Beyond Neel's theories: thermal demagnetization of narrow-band partial thermoremanent magnetizations

Partial thermoremanent magnetization (pTRM) was imparted over a narrow temp erature interval, T = 370-350 degreesC, to a suite of crushed and annealed natural magnetite samples, ranging in grain size from approximate to1 to 12 5-150 mum and in domain state from small pseudo-single-domain (PSD) to mult idomain (MD). In this way, effectively a single blocking temperature, T-B, of pTRM was activated. Stepwise thermal demagnetization did not erase the p TRMs sharply at T-B, as for single-domain (SD) grains. Demagnetization bega n well below 350 degreesC and continued above 370 degreesC, with a median u nblocking temperature, T-UB, close to 360 degreesC. The largest grains devi ated most from SD behavior. Their pTRM demagnetized over the entire interva l from room temperature to the Curie point, in accordance with predictions for MD grains. In terms of the unblocking temperature distribution f(T-UB) or slope dM/dT of the thermal demagnetization curve, SD grains have a sharp spectrum, T-UB = T-B; MD grains were observed to have a broad, roughly sym metrical spectrum centered on T-B; and intermediate size grains in the PSD range had non-Gaussian spectra, combining a central peak near T-B with broa d tails above and below T-B. In this respect, PSD grains display a superpos ition of SD and MD remanences, not a blend between the two. Practical impli cations of these observations are that Thellier's law of reciprocity (T-UB = T-B) will be increasingly violated as grain size increases in the PSD ran ge. The low-T-UB part of f(T-UB) produces anomalously large demagnetization of NRM in low-temperature heating steps of Thellier-type paleointensity de terminations and a sagging shape of the Arai plot. The high-T-UB part of f( T-UB) results in undemagnetized remanence at and above T-B in thermal demag netization. Among pre-treatments designed to make remanence more SD-like in subsequent thermal cleaning, alternating field (AF) pre-cleaning sharpened f(T-UB) more effectively than low-temperature demagnetization for the 20 m um sample. (C) 2001 Elsevier Science B.V. All rights reserved.