The ThellierTool4.0 is an intuitive and easy‐to‐use software which provides the possibility to analyze a wide range of different modifications of the Thellier absolute paleointensity experiment (available at http://earthref.org/tools/). Besides the Arai plot for paleointensity determination, orthogonal projections of the direction, decay of NRM during thermal demagnetization, and additional plots regarding alteration and multidomain checks enable the user to visualize the quality of individual determinations. Experimental checks for magnetomineralogical changes, either in‐field or zero‐field pTRM* checks, are evaluated regarding their differences to the corresponding pTRM* acquisition in two most commonly used ways. Furthermore, a measure for the cumulative alteration differences beginning at room temperature is calculated, and the possibility to correct for magnetomineralogical changes is provided. Two different experimental methods to check for multidomain bias are supported and analyzed by the software. Intensity differences recorded by pTRM*‐tail checks are calculated. Accounting for the directional difference between applied laboratory field and magnetization of the sample, the effective pTRM*‐tail is determined, and thus failures of Thellier's law of independence are monitored. Failures of the law of additivity, experimentally observed by additivity checks, are also evaluated by the software. The vectorial character of individual measurements is fully considered for all calculations. Uniform selection criteria for acceptance and rejection of determinations can be applied, and a set of such criteria with emphasis on minimal bias due to alteration, multidomain remanence, and analysis/experimental inaccuracies is suggested.

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S U M M A R YA range of basaltic samples from Olby (France) and Vogelsberg (Germany) displaying the phenomenon of partial and complete self-reversal was studied in order to decipher the physical mechanism responsible for self-reversed magnetic remanence in basalts. Microscopic observations and rock magnetic measurements show titanomagnetite to be the carrier of remanence. Due to low-temperature oxidation the titanomagnetites are present in two magnetic phases forming close side-by-side phase assemblages: The ore grains consist of a non-oxidized magnetically soft titanomagnetite part with Curie temperatures between 140 and 300 • C and a magnetically hard low-temperature oxidized part with Curie temperatures between 410 and 590 • C. Magnetic force microscopy observations present evidence that the oxidation process does not only influence the Curie temperatures, and thus the blocking temperature spectrum, but also the domain configuration. During acquisition of a thermoremanence the two phases are magnetically coupled, leading to a remanent magnetization of the low Curie temperature phase which is antiparallel to the applied external magnetic field. Computational modelling of the remanence acquisition process explains the coupling of the two phases by magnetostatic interaction. As the observed partial oxidation of ore grains is not uncommon in subaerial basalts, partial and complete self-reversal is probably a frequently occurring phenomenon. Nevertheless, it remains unnoticed in most cases as it cannot be detected by the standard stepwise thermal demagnetization technique. Despite the complex remanence properties, our investigations indicate that the upper part of the blocking temperature spectrum still carries reliable palaeodirectional information.The term self-reversal describes the phenomenon in which the remanent magnetization of a certain magnetic material is directed opposite to the external field in which this magnetization was acquired. Nagata et al. (1952) were the first to report on an impressive example of self-reversal of the natural remanent magnetization in hypersthene hornblende dacite samples from Mount Haruna (Japan). A year before the work of Nagata et al. was published, Néel (1951) predicted the possibility of self-reversal of remanent magnetization and proposed theoretical mechanisms leading to this phenomenon. These mechanisms can be subdivided into two classes: single-phase models and models with two interacting magnetic mineral phases. For the case of only one magnetic phase, Néel noted that a particular thermomagnetic behaviour of certain ferrimagnetic minerals, which he called N-type behaviour (Néel 1948), can lead to a selfreversed remanent magnetization. The first example for this type of self-reversal above room temperature in a natural rock sample was described by Schult (1976) for the weathered rim of an alkali basaltic hand sample. Recently, Doubrovine & Tarduno (2004) reported on partial and complete self-reversal due to N-type behaviour in some oceanic basalts. All other cases of...