demonstrates that some oceanic gabbros are quite vulnerable to hydrothermal remagnetization, raising concern on the acquisition timing of bulk magnetic signals of intrusive rocks recovered by dredge or drilling. We propose potential use of paleomagnetic microanalysis of plagioclase and clinopyroxene to check the coherence of magnetization in oceanic gabbros.
Magnetic Mineralogy of mid-Pleistocene Sediments From the Valles Caldera, New Mexico
Ferromagnetic resonance and magnetic carriers in Archean Buck Reef Chert from the Barberton Greenstone Belt, South Africa
The nature of the magnetic carriers in the ∼3.4 billion year old Buck Reef Chert of the Barberton Greenstone Belt, South Africa, has been studied as it has been suggested that the chert may contain potential biogenic microstructures. Depositional environments of the Buck Reef Chert may have included shallow water environments and, as such, biogenic magnetite might be associated with them. Scanning Electron Microscope observations of magnetic separates and thin sections show cubo-octahedral to quasi-rectangular and hexagonally shaped grains that fall within a stable single domain range typical of biogenic magnetite. Preliminary transmission electron microscope and Energy Dispersive Spectrometer data have also been collected. Magnetic hysteresis properties of bulk samples show a variety of hysteresis behaviors (including multi-domain, pseudo-single domain and wasp-waisted curves). Analyses of bulk magnetic susceptibility versus temperature suggest alteration on heating (100-400 degrees C). Ferromagnetic resonance (FMR) has been used to investigate the possibility of magnetite produced by magnetotactic bacteria. Preliminary FMR spectra of some bulk Buck Reef Chert samples appear asymmetrical and skew towards low fields, suggesting a magnetic anisotropy that is similar to the spectra seen with some strains of magnetotactic bacteria. However, results to date should be interpreted in terms of the original depositional environment, and the subsequent geologic history. While some of the magnetic minerals within the separates observed in SEM may be primary, it is important to recognize low grade metamorphism has occurred to peak temperatures of 250 to 320 degrees C in the region. Depositional sulfides (e.g. greigite) in the originally sulfur rich environment may have been converted to other iron oxides during subsequent metamorphic events. We will discuss attempts to better distinguish primary from secondary magnetic minerals in the Barberton rocks.