Description: The past ∼200 million years of Earth's geomagnetic field behavior have been recorded within oceanic basalts, many of which are only accessible via scientific ocean drilling. Obtaining the best possible paleomagnetic measurements from such valuable samples requires an a priori understanding of their magnetic mineralogies when choosing the most appropriate protocol for stepwise demagnetization experiments (either alternating field or thermal). Here, we present a quick, and non‐destructive method that utilizes the amplitude‐dependence of magnetic susceptibility to screen submarine basalts prior to choosing a demagnetization protocol, whenever conducting a pilot study or other detailed rock‐magnetic characterization is not possible. We demonstrate this method using samples acquired during International Ocean Discovery Program Expedition 391. Our approach is rooted in the observation that amplitude‐dependent magnetic susceptibility is observed in basalt samples whose dominant magnetic carrier is multidomain titanomagnetite (∼TM 60–65 , (Ti 0.60–0.65 Fe 0.35–0.40 )Fe 2 O 4 ). Samples with low Ti contents within titanomagnetite or samples that have experienced a high degree of oxidative weathering do not display appreciable amplitude dependence. Due to their low Curie temperatures, basalts that possess amplitude‐dependence should ideally be demagnetized either using alternating fields or via finely‐spaced thermal demagnetization heating steps below 300°C. Our screening method can enhance the success rate of paleomagnetic studies of oceanic basalt samples. Plain Language Summary Oceanic basalts are ideal recorders of the Earth's magnetic field. To decipher magnetic histories recorded in rocks, paleomagnetists need to isolate the magnetization directions and intensities within rocks by one of two possible methods. One method typically involves progressively heating the samples to high temperatures. The other method involves exposing samples to alternating magnetic fields with increasing peak field intensities. Both of these methods are ultimately destructive to the original magnetization preserved within rocks. However, without knowledge of a given rock's magnetic mineralogy, randomly choosing thermal or alternating field demagnetization methods may result in high failure rates. We developed a pre‐screening method to help decide which cleaning method will likely be more successful for a given sample based on low‐field magnetic susceptibility measurements. These measurements do not affect the original magnetic information recorded in a rock, thereby permitting subsequent paleomagnetic studies on the same sample. Our technique can be performed as rapidly as 2 min per sample, is non‐destructive, and does not require complicated sample preparation. Key Points Paleomagnetic studies utilize either alternating field or thermal demagnetization, but it is difficult to choose the best protocol a priori Amplitude‐dependence of magnetic susceptibility measurements permits preliminary magnetic mineralogy characterization in submarine basalts Rapid amplitude‐dependence measurements may aid in deciding upon the best demagnetization protocol for submarine basalt samples

Description: Since King presented the ‘plates and spheres’ model in an attempt to investigate the origin of the inclination error in sediments, no one to date has conducted specific experiments designed to separate the individual contribution of platy and spherical particles to depositional remanent magnetizations (DRMs). It is commonly accepted that it is the flattening of plates, rather than the rolling of spheres that is the main source of inclination error in sediments. Recently, however, Bilardello et al. have shown that spheres alone may lead to significant amounts of shallowing. A comparison of experiments run in parallel using synthetic platy and spherical particles is presented. Experiments of the duration of 24 hr were run in 100 μT field intensity ( μ 0 H ) and varying field inclinations ( I F ) from vertical to horizontal. A systematic dependence of the magnetization on field inclination is apparent. Results indicate that magnetic moment measurements are more repeatable for spherical particles than for plates, yielding smaller uncertainties. Inclination measurements, however, are more repeatable for platy particles, with a more linear relationship of inclination error to applied field inclination. Moreover, plates yield smaller inclination error than spheres. A clear field inclination dependency of the inclination error also exists, with the error decreasing through field inclinations of 30°, 60° and 90°. A continuous acquisition experiment involving plates was also run up to 10 d of deposition in μ 0 H = 100 μT and I F = 60°. The acquisition curves for moment, inclination and thickness of depositing sediment are compared to the mean curves measured for spheres by Bilardello et al. under the same field conditions. No unequivocal evidence of compaction of the platy particles is observed, while the inclination error is acquired virtually instantaneously for all particles. These preliminary results contradict the widespread understanding that inclination shallowing is more prominent for platy particles (e.g. hematite) than it is for more spherical particles (e.g. magnetite). It is true that larger amounts of shallowing have been commonly observed in natural hematite-bearing rocks, but the overall ranges of shallowing are also larger. The particles used in these experiments may not be a reliable proxy for natural crystals and one must exercise caution when extrapolating to the natural scenarios; however, the results provide insight into the behaviour of differently shaped particles.

Description: High-quality palaeomagnetic data for the early Carboniferous of Central Asia are scarce and the palaeogeographic evolution of this area prior to final amalgamation of the region east of the Ural mountains is still rather obscure. Here, we present palaeomagnetic data for early Carboniferous deposits from two areas in the Kyrgyz North Tianshan (NTS). Detailed rock-magnetic analysis indicates the presence of magnetite and haematite as magnetic carriers in these red sediments. In the Kazakh basin section (KEL), we identify a high-temperature component (HTC) of magnetization during stepwise thermal demagnetization at temperatures of up to ~680 °C yielding a site mean direction of D  = 176.2°, I  = –36.4°, k  = 57.4 and α 95  = 8.9° after tilt correction. Two HTCs of magnetization were identified in samples from the Sonkul Basin (DUN) with maximum blocking temperatures of ~600 °C (magnetite) and ~680 °C (haematite). The magnetite component was also identified with alternating field demagnetization. The resulting site mean directions for these two components identified in 16 and 14 sites, respectively, are D  = 149.3°, I  = –50.3°, k  = 73.6 and α 95  = 4.3° for the magnetite and D  = 139.6°, I  = –35.1°, k  = 71.6 and α 95  = 4.7° for the haematite component. All three mean directions show a significant increase of the precision parameter k after tilt correction indicating acquisition of the high-temperature magnetization prior to the main folding event in the Jurassic. We explain the difference of the two components of DUN by a process of inclination bias due to compaction to which the platy haematite particles are more susceptible. Applying the elongation-inclination (E/I) method to directional data from over 100 individual samples from location DUN results in a negligible correction for the magnetite component (〈5°), whereas the inclination of the haematite component corrects from –35.0° to –50.3° ( f  = 0.6, error interval –41.4° to –57.9°), which is then equal to the uncorrected magnetite inclination. The small number of samples from section KEL does not allow application of the E/I technique and inclination correction based on high field anisotropy of isothermal remanent magnetization was applied, yielding a corrected inclination of –75.2° ± 4°. Assuming comparable degrees of compaction for both study areas and applying the flattening factor obtained in DUN on samples from KEL, however, would result in comparable inclinations. The identification of inclination shallowing at both sections indicates that the age of magnetization is close to the deposition age. Assuming a reversed polarity of the directions from both areas results in palaeolatitudes of ~30°N for section DUN and ~60°N for the anisotropy-based correction of section KEL. The large difference, however, is geologically very unlikely. The inclination of the magnetite component of DUN (unaffected by inclination shallowing) favours a palaeoposition of ~30°N. This is supported by the inclination shallowing corrected haematite component of DUN yielding a comparable inclination. Therefore, our results indicate that the NTS domain was situated at ~30°N in the early Carboniferous. Furthermore, the NTS zone was probably not connected to Baltica or Siberia prior to the late Palaeozoic.

Description: Separating the contribution of different hematite coercivity grains to the magnetic fabric is a standing problem in rock magnetism because of the common occurrence of thermochemical alterations when measuring the anisotropy of thermal remanence. A technique that eliminates this bias is presented, which is useful when there is a need to separate the fabric of detrital from pigmentary hematite, for example. The method is based on stepwise thermal demagnetization of saturation isothermal remanent magnetizations (IRMs) applied orthogonally on three sister specimens, allowing calculation of the anisotropy tensor from the three components of each demagnetized IRM vector, avoiding the necessity of having to apply IRMs to thermochemically altered specimens. Vector subtraction allows determining the anisotropy tensor for specific unblocking-temperature ranges. The anisotropies of the pigmentary, specular and total hematite of the Mauch Chunk Formation red beds of Pennsylvania have been measured from an oriented block sample and results are compared to previous anisotropy measurements performed using the high-field anisotropy of isothermal remanence technique (hf-AIR), which measures total undifferentiated hematite. Experiments were conducted using non-saturating 1 T and fully saturating 5.5 T fields: both experimental sets seem capable of measuring the orientation of the specularite anisotropy principal axes, but 5.5 T are needed to capture the orientation of the higher coercivity pigmentary grains. The magnitudes of the principal axes, instead, are only faithfully measured using 5.5 T fields and yield somewhat higher anisotropies than those measured by hf-AIR. The fundamental requirement for this technique is homogeneous material among the three sister specimens, which is a significant limitation; homogeneity tests allow assessment of applicability of the method and reliability of the results.