Detrital Remanence, Inclination Errors, and Anhysteretic Remanence Anisotropy: Quantitative Model and Experimental Results

Abstract

S U M M A R YWe suggest that inclination errors in detrital remanent magnetization (DRM) may be recognized and corrected by measurement of the anisotropy of anhysteretic remanent magnetization (ARM). ARM anisotropy reflects directional variations in the remanence capacity of relatively fine-grained magnetic particles in rocks or sediments, generally the same particles that carry the stable component of DRM.Relative vertical and horizontal DRM magnitudes are controlled by this directional remanence capacity, as well as by the alignment efficiency of the particle magnetic moments, which in turn is governed by the relative intensities of the vertical and horizontal components of the ambient magnetic field. Thus the respective vertical and horizontal components of palaeointensity , and therefore palaeofield inclination, may be obtained by normalizing the measured DRM components by parallel ARM intensities.Synthetic sediments containing silica, kaolin, and sized magnetite powders have previously been found to acquire experimental DRM with an inclination error that is a direct function of kaolin concentration. Further experiments have now shown that ARM anisotropy in the synthetic sediments is also a direct function of kaolin content, and thus a correlation exists between DRM inclination errors and ARM anisotropy. Multiplying the measured DRM vector by the inverse of the ARM anisotropy matrix yields an improved estimate of the actual laboratory depositional field in all cases. For samples with acicular 0.5 p m magnetite, this procedure reduces a mean inclination error of over 5" to less than 0.5". Samples with subequant magnetite of slightly larger grain size (0.75 pm) exhibit similar ARM anisotropy (-10 per cent), but somewhat larger inclination errors (-10'). We interpret this in terms of stronger horizontal alignment but weaker particle anisotropies for the larger, more equant magnetites.

Compaction‐corrected inclinations from southern California Cretaceous marine sedimentary rocks indicate no paleolatitudinal offset for the Peninsular Ranges terrane

“…The extracted magnetic minerals were mixed with epoxy, and the mixture was allowed to cure in an electromagnet maintaining a DC magnetic field with intensity between 20 and 70 roT. A suite of epoxy drying experiments was conducted with increasingly stronger magnetic fields [Jackson et al, 1991;Kodama, 1997] to determine if magnetic grain alignment was saturated. After the epoxy samples hardened, their pARM spectra and AAR were measured.…”

A successful rock magnetic technique for correctng paleomagnetic inclination shallowing: Case study of the Nacimiento Formation, New Mexico

“…Both thermal and af demagnetization were used, as well as modern principal component analysis techniques for demagnetization data analysis [Kirschvink, 1980], since the previous Nacimiento Formation paleopole was only based on blanket af demagnetization data. The AAR of af demagnetized samples was measured, and the individual magnetic particle anisotropies were determined either by measurement of magnetic separates [Jackson et al, 1991] or by compaction experiments with disaggregated Nacimiento material [Kodama and Davi, 1995]. These dam were used to correct the Nacimiento Formation inclination, and the result will be compared to the inclination predicted from the Montana alkalic province paleopole [Diehl et al, 1983].…”

Theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy

“…At the same time, the ARM has been used as a grain size indicator in rock magnetism [e.g., Johnson et al, 1975;King et al, 1982], and as a normalization factor in paleointensity studies [e.g., Shaw, 1974;Rolph and Shaw, 1985]. The anisotropy of ARM (AARM) has found application in sediments for correction of inclination shallowing of detrital remanent magnetization [e.g., Jackson et al, 1991;Kodama and Sun, 1992;Kodama, 1997]. More recently, ARM magnetization curves have been used to quantify magnetofossils in freshwater and marine sediments [Egli, 2004].…”