Theory and application of the Day plot ( M rs / M s versus H cr / H c ) 1. Theoretical curves and tests using titanomagnetite data

Abstract

Although most paleomagnetic and environmental magnetic papers incorporate a Day plot of the hysteresis parameters M rs / M s versus H cr / H c , a comprehensive theory covering superparamagnetic (SP), single‐domain (SD), pseudo‐single‐domain (PSD), and multidomain (MD) (titano)magnetites is lacking. There is no consensus on how to quantify grain‐size trends within the Day plot, how to distinguish MD from SP trends/mixtures, or whether magnetite, titanomagnetites, and other minerals have distinctive trends by which they might be identified. This paper develops the theory of the Day plot parameters for MD, MD + SD, PSD, and SP + SD grains of titanomagnetite (Fe 3 – x Ti x O 4 ) with compositions x = 0 (TM0 or magnetite) and x = 0.6 (TM60). MD grains have a separate trend that intersects the curve for SD + MD mixtures. SP + SD mixtures generate a variety of trends, depending on the SP grain size. All SP + SD curves lie much above those for MD or SD + MD trends, as has been proposed, but not demonstrated, previously. Data for PSD‐size magnetites of many different origins fall along a single trend, but different levels of internal stress shift points for similar grain sizes along the “master curve.” In order to use the Day plot to determine grain size, one must have independent information about the state of internal stress. Theoretical model curves for SD + MD mixtures match the PSD magnetite and TM60 data quite well, although the SD→MD transition region in grain size is much narrower for TM60 than for magnetite. The agreement between PSD data and SD + MD mixing curves implies that PSD behavior is due to superimposed independent SD and MD moments, either in individual or separate grains, and not to exotic micromagnetic structures such as vortices. The theory also matches M rs and H c values in mechanical mixtures of very fine and very coarse grains, although nonlinear mixing theory is required to explain some H cr and H cr / H c data.