Natural Remanent Magnetization Research Articles

A Non‐Magnetized Chondrite Parent Body Revealed by Paleomagnetic Investigation of LL6 Chondrite NWA 14180

AbstractMagnetic records from meteorites provide valuable information about the formation and evolution of the solar system and planets. The parent planetesimals of chondrites are typically considered to be undifferentiated based on their primary chemical composition and texture. However, recent paleomagnetic investigations of various chondrites indicate that they carry a primary remanence generated by a dynamo, suggesting partial differentiation of their parent planetesimals. The presence of a dynamo within the parent planetesimal of LL chondrites remains uncertain due to the ambiguous origin of the remanent magnetism. Here, we report petrographic, paleomagnetic, and rock magnetic properties for the novel LL6 chondrite NWA 14180. The high metamorphic temperature experienced by NWA 14180 could have removed the pre‐accretionary remanence. The fusion crust baked‐contact test suggests that NWA 14180 preserves primary magnetic information about its parent body. Alternating field demagnetization results from interior subsamples reveal distinct low‐ and medium‐coercivity components that may represent a viscous remanent magnetization acquired in the geomagnetic field. No natural remanent magnetization was unblocked in the high coercivity range, implying that NWA 14180 cooled in zero‐field conditions. Therefore, we suggest that the parent body of NWA 14180 did not have a dynamo. Furthermore, this result suggests that the LL chondrite parent planetesimal accreted later and was smaller in size than other chondrite classes.

Petromagnetism and paleomagnetism of kimberlite pipesof the Verkhnemunskoe deposit (Yakutsk diamondiferous province)

The purpose of the studies of kimberlite pipes of the Verkhnemunskoe diamond field is to calculate the Middle Paleozoic paleomagnetic pole to clarify the trajectory of the apparent migration of the pole and reconstructions of the paleogeographic position of the Siberian platform at the time of the manifestation of active tectono-magmatic processes. The Verkhnemunskoye deposit is located within the Verkhnemunskoye kimberlite field of the Yakutsk diamondiferous province and includes five kimberlite pipes (Deimos, Zapolyarnaya, Komsomolskaya-Magnitnaya, Novinka and Poiskovaya), the age of which according to geological and isotopic data is estimated as Late Devonian-Early Carboniferous (372–347 Ma). For the first time scalar and vector physical parameters of kimberlites and captured xenoliths from different structural-material complexes of the Earth’s crust, as well as the host terrigenous sedimentary rocks of the Early Paleozoic were obtained, which are necessary for the development of physical-geological models of the Verkhnemunskoe field deposits. A relatively deep level of erosional shearing of the field has been established. The primary (synchronous with the formation of the field) natural residual magnetization was preserved in the kimberlite cohesive mass. The main carrier minerals of natural remanent magnetization vectors of kimberlites are unaltered magnesioferrite and magnetite, which indicates their thermo-sufficient nature. The natural remanent magnetization vectors of captured xenoliths indicate that the influence of hypergenic processes did not strongly affect the NRM vectors of kimberlites. Firing test is positive. The paleomagnetic pole with coordinates Φ = 26.5°N, Λ = 142.2°E, dp/dm = 6.2/7.8° was calculated from the obtained clusters of N = 10 vectors of the primary natural remanent magnetization of kimberlite pipes. On its basis, we reconstructed the paleogeographic position of the Siberian Platform, which at the time of the kimberlite intrusion was located in the middle latitudes of the northern hemisphere and was facing north with its southern edge.

Jurassic–Cretaceous boundary in the Dedina section (Serbian Carpathians): Effects of remagnetization on magnetostratigraphy

The Upper Tithonian to Lower Berriasian carbonate sequence of the Getic Nappe system was studied near Golubac (eastern Serbia) using rock-magnetic and paleomagnetic methods to verify the age of the magnetization and to correlate magnetostratigraphy with biostratigraphy. A major part of the Dedina section shows the presence of authigenic goethite, hematite and magnetite as carriers of remagnetization. The youngest overprint, residing in goethite, sometimes carrying up to 90 % of natural remanent magnetization, was probably received after 18 Ma. The remagnetization residing in hematite and magnetite, attributed to the late Early Cretaceous collision, was obtained during long normal polarity Chron C34 (118–82 Ma). The mean direction implies a clockwise post-remagnetization rotation by about 57°. The normal (Dn) and reverse (Er) polarity components, heavily affected by the chemo-remanent magnetization overprint, can be tentatively interpreted in terms of polarity zones. Thus, the obtained data enabled a preliminary identification of M17r to M19n.2n magnetochrons. The correlation of magnetostratigraphy with biostratigraphy of the Dedina section contributes to the stratigraphic framework necessary for the definition of the Berriasian Global Boundary Stratotype Section and Point.

Salient Changes of Earth's Magnetic Field Toward the End of Cretaceous Normal Superchron (CNS)

AbstractChanges in Earth's magnetic field during the Cretaceous Normal Superchron (CNS) spanning ∼121 Ma to ∼84 Ma hold important clues about the geodynamo evolution. Canonical models predict a persistently strong geomagnetic field with low variability during CNS, which, however, has not been observed in the available absolute paleointensity data and seafloor marine magnetic anomaly (MMA) records. The lack of relative paleointensity (RPI) data across CNS further impedes tests of model predictions. Here, we present a ∼9‐Myr (∼94–∼85 Ma) RPI record from a Turonian to Santonian hemipelagic succession from IODP Site U1512 offshore southern Australia. Detailed paleomagnetic and rock magnetic analyses demonstrate that the ratio of natural remanent magnetization (NRM) demagnetized at 20 mT over magnetic susceptibility (MS), that is, NRM20mT/MS, as a reliable proxy for the RPI of the Upper Cretaceous succession. The new RPI record shows marked changes in both intensity and variability at ∼90.8 Ma. Also, the 6 Myr‐long (∼94–∼88 Ma), near‐continuous, ∼1.2 kyr‐resolution RPI record exhibits a strong positive correlation between field intensity and variability. Assuming this correlation holds for the entire CNS, an extrapolated RPI curve for the entire CNS is obtained by integrating the positive correlation with field variability estimates from the MMA data. The extrapolated RPI curve shows a strong and highly variable field in the middle CNS but a weak and stable field at its beginning and ending. These features imply a much more dynamic geodynamo than previously thought, and provide crucial benchmarks for unraveling the geodynamo evolution during CNS.

High‐Resolution Paleomagnetic Secular Variation Since ∼13 ka From a Loess Section in Northwest China and a Regional Geomagnetic Directional Model for East Asia

AbstractPaleomagnetic secular variation (PSV) records provide important information for the dynamic processes of the Earth's geomagnetic field, and also can be used for regional stratigraphic correlation. We conducted a paleomagnetic study on a high sedimentation rate Holocene loess section (the Minle section) with precise 14C age constraints in Northwest China. Rock magnetic results indicate that single domain and pseudo‐single‐domain magnetites are the main magnetic carriers of the natural remanent magnetization. Combining the alternating field demagnetization and thermal demagnetization results, we obtained a high‐resolution PSV record with both absolute declination and inclination since ∼13 ka. Combining with previously published sedimentary records in East Asia, we developed an East Asian PSV reference curve over the past 14 ka (EASed14k) by relocating all the data to a common location (30ºN, 108ºE) and averaging them through Fisher statistics. We further generated a regional geomagnetic directional model spanning the past 14 ka for East Asia (SCHA.EAS14k), using the in situ PSV records and applying the revised spherical cap harmonic analysis in space and cubic B‐spline in time. The predicted directional curves from this model at the common location are consistent with the EASed14k curves, which represent the first‐order variation of the geomagnetic field in this area. The established reference curve and regional model for East Asia in this study will be used for regional and global comparison of the geomagnetic field.

Remagnetization of magnetite-bearing rocks in the Eastern Qiangtang Terrane, Tibetan Plateau (China): Mechanism and diagnosis

Remagnetization is a common yet notorious phenomenon that interferes with paleogeographic reconstruction. Classical paleomagnetic field tests are helpful in detecting remagnetization but their diagnostic power is limited: remagnetization may occur before folding, the tilting age may be ambiguous, or protracted remagnetization may yield dual polarities. Rock magnetic information can provide other constraints on our understanding of the origin of natural remanent magnetization (NRM). Here we focus on the rock magnetic properties of acknowledged remagnetized limestones and unremagnetized rocks of the Zaduo area in the Eastern Qiangtang Terrane, Tibetan Plateau (China). Chemical remanent magnetization is suggested as a more frequent mechanism than the thermoviscous resetting of the NRM. The secondary NRM resides in authigenic magnetite of stable single domain and superparamagnetic (SP) size which grew during post-depositional burial processes. Both high-field and low-field thermomagnetic runs reveal the alteration of existing iron sulfides to magnetite in the remagnetized limestones. NRM decay curves show that the maximum unblocking temperature of the remagnetized samples is significantly lower than that of the unremagnetized samples. Component analysis of acquisition curves of the isothermal remanent magnetization (IRM) reveals a hard component that represents SP magnetite in remagnetized limestones. This component is absent in unremagnetized rocks. End-member modelling reveals a convex curve in the coefficient of determination versus the number of end-members plot for the unremagnetized limestones, whereas the remagnetized rocks exhibit both near-linear and convex shapes. In addition, quantitative analysis of the hysteresis loop shape for different lithologies indicates its validity in detecting remagnetization. Furthermore, we show the differences in the hysteresis data distributions of the two rock types on the Day plot, the Néel diagram, the Borradaile diagram, and the Fabian diagram. Our research emphasizes that rock magnetic properties can serve as tools to diagnose remagnetization in magnetite-dominated rocks. We recommend a comprehensive rock magnetic study to discriminate remagnetization, involving the Day plot, Fabian diagram, thermal demagnetization curves, IRM component analysis and end member modelling.

Determination of the magnetization direction via correlation between reduced‐to‐the‐pole magnetic anomalies and total gradient of the magnetic potential with vertical magnetization

AbstractThe total magnetization of an underground magnetic source is the vector sum of the induced magnetization and the natural remanent magnetization. The direction of the total magnetization serves as important a priori information in the inversion and processing of magnetic data. We demonstrated that the total gradient of the magnetic potential with vertical magnetization constitutes the envelope of the vertical component of the magnetic field for all directions of the Earth's field and source magnetization. The total gradient of the magnetic potential with vertical magnetization and the reduction‐to‐the‐pole field simultaneously tend to achieve maximum symmetry near the correct total magnetization direction. As a result, the total magnetization direction can be estimated by computing the correlations between the reduction‐to‐the‐pole and the total gradient of the magnetic potential with vertical magnetization. The proposed method yields accurate magnetization directions in synthetic model examples. The total gradient of the magnetic potential with vertical magnetization is less susceptible to data noise than transforms which are derived from the high‐order magnetic field derivatives or tensors. The estimation results are slightly affected by changes in the source magnetization direction. In a field example in the Weilasito region (North China), the reduction‐to‐the‐pole fields calculated using the estimated magnetization directions are well centred over the source. The proposed method obtained a more focused magnetization direction than that of a three‐dimensional magnetization vector inversion. The total gradient of the magnetic potential with vertical magnetization therefore provides a novel and accurate approach to determine the total magnetization direction from the total field anomaly in a variety of situations.

Magnetic micro-archaeology: a method for conducting rock magnetic microfacies analysis on archaeological soil micromorphology samples, with a case study from El Salt, Alcoy, Spain

For geoarchaeologists the use of multiproxy and multimethod integrative approaches has become common for studying the sedimentary context of archaeological materials. In this paper, we introduce a novel approach that combines rock magnetic analysis with micromorphological analysis. We first conduct a brief examination of the magnetic properties of organic resins used in micromorphology to understand the potential impact of the resin and manufacturing process on the magnetic signature. It is crucial to understand the original production of the samples to ensure that endothermic reactions do not compromise the magnetic data. We illustrate the advantages of this approach through a case study of a pit hearth from the Neanderthal archaeological site of El Salt in Alcoi, Spain. This study utilises archived micromorphological samples of a pit hearth (combustion structure H77) initially described by Leierer et al. (J Archaeol Sci 123:105237, 2020). Rock magnetic analysis of multiple magnetic parameters—including magnetic susceptibility, natural remanent magnetization, and progressive isothermal remanent magnetization—reveals that each microfacies type exhibits a unique magnetic pattern, even if there is overlapping in signatures across some magnetic parameters. The interpretation of the magnetic data aligns with the results and microfacies descriptions previously provided by Leierer et al. (J Archaeol Sci 123:105237, 2020), further supporting the interpretation of the combustion feature as being in situ with a history of repeated use. The approach presented here offers an enhanced tool for micromorphologists to better understand the magnetic characteristics of microfacies units in higher resolution and establish direct connections to the micromorphological results.

Geomagnetic paleointensity dating of mid-ocean ridge basalts from the neo-volcanic zone of the Central Indian Ridge

Describing the evolution of the neo-volcanic zone in the spreading ridge is essential for understanding the dynamics and environments of abyssal basins. However, the absolute dating of ocean floor basalts is generally difficult. As a characteristic age indicator, absolute intensity of past geomagnetic field (absolute paleointensity, API) is useful to date ocean floor basalts. In this study, we adopted the Tsunakawa–Shaw method to measure APIs of whole-rock seafloor basalts collected from a conical cone on the Central Indian Ridge and performed rock magnetic experiments. We conducted the experiments on a total of 18 specimens (two or three specimens from each of eight lava sites). Six specimens from two lava sites with different morphologies (pillow and sheet), three for each, passed the acceptance criteria. API means at site level are 33.0 ± 1.0 and 35.8 ± 1.7 μT, respectively. The similarity of API site means suggests that they erupted within a short period. These site-level API means are approximately 0.7 to 0.8 times the present geomagnetic intensity of 46.0 μT at the sampling sites. The accepted specimens show higher Curie temperature, lower initial intensity of natural remanent magnetization, higher ratio of saturation remanence to saturation magnetization (Mrs/Ms), and signal of harder magnetic mineral than rejected ones. Our primary comparison between the two site-level API means and the 1590–present high-resolution IGRF-13 + gufm1 model constrains that the eruption timing of the conical cone to be < 1590 CE. When we compared the two site-level API means with the paleointensity curves calculated from the BIGMUDI4k.1 and ArchKalmag14 k.r, we found that they overlap in the period of − 7575 to −1675 CE or − 25 to 1590 CE, which may be the eruption timing of the conical cone. We concluded that timing of recent volcanic eruption in abyssal environment could be investigated by using appropriate rock magnetic selection and carefully examined API.Graphical

Hayabusa 2 returned samples reveal a weak to null magnetic field during aqueous alteration of Ryugu's parent body

The JAXA Hayabusa 2 mission returned 5.4 g of material from the C-type asteroid Ryugu. The Mn-Cr ages of dolomite in the returned samples indicate that Ryugu's parent body experienced aqueous alteration sometimes between <1.8 and 6.8 Myr after CAI formation. Because this time range overlaps with the lifetime of the solar nebula, we investigate the possibility that magnetite and pyrrhotite, which are aqueous alteration products found in Ryugu samples, acquired a remanent magnetization reflecting the nebula field intensity. We analyze the intrinsic magnetic properties and paleomagnetic record of three Ryugu samples of 0.82, 0.97 and 21.87 mg. None of the samples exhibit a stable natural remanent magnetization. This indicates that the aqueous alteration of Ryugu's parent body took place either in a field of a few µT, or in a very weak to null field. In the former scenario, the solar nebula field is the most likely magnetizing field, implying that aqueous alteration occurred before its dissipation, i.e., before ∼5 Myr after CAI formation. In the latter scenario, aqueous alteration must have occurred either after the dissipation of the nebula, or at an earlier epoch and a large heliocentric distance (> 5 au). The similarities between Ryugu samples and CI chondrites favor this second hypothesis. Our results contrast with another paleomagnetic study of two Ryugu samples, arguing for a paleofield intensity of 40 to 390 µT. Our interpretation of this discrepancy is that these samples were exposed to artificial magnetic fields (> mT) during preceding experiments. This highlights the importance of conducting, as much as possible, the paleomagnetic investigations of returned samples before any other experiment. We also demonstrate that the ratio of NRM over low-field magnetic susceptibility is a powerful, non-destructive indicator of magnetic contamination. We recommend measuring this ratio routinely before paleomagnetic investigations of meteorites and returned samples.

First Results of the Intraformational Conglomerate Test of the Late Riphean Katav Formation (Southern Urals)

Detailed petrographic, electron-microscopic, and paleomagnetic studies were conducted on flatbedded synsedimentary carbonate breccias from three blocks of rocks (one block from Minyar, two blocks from Katav-Ivanovsk) to verify the ideas about the time of formation of the high-temperature component (HTC) of natural remanent magnetization (Jn) of limestones from the Upper Riphean Katav Formation, Southern Urals. Petrographic and electron-microscopic observations revealed that the composition of pebbles corresponds to that of the host matrix rocks, thus enabling the use of an intraformational conglomerate test. In all the studied blocks, the distribution of paleomagnetic directions for pebbles is generally chaotic, with the concentration parameter not exceeding 3. These test results meet the Graham criterion. The pebbles’ average Jn have a larger confidence oval and differ significantly from the matrix’s directions with a much smaller confidence oval. The Hodges-Ajne test was applied to the blocks from Katav-Ivanovsk. The Rayleigh test was used for the block from Minyar. The results obtained at this stage indicate that the HTC of magnetization of the Katav limestones may have a primary origin. If this is confirmed, the Katav Formation would be a good paleomagnetic record of the Earth’s Neoproterozoic history and provide valuable insights into the geomagnetic field behavior in the Late Precambrian.

Inverse magnetic fabric of remagnetized limestones in the Zaduo area, Eastern Qiangtang Terrane: Implications for oroclinal bending in the Eastern Himalayan Syntaxis

Magnetic fabric analysis is a common technique to assess the strain regime during mountain building processes. Here, we use this approach to evaluate the tectonic evolution of the Tibetan Plateau and the Eastern Himalayan Syntaxis by analyzing the limestones of the Jurassic Buqu Formation in the Zaduo area, Eastern Qiangtang Terrane (China). These limestones were chemically remagnetized during the Cenozoic. For a proper assessment, it is relevant to understand how the mineralogy of the remagnetized limestones affects their magnetic fabric and how the magnetic fabric can improve our understanding of the tectonic strain and regional deformation. The role of the authigenic magnetite in the development of the magnetic fabric should thus be explored. Comparison of the bulk susceptibility (Km) with various natural and laboratory rock magnetic properties (Km versus natural remanent magnetization, Km versus saturation isothermal remanent magnetization, and Km versus saturation magnetization) indicates that susceptibility and remanences are both carried by authigenic magnetite. Most of the magnetite grains show axial ratios <1.3:1 according to the Néel diagram, and fall within the single-domain range based on the mass magnetic susceptibility (χ) and DC field-normalized anhysteretic remanent magnetization (χARM) ratio, giving rise to the inverse magnetic fabric observed. Twelve sites (120 specimens) are divided into four groups based on the magnetic fabric and rock magnetic behaviors. Overall, there is a clear trend of decreasing Km, natural remanent magnetization, saturation isothermal remanent magnetization, ferromagnetic percentage and shape parameter from Group I to IV. The K1 axis of all four groups documents a NNE-SSW oriented compression during remagnetization, contrasting with the Eocene NE-SW compression in the Gongjue area farther east. This different compressional regime likely resulted in different rotations and structural trends surrounding the Eastern Himalayan Syntaxis.

Magnetism of the Acapulco Primitive Achondrite and Implications for the Evolution of Partially Differentiated Bodies

AbstractPrimitive achondrites like the acapulcoites‐lodranites (AL) clan are meteorites that formed on bodies in the process of forming a metallic core, providing a unique window into how early solar system processes transformed unmelted material into differentiated bodies. However, the size and structure of the parent body of ALs and other primitive achondrites are largely unknown. Paleomagnetism can establish the presence or absence of a metallic core by looking for evidence of a dynamo field. We conducted a magnetic study of the Acapulco acapulcoite to determine its ferromagnetic minerals and their recording properties. This is the first detailed rock magnetic and first paleomagnetic study of a primitive achondrite group. We determined that metal inclusions inside silicate grains consist of two magnetic minerals, kamacite and tetrataenite, which have robust recording properties. However, the mechanisms and timing by which these minerals acquired any natural remanent magnetization are unknown. Despite this, Acapulco has not been substantially remagnetized since arriving on Earth and therefore should retain a record dating to 4.55 billion years ago. Future studies could characterize this record by using high‐resolution magnetometry measurements of individual populations of grains and developing an understanding of how and when they became magnetized. Our discovery of tetrataenite in ALs provides the first mineralogical evidence for slow cooling [&lt;∼5–10 × 103°C per million years (Ma−1)] of the AL parent body at low temperatures (∼320°C). Its presence suggests the AL parent body is unlikely to have been catastrophically disrupted at AL peak temperatures (∼1,200°C) without subsequent reaccretion.

On the early development of Huastecs (Gulf of Mexico) revealed by the Earth's magnetic field recorded in domestic hearths

AbstractA detailed magnetic mineralogy and archaeomagnetic study was carried out on recently discovered domestic hearths and burned floors at the Chak Pet archaeological settlement (Tamaulipas, Mexico). The study aimed to obtain reliable absolute chronological constraints on the early development of Huastecs during the Formative period. Oriented hand samples corresponded to four domestic hearths and one burned floor. Continuous thermomagnetic curves revealed mostly irreversible behavior, while titanomagnetites, titanomaghemites, and goethites are assumed to carry the remanent magnetization. In total, 87 specimens were subjected to stepwise demagnetization of natural remanent magnetization using an alternating field procedure. Characteristic remanent magnetization directions were obtained for 29 samples of two hearths and one burned floor. No single, technically acceptable paleointensity determination was obtained. The new archaeomagnetic age intervals for Chak Pet allow locating the origin of this settlement at the Gulf of Mexico within the Middle Formative (900–600 BCE) continuing until the Late Formative period (350–100 BCE). New archaeomagnetic ages are in accordance with the diagnostic pottery analysis. Dated archaeological elements are associated with both ceramic types and different sets of burials, providing a reliable tool to calibrate their chronological and stratigraphic positions.

Dynamic physical-geological model of complex folding according to paleomagnetic data

The purpose of the study is to evaluate the possibility of using the mathematical apparatus of spatial rotations to solve the determination issues of the nature and age of natural remanent magnetization vectors under complex tectonic dislocations of rocks manifested in the areas of modern folded structures of platform framing. Dynamic physical-geological models are shown to be useful when forming complex tectonic-magmatic systems. Mathematical (computer) modeling in comparison with other methods of studying geological processes features higher accuracy, cost-effectiveness and unambiguity of data interpretation in achieving the set goal. On the basis of dynamic physical-geological model of complex folding, an algorithm is developed and the results of mathematical modeling of natural remanent magnetization vectors are given for solving direct and inverse problems on correct application of the fold test under complex rock deformations. The dynamic physical-geological model of complex folded structure formation shows that the characteristic natural remanent magnetization vectors identified in laboratory experiments on demagnetization can be used to determine their age relative to the folding stages, and, depending on this, fully or partially restore the number, sequence and direction of tectonic dislocations. This will enable us to solve the mineralogenic and geodynamic problems of folded region development more effectively on the basis of paleomagnetic data.

New insights on faulting in southwest New Brunswick and northeast Maine based on marine magnetic mapping of the Ministers Island dyke

The Passamaquoddy Bay region of southwest New Brunswick and northeast Maine is known for its moderate seismicity, yet the structures responsible for these earthquakes are not well understood. To gain insight on the tectonic history of this region, marine magnetic surveys were conducted at speculated intersections of faults and the Early Jurassic Ministers Island dyke. Analysis of the vertical gradient of the magnetic field strongly suggests that the 189 ± 8 Ma Ministers Island dyke is dextrally offset 370 m by the Oak Bay Fault below the St. Croix River. This sense of motion points to a reversal of fault movement compared to that recorded in much older (Silurian) rocks nearby. The interpretation is supported by magnetic modelling, constrained by direct measurements of the magnetic susceptibility and natural remanent magnetization of the dyke. In Passamaquoddy Bay, however, farther to the east, there is no evidence that slight deviations in the trend of the dyke anomaly or its termination result from post-emplacement faulting. Furthermore, the data suggest that a sub-parallel mafic dyke on the northern shore of Passamaquoddy Bay is either a distinct dyke system or a left stepover emplacement of the Ministers Island dyke, as opposed to a faulted offset as previously speculated. This study illustrates how focused high-resolution magnetic surveys can strengthen interpretations of faulting in areas of geological ambiguity.

Magnetite in Muong Nong‐Type Australasian Tektites From South China

AbstractSouth China belongs to the northern geographic branch of the Australasian strewn field (AASF) of tektites and microtektites, and this area is assumed to be part of the uprange region of the putative impactor trajectory that formed the yet undiscovered source crater. Ferromagnetic minerals in impact glass may record the magnetization process and thermal history of impact melt, but the possible identity of ferromagnetic minerals in AASF tektites from South China is elusive. Here, we perform a rock magnetism and crystallography study of iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant single‐domain and pseudo single‐domain nano‐grains of magnetite within the spherules. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Compared to the large value ranges of magnetic properties of tektites in the entire strewn field, individual specimens of both Muong Nong‐type and single splash‐form tektites have relatively homogeneous magnetic properties in terms of magnetic susceptibility, natural remanent magnetization (NRM), saturation isothermal remanent magnetization (SIRM) and NRM/SIRM ratio. The results indicate that the impact melt that formed each tektite specimen experienced similar thermal history, and most AASF tektites from South China were cooled in an ambient magnetic field that is the same order of magnitude as the Earth's magnetic field.

An end-member modelling approach to pseudo-Thellier palaeointensity data

SUMMARY Absolute palaeointensities are notoriously hard to obtain, because conventional thermal Thellier palaeointensity experiments often have low success rates for volcanic samples. The thermal treatments necessary for these experiments potentially induce (magnetic) alteration in the samples, preventing a reliable palaeointensity estimate. These heating steps can be avoided by pseudo-Thellier measurements, where samples are demagnetized and remagnetized with alternating fields. However, pseudo-Thellier experiments intrinsically produce relative palaeointensities. Over the past years, attempts were made to calibrate pseudo-Thellier results into absolute palaeointensities for lavas by mapping laboratory induced anhysteretic remanent magnetizations (ARMs) to the thermally acquired natural remanent magnetizations (NRMs). Naturally occurring volcanic rocks, however, are assemblages of minerals differing in grain size, shape and chemistry. These different minerals all have their own characteristic mapping between ARMs and thermal NRMs. Here, we show that it is possible to find these characteristic mappings by unmixing the NRM demagnetization and the ARM acquisition curves into end-members, with an iterative method of non-negative matrix factorization. In turn, this end-member modelling approach (EMMA) allows for the calculation of absolute palaeointensities from pseudo-Thellier measurements. We tested our EMMA using a noise-free numerical data set, yielding a perfect reconstruction of the palaeointensities. When adding noise up to levels beyond what is expected in natural samples, the end-member model still produces the known palaeointensities well. In addition, we made a synthetic data set with natural volcanic samples from different volcanic edifices that were given a magnetization by heating and cooling them in a controlled magnetic field in the lab. The applied fields ranged between 10 and 70 µT. The average absolute difference between the calculated palaeointensity and the known lab field is around 10 µT for the models with 2–4 end-members, while the palaeointensity of almost all flows can be retrieved within a deviation of ±20 µT. The deviations between the palaeointensities and the known lab fields are almost Gaussian distributed around the expected values. Although the two data sets in our study show that there is potential for using this end-member modelling technique for finding absolute palaeointensities from pseudo-Thellier data, these synthetic data sets cannot be directly related to natural samples. Therefore, it is necessary to compile a data set of known palaeointensities from different volcanic sites that recently cooled in a known magnetic field to find the universal end-members in future studies.

Visualization of nanoscale magnetic domain states in the asteroid Ryugu

In the samples collected from the asteroid Ryugu, magnetite displays natural remanent magnetization due to nebular magnetic field, whereas contemporaneously grown iron sulfide does not display stable remanent magnetization. To clarify this counterintuitive feature, we observed their nanoscale magnetic domain structures using electron holography and found that framboidal magnetites have an external magnetic field of 300 A m−1, similar to the bulk value, and its magnetic stability was enhanced by interactions with neighboring magnetites, permitting a disk magnetic field to be recorded. Micrometer-sized pyrrhotite showed a multidomain magnetic structure that was unable to retain natural remanent magnetization over a long time due to short relaxation time of magnetic-domain-wall movement, whereas submicron-sized sulfides formed a nonmagnetic phase. These results show that both magnetite and sulfide could have formed simultaneously during the aqueous alteration in the parent body of the asteroid Ryugu.

Difference in Relative Paleointensity Recording Efficiency of Magnetic Mineral Constituents in a Sediment Core Off Chile

AbstractProgress of relative paleointensity (RPI) estimations using marine sediments has greatly contributed to better understanding of the behavior of the past geomagnetic field. To enhance further the reliability of RPI estimations, we must overcome the problem that climatically induced variations of magnetic‐mineral assemblages in sediments may influence RPI records. Two major constituents of magnetic‐mineral assemblages in marine sediments are magnetofossils and detrital magnetic minerals, and the latter consists of silicate‐hosted magnetic inclusions and unprotected magnetic minerals. It is necessary to understand different RPI recording efficiencies among those constituents to tackle the problem, but previous evaluations were inconclusive. We studied this issue using a sediment core taken from the southeast Pacific Ocean. Rock‐magnetic investigations revealed that the magnetic mineral assemblage of this core during the last ∼900 Kyr is a mixture of low‐coercivity magnetofossils and middle‐coercivity detrital unprotected partially oxidized magnetite. Natural remanent magnetization versus isothermal remanent magnetization demagnetization diagram showed strong convex curvature, and RPI signals carried by the two components could be separated by calculating RPI from the gradients of 20–40 and 70–160 mT segments. We confirmed that RPI recording efficiency of magnetofossils is lower than that of detrital unprotected magnetites/maghemites. Because of the marginal overlap between the coercivity ranges of the two components, changes in their relative abundance do not influence RPI estimations. This condition is ideal for RPI estimations, and the resulted RPI curve closely coincides with that of the PISO‐1500 stack despite changes in the relative abundance of the two components.