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Abstract The study of sedimentary magnetism in the intermontane Tarom Basin (northern Iran) offers insights into local paleoenvironmental conditions during global middle‐late Miocene climate changes and the topographic growth triggered by the Arabia‐Eurasia continental collision. Rock magnetic results reveal that the ∼16.2 to ∼10–9 Ma coarse‐grained deposits at the basin's southern margin present a homogenous magnetic mineral assemblage, reflecting sediment provenance. Conversely, the ∼13.2 to ∼7.6 Ma, fine‐grained deposits in the basin's depocenter include alternating playa‐lake and lacustrine deposits, recording dry, evaporative conditions, leading to hematite formation in a low‐temperature oxidizing environment, and wetter conditions that preserve the original detrital signal, respectively. Time series analyses show cyclicity in different period bands for magnetic susceptibility, but precession and obliquity cycles can hardly be resolved in the record. Comparison with deep‐sea oxygen isotope data suggests that from ∼13.2 to ∼10.8 Ma environmental conditions likely mirrored global climatic forcing, with lacustrine and playa‐lake deposits associated with increased and decreased global temperature, respectively. At ∼10.8 Ma, the basin likely recorded the Tortonian Thermal Maximum with the establishment of a lacustrine system. From ∼10.4 Ma, the magnetic susceptibility signal departed from the global climate record, possibly due to basin margin (western Alborz and Tarom mountains) and regional (Anatolian‐Iranian plateau) topographic growth, accompanied by increased precipitation seasonality, focused rainfall and augmented erosion rates. Finally, we suggest that before ∼10.8 Ma, the Hadley cells expanded northward, leading to a trade‐dominated system with moist air masses sourced from the Caspian, while from ∼10.8 Ma, westerlies dominance progressively prevailed. 

Abstract Some rock and soil samples exhibit significant loss of magnetic susceptibility (χ) with increasing applied field amplitude even at relatively low (10–100s of A/m) fields, a behavior which remains unexplained. Exceptionally strong negative field‐dependence of susceptibility (χ_HD) is present in sandstones and altered intermediate‐felsic igneous rocks in several cores from the northeastern Oklahoma subsurface. These same rocks also show elevated frequency‐dependence of susceptibility (χ_FD), with reasonable correlation ofχ_HDtoχ_FD, and frequency‐dependentχ_HD. Results from multiple characterization methods indicate that strongly negativeχ_HDin these rocks is linked to a yet‐unidentified phase which begins the approach to magnetic saturation in low fields (<1 mT/800 A/m), shows elevatedχ_FDto low temperatures, is unstable at high temperatures, possesses significant anisotropy of magnetic susceptibility, and becomes paramagnetic above ∼83°C. Clear associations with fluid alteration features indicate that this material may be highly relevant to rock alteration, diagenetic, and environmental studies. 

SUMMARY The Verwey transition in magnetite is a crystallographic phase transition occurring in the temperature range 80–125 K and depends on stoichiometry and cation substitution, which may in turn serve as an indicator of the conditions under which magnetite was formed or altered in nature. We have analysed the distribution of Verwey transition temperatures (TV) in a large set of samples (N = 1110) from a wide variety of rocks, sediments, and other natural and synthetic materials containing magnetite, mined from the database of the Institute for Rock Magnetism and from published studies. The analysis is restricted to measurements of remanence while warming through the transition from which TV was determined by the derivative method. Our analysis showed that the TV distribution exhibited a generally bimodal distribution of Verwey transition temperatures, both for the entire data set and for almost all of the lithological subsets. There is a sharp peak for values in the range 118–120 K, and a broad, relatively flat or polymodal distribution from about 98 to 118 K. The upper end of the distribution was sharp, with only a few values exceeding 124 K, and the tail on the lower end extended down to about 80 K. Virtually all of the sample types exhibited polymodal distributions, almost always with one peak near 120 K, and with one or more additional peaks at lower temperatures. Biogenic magnetites produced by magnetotactic bacteria had the lowest modal value of TV (100 K). Loesses (103.5 K) and igneous extrusives (102.5 K) also had low modal transition temperatures and distributions with dominant low-TV peaks. Lithological groups with the highest modal transition temperatures were modern soils (119.5 K), silicate minerals with exsolved magnetite (119 K) and sedimentary rocks (119 K). Numerical experiments confirmed that the derivative method for the determination of TV was reasonably robust and that the observed distributions cannot be explained as an artefact related to the determination of TV from individual thermomagnetic runs but rather is a general characteristic of natural magnetites. The results provide context for studies that interpret TV in particular samples in terms of natural processes or conditions during formation or alteration of magnetite. 

SUMMARY Exsolved iron oxides in silicate minerals can be nearly ideal palaeomagnetic recorders, due to their single-domain-like behaviour and the protection from chemical alteration by their surrounding silicate host. Because their geometry is crystallographically controlled by the host silicate, these exsolutions possess a shape preferred orientation that is ultimately controlled by the mineral fabric of the silicates. This leads to potentially significant anisotropic acquisition of remanence, which necessitates correction to make accurate interpretations in palaeodirectional and palaeointensity studies. Here, we investigate the magnetic shape anisotropy carried by magnetite exsolutions in pyroxene single crystals, and in pyroxene-bearing rocks based on torque measurements and rotational hysteresis data. Image analysis is used to characterize the orientation distribution of oxides, from which the observed anisotropy can be modelled. Both the high-field torque signal and corresponding models contain components of higher order, which cannot be accurately described by second-order tensors usually used to describe magnetic fabrics. Conversely, low-field anisotropy data do not show this complexity and can be adequately described with second-order tensors. Hence, magnetic anisotropy of silicate-hosted exsolutions is field-dependent and this should be taken into account when interpreting isolated ferromagnetic fabrics, and in anisotropy corrections. 

SUMMARY Understanding the temporal changes of the Earth’s magnetic field intensity is one of the main goals of modern palaeomagnetism. For most palaeointensity methods to yield reliable results, the magnetic minerals must obey a set of rules. One of these rules is the additivity of partial thermal (TRM) or anhysteretic remanent magnetizations (ARM). Additivity was previously shown for partial TRM in single-domain particles and more generally for ARMs. Additivity between these two low-field remanences, however, has not been investigated, yet. This paper presents a series of rock magnetic experiments on natural low Ti titanomagnetites (Curie temperature between 534 °C and 561 °C) examining the effects of high temperatures on alternating field (AF) demagnetization and acquisition of an ARM. One of our sample sets comes from a borehole drilled through the impact melt sheet of the Manicouagan crater (Canada), the other from the Rocche Rosse lava flow on the island of Lipari (Italy). Hysteresis parameters indicate the magnetic carriers in the pseudo-single-domain range showing no evidence for oxidation. Thermal demagnetization at 300 °C and 500 °C before AF demagnetization shifts the coercivity spectra towards higher fields. AF demagnetization experiments at 500 °C show a significant (by a factor between 1.4 and >7.6) reduction in median destructive field and a shift towards lower coercivities. A linear relationship was found between the peak magnetic field required to demagnetize a fraction of a full TRM of a sample at a specific temperature and the one necessary to demagnetize the same fraction at room temperature after heating to that temperature. The comparison of full ARM and partial TRM at successively higher temperatures with a hybrid hTARM reveals that combined additivity between the two kinds of remanences is fulfilled. These results open the possibility to demagnetize highly coercive minerals, such as hematite and goethite, which is often not achievable at elevated temperatures. Furthermore, the additivity of TRM and ARM remanences may be used to develop novel hybrid TRM/ARM palaeointensity methods for samples, where heating is problematic (e.g. in meteorites).