Remanence Carriers Research Articles

Reliability of the palaeomagnetic signal recorded in a lava flow erupted on 4 December 2021 in La Palma (Canary Islands, Spain)

SUMMARY A basaltic lava flow erupted from the Tajogaite volcano on 4 December 2021, in La Palma (Canary Islands, Spain) was sampled to find out to what extent reliable and correct information on both intensity and direction of the Earth's magnetic field can be obtained from the palaeomagnetic signal recorded in a lava flow which erupted under known conditions. Samples were taken every few centimetres across a flow up to a total of 27 oriented cores. Palaeomagnetic experiments showed a strong viscous overprint in many samples. Nevertheless, the mean palaeomagnetic direction obtained agrees well with the actual value from IGRF-13. Rock magnetic experiments were performed to obtain additional information about the quality and reliability of the results and the reasons for unsuccessful determinations. Analysis of mostly irreversible thermomagnetic curves showed that the carriers of remanence were magnetite and titanomagnetite of low and/or intermediate Curie-temperature. Hysteresis parameter ratios showed a pronounced variability across the flow. Analyses of frequency dependent susceptibility, IRM acquisition coercivity spectra and FORCs showed a noticeably presence of very low coercivity grains (multidomain and superparamagnetic-single domain boundary). Multimethod palaeointensity experiments were performed with the Thellier-Coe, multispecimen and Tsunakawa-Shaw methods. Only three of 25 cores from the flow yielded successful Thellier-Coe determinations, in agreement with the expected field value of 38.7 μT (IGRF-13). However, palaeointensities of 60 per cent of the specimens agree with the expected value performing an informal analysis without considering criteria thresholds. Four of six Tsunakawa-Shaw determinations performed on samples from the flow yielded correct results, but three multispecimen determinations providing apparently successful determinations largely underestimate the expected field intensity. Combination of three Thellier-Coe and four Tsunakawa-Shaw successful determinations yields a multimethod palaeointensity result B = (36.9 ± 2.0) μT in good agreement with the expected field intensity.

Magnetization related to late-Variscan extensional collapse in the Lugo Dome (NW Iberian Massif): a metamorphic petrology approach

The Eastern Galicia Magnetic Anomaly (EGMA) is one of the most distinctive and best studied magnetic anomalies in Iberia. It is located in NW Spain and overlaps the Variscan gneissic domes of Lugo and Sanabria. The Lugo Dome, in the north of the EGMA, deforms a large thrust sheet, the Mondoñedo Nappe, allowing the outcropping of its relative autochthon in the Xistral Tectonic Window, where extensional detachments resulting from late-Variscan tectonics crop out. A younger major detachment, the Viveiro Fault, developed on the western limb of the dome. Detailed ground-based magnetic mapping and geological charts show a clear spatial relationship between magnetic maxima and the extensional structures. Magnetic and paleomagnetic studies carried out on samples from this region indicate that rocks in the detachments commonly bear magnetite and hematite carrying induced but also remanent magnetizations and a very well-defined anisotropy of the magnetic susceptibility with directions matching those of the late-Variscan extensional fabrics.Three pairs of metasedimentary samples in equivalent structural positions display variously developed medium-pressure Barrovian parageneses attributed to the early compressional phases of the Variscan orogeny, and low-pressure Buchan-type parageneses associated with the late Variscan extension. Our phase diagram-based petrological study suggests that the non-magnetic samples preserve the Barrovian conditions of 560–640 °C, 5–8.7 kbar. The lithologically and structurally equivalent samples, originally non-magnetic, developed magnetite/hematite-bearing mineral assemblages during decompression and cooling to 500–620 °C, 1.5–6 kbar. This recrystallization was probably assisted by metasomatic oxidizing fluids. The induced component of magnetization is essentially carried by magnetite, while hematite seems the main carrier of remanence. These results support the existing cartographic, geophysical and paleomagnetic findings indicating that the magnetization in the Lugo Dome is related to late-Variscan extension.

Vortex Magnetic Domain State Behavior in the Day Plot

AbstractThe ability of rocks to hold a reliable record of the ancient geomagnetic field depends on the structure and stability of magnetic domain‐states contained within constituent particles. In paleomagnetic studies, the Day plot is an easily constructed graph of magnetic hysteresis parameters that is frequently used to estimate the likely magnetic recording stability of samples. Often samples plot in the region of the Day plot attributed to so‐called pseudo‐single‐domain particles with little understanding of the implications for domain‐states or recording fidelity. Here we use micromagnetic models to explore the hysteresis parameters of magnetite particles with idealized prolate and oblate truncated‐octahedral geometries containing single domain (SD), single‐vortex and occasionally multi‐vortex states. We show that these domain states exhibit a well‐defined trend in the Day plot that extends from the SD region well into the multi‐domain region, all of which are likely to be stable remanence carriers. We suggest that although the interpretation of the Day plot and its variants might be subject to ambiguities, if the magnetic mineralogy is known, it can still provide some useful insights about paleomagnetic specimens' dominant domain state, average particle sizes and, consequently, their paleomagnetic stability.

Paleomagnetism of mafic dykes in South Rewa Basin, India: Constraints to extension of Deccan volcanism

The magma source for mafic dykes from South Rewa Basin (SRB) in central India is invariably linked to Rajmahal volcanism (117 Ma) for Damodar, Raniganj-Jharia, and Bengal basin (Permian - Quaternary) to the east, and Deccan volcanism (66 Ma) for Satpura Basin to the west of SRB. However, the magmatic events linked to the SRB are not explicit. To ascertain the dyke association with these volcanic events, we performed a comprehensive paleomagnetic study on the exposed dykes and basalts from Shahdol region in SRB. Rock magnetism indicate that magnetite or titano-magnetite is the main remanence carrier mineral in these dykes. The measured directions produce a mean declination (Dm) of 338° and mean inclination (Im) of - 35° (α95= 8.4°, k = 25.3, N = 13), is close to Deccan normal directions. The calculated Pole position (λp) is at 42.02°N, and (Lp) is at 289.33°E, suggesting that the studied dykes are emplaced simultaneously along with Deccan Traps (36.96°N/78.70°W) and not of Rajmahal Traps (11.37°N/297.58°E). These dykes can be the result of multiple Deccan magma intrusions along the Narmada-Tapti lineament and intra-basinal faults in the SRB of central India.

Assessing lunar paleointensity variability during the 3.9 - 3.5 Ga high field epoch

Numerous paleomagnetic studies suggest that a lunar dynamo, with surface field intensities potentially as high as 40-100 µT, existed between ∼3.9 Ga and ∼3.5 Ga. This period is referred to as the High Field Epoch (HFE). However, the debate over the origin of magnetization recorded in lunar rocks still persists. In addition, whether the Moon could have sustained a continuously strong dynamo during the HFE remains unclear. To unravel the origin of magnetization preserved in lunar rocks and to better characterize the evolution of the ancient lunar dynamo, we conducted a comprehensive set of experiments including rock magnetic tests, electron microscopy, and paleomagnetic investigations on four HFE-aged Apollo 11 mare basalt samples: 10003, 10044, 10069, and 10071. Rock magnetic experiments and electron microscopy indicate that the remanence carriers are kamacite grains of varying sizes and domain states. Sample 10003 recorded a paleointensity of 54.10 ± 4.66 µT. Sample 10044, which was shocked (peak pressure >5 GPa), did not preserve a stable high coercivity remanent magnetization. Samples 10069 and 10071 recorded paleointensities of 61.46 ± 26.09 µT and 10.69 ± 2.87 µT, respectively. A series of hydrostatic pressure experiments, isothermal remanent magnetization (IRM) acquisition experiments, and viscous remanent magnetization (VRM) tests preclude the possibility of our samples containing shock remanent magnetization from transient impact-generated fields, IRM acquisition from exposure to spacecraft fields, and VRM acquisition from exposure to the Earth magnetic field. Overall, our study suggests that the source of these magnetizations was likely the lunar dynamo and may indicate nearly order-of-magnitude magnetic field fluctuations during the HFE.

Early–Middle Devonian paleomagnetic results from the Zhongba microterrane, Tibetan Plateau: Evidence for its origin from the northern margin of Greater India

Abstract The initial disintegration of Gondwana during the Paleozoic laid the foundation for the formation of the Tibetan Plateau in the Cenozoic. Determining the relative positions of the microterranes in Gondwana during the Paleozoic not only informs the subsequent drift and accretion processes of these microterranes but is also crucial to the paleogeographic reconstruction of Gondwana. However, the lack of paleomagnetic constraints on the Devonian paleogeography of the microterranes in the northern part of Gondwana makes this effort challenging. Here, we report paleomagnetic results for the first time from the Early–Middle Devonian sediments of the Zhongba microterrane. The site-mean direction is declination (Ds) = 310.7°, inclination (Is) = −67.2°, ks = 31.2, α95 = 8.3°, and n = 11, in stratigraphic coordinates. Positive fold and reversal tests, together with rock magnetism results and microscopic observations, strongly suggest that the remanence carriers are of depositional origin. The paleomagnetic results meet the paleomagnetic reliability criteria and therefore can be used for tectonic reconstructions. Our results constrain the paleolatitude of the Zhongba microterrane to be 50.0°S ± 11.7°S in the Early–Middle Devonian. Combined with published detrital zircon ages as well as paleomagnetic results and geological data, our data indicate that the Zhongba microterrane, which had no tectonic affinity with the Lhasa terrane, was part of the northern margin of Greater India during 408–388 Ma and coupled with the South Qiangtang terrane, Tethyan Himalayas, and other terranes to form the continuous northern continental margin of East Gondwana.

Nanoscale imaging of Fe-rich inclusions in single-crystal zircon using X-ray ptycho-tomography

We apply X-ray ptycho-tomography to perform high-resolution, non-destructive, three-dimensional (3D) imaging of Fe-rich inclusions in paleomagnetically relevant materials (zircon single crystals from the Bishop Tuff ignimbrite). Correlative imaging using quantum diamond magnetic microscopy combined with X-ray fluorescence mapping was used to locate regions containing potential ferromagnetic remanence carriers. Ptycho-tomographic reconstructions with voxel sizes 85 nm and 21 nm were achievable across a field-of-view > 80 µm; voxel sizes as small as 5 nm were achievable over a limited field-of-view using local ptycho-tomography. Fe-rich inclusions 300 nm in size were clearly resolved. We estimate that particles as small as 100 nm—approaching single-domain threshold for magnetite—could be resolvable using this “dual-mode” methodology. Fe-rich inclusions (likely magnetite) are closely associated with apatite inclusions that have no visible connection to the exterior surface of the zircon (e.g., via intersecting cracks). There is no evidence of radiation damage, alteration, recrystallisation or deformation in the host zircon or apatite that could provide alternative pathways for Fe infiltration, indicating that magnetite and apatite grew separately as primary phases in the magma, that magnetite adhered to the surfaces of the apatite, and that the magnetite-coated apatite was then encapsulated as primary inclusions within the growing zircon. Rarer examples of Fe-rich inclusions entirely encapsulated by zircon are also observed. These observations support the presence of primary inclusions in relatively young and pristine zircon crystals. Combining magnetic and tomography results we deduce the presence of magnetic carriers that are in the optimal size range for carrying strong and stable paleomagnetic signals but that remain below the detection limits of even the highest-resolution X-ray tomography reconstructions. We recommend the use of focused ion beam nanotomography and/or correlative transmission electron microscopy to directly confirm the presence of primary magnetite in the sub 300 nm range as a necessary step in targeted paleomagnetic workflows.

Unmixing the hydrothermal and detrital components from a metalliferous sediment core from the eastern Southwest Indian Ridge – Insights from mineral magnetism

Mineral magnetic and geochemical investigations were carried out on a short sediment core collected from the eastern Southwest Indian Ridge (ESWIR) to study the variations in the source components. The magnetic concentration parameter χarm (representing the concentration of stable single domain (SSD) sized grains), magnetic grain size ratios χarm/χlf, χarm/SIRM, geochemical ratios Fe/Ti, Co/Zn, and Zn/Fe and Ba content serve as hydrothermal indicators, all of which show a distinct top - bottom variability. The magnetomineralogical parameters S-ratio and coercivity (Hc) also follow these variations, suggesting an increased contribution of hydrothermal components in the top 15 cm of the core. Significant enrichments of Fe, Mn, and Mg in the samples, along with depletions of detrital elements Al, Si, Rb, Zr, Hf, and Nb in the element/Upper Continental Crust (UCC) values, show that the majority of the detrital component is locally sourced. Thermomagnetic measurements of samples indicate that the major remanence carrier is magnetite in the core, along with minor contributions from titanomagnetite, hematite, and iron sulfides.

Quaternary Magnetic Stratigraphy of Deep‐Sea Sediments in the Western North Pacific: Influences of Paleomagnetic Recording Efficiency and Lock‐In Delay

AbstractUnderstanding past environmental changes in the western North Pacific is rather limited due to the general paucity of robust chronology for carbonate‐lacking pelagic clay sediments. Here, we present a new magnetostratigraphy of Quaternary deep‐sea sediments in the northern Mariana Basin. A comprehensive set of paleomagnetic, rock magnetic, and microscopic results reveal that paleomagnetic signals are mainly carried by detrital magnetic minerals of Asian eolian dust and volcanic origins. The volcanic materials, most likely from the Mariana Arc, are characterized by low coercivity (∼30 mT) interacting single domain (SD) and pseudo‐SD grains, associated with abundant dendritic magnetic crystals/inclusions. This volcanic component is a suitable remanence carrier but is less efficient in recording geomagnetic intensity than the higher coercivity eolian component. The relative paleointensity record derived with minimum rock magnetic artifacts shows comparable patterns with global templates. At the Matuyama‐Brunhes transition, a ∼2‐cm remanence lock‐in depth offset is identified from beryllium isotope records, and the estimated magnetostratigraphic age delay is small (∼8 kyr). Our results provide an improved understanding of remanence recording processes in carbonate‐poor deep‐sea clays and demonstrate magnetic stratigraphy as a valuable chronology in the western North Pacific region.

Rock Magnetic Characterization of Returned Samples From Asteroid (162173) Ryugu: Implications for Paleomagnetic Interpretation and Paleointensity Estimation

AbstractIn this study, systematic rock magnetic measurements and saturation isothermal remanent magnetization (SIRM) paleointensity calibration experiments were conducted for the returned samples from C‐type asteroid (162173) Ryugu and two carbonaceous chondrites (Orgueil and Tagish Lake) to evaluate the remanence carriers of the Ryugu sample and its ability as a paleomagnetic recorder. Our magnetic measurements show that Ryugu samples exhibit signatures for framboidal magnetite, coarse‐grained magnetite, and pyrrhotite, and that framboidal magnetite is the dominant remanence carrier of Ryugu samples in the middle‐coercivity range. The SIRM paleointensity constant was obtained for two Ryugu samples, and the median value was 3,318 ± 1,038 μT, which is close to the literature's value based on the average among magnetite, titanomagnetite, pyrrhotite, and FeNi alloys and is widely used for SIRM paleointensity experiments. The paleointensity values estimated using the obtained SIRM paleointensity constant indicate a strong magnetic field of the protoplanetary disk, suggesting that Sun's protoplanetary disk existed at the disk location of Ryugu's parent planetesimal when framboidal magnetite precipitated from the aqueous fluid.

Anisotropy-induced spin disorder in intergrown, ferrimagnetic Fe7S8 polytypes

The monosulfides of the pyrrhotite omission series (Fe1−xS, 0<x≤0.125) are important remanence carriers for paleomagnetic reconstruction of the Earth's crust and extraterrestrial materials. The ferrimagnetic Fe7S8 polytypes are the endmembers, and their stacking modulations of full and vacant layers generate different magnetic anisotropy properties due to the cation-vacancy configurations. In this study, intergrown long-range ordered polytypes with four- and threefold modulation, i.e., 4C and 3C pyrrhotite, were prepared in a diffusion-driven process by quenching of a natural pyrrhotite crystal with randomized vacancies. In addition, a third constituent with coherence lengths of a few nanometers, denoted 3C*, was found that exhibits spin-glass behavior at about 10 K due to local magnetic anisotropies arising from vacancy-density variations. The concomitant occurrence of this nano-scale constituent with spin disorder and the long-range ordered polytypes indicate competitive diffusion-driven processes during Fe7S8 formation. Such information provides insight into the provenance and genesis of ferrimagnetic pyrrhotite in Earth and extraterrestrial systems and in a broader sense into vacancy-induced materials.

Tectonic evolution of the Eocene–Oligocene Lushi Basin in the eastern Qinling belt, Central China: Insights from paleomagnetic constraints

Abstract The eastern Qinling orogenic belt, located in the conjoint region between the Neo-Tethys tectonic domain and the western Pacific tectonic domain, is key to understanding the tectono-sedimentary evolution of the East Asian interior. Extensive Late Cretaceous–Cenozoic fluvial and lacustrine deposits within the eastern Qinling orogen provide ideal archives of tectono-sedimentary evolution in Fenwei, the eastern Qinling orogen. We present an integrated rock magnetism and anisotropy of magnetic susceptibility (AMS) study of a Middle Eocene–Early Oligocene succession in the Lushi Basin, eastern Qinling region. Our rock magnetic results suggest that hematite and magnetite are the main magnetic carriers of remanence, while χ−T curves, hysteresis loops, and low values of bulk susceptibility indicate that paramagnetic minerals are major contributors to AMS in the Lushi Basin. Nevertheless, the anomalous magnetic fabrics in the Zhangjiacun and Lushi Formations may result from the contribution of the iron-bearing carbonates in deposits. The clustering of the minimum principal axes nearly vertical to the bedding plane and the well-defined NW–SE magnetic lineation almost parallel to the dip of the bounding fault suggest that the AMS of the Dayu Formation is not a sedimentary fabric but an incipient deformation magnetic fabric. The pronounced NW–SE magnetic lineation indicates the NW–SE stretching of the Lushi Basin during the Late Eocene–Earliest Oligocene, which may have been caused by the combined effects of the India–Eurasia collision and the subduction of the western Pacific plate.

Occurrence and Distribution Patterns of Magnetic Particles Within Stalagmite Growth Laminae

AbstractThe concentrations of magnetic particles in stalagmites depend greatly on the regional hydrologic condition, and thus can act as an important proxy for paleoclimate reconstruction. The central axis region of a stalagmite is usually selected for such magnetism study. However, little is known about the distribution and influence factors of exogenetic magnetic particles within the growth laminae. In this study, the occurrence and variations of magnetic particles along the growth laminae of three shape stalagmites were investigated by a set of magnetic measurements, including isothermal remanent magnetization (IRM) acquisition curves and low‐temperature magnetic analysis. The results show the main magnetic remanence carrier is detrital magnetite/maghemite, coexisting with small amounts of hematite and goethite. The distribution of soft magnetic particles along the growth laminae showed different patterns among the three shape stalagmites (dome, cone, and candle). For the dome shape stalagmite, the soft magnetic particle concentrations in all studied growth layers showed an increasing trend from center to edge. Similar patterns were also appeared in the bottom growth layers of the cone and candle shape stalagmites. This edge enrichment of magnetic particles is caused by drip water washing effect. However, such enrichment pattern became weaker or even inverse in the top growth layers of the cone and candle shape stalagmites, indicating that the shapes of stalagmites could also affect the surface distribution of soft magnetic minerals. These uneven distributions of magnetic particles imply that the central axis region of a stalagmite may not always be the optimal specimen for environmental magnetism study.

Rock magnetism and AMS studies in Kondapalle-Pangidi layered complex, Eastern Ghats Belt, India: Remanence carriers and tectonic implications

The present study is an integrated approach towards the rock magnetism and Anisotropy of Magnetic Susceptibility (AMS) of the high-grade metamorphic rocks in the Kondapalle Pangidi Layered Complex (KPLC), Eastern Ghats Belt (EGB), India. Our study aims at determining the petrography, rock magnetic properties and the magnetic fabrics. The rock magnetic properties and the magnetic fabrics from AMS studies are used to determine their tectonic implications. Petrography reveals that the chief remanence carrier mineral is Ti-magnetite. Rock magnetic studies including Isothermal Remanent Magnetization (IRM), back-field IRM, hysteresis loop studies, and thermo-magnetic studies reveals the same. The magnetic hysteresis parameters (remanence and coercivity) are used to determine the magnetic domains from a modified Day plot. The magnetic domains lie within Stable Single Domain and Pseudo Single Domain ranges, thereby explaining their potential to record an ancient magnetic field. Besides the ferrimagnetic remanence carriers, paramagnetic silicates are also dominant which controlled the development of magnetic fabric. The magnetic fabric bears analogy with the regional structures and bear tectonic implications related to the post-rifting closure of the ocean followed by subduction of the ocean basin, accretionary orogeny and felsic magmatism in the Indo-Antarctic continental blocks.

Rock Magnetism of Lapilli and Lava Flows from Cumbre Vieja Volcano, 2021 Eruption (La Palma, Canary Islands): Initial Reports

We present initial rock magnetic results for both lava flows and lapilli produced by the 2021 eruption of the Cumbre Vieja, La Palma (Canary Islands). Samples were taken during the eruption to minimize early alteration and weathering of the rocks and tephra. Standard procedures included progressive alternating field and thermal demagnetization, hysteresis curves, thermomagnetic experiments, progressive acquisition of isothermal remanent magnetization (IRM), and First-Order Reversal Curves (FORCs). Overall, our observations, including low to medium unblocking temperatures, isothermal remanent magnetization to 1 Tesla, and the abundance of wasp-waist hysteresis loops, strongly suggest the presence of Ti-rich titanomagnetites as the main remanence carriers in both lava flows and lapilli, in addition to some hematite as well. Whereas the former has been directly seen (SEM), hematite is elusive with nonmagnetic-based methods. Rock magnetic data, on a Day plot, also reveal that the magnetic grain size tends to be larger in the lava flows than in the lapilli.

An enigma in rock magnetism: can microstructures in magnetite cause a threefold increase in the efficiency of NRM acquisition in the Stardalur Basalts?

SUMMARY Quaternary lavas of the Stardalur Caldera, 20 km northeast of Reykjavik, Iceland, create a 27 300 nT magnetic anomaly visible in both ground and aeromagnetic surveys. Here, we provide a comprehensive mineralogical and rock magnetic data set to analyse NRM intensities and Koenigsberger ratios of 57 drill-core samples from the critical zone (CZ) of the anomaly high at depths between 41 and 131 m. This extends previous studies and verifies that the anomaly is due to an unusually high intensity of remanent magnetization carried by magnetite. The NRM of the CZ samples was acquired during the Olduvai subchron in a field of at most today’s strength. NRM intensities range from 20 to 128 A m–1 with a median of 55 A m–1, and an average of 61 A m–1, respectively, approximately 13–15 times higher than in typical Icelandic basalts (AIB) with an NRM intensity of 4 A m–1. Our new data set shows that the magnetite concentration throughout the CZ basalts is at most twofold higher than in AIB lavas. New data on domain state and TRM efficiency prove that these properties account for an additional factor of at most 2.3. Because magnetite is the most abundant remanence carrier in rocks on Earth, and its remanence acquisition is considered to be extremely well understood, we assert that the remaining discrepancy is a critical enigma in rock magnetism. Results from scanning electron microscopy show that a significant fraction of all CZ magnetite particles have dendritic shapes with grain sizes <1 μm, indicating rapid crystallization. Most large magnetite grains are heavily subdivided by very fine oxidation-exsolution lamellae of ilmenite, and subordinate amount of exsolved spinel as needles, blebs and blades. These common microstructures found throughout the CZ subdivide the initially homogeneous mineral into separate cubicles, here denoted as compartments. The magnetite compartments then have sizes below 1 μm. Hysteresis data, Preisach maps and FORC data consistently confirm that the coercivity distribution is dominated by values above 10 mT, such that multidomain behaviour is of little relevance in the CZ. Between 5 and 20 per cent of the IRM is carried by coercivities above 100 mT, which for magnetite indicates unusually high anisotropy effects in the individual particles. Based on the quantitative analysis of all magnetic contributions to the NRM, we can demonstrate that the average efficiency of NRM acquisition in the CZ Stardalur basalts must be at least a factor 3 higher than in typical basalts. We speculate that this is related to the observed focused compartment size distribution <1 μm, and indicates thermochemical remanence acquisition below the Curie temperature of magnetite. Yet, a detailed physical mechanism for the extreme overefficiency of NRM acquisition remains enigmatic.

Permian Magnetostratigraphy and End of the Kiaman Reverse Polarity Superchron From the Southeast Karoo Basin, South Africa

AbstractPaleomagnetic results for a ∼2,353‐m‐thick magnetostratigraphic section for undeformed middle to late Permian rocks in the southeast of the Karoo Basin of South Africa are reported. Predominantly pseudo‐single‐domain or single‐domain titanomagnetite (with possible minor contributions by pyrrhotite and multi‐domain magnetite) were identified as remanence carriers of a dual polarity magnetization interpreted as the record of the Permian geomagnetic field during the Kiaman Reverse Polarity Superchron and subsequent Illawarra mixed polarity interval. The timing of remanence acquisition is further constrained by the effect of Jurassic‐aged dolerite intrusions, which either partially or wholly overprint the Permian remanence in their immediate vicinity. A paleopole at 62.0°S; 64.3°E and dp/dm = 4.9°/5.8° is calculated from the bedding‐corrected primary remanence (when corrected for inclination shallowing at f = 0.6 the paleopole is located at 53.2°S; 46.9°E and dp/dm = 5.9°/6.3°). This is comparable to other Permian paleopoles from the southwest section of the Karoo Basin. The end of the Kiaman Reverse Polarity Superchron can be correlated between the E‐W extremes of the basin to reveal a diachronous boundary between the Ecca and the Beaufort groups, with diachronicity calibrated to 1.1 million years for the first time.

Eocene relative paleointensity of the geomagnetic field from Integrated Ocean Drilling Program Site U1403 and U1408 sediments in the northwest Atlantic

Published relative paleointensity (RPI) records older than ∼3Ma are extremely limited in time and space. To develop a more robust understanding of RPI variations, we have conducted rock magnetic and paleomagnetic measurements on Eocene marine sediments recovered at Integrated Ocean Drilling Program (IODP) Sites U1403 and U1408 in the northwest Atlantic. A series of rock magnetic measurements indicate that the main remanence carrier is single domain biogenic magnetite. Paleomagnetic measurements yielded RPI records for Chrons C18–C21 and C22n, which correlates to ∼38.4–49.6Ma. The record is compromised in a few short intervals by inhomogeneous rock magnetic properties. RPI minima always occur at chron boundaries and RPI fluctuates between highs and lows within each chron. This record is the first to show that these characteristics persist at least since the onset of Chron C22n at ∼49.3Ma. We conclude that these are intrinsic and fundamental features of the geomagnetic field regardless of the polarity reversal rate. We produce a stacked RPI curve for Chron 18, named PIS-C18, on the basis of the RPI records obtained in this study and those from IODP Sites U1331 and U1332 in the equatorial Pacific that are matched by visual inspection. The PIS-C18 RPI of the stack is generally high with no prominent lows during Chron C18n.2n, whereas it is not as high and has several prominent lows almost equivalent to the RPI minima at the chron boundaries during Chrons C18n.1n and C18r. A histogram of RPI during Chron 18 is slightly skewed to the right, and the ratio of the standard deviation to the mean paleointensity is 0.38. These characteristics are similar to a histogram of the RPI stack for the last 1.5 million years. We interpret this to imply that character of the geodynamo for 104–106 years timescales has been unchanged since the Eocene.

Magnetostratigraphy of the Upper Cretaceous Nenjiang Formation in the Songliao Basin, northeast China: Implications for age constraints on terminating the Cretaceous Normal Superchron

We developed an integrated chronology for the non-marine Upper Cretaceous Nenjiang Formation based on high-resolution magnetostratigraphic results and previously published secondary ion mass spectrometry (SIMS) U–Pb zircon analyses of the eastern borehole of the Cretaceous Continental Scientific Drilling (CCSD-SK-II) borehole and two outcrop sections located in different structural provinces of the Songliao Basin, China. Detailed rock magnetic results demonstrated that pseudo-single-domain magnetite and single-domain greigite coexisted in the lacustrine black shales of the Nenjiang Formation, with the latter dominating remanence carriers. The reliable primary remanent magnetizations were isolated, which passed a class A positive reversal test and positive bootstrap reversal test, after correction for inclination shallowing, yielding a high-quality paleopole of 79.6°N/208.4°E, A95 = 2.3°. The borehole sequence and outcrop sections were stratigraphically correlated by combining lithostratigraphy, biostratigraphy, and SIMS U–Pb zircon geochronology. The correlation of the recognized magnetic polarity sequences to the geomagnetic polarity timescale suggests that the Nenjiang Formation from the CCSD-SK-II borehole sequence and the two outcrop sections span from very late chron C34n to very early chron C33r. Furthermore, the age of the Cretaceous Normal Superchron (CNS) termination can be constrained to 82.7 ± 0.6 Ma based on magnetostratigraphy, radiometric dating, and the perfect/typical averaged sediment accumulation rate for Member 1 of the Nenjiang Formation of the borehole sequence. The estimated age obtained in this study accurately represents the age at CNS termination.

Anisotropy of magnetic susceptibility study and its significance in the Late Cretaceous-Cenozoic Sanmenxia Basin in the southeastern Shanxi rift, Central China

The Sanmenxia Basin, located on the southeastern margin of the Shanxi rift and filled with Cretaceous-Paleogene fluvial and lacustrine sediments, is a faulted basin bounded by a series of normal strike-slip faults. This provides a valuable opportunity to investigate the early tectonic deformation in the Fenwei graben. In this study, we report an integrated rock magnetism and AMS analysis of two sections from the Sanmenxia Basin spanning an interval from the Late Cretaceous to the Eocene. The rock magnetic results demonstrate that magnetite and hematite are the main magnetic carriers of remanence, and paramagnetic minerals and hematite are major contributors to the AMS in both sections of the Sanmenxia Basin. Together with the relatively low-corrected anisotropy values, the tightly grouped minimum principal axes are almost perpendicular to the bedding plane, and the well-defined magnetic lineation is generally parallel to the bedding trend, indicating that the primary sedimentary fabrics in the Sanmenxia Basin were overprinted by the initial deformation. The NW–SE magnetic lineation denotes the NW–SE stretching during the Late Cretaceous-Eocene. The stretching process may have been controlled by the Indian-Eurasian convergence and/or the subduction of the western Pacific plate.