Switching field angular dependence and the measurement of anhysteretic remanence magnetization (ARM)

Abstract:

We describe a procedure for measuring ARM coercivity spectrum with a three-axis static alternating field demagnetization (AFD). The approach is uniquely designed to mitigate the effects of switching field angular dependence (SFAD) and gyroremanence acquisition. The measurement of ARM is important for applications to tectonics, volcanology, sedimentology, environmental and paleoclimate studies and geomagnetic field behavior. Often, however, too little attention is paid to the effects of SFAD which govern the coercivity and angular distributions of grains that actually carry the ARM. As a result, commonplace methods of ARM measurement, such as turning on the direct field within a narrow AF window (i.e, partial ARM), are not ideally designed for their intended purpose of accurately isolating the ARM contribution from a specified coercivity grain fraction. We show that the best method of measuring an ARM is to differentiate a progressive tumble AFD of a total ARM generated with a peak AF high enough to fully activate the coercivity range of interest. Fortunately for experimental convenience, a static AFD along 3 or more axes can reasonably substitute for a tumble method. The size of the fully activated coercivity range from a given AF is governed by grain SFAD, typically being only ~1/3 to 1/2 the peak AF intensity for a large range of magnetite grain sizes. Only this lower-coercivity, fully activated portion of a progressive ARM demagnetization should be used to characterize the ARM coercivity spectrum. Using ARM removed above this range will cause errors in intensity (and direction for static AF methods), ultimately leading to overestimation of relative paleointensity, smearing of coercivity distribution, amplification of ARM anisotropy with concomitant error in natural remanence corrections, and unwanted mixing of anisotropy fabric components held in separate coercivity grain fractions. Concepts we introduce for measuring ARM also apply to measurement of isothermal remanence. Additionally, we will describe our preferred procedure (i.e. “the leap-frogged double permutation”) for choice of AF axes and the order in which they are applied during progressive three-axis static demagnetization. This technique can be used as standard protocol for removal of both natural and laboratory imparted remanences.