Increasing the efficiency of paleointensity analyses by selection of samples using first‐order reversal curve diagrams

Abstract

[1] The global paleointensity database is restricted by the high failure rate of paleointensity analyses. Excluding thermal alteration, failure is usually caused by the presence of multidomain grains and interactions among grains, two properties that can be identified using first-order reversal curve (FORC) diagrams. We measured FORC diagrams on sister samples of about 200 samples that had been used for Thellier paleointensity determinations and determined criteria to discriminate samples that gave acceptable paleointensity results from those that did not. The three most discriminating criteria are the vertical spread of the FORC distribution (indicative of interactions), expressed as the full width at half maximum (FWHM), the spread of the FORC distribution along the H c = 0 axis (width), and the bulk coercivity H c (both indicative of domain state). Setting thresholds at 132 mT for the width of the distribution and 29 mT for the FWHM maximizes the number of unsuccessful rejected samples. Using an additional threshold of H c = 5.4 mT results in rejection of 32% of unsuccessful samples. Seven samples that barely satisfy the paleointensity selection criteria would also be rejected using these selection criteria. Most of the samples that fail the paleointensity experiment without being detected by our selection criteria have ideal noninteracting single-domain magnetic properties but fail because of the thermal alteration that results from repeated heating. Being able to eliminate at least one third of unsuccessful samples using our FORC diagram-based prescreening procedure should provide a significant improvement in efficiency of paleointensity measurements.Citation: Carvallo, C., A. P. Roberts, R. Leonhardt, C. Laj, C. Kissel, M. Perrin, and P. Camps (2006), Increasing the efficiency of paleointensity analyses by selection of samples using first-order reversal curve diagrams,

Paleointensity estimates from oceanic gabbros: Effects of hydrothermal alteration and cooling rate

“…This is in sharp contrast to most igneous rocks (Carvallo et al, 2006) and to the altered olivine. Therefore, FORC diagrams can be a very effective pre-selection tool for paleointensity studies on plutonic rocks which contain exsolved magnetite.…”

“…1(b)) carries a low NRM/TRM ratio. Although the FORC diagrams indicate some magnetostatic interactions, the interaction seems to be low compared to volcanic rocks which yielded successful Thellier paleointensity results (Carvallo et al, 2006); thus, the magnetostatic interactions are unlikely to explain the low NRM/TRM. The low NRM/TRM is considered to simply reflect the low efficiency of chemical remanence (e.g., Stacy and Banerjee, 1974).…”

“…1(a)) which is composed of non-interacting SD particles. The near-ideal horizontal ridges are rare even in volcanic samples (Carvallo et al, 2006), so igneous magnetic minerals are unlikely to explain the horizontal ridges observed in successful samples. The similarity between the FORC diagram for low NRM/TRM samples ( Fig.…”

Nanofabrication of two‐dimensional arrays of magnetite particles for fundamental rock magnetic studies

“…However, rocks rarely contain ideal assemblages of monodisperse SD particles. Techniques used to determine the magnetic domain state of remanence carriers, such as plotting the ratio of saturation remanence over saturation magnetization (M RS /M S ) versus the ratio of coercivity of remanence over coercive force (B CR /B C ) [Day et al, 1977], or determining first-order reversal curve (FORC) diagrams [Pike et al, 1999;Roberts et al, 2000], often reveal the presence of a mixture of SD and multidomain (MD) particles, or of pseudosingle-domain (PSD) particles [e.g., Dunlop, 2002;Carvallo et al, 2006]. These larger particles generally have a lower magnetic stability than SD particles and can lead to erroneous determinations of absolute paleointensity [e.g., Krása et al, 2003;Leonhardt et al, 2004].…”

Investigating the archaeointensity determination success of prehistoric ceramics through a multidisciplinary approach: new and re-evaluated data from Greek collections

“…In the literature, several pre-selection procedures before archaeointensity determinations have been proposed (e.g. Cui & Verosub 1995;Cui et al 1997;Jordanova et al 1997Jordanova et al , 2001Leonhardt et al 2004;Carvallo et al 2006). Nevertheless, some of these procedures are almost as time consuming as a Thellier experiment and a full success cannot still be guaranteed.…”