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[en] Complete text of publication follows. Extremely high coercivity, occasionally exceeding 100 mT at room temperature, can be found for some kinds of basalts or andesites. These samples contain titanomagnetite (or slightly altered to titanomaghemite) and usually come from quenched facies, e.g., a cryptocrystalline portion of pillow basalt or a surface clinker of subaerial andesite, suggesting that fine-grained titanomagnetite is responsible for the high coercivity. Low-temperature variations of saturation magnetization (Ms), saturation remanence (Mr) and coercivity (Hc) were measured for pillow basalts and subaerial andesites (originally containing ∼TM60 and ∼TM10, respectively) down to 20 K using MPMS in order to infer governing magnetic anisotropies. Ms and Mr values were calculated after subtracting paramagnetic contributions by applying a maximum field of 5 T. For all the samples Mr/Ms ratios at room temperature were less than 0.5, which is expected for randomly oriented single-domain grains with uniaxial anisotropy. Cooling down the samples below room temperature, Mr/Ms ratios increase somehow beyond 0.5 but still remain well below 0.831 or 0.866 expected from multi-axial magnetocrystalline anisotropy. This suggests that multi-axial anisotropy might govern the ratios or that the maximum field of 5 T is still insufficient to saturate in the low temperature range. Coercivity at room temperature ranges from ?10 mT of coarse-grained portions of pillow basalts and andesites to ∼90 mT of the cryptocrystalline portion. Basalts exhibit lower coercivity values than andesites with comparative Mr/Ms ratios. Upon cooling coercivity gradually increased up to several hundreds of mT especially for pillow basalts, implying that such magnetic hardening is likely magnetostrictively controlled.
[en] Complete text of publication follows. The magnetic domain state of the remanence carrying fraction affects absolute paleointensity determinations in various different ways. Multidomain magnetic behaviour produces magnetization tails and ongoing stabilization during repeated heating steps. As a consequence, concave and s-shaped curves are observed in Thellier-type experiments and multispecimen protocols (MSP) have been suggested to avoid these problems. Single domain behaviour leads to a strong cooling rate dependency and false results if natural cooling rates differ from the ones used during laboratory treatment. Usually overestimates of field intensity result. In order to overcome such single domain bias due to cooling rate differences, a correction technique is introduced and tested for synthetic volcanic glass. These remelted glasses are produced under controlled laboratory conditions facilitating SD behaviour, known field values, and a variety of different cooling rates. Uncorrected paleointensity values overestimate the field by up to 35% for usually observed cooling rate differences. Only after correction the correct field value is predicted. The effect of multidomain remanence on a multispecimen protocol, leading to field overestimates and scatter, is demonstrated. This protocol is then extended by experimental tests to recognize any multidomain bias and to significantly reduce related scatter in the MSP diagram. Even a correction for multidomain contribution is possible, which in case of the investigated synthetic magnetites of well defined grain size allows for reconstructing the originally applied field strength.
[en] Detailed description of the characteristics of bulk and surface properties of synthetic magnetites employed in simulation studies is given. Different synthetic routes are discussed, and the products obtained are assessed. The relevance of composition and of particle size and shape for meaningful extrapolation to plant situation is discussed. (author)
[en] Complete text of publication follows. There are two types of commonly used methods for determining absolute paleointensity from volcanic rocks: the Thellier-type method (Thellier and Thellier, 1959) and the Shaw-type method (Shaw, 1974). Most paleomagnetists have regarded the former method as the most reliable, but evidence is accumulating which indicates that the Thellier-type method is not always robust for historical basaltic samples. There have been a relatively increasing number of studies to clarify possible causes of incorrect Thellier paleointensity determinations obtained from historical lava flows. It seems that many authors prefer 'multi domain (MD) grains' as the predominant cause. However, apart from the MD grains, we think high-temperature (HT) oxidation states of titanomagnetite (TM) grains might also influence to resultant Thellier paleointensities: (1) low to middle degree of magnetostatic interactions are expected for low to middle HT oxidized TM grains (low to middle numbers of ilmenite lamellae) possibly resulting in about 10∼30 % high Thellier paleointensity; (2) high degree of magnetostatic interactions as well as acquisitions of thermochemical remanent magnetization (TCRM) are expected for middle to high HT oxidized TM grains (abundant ilmenite lamellae) possibly resulting in about 20∼70 % high Thellier paleointensity. This conjecture is based on the thorough experimental results obtained from the Hawaiian (Kilauea 1960) and Japanese (Sakurajima 1914 and 1946) historical lava flows. We will introduce the detail in the talk.