HR: 11:05h
AN: GP12A-04    [Abstracts]
TI: Characterizing the Superparamagnetic Grain Distribution f(V,Hk) by Thermal Fluctuation Tomography
AU: * Jackson, M J
EM: irm@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, 291 Shepherd Labs 100 Union St SE, Minneapolis, MN 55455 United States
AU: Carter-Stiglitz, B S
EM: cart0196@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, 291 Shepherd Labs 100 Union St SE, Minneapolis, MN 55455 United States
AU: Solheid, P A
EM: peat@umn.edu
AF: Institute for Rock Magnetism, University of Minnesota, 291 Shepherd Labs 100 Union St SE, Minneapolis, MN 55455 United States
AB: Increasing absolute temperature T causes the progressive unblocking of single-domain (SD) magnetic grains with increasing volume V, thereby providing the primary means of quantifying the SD grainsize distribution f(V). According to Néel theory, unblocking temperatures T_UB depend not only on V but also on the thermal variation of spontaneous magnetization M_s(T) and of microcoercivity H_k(T). Weak-field and zero-field thermomagnetic granulometry methods (k_fd(T) and thermal demagnetization) thus require some assumptions about these properties in order to estimate f(V). Dunlop [1965, J. Geomag. Geoelectr.] showed that the joint distribution f(V,H_k0) can be determined for magnetically monomineralic, thermally-stable SD (SSD) ensembles by taking advantage of the joint temperature- and field-dependence of relaxation time. Dunlop's method involves imprinting pTRMs over narrow intervals of temperature (thus magnetizing sets of grains with narrow ranges of the product VH_k) followed by stepwise AF demagnetization at room temperature. It is elegant but not easily adaptable to the study of ultrafine particles that are superparamagnetic (SP) at room temperature. We have developed and tested a procedure that follows Dunlop's strategy of using the joint temperature- and field-dependence of relaxation time to obtain f(V,H_k0) for ensembles containing both SP and SSD grains. We measure backfield remanence curves over a range of temperatures and calculate the derivative dM/dH curves. Each point on the derivative curves represents the integrated contribution from grains that lie along a corresponding (H,T) blocking contour on the Néel plot. Tomographic reconstruction methods use line-integral samples to reconstruct 2- or 3-dimensional spatial distributions of physical properties such as seismic wave velocity, and we apply the same approach to reconstruct the 2-D f(V,H_k0) distribution. Samples of the base of the Tiva Canyon Tuff have narrow size distributions of elongate Ti-poor titanomagnetite. Tomographic inversion of the low-temperature backfield spectra yield sharply-peaked f(V,H_k0) distributions, from which we calculate modal grain dimensions in excellent agreement with those observed in previous studies [Schlinger et al., 1991, JGR] by TEM.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005