Comparison and calibration of nonheating paleointensity methods: A case study using dusty olivine

Abstract

[1] We present a comparative study of nonheating paleointensity methods, with the aim of determining the optimum method for obtaining paleointensities from ''dusty olivine'' in chondritic meteorites. The REM method, whereby thermoremanent magnetization (TRM) is normalized by saturation isothermal remanent magnetization (SIRM), is shown to ''over normalize'' TRM in dusty olivine due to the transformation of stable single-vortex (SV) states to metastable single-domain (SD) states in a saturating field. The problem of over normalization is reduced in the REMc and REM' methods, which more effectively isolate the high-coercivity stable SD component of remanence. A calibration factor of f ¼ 1600 (1000 < f < 2900) is derived for the REM' method. Anhysteric remanent magnetization (ARM) is shown to be a near perfect analogue of TRM in the stable SD component of dusty olivine. ARM normalization of the high-coercivity (100-150 mT) remanence with a calibration factor f ARM ¼ 0.91 (0.7 < f ARM < 1.2) yields paleofield estimates within 6 30% of the actual field values for SD dominated samples. A Preisach method for simulating TRM acquisition using information extracted from first-order reversal curve (FORC) diagrams is shown to work well for SD dominated samples, but fails when there is a large proportion of SV remanence carriers. The failure occurs because (1) SV states are not properly incorporated into the Preisach distribution of remanence carriers, and (2) the acquisition of TRM by SV states is not properly modeled by the underlying SD thermal relaxation theory.

Solar nebula magnetic fields recorded in the Semarkona meteorite

“…Finally, our applied bias field of 100 µT for the ARM acquisition experiments is well within the range where ARM intensity varies linearly with the bias field. For these reasons, we expect similar performance of the ARM normalization method for our dusty olivinebearing chondrules compared to the SV grains dominated synthetic dusty olivine grains used by Lappe et al (29), implying that 1σ uncertainty due to the ARM calibration factor is ~20%.…”

“…The high coercivities above 170 mT indicate that SD kamacite grains are responsible for carrying part of the HC magnetizations (29). The field geometry observed in SQUID Microscope and NV diamond magnetometer maps confirm that high coercivity magnetization is carried by dusty olivine grains (Fig.…”

“…6 HC/HT magnetizations were indeed acquired parallel to the spin axis. Seventh, the coercivity 154 spectrum of the HC component of dusty olivine-bearing chondrules is very similar to that of an 155 ARM and dissimilar to that of an IRM, which suggests that the HC component was acquired as a 156 TRM (29). We therefore conclude with high confidence that the HC/HT magnetizations 157 observed in dusty olivine-bearing chondrules are TRMs acquired in the solar nebula.…”

“…TRM acquisition experiments on analogs of dusty olivine chondrules have shown that the 159 ARM normalization method potentially produces paleointensities accurate to ~40% [2σ; (29)]. 160…”

Magnetic domain state diagnosis using hysteresis reversal curves

“…For example, Pike and Fernandez [] studied submicron Co dots and suggested that the magnetic vortex state should give rise to two positive regions and a possible extra butterfly pattern in FORC diagrams. However, these features have only been observed so far in synthetic samples with a narrow grain‐size range [ Pike and Fernandez , ; Dumas et al , , ; Lappe et al , , ] and in low‐temperature FORC measurements [ Smirnov , , ; Carvallo and Muxworthy , ]. Hysteresis mechanisms in multidomain (MD) samples have also been analyzed using FORC diagrams [ Pike et al , ], and it has been concluded that the classical domain wall pinning model of Néel [] does not explain the behavior of natural MD samples.…”