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Mineral fabric analyses of Permian La Plata and Jurassic Paez arc plutons provide information about the interplay of tectonic deformation and magma emplacement in the southern Colombian Andes. We combined field mapping, microstructural data, AMS fabrics, rock magnetic properties, vorticity analysis, and thermobarometry to study the internal fabrics of the plutons. Our results indicate that the studied plutons have well-developed NE magmatic and mylonitic foliations, with the latter being distinctive for the Permian pluton. These foliations are consistent with the N65°E/63°W Permian and N43°E/82°W Jurassic AMS magnetic fabrics. Magnetic lineations record subhorizontal (Permian) and vertical (Jurassic) magmas upwards in the NE–SW trajectories. The orientations of all the plutonic fabrics indicate structurally controlled magma emplacement consistent with compressional tectonics and bulk NW–SE crustal shortening. Microstructural features recorded in the plutons reveal magmatic to sub-magmatic and solid-state deformation developed at high- to low-temperatures during their syn-tectonic emplacement history. Thermobarometric calculations indicate similar shallow emplacement conditions (~10 km depth) in the La Plata and Paez plutons. The application of vorticity analysis shows that the magnetic fabrics and syn-emplacement of the Permian pluton developed by simple-shear dominated dextral transpression, whereas the AMS fabrics for the Jurassic syn-tectonic intrusion may have been associated with pure-shear contractional deformation. [ABSTRACT FROM AUTHOR]

The present study deals with the Fe–Ti oxides, rock magnetic and anisotropy of magnetic susceptibility (AMS) carried out to determine the magneto-mineralogical characteristics and the nature of the magnetic fabrics of the high-grade metamorphic rocks from Chilka Lake area, Eastern Ghats Mobile Belt (EGMB), India. Petrography depicted the presence of both primary and secondary magnetite and titano-magnetite as the chief magnetic minerals. Different generations of magnetite were found in these rocks corresponding to different conditions of temperature that prevailed during their oxidation, having tectonic implications depicting the crustal upliftment. Thermomagnetic analysis, isothermal remanent magnetization (IRM), hysteresis loops and backfield IRM demagnetisation show the presence of ferrimagnetic minerals of different origin. The magnetic domain is determined by Day plot, using the remanence and coercivity values from hysteresis curves, dominated by stable single domain (SSD), which reveals the potentiality to record the ancient magnetic field. AMS studies unravelled the nature of the magnetic fabrics in the region. The magnetic susceptibility ellipsoids are dominantly oblate as revealed from the Pj–Tj shape plots. The magnetic lineation is highly variable which states the multiple phased tectono-metamorphic conditions. The similarity between the magnetic and mesoscopic fabrics in the granulite is significant, whereas the anorthosites result from felsic magmatism, which occurred after the deformational phases and thus did not record any mesoscopic tectonic significance. [ABSTRACT FROM AUTHOR]

Multidisciplinary studies of volcanic processes and their timescales are vital for the assessment and mitigation of the risk of future volcanic disasters. Paleomagnetic and rock magnetic approaches have long been applied in diverse volcanological studies worldwide. In this review, we provide conceptual and methodological explorations of a representative suite of paleomagnetic and rock magnetic applications, and discuss their future perspectives in Korean volcanological research. These paleomagnetic and rock magnetic applications will contribute to a better understanding of volcanic and associated processes during the pre-, syn-, and post-eruption periods, and their timing and duration, and also contribute to the improvement of spatiotemporal correlations of volcanic and associated units in Korea. Thus it will be connected to meaningfully lead future volcanic hazard analyses. [ABSTRACT FROM AUTHOR]

We present new results of geochronological, rock magnetic, paleomagnetic and paleointensity studies of the olivine gabbro dyke located at the northern part of the Murmansk craton, NE Fennoscandia (the Kola Peninsula). According to its geochemistry, petrographic and geochronology features, the dyke belongs to the 2.68 Ga dyke swarm, as confirmed by Sm-Nd mineral isochron. We find a significant difference in the rock magnetic and paleomagnetic characteristics of the central and marginal parts of the dyke, which is independently supported by petrography and geochemistry. It is shown that the rocks of the central part of the dyke retained not only their primary mineral composition, but also the primary component of the natural remanent magnetization. We use its direction to determine the 2.68 Ga virtual geomagnetic pole for the Murmansk craton: Slat = 68.64292° N, Slong = 37.7945° E, N = 41 specimens, Plat = –73.5°, Plong = 138.9°, dp/dm = 3.2°/3.4°, paleolat = –65.9°. We also obtain reliable estimates (17 samples) of the Earth's magnetic field intensity at ca. 2.68 Ga: VDM value is found to be 1.85 × 1022 A m2 corresponding to the geomagnetic field several times weaker than the present-day field. [ABSTRACT FROM AUTHOR]

Iron oxide assemblages are central to many pursuits, ranging from Mars exploration to environmental remediation. Oxides and oxyhydroxides of iron both carry the special properties of color and magnetism. In this paper, we use visible-light spectroscopy and rock magnetic data collected at varying temperatures (~77–973 K) to analyze the concentrations and identities of iron oxides found in natural hematite-dominated samples that were obtained from a scientific drill core of Late Triassic red beds in the American Southwest. Our results suggest that hematite colorization of Earth materials varies from red to blue/purple as crystal size increases. Second-derivative analysis of the collected visible-light spectra allows this variation to be measured through the characteristic wavelength band position. Magnetic coercivity data indicate "hardness" differences that also may suggest smaller grain sizes are associated with redder colors. Yellowish maghemite and goethite have overlapping characteristic wavelength band positions that make it challenging to distinguish their contributions to mixed assemblages from visible-light data alone. Remanent magnetizations acquired at ~77 K and room temperature suggest the presence of hematite and a low-coercivity phase that may be maghemite and/or oxidized magnetite. However, we interpret this phase as maghemite in order to explain the changes in iron oxide concentrations indicated by visible-light intensities near ~425 nm and because the thermal demagnetization data suggest that goethite is absent from the samples. Future research that increases the resolution of hematite, maghemite, and goethite detection in experimental and natural samples will provide opportunities to refine the study of past climates and constrain soil iron availability under future changes in global moisture and temperature. Multimethod datasets improve understanding of environmental conditions that cause iron oxides assemblages to shift in phase dominance, grain size, and crystallinity. [ABSTRACT FROM AUTHOR]

The minerals carrying the magnetic remanence in geological samples are commonly a solid solution series of iron‐titanium spinels known as titanomagnetites. Despite the range of possible compositions within this series, micromagnetic studies that characterize the magnetic domain structures present in these minerals have typically focused on magnetite. No studies systematically comparing the domain‐states present in titanomagnetites have been undertaken since the discovery of the single vortex (SV) structure and the advent of modern micromagnetism. The magnetic properties of the titanomagnetite series are known to vary strongly with composition, which may influence the domain states present in these minerals, and therefore the magnetic stability of the samples bearing them. We present results from micromagnetic simulations of titanomagnetite ellipsoids of varying shape and composition to find the size ranges of the single domain (SD) and SV structures. These size ranges overlap, allowing for regions where the SD and SV structures are both available. These regions are of interest as they may lead to magnetic instability and "partial thermal remanent magnetization (pTRM) tails" in paleointensity experiments. We find that although this SD + SV zone occupies a narrow range of sizes for equidimensional magnetite, it is widest for intermediate (TM30‐40) titanomagnetite compositions, and increases for both oblate and prolate particles, with some compositions and sizes having an SD + SV zone up to 100s of nm wide. Our results help to explain the prevalence of pTRM tail‐like behavior in paleointensity experiments. They also highlight regions of particles with unusual domain states to target for further investigation into the definitive mechanism behind paleointensity failure. Plain Language Summary: Rocks that record Earth's magnetic field often contain the mineral magnetite. The crystal structure of magnetite allows titanium atoms to substitute for iron, giving rise to a range of minerals known as titanomagnetites. The internal magnetic structure of titanomagnetite particles in rocks, known as the "domain structure," controls the ability of that particle to record magnetic fields. Particles with certain kinds of domain structure are unstable magnetic recorders, which can cause problems for experiments trying to determine Earth's magnetic field strength in the past (paleointensity experiments). Although the domain structures in magnetite are well understood, there are no recent studies which describe them in titanomagnetites. In this paper, we simulate the domain structures in small titanomagnetite particles and map these out as a function of size, shape and chemical composition. In doing so, we identify types of magnetic particles with multiple possible domain structures that may give rise to unstable magnetizations. Our results indicate that some titanomagnetite particles may have unstable magnetizations over a much larger range of sizes than has previously been seen in magnetite. This wide range of sizes could explain the high failure rates of paleointensity experiments. Key Points: We systematically map out the domain states in titanomagnetite as a function of shape and compositionOur results highlight ranges of compositions, shapes and sizes which contain unreliable paleomagnetic recordersFor certain shapes and compositions, these regions span hundreds of nanometers, representing a significant proportion of remanence carriers [ABSTRACT FROM AUTHOR]

Objective Palaeomagnetic methods can be used to quantitatively constrain the Early Palaeozoic palaeo-position of the North China Block (NCB), which provides a crucial scientific basis for researching the evolution of the Proto-Tethyan Orogeny and the affinity of the NCB for Gondwana. However, the NCB has undergone considerable tectonic activity since the Early Palaeozoic, and the Ordos Basin in the western part of the NCB is rich in hydrocarbon resources. Late tectonic activity and the migration of hydrocarbon resources may have caused remagnetization of Early Palaeozoic strata. Methods Thus, in this paper, to determine the main magnetic minerals in various rocks and their ability to record the primary remanent magnetization, we carry out a series of detailed petrography, rock magnetism and stepwise demagnetization experiments on the limestone and tuff samples of the Upper Ordovician Zhaolaoyu Formation in the Fuping area of the southern Ordos Basin, including optical microscope identification, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), saturated isothermal remanent magnetization (SIRM) acquisition curve, three-axis isothermal thermal demagnetization test, κ-T curve test, hysteresis loop experiment, first-order reversal curve (FORC), and thermal demagnetization and hybrid (TD+AF) demagnetization. Results The results indicate that the main magnetic minerals in the limestone and tuff samples are magnetite, pyrrhotite and/or greigite, and some samples also contain a tiny quantity of haematite with impurities and goethite. Mineral particles possess single domains (SDs) as well as multiple domains (MDs) within their magnetic domains. Combining petrographic and demagnetization data, this paper suggests that the main magnetic minerals in the samples are likely secondary minerals associated with later fluid alteration (maturation and migration of organic matter). Conclusion The remanent magnetic signals conveyed by these rocks cannot be directly used to constrain the Early Palaeozoic palaeoposition of the NCB but may be used to constrain hydrocarbon migration and accumulation in the Lower Palaeozoic strata of the southern Ordos Basin.

New archaeointensity results from the Indian archaeological excavation at the historical Harappa Rakhigarhi site, India, dating from the third millennium BC, are reported. Fifteen samples from nine potsherd fragments were collected and subjected to the IZZI Thellier palaeointensity method. Detailed rock magnetic measurements were performed on representative potsherds. Rock magnetic properties indicate that magnetite is the dominant carrier of remanent magnetization. Archaeointensity values range from 23.65 ± 1.23 µT to 41.22 ± 1.82 µT (mean 32.64 ± 0.81 µT), while corresponding virtual axial dipole movements range from 5.06 to 10.60 x 1022 Am² (mean 8.11 x 1022 Am²). [ABSTRACT FROM AUTHOR]

The Qaidam Basin is the largest Cenozoic sedimentary basin on the northern part of Qinghai-Xizang Plateau, with a stratum thickness exceeding 10 km. By revealing the magnetic fabric information preserved in Cenozoic within the basin, it is possible to reconstruct the regional tectonic stress orientation, which is of great significance for exploring the tectonic deformation and dynamic mechanism in the northern part of Qinghai-Xizang Plateau. The anisotropy of magnetic susceptibility in various directions of Cenozoic strata samples from Lulehe section in the northern Qaidam Basin was measured. Subsequently, magnetic fabric information was obtained from 1 070 samples, and an analysis was conducted on the morphology of the magnetic fabric and its related parameters. The results show that the magnetic fabric of strata during 23.7-9.9 Ma in Lulehe section shows an initial deformation magnetic fabric, with its magnetic lineation indicating the direction of compression stress. The magnetic fabric of strata during 9.9-5.2 Ma is characterized by sedimentary fabric, and no obvious tectonic stress direction is recorded. After removing the effects of structural rotation using magnetic declination, variations are observed in the orientation of the compressive stress recorded by the magnetic fabric in Lulehe section over different periods, respectively. During 23.7-18.1 Ma, it gathers at ～25°, revealing that the northern Qaidam Basin suffers from near SN compression stress in the Early Miocene. During 18.1-15.2 Ma, the direction of compressive stress rotates counterclockwise from ～25° to ～-20°. During 15.2-9.9 Ma, the direction of compressive stress rotates clockwise from ～-20° to ～20°. These results reveal that the compression stress direction of the northern Qaidam Basin first rotates counterclockwise (18.1-15.2 Ma) and then rotates clockwise (15.2-9.9 Ma) in the Middle Miocene. Based on the surrounding tectonic framework and the previous analysis of tectonic deformation, it is considered that the rotation of tectonic stress on the northern margin of Qaidam Basin since the Middle Miocene may be closely related to the superposition of bidirectional compression stress from East Kunlun and Qilian Mountains [ABSTRACT FROM AUTHOR]

The Qaidam Block, located at the northern Qinghai–Tibet Plateau, is a pivotal area in unraveling the closure time of the Kunlun Ocean basin which might have recorded the transformation process between the Proto-Tethys and Paleo-Tethys Ocean basins. However, the late Triassic position of the Qaidam Block remains enigmatic, largely due to the scarcity of paleomagnetic data essential for quantitatively determining its paleolatitude. The widespread presence of the Elashan formation, particularly along the southern periphery of the Qaidam block, presents good material for conducting paleomagnetic work. Nevertheless, the primary magnetic carriers preserved within the Elashan formation might be influenced by multiple tectonic thermal events, particularly those associated with collisions between southern blocks and the Qaidam Block. Here we present rock magnetism and magnetic fabrics studies to identify the content and composition of magnetic minerals within the Elashan formation. The rock magnetic and petrologic results show that the magnetic carriers in the samples from the Elashan formation are dominated by magnetite with a small amount of goethite, pyrrhotite, and hematite. The results of Anisotropy in Magnetic Susceptibility indicate that the south of the Longwalangku section might not be obviously influenced by the tectonic events. Our results also provided guidance for future paleomagnetic research, emphasizing the importance of conducting further sampling away from adjacent faults, particularly in the southern Longwalangku area. [ABSTRACT FROM AUTHOR]

Volcanic rocks are considered reliable recorders of past changes in the Earth's magnetic field. Recent flows, however, sometimes fail to produce the known magnetic field at the time of cooling. Previous research on Mt. Etna suggests paleomagnetic data might not be accurately recorded. Here we test the accuracy of paleomagnetic data obtained from Mt. Etna lavas by comparing paleomagnetic data from historical flows to direct measurements of the magnetic field above the current topography. The inclinations and intensities in both data sets are biased toward lower values, while there is no such trend for the declination. Inclinations are on average 2.9° lower than expected; intensities are on average 8.8 µT lower. The deviations from the expected values depend on the height above the flow. Moreover, the inclinations and intensities vary as a function of topography. Both are higher above ridges and lower in gullies; the variations within a site are up to 14.1° in inclination and 12.9 µT for intensity. To suppress this paleomagnetic data bias it is important to take samples several meters apart and from different parts of the flow whenever possible. While this leads to a higher degree of scatter in paleodirections, the results better represent the Earth's magnetic field at the time of cooling. This emphasizes the importance of reporting paleomagnetic sampling strategies in detail. Plain Language Summary: Paleomagnetic data from lavas is routinely used in the Earth Sciences to for example, reconstruct the past behavior of the Earth's magnetic field, or make models of past plate motions. Very young flows for which the ambient magnetic field at the time of cooling is known, however, sometimes fail to produce the known reference values. Lava flows from Mt. Etna are extensively studied in the past and the paleomagnetic data obtained does often not agree with the known magnetic field value in which the lavas cooled. Here we show that the topography of volcanic terrain may influence the magnetic signal of new, overlying, flows, and we make recommendations for sampling strategies that suppress these terrain effects as much as possible. Key Points: Paleomagnetic data from Mt. Etna does often not reproduce the known geomagnetic field wellLocal magnetic anomalies explain bias in paleomagnetic data as function of topographyOptimizing the paleomagnetic sampling strategy may suppress this bias in paleomagnetic data [ABSTRACT FROM AUTHOR]

Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, <50 MPa), and maximized when no nucleation substrate is present (homogeneous nucleation, >100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (<100 MPa) depths represents an apparent paradox that hints at the presence of a cryptic nucleation substrate. In a pair of studies focusing on Glass Mountain eruptive units from Medicine Lake, California, we characterize titanomagnetite nanolites and ultrananolites in pumice, obsidian, and vesicular obsidian (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336), calculate titanomagnetite crystal number densities, and compare titanomagnetite abundance with the physical properties of pumice to evaluate hypotheses on the timing of titanomagnetite crystallization. Titanomagnetite crystals with grain sizes of approximately 3–33 nm are identified in pumice samples from the thermal unblocking of low‐temperature thermoremanent magnetization. The titanomagnetite number densities for pumice are 1018 to 1020 m−3, comparable to number densities in pumice and obsidian obtained from room temperature methods (Brachfeld et al., 2024, https://doi.org/10.1029/2023GC011336). This range exceeds reported bubble number densities (BND) within the pumice from the same eruptive units (average BND ∼4 × 1014 m−3). The similar abundances of nm‐scale titanomagnetite crystals in the effusive and explosive products of the same eruption, together with the lack of correlation between pumice permeability and titanomagnetite content, are consistent with titanomagnetite formation having preceded the bubble formation. Results suggest sub‐micron titanomagnetite crystals are responsible for heterogeneous bubble nucleation in this nominally aphyric rhyolite magma. Key Points: Aphyric rhyolite eruptions staged from shallow magma reservoirs lack the overpressure needed for homogeneous bubble nucleationHeterogeneous bubble nucleation may occur on sub‐µm titanomagnetite crystals, which are undetectable using standard analytical techniquesSub‐µm titanomagnetite crystals can be detected and quantified with low temperature magnetic analyses [ABSTRACT FROM AUTHOR]

We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (<50–80 nm) to multidomain (>10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 1012 to 1020 m−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 1018 to 1019 m−3. These ranges agree well with TND determinations of 1018 to 1020 m−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation. Plain Language Summary: We use magnetism experiments to prove that nanometer‐sized magnetic particles are present in volcanic rocks with low iron content and few visible crystals. Nanolites (particles between 30 and 1,000 nm) and ultra‐nanolites (particles smaller than 30 nm) are extremely difficult to detect in volcanic rocks composed mainly of glass using conventional methods such as optical and electron microscopy. Titanomagnetite nano‐particles may play a role in controlling the explosiveness of volcanic eruptions. The magnetic signatures of minerals can be used to determine their chemical composition, particle size range, and particle abundance. Pumice and obsidian contain the mineral titanomagnetite, with no evidence of prolonged crystallization at high oxygen levels at the Earth's surface. Observed magnetic behaviors are very similar to those of previously published studies of titanomagnetite in the 10–1,000 nm size range, and similar to mathematical models that simulate this size range. We find that pumice clasts have a magnetic fabric, suggesting that the nanolites and ultra‐nanolites were aligned in spatial patterns before the magma solidified, with stronger alignment coinciding with high degrees of vesicularity. Our results indicate that titanomagnetite crystals are highly abundant, and had crystallized in the magma chamber before the eruption. Key Points: Magnetic methods document titanomagnetite nanolites in rhyolitic materials from Glass Mountain, Medicine Lake Volcano, CaliforniaTitanomagnetite number densities for pumice, obsidian, and vesicular obsidian span 1012 to 1020 m−3 of solid materialTitanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation [ABSTRACT FROM AUTHOR]

Abstract Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve. Graphical abstract

In this article, the results of the rock-magnetic measurements (magnetic susceptibility and its anisotropy before and after the heating of rocks, magnetic saturation) and paleomagnetic studies of the Lower Syzran Subformation (Paleocene) in the Peschanyi Umyot section (Saratov, Russia) are discussed. The section is composed of weakly magnetic silicites and has the paleomagnetic parameters that are suitable for magnetostratigraphic purposes. The presence of a magnetic zone with predominantly reverse polarity, which is an analogue of either chron C27 or C26 or their combination, was found in the section. The results obtained, along with previously collected data from coeval sections in the south of the Saratov right bank and the city of Saratov, made it possible to draft a magnetostratigraphic chart for the Paleocene of the Volga region, provide a detailed correlation of the Syzran Formation, estimate the duration of the pre-Paleogene erosion, and more accurately date the age of deposits.

Construction of regional geomagnetic secular variation curves for the last several tens of thousands of years is important for understanding the behavior of non-dipole fields and applications to geochronology. Around Japan, secular variation records of older than 10 ka was scarce, in particular for relative paleointensity (RPI). Here, we conducted a paleomagnetic study of a sediment core covering the last ~ 40 kyr taken from a small basin in the Nankai Trough. The core consists of homogenous hemipelagic sediments except for turbidites and volcanic ashes. The age model was constructed based on seven 14C datings and two volcanic ashes. Turbidites and volcanic ashes were excluded from the construction of secular variation curves because of geologically instantaneous deposition. It was revealed that the magnetization of this core is carried largely by detrital magnetic minerals, although magnetofossils are also contained. Bulk magnetic properties show some temporal changes in magnetic concentration and grain size, but still homogeneous enough for reliable RPI estimations except for turbidites and volcanic ashes. The resultant RPI shows no correlation with the normalizer, anhysteretic remanent magnetization, of the RPI estimations or with a proxy for a magnetic grain size and/or the proportion of magnetofossils to detrital magnetic minerals. The obtained RPI record shows a long-term increasing trend since ~ 40 ka, which coincides with global stack curves. On the other hand, there are some differences in shorter timescale variations, which may reflect non-dipole fields. This study demonstrated that hemipelagic sediments in the Nankai Trough have potential for recovering high-quality RPI records when turbidites and volcanic ashes were excluded and are useful for accumulating records to construct a regional master curve. [ABSTRACT FROM AUTHOR]

The lithological, paleontological and biostratigraphic characteristics of the Turonian–Coniacian deposits of the Lower Bannovka section, the stratotype of the Bannovka Formation (Volga region, Turonian) are presented. Magnetostratigraphic characteristics of thе sediments have been supplemented and elaborated. Petromagnetic data contribute to more refined stratigraphy of the section and facilitate the revealing of depositional rhythmicity. Paleomagnetic data combined with benthic foraminifera data enable the most detailed correlation of the Turonian–Coniacian deposits in the Volga region. Results of a comprehensive study of this stratigraphic unit within the southeastern arch of the Ulyanovsk-Saratov trough revealed patterns in the distribution of the Gubkino Horizon, determined by regional tectonics and processes preceding deposition in the Coniacian and Santonian. [ABSTRACT FROM AUTHOR]

Abstract Volcanic rocks are considered reliable recorders of past changes in the Earth's magnetic field. Recent flows, however, sometimes fail to produce the known magnetic field at the time of cooling. Previous research on Mt. Etna suggests paleomagnetic data might not be accurately recorded. Here we test the accuracy of paleomagnetic data obtained from Mt. Etna lavas by comparing paleomagnetic data from historical flows to direct measurements of the magnetic field above the current topography. The inclinations and intensities in both data sets are biased toward lower values, while there is no such trend for the declination. Inclinations are on average 2.9° lower than expected; intensities are on average 8.8 µT lower. The deviations from the expected values depend on the height above the flow. Moreover, the inclinations and intensities vary as a function of topography. Both are higher above ridges and lower in gullies; the variations within a site are up to 14.1° in inclination and 12.9 µT for intensity. To suppress this paleomagnetic data bias it is important to take samples several meters apart and from different parts of the flow whenever possible. While this leads to a higher degree of scatter in paleodirections, the results better represent the Earth's magnetic field at the time of cooling. This emphasizes the importance of reporting paleomagnetic sampling strategies in detail.

Abstract Nucleation of H2O vapor bubbles in magma requires surpassing a chemical supersaturation threshold via decompression. The threshold is minimized in the presence of a nucleation substrate (heterogeneous nucleation, 100 MPa). The existence of explosively erupted aphyric rhyolite magma staged from shallow (

Abstract We document the presence, composition, and number density (TND) of titanomagnetite nanolites and ultra‐nanolites in aphyric rhyolitic pumice, obsidian, and vesicular obsidian from the 1060 CE Glass Mountain volcanic eruption of Medicine Lake Volcano, California, using magnetic methods. Curie temperatures indicate compositions of Fe2.40Ti0.60O4 to Fe3O4. Rock‐magnetic parameters sensitive to domain state, which is dependent on grain volume, indicate a range of particle sizes spanning superparamagnetic (10 μm) particles. Cylindrical cores drilled from the centers of individual pumice clasts display anisotropy of magnetic susceptibility with prolate fabrics, with the highest degree of anisotropy coinciding with the highest vesicularity. Fabrics within a pumice clast require particle alignment within a fluid, and are interpreted to result from the upward transport of magma driven by vesiculation, ensuing bubble growth, and shearing in the conduit. Titanomagnetite number density (TND) is calculated from titanomagnetite volume fraction, which is determined from ferromagnetic susceptibility. TND estimates for monospecific assemblages of 1,000 nm–10 nm cubes predict 1012 to 1020 m−3 of solid material, respectively. TND estimates derived using a power law distribution of grain sizes predict 1018 to 1019 m−3. These ranges agree well with TND determinations of 1018 to 1020 m−3 made by McCartney et al. (2024), and are several orders of magnitude larger than the number density of bubbles in these materials. These observations are consistent with the hypothesis that titanomagnetite crystals already existed in extremely high number‐abundance at the time of magma ascent and bubble nucleation.

Objective Palaeomagnetic study of the early Palaeozoic North Qiangtang Terrane can quantitatively constrain its palaeogeographic location since the Palaeozoic, which serves as an important foundation and key constraint for studying of the palaeotectonic pattern of the Qinghai-Tibet Plateau, the evolution of the Phanerozoic Tethys and palaeogeographic reconstruction. Methods Prior to palaeomagnetic study, it is essential to investigate the magnetic properties of rocks to identify the types and assemblages of magnetic carriers present in rocks, which will facilitate the selection of demagnetization experimental protocols and discussions on the primary nature of remanent magnetization. This paper focuses on the magnetic properties of limestone and sandstone from the Late Silurian Longmu Co Upper Formation in the North Qiangtang Terrane, through analyses of isothermal remanence acquisition curves, magnetic susceptibility with temperature(χ-T) curves, triaxial thermal demagnetization experiments, low-temperature magnetic properties tests, scanning electron microscopy (SEM) and energy dispersive spectral (EDS) analysis. Results Results show that magnetite is the predominant magnetic carrier in the limestone samples of the Longmu Co Upper Fm, with small amounts of pyrrhotite. The magnetic assemblage in the sandstone samples is more complex, dominated by magnetite, possibly with other magnetic minerals such as pyrrhotite. Conclusion Moreover, it demonstrates that stable high-temperature remanent magnetization components can be isolated from the limestone samples in the Longmu Co Upper Formation, making them suitable for further palaeomagnetic researches.

Abstract Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α95 (

Iron oxide assemblages are central to many pursuits, ranging from Mars exploration to environmental remediation. Oxides and oxyhydroxides of iron both carry the special properties of color and magnetism. In this paper, we use visible-light spectroscopy and rock magnetic data collected at varying temperatures (~77–973 K) to analyze the concentrations and identities of iron oxides found in natural hematite-dominated samples that were obtained from a scientific drill core of Late Triassic red beds in the American Southwest. Our results suggest that hematite colorization of Earth materials varies from red to blue/purple as crystal size increases. Second-derivative analysis of the collected visible-light spectra allows this variation to be measured through the characteristic wavelength band position. Magnetic coercivity data indicate “hardness” differences that also may suggest smaller grain sizes are associated with redder colors. Yellowish maghemite and goethite have overlapping characteristic wavelength band positions that make it challenging to distinguish their contributions to mixed assemblages from visible-light data alone. Remanent magnetizations acquired at ~77 K and room temperature suggest the presence of hematite and a low-coercivity phase that may be maghemite and/or oxidized magnetite. However, we interpret this phase as maghemite in order to explain the changes in iron oxide concentrations indicated by visible-light intensities near ~425 nm and because the thermal demagnetization data suggest that goethite is absent from the samples. Future research that increases the resolution of hematite, maghemite, and goethite detection in experimental and natural samples will provide opportunities to refine the study of past climates and constrain soil iron availability under future changes in global moisture and temperature. Multimethod datasets improve understanding of environmental conditions that cause iron oxides assemblages to shift in phase dominance, grain size, and crystallinity.

Vulcanian activity is one of the most common eruption styles of arc andesitic volcanism on Earth. It ejects and deposits volcanic bombs around the source crater. Although paleomagnetic studies of volcanic bombs are limited, such studies can potentially provide more opportunities for high-resolution paleomagnetic dating of volcanic activity. In this study, paleomagnetic dating was applied to large (> 1 m) volcanic bombs around active craters in the Azuma volcano group, NE Japan. Oriented samples were collected from the interior parts of five large volcanic bombs situated on gentle slopes, a few hundred meters from the source crater. More than six core samples were collected from each bomb and all samples were subjected to a range of rock magnetic experiments, including anisotropy of magnetic susceptibility (AMS) and thermal/alternating field demagnetization (THD/AFD) analyses. The Characteristic Remanent Magnetization (ChRM) directions for specimens from all bombs were well-defined, have small α95 (< 2.5º), and are in close agreement with each other. Comparing our measured overall mean direction (Dm = 355.5º, Im = 49.8º, α95 = 1.6º) with modeled geomagnetic field estimates and a reference secular variation curve for this area (using MATLAB-based archaeomagnetic dating tool), we suggest that the volcanic bombs were produced in the historical Meiji period (1893–1895 CE) eruption. In addition, a combination of the data of ChRM, AMS, thermomagnetic analyses, hysteresis measurement, and XRF analysis indicates that the volcanic bombs were derived from a plug of lava in the conduit under the solidification point (ca. 800 °C), but above the Curie point of the titanomagnetite remanence carrier (around 300 °C). We show that volcanic bombs can be powerful for paleomagnetic dating if certain sampling conditions, such as quantity, situation, size and portion are satisfied. [ABSTRACT FROM AUTHOR]

Coseismic frictional heating and associated hydrothermal fluid circulation play an essential role in the dynamic weakening of seismic faults. However, temperature rise induced by frictional heating during an earthquake is still difficult to constrain. Magnetic properties of fault rocks convey abundant information on faulting processes. In this study, detailed rock magnetic measurements in combination with electron microscopic observations are conducted on fault rocks and protoliths from the Shaba outcrop (Beichuan County) on the Yingxiu‐Beichuan Fault ruptured during the 2008 Wenchuan Mw 7.9 earthquake. Results show that protoliths and the majority of fault rocks are dominated by paramagnetic pyrite and/or Fe‐bearing clay minerals; in contrast, the presence of pyrrhotite and goethite is confined to the fault gouges just next to the principal slip surface. Pyrrhotite is a product of pyrite alteration at high temperatures (>500°C) induced by seismic frictional heating during earthquake slip. Meanwhile, the goethite implies the presence of coseismic hot fluids within the fault zone. All these observations strongly suggest the occurrence of thermal pressurization as a plausible mechanism of coseismic fault weakening during the Wenchuan earthquake.Plain Language Summary: As the products of faulting processes, physical and chemical properties of fault rocks convey important information for understanding the faulting processes. Iron, as the fourth most abundant element in the earth's crust, widely occurs in fault zones as iron‐oxides, ‐hydroxides, and/or ‐sulfides. Moreover, these Fe‐bearing (magnetic) minerals are sensitive to stress‐strain, temperature change, and fluid activity. Here, detailed rock magnetic analyses in combination with electron microscopic observations are conducted on the fault rocks and protoliths from the Shaba outcrop (Beichuan County) on the Yingxiu‐Beichuan Fault hosting the 2008 Wenchuan Mw 7.9 earthquake. It is found that pyrrhotite and goethite are only present and confined to the fault gouges just next to the principal slip surface. Their formation is resulted from the alteration of the pre‐existing pyrite and Fe‐bearing clay minerals driven by coseismic frictional heating and concomitant hot fluids. These results suggest the occurrence of thermal pressurization that played an essential role in coseismic weakening during the earthquake. Thus, it demonstrates that the rock magnetic properties of fault rocks could provide valuable insights into faulting‐related physical and chemical processes.Key Points: Pyrite thermally altered to pyrrhotite at >500°C induced by coseismic frictional heating in fault gouges from the Yingxiu‐Beichuan FaultGoethite precipitated as a result of hydrothermal leaching of the pre‐existing pyrite and Fe‐bearing clay minerals by coseismic hot fluidsPresence of pyrrhotite and goethite reveals the essential role of thermal pressurization in fault weakening during the Wenchuan earthquake [ABSTRACT FROM AUTHOR]

This paper presents an interpretation of sedimentologic, paleomagnetic, and geochemical data collected in the Upper Kimmeridgian-Valanginian carbonates of the Giewont series (Giewont and Mały Giewont sections, High-Tatric succession, Western Tatra Mountains, Poland). The studied succession provides insight into the sedimentary conditions prevailing in the South Tatric Ridge (Tatricum), a submarine elevation located between the Zliechov Basin (Fatricum) and the Vahic (=South Penninic) Ocean. The sedimentary sequence includes micrites, pseudonodular limestones, cyanoid packstones, lithoclastic packstone, and encrinites. The results are discussed with regards to their significance for detrital input, paleoclimate, and paleoproductivity, which in turn are considered in the context of both local and regional paleoenvironmental trends and events. The greatest depositional depths during the latest Kimmeridgian-earliest Tithonian are documented by the occurrence of pseudonodular limestones. A Tithonian shallowing trend is demonstrated via the increasing size and roundness of cyanoids, while the final (1)emergence and erosion in the South Tatric Ridge is documented by earliest Cretaceous disconformities. This process might have been related to both falling sea-level during the major eustatic regressive cycle and tectonic uplift caused by the mutually related (re)activation in the Neotethyan Collision Belt and rifting in the Ligurian-Penninic-Vahic Oceans. The highest lithogenic influx (although still low; max 0.5% of Al content) during the Late Kimmeridgian is considered as associated with relatively humid climate conditions, whereas a subsequent decreasing trend is thought to result from aridification during the latest Kimmeridgian-earliest Tithonian. Ultimately, deposition in the High-Tatric zone was affected by both large-scale environmental perturbations characteristic of the latest Jurassic (climate changes, variations in seawater pH, monsoonal upwelling, lithogenic input, etc.), as well as local sedimentary controls, predominantly the oxygenation state of bottom waters and tectonic movements. [ABSTRACT FROM AUTHOR]

South 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. Plain Language Summary: The vast Australasian strewn field (AASF) of tektites and microtektites was formed at 0.788 million years ago by an enormous impact that may have occurred somewhere in the Indochina Peninsula. The magnetic properties of tektites may record the cooling history and post‐ejection magnetization process of the tektite‐forming melt, which could provide additional insights into the possible impact location. AASF tektites from South China are a unique portion of this strewn field regarding the hypothesized impactor trajectory, and our earlier rock magnetism study suggested that tektites in this region exhibit systematically different magnetic properties. This work performs detailed geochemical and crystallographic analyses for iron‐sulfur spherules in Muong Nong‐type tektites from South China, revealing abundant nano‐sized magnetite grains in the interior that are consistent with being single‐domain and pseudo single‐domain magnetite. This result is consistent with the detection of magnetic signals caused by single‐domain and pseudo single‐domain magnetite in these samples. Most tektites from South China have relatively homogeneous magnetic properties in the interior. Their paleointensity has the same order of magnitude compared to the Earth's magnetic field. Key Points: Nano‐sized euhedral magnetite grains were discovered in Muong Nong‐type tektitesIndividual tektite specimens from South China usually exhibit relatively homogenous magnetic propertiesAustralasian tektites in South China have been magnetized in an Earth‐strength magnetic field [ABSTRACT FROM AUTHOR]

Ancient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations. Plain Language Summary: Minerals such as hematite are capable of recording the direction of the Earth's magnetic field during formation and this paleomagnetic information helps geoscientists estimate the age of rocks and the past motions of tectonic plates. During geologic events like mountain‐building, primary paleomagnetic signatures in rocks may be modified or replaced by new, secondary paleomagnetic recordings. Such secondary magnetizations can be challenging to interpret, especially if the age of these magnetizations is unknown. (U‐Th)/He analysis, a form of radiometric dating, may aid paleomagnetic studies by revealing the age of minerals that carry secondary magnetizations. In this study, we compare hematite (U‐Th)/He data and a secondary magnetization carried by hematite from Colorado's Front Range and find that (U‐Th)/He dates are scattered and considerably younger than the reported age of the secondary magnetization. We interpret the variability of hematite (U‐Th)/He dates to reflect differences between aliquots relating to crystal size and the presence of non‐hematite minerals. Our results suggest that the (U‐Th)/He system was partially to fully reset by a more recent phase of burial and associated reheating that was not hot enough to fully reset the paleomagnetic system. We conclude that (U‐Th)/He and paleomagnetic investigations in hematite may not record the same geologic events. Key Points: We compare new hematite (U‐Th)/He dates with previously published paleomagnetic data from the same exposure of crystalline basement rockHematite (U‐Th)/He dates are younger than the inferred timing of magnetization and imply partial He loss since magnetization acquisition(U‐Th)/He dates may not relate to remanence acquisition because of differences in sampling requirements and temperature sensitivity [ABSTRACT FROM AUTHOR]

Red Ereño is a red-stained ornamental and construction limestone with characteristic white fossil shells. Although exploited since Roman times, marketed worldwide and that the rock itself and its outcrop areas have been included in geological heritage inventories, the origin of its characteristic reddish colour remained unresolved. The aim of this work is to deepen the scientific knowledge of Red Ereño as a basis for understanding the characteristics of this stone and to make this information available for geoconservation actions. The mineralogical and petrological study, mainly based on optical and electron microscopy, X-ray diffraction, and rock magnetism and paleomagnetic techniques, concluded that the red-staining mineral is pigmentary hematite. Moreover, the analysis stated that hematite precipitated after sedimentation but prior to burial diagenesis and before alpine inversion. Based on palaeomagnetic studies, it can be stated that mineralisation occurred during the Late Cretaceous. This work illustrates how scientific research on this potential heritage stone provides key information for geoconservation. [ABSTRACT FROM AUTHOR]

The Mersin ophiolite of southern Turkey is a well-exposed, Late Cretaceous, Neo-Tethyan suprasubduction zone ophiolite. It is underlain by metamorphic sole rocks inferred to have formed at the top of a down-going plate during subduction. These have a well-developed foliation and lineation observable in the field (defined by the preferred orientations of hornblende and plagioclase crystals). Here we present the first magnetic fabric data reported from the Mersin ophiolite with such settings. Anisotropy of low field magnetic susceptibility ellipsoids in sampled amphibolites have clustered, NW-plunging minimum principal axes representing poles to a SE-dipping magnetic foliation that aligns with the macroscopic metamorphic foliation plane seen in the field. Maximum AMS principal axes define a SE-plunging magnetic lineation that is parallel to the macroscopic metamorphic lineation. Oblate magnetic fabrics at specimen-level and an overall triaxial fabric at localitylevel in these rocks are consistent with the development of the dominant metamorphic fabric by a combination of pure shear flattening and simple shearing during the formation and exhumation of the Mersin sole rocks. These observations are compatible with a recent tectonic model for the evolution of the ophiolite based on paleomagnetic data that invokes flattening and exhumation of the down-going slab in an incipient subduction zone during supra-subduction zone spreading.

Abstract Coseismic frictional heating and associated hydrothermal fluid circulation play an essential role in the dynamic weakening of seismic faults. However, temperature rise induced by frictional heating during an earthquake is still difficult to constrain. Magnetic properties of fault rocks convey abundant information on faulting processes. In this study, detailed rock magnetic measurements in combination with electron microscopic observations are conducted on fault rocks and protoliths from the Shaba outcrop (Beichuan County) on the Yingxiu‐Beichuan Fault ruptured during the 2008 Wenchuan Mw 7.9 earthquake. Results show that protoliths and the majority of fault rocks are dominated by paramagnetic pyrite and/or Fe‐bearing clay minerals; in contrast, the presence of pyrrhotite and goethite is confined to the fault gouges just next to the principal slip surface. Pyrrhotite is a product of pyrite alteration at high temperatures (>500°C) induced by seismic frictional heating during earthquake slip. Meanwhile, the goethite implies the presence of coseismic hot fluids within the fault zone. All these observations strongly suggest the occurrence of thermal pressurization as a plausible mechanism of coseismic fault weakening during the Wenchuan earthquake.

Abstract This study aims to elucidate the control of source‐to‐sink processes on the sediment dynamics of a complex mesotidal tropical estuarine system using rock magnetic, sedimentological, mineralogical, and geochemical methods. Integration of multiproxy data of catchment rocks, riverbank and hinterland soils, and bedload sediments from various depositional environment of Mandovi estuary revealed a marked change in the magnetic mineral composition and accumulation pattern. The magnetic mineralogy mainly comprised ferri‐ (magnetite, titanomagnetite) and antiferromagnetic (hematite, goethite) minerals. Higher accumulation of coarser magnetic particles in the catchment area followed by a systematic trend of progressive fining of magnetic grain size in the down reaches of the estuary can be attributed to the decrease in transport energy. Enhanced accumulation of coarser magnetic particles in the estuarine region can be reconciled with the strong tidal and wave‐induced bottom currents, which preferentially favored mineral‐density selective fractionation resulting in higher deposition of heavy (magnetic) particles. A clear trend of loss of fluvial‐derived fine silt‐size clastic sediments and selective retention of coarser and heavy magnetic particles within the estuary can be attributed to the hydrodynamic driven sediment partitioning regime, which inhibited the permanent settling of fine sediment fraction and are therefore frequently flushed out to the sea. A marked drop in S‐ratio in different zones of the estuary can be linked to the increased input of high coercive magnetic particles derived from pedogenic soils along eroding riverbanks. We further demonstrate that the S‐ratio is an effective parameter to track the riverbank erosion in an estuarine system.

Abstract South 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.

Fired archaeological objects have been widely studied in palaeomagnetic research. However, the uneven heating during the firing process can affected the type, grain size and magnetic domain characteristics of the magnetic carrier, which in turn can led to notable discrepancies in the paleomagnetic information that they captured. In this study, we conducted a detailed rock magnetic study on two Chinese archaeological (one grey and one red) to reveal the properties of magnetic minerals subjected to different firing temperatures. Our results show that single domain (SD) magnetite with strong magnetostatic interactions is the dominant magnetic carrier in grey bricks, while SD magnetite with weak magnetostatic interactions is the dominant magnetic carrier in the red bricks. And stepwise heating results show that the firing temperature of the grey brick and the outer layers of red brick (R-1, R-3) is above 700°C, while the inner layer (R-2) of red brick shows a thermal transformation process at ∼ 350°C. This uneven heating may cause the spatial discrepancies in magnetic parameters such as the mass-normalized magnetic susceptibility (χ) and natural remanent magnetization (NRM) in red brick. Nevertheless, the direction of the geomagnetic field can be accurately obtained even if the brick is not heated evenly. However, more work is needed to explore the paleointensity behavior at different firing temperatures.

Understanding the evolution of magma storage conditions on volcanoes which have had more than one caldera-forming eruption (CFE) is important to know about past and present conditions, as a key to forecast future potential hazards. Krakatau volcano is characterized by cyclic phases of growth and destruction of the edifice. A volcanostratigraphic study identified three eruptive periods: Old Krakatau, Young Krakatau, and Anak Krakatau. The Old and Young Krakatau periods ended with the first and second CFE respectively. Due to its permanent activity and edifice evolution, Krakatau poses a high risk on the surrounding inhabited islands. In this study, we combined geochemistry, rock magnetic, and petrology to infer the evolution of magma storage conditions from Old to Anak Krakatau periods. This study is the first to report on the chemical and rock magnetic characteristics, as well as storage system conditions of Old Krakatau and its relation to the ongoing evolution of Krakatau. Our data show that: 1) Old and Young Krakatau magma storage regions are shallow (within the upper 3 km), contain more differentiated magmas, from which the Old Krakatau magmas may be less oxidized and had lower temperatures than Young Krakatau; 2) Anak Krakatau magma storage is deeper (up to 26 km), less differentiated, and erupted hotter but more reduced compared to Old and Young Krakatau. The Old and Young Krakatau lavas were the products of pre-CFE and their chemical characteristics are included at maturation phase, whereas the Young Krakatau pumice samples were the product of the second CFE. Lastly, the post-second CFE activity of AK is currently in an incubation phase and represented by mafic products of frequent and small eruptions. Knowing that the volcano has experienced maturation and CFE phases in the past, the current AK may evolve to those phases in the future.

Abstract Ancient magnetization(s), often recorded by hematite (Fe2O3), provide key paleomagnetic constraints on plate interactions through time. Primary remanent magnetizations may be modified or overprinted by secondary processes that complicate interpretations of paleomagnetic data. Hematite (U‐Th)/He (hematite He) dating has the potential to resolve when secondary magnetizations were acquired. Here, we compare hematite He data and paleomagnetic results in Paleoproterozoic crystalline rocks, meters below a major nonconformity in the Colorado Front Range, USA. Prior work and new rock magnetic data indicate that pervasive hematite alteration records a secondary chemical remanent magnetization (CRM) during the Permo‐Carboniferous Reverse Superchron, coincident with the Ancestral Rocky Mountain orogeny. We target minor hematite‐coated faults cutting basement for (U‐Th)/He analyses because they are of sufficient hematite purity to yield geologically meaningful dates. Two samples yield overlapping and scattered individual hematite He dates ranging from ∼138 to 27 Ma (n = 33), significantly younger than the age of the late Paleozoic CRM. Scanning electron microscopy, electron probe microanalysis, and Raman spectroscopy indicate that aliquots have variable grain size distributions and fluorocarbonate impurities. Thermal history models support hematite on fault surfaces mineralized coeval with CRM acquisition during the late Paleozoic, and hematite He data scatter reflects variable He loss during Mesozoic burial owing to differences in grain size distribution from fault slip comminution and in chemistry among aliquots. Our results underscore the differences in temperature sensitivity and sampling requirements between paleomagnetic and hematite He investigations and illustrate that hematite He dates will usually be younger than preserved remanent magnetizations.

Progress 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. Plain Language Summary: Marine sediments potentially preserve records of geomagnetic field intensity variations in the past (paleointensity), which may be important for life on the Earth to shield influx of high‐energy particles from the universe. A serious problem recognized by previous studies is that paleointensity records from sediments sometimes include artificial changes induced by sediment lithology changes reflecting varying depositional environments. Two major constituents of magnetic minerals in sediments, which records paleointensity variations, are magnetites produced by magnetotactic bacteria, called magnetofossils, and inorganic magnetic minerals originated from weathering of rocks on land or materials of volcanic eruptions. Our study using a sediment core taken from the southeast Pacific Ocean revealed that magnetofossils yield lower paleointensity values than inorganic magnetic minerals. This implies that temporal changes in the relative abundance of magnetofossils and inorganic magnetic minerals in sediments may cause the artificial paleointensity variations. We could obtain a reliable paleointensity record from the studied sediments by evaluating separately the magnetizations carried by the two magnetic‐mineral components, which utilized the observations that the two magnetic‐mineral components have different resistance to an external magnetic field. Key Points: Magnetofossil and detrital partially oxidized unprotected magnetite are the major constituents of magnetic mineral assemblageLittle overlap in coercivity distribution of the two components enabled separate evaluation of relative paleointensity (RPI) recording efficiencyRPI recording efficiency of magnetofossil is lower than that of detrital unprotected magnetite/maghemite [ABSTRACT FROM AUTHOR]

Demagnetizing effects and internal stress are difficult to distinguish in natural magnetite samples, but quantitative stress estimates can provide valuable information about microstructure formation, surface oxidation, impacts, tectonic stresses, or interface properties in exsolution structures. Quantifying demagnetizing effects informs about magnetite particle shape, magnetostatic interaction, or anisotropic texture. Here, we establish an improved measurement workflow to separate demagnetizing effects from internal stress for natural magnetite. The method is based on temperature‐dependent hysteresis measurements, and for natural samples require accurate estimates of Curie temperature and temperature‐dependent saturation magnetization to ensure that near‐end‐member magnetite is the dominant magnetic mineral, and to calibrate the temperature‐dependent scaled reversible work (SRW). SRW is the fundamental quantity to determine stress and demagnetizing factor. The improved SRW method is applied to three natural samples with different stress histories where it proves that large magnetite crystals in the metamorphosed Modum complex (Norway) have low internal stress (<100 MPa), while in highly exsolved magnetite‐ilmenite intergrowths from Taberg (Sweden) and Bushveld (South Africa) the magnetite component is highly stressed (>230 MPa). This confirms experimentally that interface strain in complex microstructures due to spinodal decomposition and partial oxidation creates large average internal stress in the magnetite minerals. Because sister specimens have similar internal stress but noticeably (>20%) different demagnetizing factors, textural, and shape anisotropy contribute substantially to SRW in these samples. Plain Language Summary: Magnetite is one of the most abundant magnetic minerals in nature and an important magnetic recorder over the entire history of the Earth. In natural rocks even chemically pure magnetite particles vary considerably in size, shape, and internal stress. Hitherto, mainly the variation in size has been systematically studied, partly because there was no experimental way to reliably quantify stress and shape effects. We now use a new technique to quantify stress and shape effects in three natural magnetite mineral types. One which formed millions of years ago at elevated temperature shows very little shape and stress influences. In the other two samples, slowly cooled magnetite formed a very fine network of lamellae at nanometer scale. In these cases, it was theoretically predicted that internal stress should reach values of up to 300–600 MPa at the interface. For the first time, we can experimentally verify that the high average stresses of up to 340 MPa indeed occur in these exsolution structures. The successful test of our new method extends the toolbox of rock magnetism by a valuable additional technique to quantify previously unknown internal properties of natural magnetite. Key Points: An experimental workflow is presented to separate stress and demagnetizing energy in natural magnetite samplesDemagnetizing energies in synthetic and natural magnetite have equivalent demagnetizing factors of ΔN = 0.33 − 0.46Average internal stress in natural nanoscale magnetite‐ilmenite exsolution structures corresponds to 230–340 MPa [ABSTRACT FROM AUTHOR]

The Mt. Melbourne Volcanic Field is a quiescent volcanic complex located in Northern Victoria Land, Antarctica, mostly covered by ice. Its inner structure has remained largely unknown, due to the paucity of outcrops and the lack of detailed multi‐disciplinary investigations. Here we present a novel high‐resolution aeromagnetic dataset, revealing strong long‐wavelength negative anomalies superimposed by short‐wavelength positive ones forming characteristic radial patterns. Automatic lineament detection, through the Hough transform technique applied to the tilt derivative of our magnetic dataset, shows prevailing NW‐SE‐to NNE‐SSW‐trending structural features, which combined with the few structural field observations contribute to define the deformation pattern. Pre‐existing and novel magnetic property measurements, coupled with available geochronological data, are used to constrain a two‐step 3D magnetic inversion. A layer‐structured Oldenburg‐Parker's inversion was utilized to model the deep and long‐wavelength components of the magnetic field, whereas a linear inversion based on a set of shallower prisms was used to model the short‐wavelength components. The final 3D model shows widespread reversely‐polarized volcanics, which are locally intruded and superimposed, respectively by swarms of normally‐polarized dikes and radial lava flows along paleo‐valleys. These results support the onset of volcanic activity in the entire field at least in the Matuyama magnetic epoch, that is, between 2.58 and 0.78 Ma. Plain Language Summary: Airborne high‐resolution magnetic data play a crucial role in the understanding of the geological structure of ice‐buried volcanoes. In fact, an interesting property of magnetic minerals contained in volcanic rocks is that of retaining, after cooling below a certain characteristic temperature (i.e., Curie temperature), a permanent magnetization called remanent. Its direction (polarization) at the South Pole can be normal if rocks are cooled when the Earth's magnetic field presents negative inclination (as the current Brunhes epoch), whereas reverse in the case the Earth's magnetic field shows positive inclination (i.e., the Earth's magnetic field poles swap places). As a consequence, at polar latitudes, positive magnetic anomalies are associated with normally‐polarized rocks and, conversely, negative anomalies with reversely‐polarized rocks. Thanks to this property, in this study we have carried out the first reconstruction of the internal architecture of the Mt. Melbourne Volcanic Field. Our results, constrained by independent geological and geophysical information, suggest the volcanic complex is composed mainly by an older reversely‐polarized unit superimposed and intruded locally by younger normally‐polarized lava flows. Being the periods of magnetic polarity reversal well known, we have been able to estimate the beginning of a widespread volcanic activity before the last Brunhes‐Matuyama polarity reversal. Key Points: First detailed geophysical model of the internal structure of the Mt. Melbourne Volcanic FieldMagnetic anomalies and rock magnetism data suggest the field is predominantly built‐up by reversely‐polarized volcanicsOur results support an establishment of the Mt. Melbourne edifice at least at the reverse polarity Matuyama magnetic epoch [ABSTRACT FROM AUTHOR]

The lack of suitable indicators of changes in such as sea‐level and circulation has been a major limit to paleoenvironmental and paleoceanographic investigations in continental shelf regions. This paper presents an environmental magnetic study by comparing two late‐Quaternary sediment cores (DH02 and DH03) from the outer shelf of the East China Sea (ECS). Late and early Marine Isotope Stage (MIS) 3 sediments were deposited in a prodelta under cold coastal currents and an open‐shelf with the Taiwan Warm Current and upwelling. The dominant iron‐bearing minerals of the late and early MIS 3 sediments are authigenic greigite (Fe3S4) and pyrite (FeS2), respectively, which were assumed to be formed nearly syndepositionally. The overlying sediments, however, are magnetically dominated by detrital magnetite. This pattern corresponds well to the temporal changes in sea‐level over this period. The widespread occurrence of greigite in the late MIS 3 sediments can also be used for future stratigraphic division and correlation in the ECS. Additionally, compared to microfossil assemblages, rock magnetic parameters based on greigite may be more sensitive to environmental changes on continental shelves. Furthermore, the inter‐borehole spatial comparisons imply not only a sedimentary hiatus/erosion of at least 30‐m thickness in core DH02, most probably during the Last Glacial Maximum, but also that core DH02 was in a more reductive environment than core DH03 during late MIS 3. The findings highlight the potential of authigenic greigite as an indicator of spatiotemporal changes in paleoenvironmental and paleoceanographic conditions on the continental shelf at orbital or even suborbital timescales. Plain Language Summary: Environmental and oceanographic conditions on continental shelves are vital for sedimentation processes and human settlements, but the reconstruction of their evolution is hindered by the lack of effective indicators. Recent studies have indicated that syndepositional formation of greigite, a precursor to pyrite, is highly likely in shallow‐water environments with rapid sedimentation. Here, two sediment cores meridionally traversing the outer shelf of the East China Sea were investigated. The magnetic mineral assemblage and sedimentary environment exhibited almost identical change patterns, with pyrite, greigite, and magnetite as the major magnetic minerals of the early and late Marine Isotope Stage 3, and Last Glacial Maximum/last deglacial deposits, respectively. This agreeably reflects the temporal fluctuations in sea‐level and ocean circulation. By comparing the two cores, the spatial characteristics of stratigraphy and redox conditions could be determined clearly. This study thus provides a good reference for paleoenvironmental and paleoceanographic research based on authigenic greigite in continental shelf sediments. Key Points: Greigite was further confirmed as the major magnetic mineral of the late Marine Isotope Stage 3 deposits widespread on the outer shelf of the East China SeaSpatiotemporal changes in environmental and oceanographic conditions on the continental shelf are well revealed by greigite‐based parametersAuthigenic greigite can serve as a sensitive indicator for paleoenvironmental and paleoceanographic studies in continental shelf regions [ABSTRACT FROM AUTHOR]

Geochemical and petrographic characterizations were carried out on five episodic effusive eruptions from Batur Volcano on the Island of Bali, Indonesia, and revealed that these lavas are basaltic to andesitic in composition. Various micro-textures were identified, reflecting pre-eruptive magmatic processes, magma mixing, and adiabatic decompression. Apart from XRF analyses, the five lava flows (L849, L926, L963, L968 and L974) were subjected to magnetic analyses in the form of magnetic susceptibility and magnetic hysteresis measurements. The presence of cyclicity in the magmatic process is suggested by the variation of micro-texture types, CaO content, and magnetic susceptibility values. Two possible models of this cyclicity are presented. In the first model, cyclicity is driven by the variation of influx from the lower magma chamber that affects the interaction of magma with the surrounding rocks. In the second model, cyclicity is affected by the appearance of a new magmatic vent, which causes intense interaction with surrounding rocks. This study infers that combined micro-texture, composition, and magnetic susceptibility analyses might provide insight into the cyclicity of lava episodes observed in active volcanos such as Batur.

Abstract—The evolution of the domain structure (DS) of cubic submicron- and micron-sized magnetite particles has been studied in detail during "cooling" of specimens from the Curie temperature Тс to room temperature Тr followed by their "reheating" to Тс in order to determine the degree of irreversibility of DS changes during heat treatment and their possible effects on the thermoremanent magnetization (TRM) properties. It is shown that typical magnetic configurations in particles up to 2 μm in size have flower or vortex shapes with one or two vortices. A model of the formation of thermoremanent magnetization (TRM) in submicron-sized pseudo-single-domain particles (PSD) is proposed based on an expansion of Neel's single-domain thermofluctuation model of TRM acquisition. According to the model, the equality of blocking and deblocking temperatures is retained for submicron PSD grains. Consequently, for this magnetic fraction, the Thellier laws of pTRM additivity and independence must also be valid. [ABSTRACT FROM AUTHOR]

Lacustrine sediments are good recorders of palaeoenvironment changes and have been widely studied in recent years. The study of lacustrine sediments in Southwest China will improve our understanding of the complex environmental evolution history of Southwest China. Therefore, this paper presents a high-resolution rock magnetism study from the Pengtun drilling hole (PT2) in the Heqing Basin, Southwest China. The results show that the dominant magnetic minerals in the PT2 drill core are magnetite, maghemite and haematite. The magnetic susceptibility (χ) of the upper part of the core (0–13.5 m) is lower than that of the lower part (13.5–33.5 m). There is no maghemite in the upper sections. The minerals with high χ values have the characteristics of pseudosingle-domain (PSD), single-domain (SD), superparamagnetic (SP) and multidomain (MD) grains. The magnetic domains are widely distributed, and the particles are coarser. When χ is low, the magnetic domains of the different samples vary greatly, and the particles are finer. Combined with the magnetic and nonmagnetic characteristics of the sediment, we infer that the change in χ in the PT2 drill core is related to clastic input, water erosion, and reductive dissolution. [ABSTRACT FROM AUTHOR]