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Lawrence, RM, Gee JS, Hurst SD.  1997.  Magnetic anisotropy in serpentinized peridotites from Site 920; its origin and relationship to deformation fabrics. Proceedings of the Ocean Drilling Program, Scientific Results. 153:419-427.   10.2973/odp.proc.sr.153.041.1997   Abstract

Anisotropy of magnetic susceptibility (AMS) measurements on serpentinized peridotites from Ocean Drilling Program Site 920 reveal a strong magnetic "fabric," typically characterized by an oblate susceptibility ellipsoid. Curie temperatures and max- imum unblocking temperatures near 580°C, as well as petrographic observations, suggest that magnetite is the sole magnetic carrier in the serpentinites. Because the magnetic mineralogy is dominated by coarse-grained magnetite, the susceptibility ellip- soid should provide a three-dimensional image of the average elongation of magnetite grains or grain clusters. Petrographic studies of three orthogonal thin sections from a limited number of samples indicate that the preferred shape orientation of mag- netite grain clusters correlates well with the apparent susceptibility maxima and minima in these planes. The magnetite long- axis preferred orientation is typically within -20° of the maximum principal axis of the susceptibility ellipsoid. The close corre- spondence between the magnetic foliation and the orientation of magnetite-bearing serpentine veins, together with the petro- graphic evidence for the distribution of magnetite, suggests that the magnetic "fabric" is primarily a reflection of the orientation of these veins. Hence, the AMS ellipsoid may be a more accurate descriptor of the integrated three-dimensional vein orienta- tions than visual orientation measurements made on the split cores.

Lawrence, RM, Gee JS, Karson JA.  2002.  Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones. Journal of Geophysical Research-Solid Earth. 107   10.1029/2000jb000007   AbstractWebsite

[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1degrees +0.8degrees/ -1.4degrees) that is shallower than either the expected geocentric axial dipole inclination (40.7degrees) or present-day inclination (41.9degrees) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (> 350 degreesC) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5degrees) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350 degreesC. Accounting for the anisotropy-related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.

Lawrence, K, Johnson C, Tauxe L, Gee J.  2008.  Lunar paleointensity measurements: Implications for lunar magnetic evolution. Physics of the Earth and Planetary Interiors. 168:71-87.   10.1016/j.pepi.2008.05.007   AbstractWebsite

We analyze published and new paleointensity data from Apollo samples to reexamine the hypothesis of an early (3.9-3.6 Ga) lunar dynamo. Our new paleointensity experiments on four samples use modern absolute and relative measurement techniques, with ages ranging from 3.3 to 4.3 Ga, bracketing the putative period of an ancient lunar field. Samples 60015 (anorthosite) and 76535 (troctolite) failed during absolute paleointensity experiments. Samples 72215 and 62235 (impact breccias) recorded a complicated, multicomponent magnetic history that includes a low-temperature (< 500 degrees C) component associated with a high intensity (similar to 90 mu T) and a high temperature (> 500 degrees C) component associated with a low intensity (2 [LT). Similar multi-component behavior has been observed in several published absolute intensity experiments on lunar samples. Additional material from 72215 and 62235 was subjected to a relative paleointensity experiment (a saturation isothermal remanent magnetization, or sIRM, experiment); neither sample Provided unambiguous evidence for a thermal origin of the recorded remanent magnetization. We test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples. Specifically, an overprint from exposure to a small magnetic field (an isothermal remanent magnetization) results in multi-component behavior (similar to absolute paleointensity results) from which we could not recover the correct magnitude of the original thermal remanent magnetization. In light of these new experiments and a thorough re-evaluation of existing paleointensity measurements, we conclude that although some samples with ages of 3.6 to 3.9 Ga are strongly magnetized, and sometimes exhibit stable directional behavior, it has not been demonstrated that these observations indicate a primary thermal remanence. Particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock. As relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities, the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable. Consequently, current paleointensity measurements do not support the existence of a 3.9-3.6 Ga lunar dynamo with 100 mu T surface fields, a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models. (c) 2008 Elsevier B.V. All rights reserved.