Stable Remanent Magnetization Research Articles

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.

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

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

Visualization of nanoscale magnetic domain states in the asteroid Ryugu

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

Magnetic Recording Stability of Taenite‐Containing Meteorites

AbstractSub‐μm taenite and tetrataenite grains observed in a number of (stony‐)iron meteorite groups are promising sources of paleomagnetic records in meteorites. While slowly‐cooled meteorites form tetrataenite—an extremely good recorder—, fast‐cooled meteorites may contain fine‐grained taenite, which was considered unsuitable for paleomagnetic studies. In this work, however, we show that nm‐sized taenite grains are stable over billion‐year timescales, indicating that taenite‐bearing meteorites are reliable sources of paleomagnetic information. We find a range of sizes for which taenite forms stable single‐domain structures, which coincides with the grain sizes observed in the cloudy zone of most fast cooled IVA meteorites. These meteorites, therefore, can provide reliable paleomagnetic information recorded as a stable crystallization remanent magnetization as taenite grains grown. Vortex states observed in larger (&gt;50 nm) grain sizes are also highly stable, indicating that coarse‐grained taenite observed in meteoritic microstructures can also provide reliable records of paleomagnetic fields.

Effects of Hydrothermal Alteration and Mineralization on the Paleomagnetic Properties of Rocks from IODP Expedition 376 at Brothers Volcano

AbstractThe 3-D subseafloor architecture of submarine hydrothermal systems is largely unknown, particularly at arc volcanoes. The alteration of volcanic rocks in these systems produces dramatic changes in their magnetic properties. Here, we present the first comprehensive study of paleomagnetic measurements from oriented samples of hydrothermally altered dacites from Brothers volcano (Kermadec arc), drilled during International Ocean Discovery Program (IODP) Expedition 376. These data have enabled insight into the progressive evolution of magnetic minerals in subseafloor volcanic rocks affected by variable types and degrees of hydrothermal alteration in response to varying fluid temperatures, chemistry, and associated mineralization; from initial chloritization typical of relatively low-temperature interaction with seawater to extremely altered rocks affected by higher-temperature, very acidic magmatic fluids.Hydrothermally altered samples show a significant reduction in natural remanent magnetization intensity (10–4 to 10–2 A/m) compared with unaltered samples (1–10 A/m), suggesting that primary titanomagnetite grains are destroyed during the hydrothermal alteration process. Except for a small region in proximity to the mineralized stockwork zone, no chemical remanent magnetization is observed in association with hydrothermal alteration, consistent with the widespread formation of diamagnetic and/or paramagnetic minerals such as pyrite, rutile, and leucoxene, which do not carry any natural remanent magnetization.Demagnetization experiments show that most of the oriented samples possess a stable characteristic remanent magnetization induced by the residual primary magnetic minerals formed at the time the rocks cooled on the sea floor. Partially chloritized dacites, however, are characterized by large magnetic susceptibilities, low Koenigsberger ratios, and very low magnetic coercivities, consistent with initial dissolution of smaller, singledomain magnetic grains, indicating that intensely hydrothermally altered rocks are better paleomagnetic indicators than initially chloritized samples at the periphery of the hydrothermal systems.The significant magnetic contrast between fresh and hydrothermally altered rocks, in addition to a thick layer (&amp;gt;300 m) of demagnetized rocks observed at Brothers volcano, confirms the empirical results that magnetic anomalies are important geophysical tools to determine the geometry of hydrothermal systems at submarine arc volcanoes.

Acquisition of natural remanence in the basaltic laterites of Deccan volcanic province (India): Implications to palaeomagnetic studies in laterites

Lateritization in the Deccan basalt province (India) represents a large spectrum of chemical weathering with up to 99% CIA values and complete ferri- to antiferromagnetic transformations. We combined rock magnetic, palaeomagnetic, XRD, and XRF analyses to investigate the mineralogical transformations and its relationship to the acquisition of natural remanent magnetization (NRM) by sampling a 25 m thick Deccan lateritic profile from the West Coast of India. Significant amount of amorphous hematite (ha), crystalline hematite (hc), goethite and their Al- substituted forms were detected along with gibbsite and kaolinite. The domain size association of ‘SP + SD and ‘SP + SD + MD’ in the ferri- as well as antiferromagnetic forms; and the combination of ha and hc yield strong NRMs amongst these laterites. Low field demagnetization (<5mT) allows removal of haNRM; and the secondary components due to ferrimagnetic oxides can be removed at < 25 mT. Thereafter, the hcNRM continued to decay up to > 150 mT. Palaeomagnetic tests (Koenigsberger ratio, MDF, intensity decay and Zijderveld diagrams) indicate stable Characteristic remanent magnetizations (ChRM) due to hc; and the Al-substitutions does not affect the ChRM stability, depicting these laterites as excellent palaeomagnetic recorders. The stable ChRMs due to Chemical/Crystallization RMs due to ‘hc’ also benchmarks the degree of maturity within the process of lateritization.

Interaction Between Mineralization and Rock Magnetization: New Constraints From a Silurian‐Lower Devonian Volcanogenic Massive Sulfide (VMS) Deposit

AbstractThe unclear relationship between mineralization, hydrothermal alteration, and rock magnetization in volcanogenic massive sulfide (VMS) deposits limits us from fully understanding the magnetic anomalies and remagnetization process in this type of deposit. We address the issue by conducting systemic paleomagnetic, rock magnetic, petrographic, and X‐Ray diffraction studies in the Dapingzhang VMS deposit on the northwestern Indochina Block. Magnetite is the dominant magnetic carrier of hydrothermally altered surrounding rocks and orebody. Magnetite consumption and secondary magnetite formation occurred at different stages and types of hydrothermal alteration. The overall decrease in magnetite concentration from chloritization via silicification to mineralization implies that the magnetite was mostly consumed during hydrothermal alterations. Secondary single domain magnetite, which can carry a remagnetization direction, is proposed to be formed during illitization via the smectite‐to‐illite transformation. Secondary superparamagnetic magnetite, which was most likely formed during the late stage of chloritization, is unable to carry the stable characteristic remanent magnetization. The site‐mean direction of high‐temperature and high‐coercivity components is Dg/Ig = 324.5°/43.1°, and kg = 35.1, with α95 = 6.8° before tilt correction, and Ds/Is = 316.2°/37.6°, and ks = 16.4, with α95 = 10.1° after tilt correction, with a negative fold test. However, plate reconstruction is limited by the uncertainty of the tilting process following mineralization and the possibility of remagnetization during burial alteration. Therefore, this study provides a mechanism for rock magnetic variation and remagnetization during VMS mineralization.

Sinistral displacement along the Tan–Lu Fault during the Cretaceous induced by Paleo-Pacific subduction: Constraints from new paleomagnetic and U–Pb geochronological data

The Jiaodong Peninsula in eastern China is separated from the main mass of the North China Craton by the Tancheng–Lujiang (Tan–Lu) Fault, the largest fault system in eastern Asia. Although accumulating lines of evidence have suggested that the Tan–Lu Fault underwent strike-slip movement in the Mesozoic, there is still debate about the timing and magnitude of the displacement. To decipher this problem, we conducted paleomagnetic and U–Pb geochronological studies of the Lower Cretaceous volcanic rocks of the Qingshan Group on the Jiaodong Peninsula, to the east of the Tan–Lu Fault on the southeastern edge of the North China Craton. LA–ICP–MS U–Pb dating of zircons from five samples yielded consistent ages of ca. 120 Ma. Stepwise thermal demagnetization conducted on 308 samples (40 sites) from two sections revealed stable characteristic remanent magnetization directions. After paleomagnetic tests and rejecting inappropriate data, the mean direction from 288 samples (31 independent units) is 200.3°/–51.4° (α95 = 4.1°), with a corresponding pole at 215.1°/73.0° (A95 = 4.8°). The paleolatitude calculated for the Jiaodong Peninsula is lower than that of the main block of the North China Craton, which indicates that latitudinal offset occurred between these two parts, and this offset can be ascribed to strike-slip movement along the Tan–Lu Fault. Our data provide new constraints on regional tectonic movement and Paleo-Pacific subduction processes during the late Mesozoic.

Paleomagnetism and geochronology of upper Eocene volcanic rocks from the western Qiangtang block: Constraints on the post-collisional shortening in western Tibet

Understanding the dynamics of the uplift of the Tibetan blocks requires constraints on the timing and magnitude of the crustal shortening accommodated by the different blocks. However, the estimates of the magnitude of post-collisional intra-Asian convergence range from >3000 km to a few hundred kilometers. Here we present new paleomagnetic and geochronological results from the Meisu Formation lavas on the southwestern margin of the western Qiangtang block, western Tibet. Zircon U/Pb data reveal that these volcanic rocks erupted during the late Eocene (∼40 Ma). Following progressive thermal demagnetization, stable characteristic remanent magnetizations (ChRMs) were successively isolated from 28 sites. These ChRMs passed the fold and reversal tests, consistent with a primary remanence. The paleopole at 50.2°N, 163.2°E with A 95 = 5.9° yields a paleolatitude of ∼29.5 ± 5.9°N at ca. 40 Ma for the southern margin of the western Qiangtang block (33.2°N, 80.9°E). A comparison of the Eocene latitudes between the western Qiangtang and Tarim block indicates a ∼ 700 km of post-40 Ma latitudinal crustal shortening between them at the longitude of ∼80°E. Our review of published Cretaceous-Paleogene paleopoles from western Tibet suggests that the western Qiangtang block was positioned at a stable latitude during ∼116–30 Ma. Our paleomagnetic compilation also indicate a discrepancy in the latitude (∼9.7 ± 4.3°) of western Lhasa and Qiangtang during the interval of 132 and 67 Ma, which vanished at ca. 30 Ma. We suggest a crustal shortening model to interpret this 1076 ± 477 km discrepancy, which was accommodated by the interior of the Lhasa block, the Bangong-Nujiang suture zone, and/or an eastward extrusion of Indochina from an original position between the Lhasa and Qiangtang blocks during ∼67–30 Ma. We conclude that the crustal shortening caused by the ongoing India-Asia collision spread progressively northwards into Asia. We emphasize that reliable paleomagnetic data from Paleocene and lower Eocene units in the western Qiangtang and Lhasa blocks have the potential to further refine these crustal shortening estimates. • We provide late Eocene paleomagnetic results from western Qiangtang. • The western Qiangtang block had a paleolatitude of 29.5 ± 5.9°N at ca. 40 Ma. • ∼700 km of post-40 Ma N-S crustal shortening between western Qiangtang and Tarim. • ∼1100 km of N-S crustal shortening between western Lhasa and Qiangtang during ∼67–30 Ma. • Crustal shortening spread progressively northwards into Asia.

Paleomagnetic study of impactites from the Karla impact structure suggests protracted postimpact hydrothermalism

AbstractWe present a paleomagnetic study of the ~10 km diameter Karla impact structure in Russia. We sampled the target carbonate rocks, and a yet undocumented fragmental melt‐bearing lithic breccia layer. This impact breccia, which contains carbonate melt, is enriched in stoichiometric magnetite by a factor of ~15 compared to the target lithologies, and carries a stable natural remanent magnetization. The weak remanent magnetization and the presence of both normal and reverse polarities down to the centimeter scale indicate that the breccia does not carry a thermoremanent magnetization (TRM), but rather a chemical remanent magnetization (CRM). The presence of stoichiometric magnetite and the absence of TRM suggest that the magnetite was formed during relatively low‐temperature postimpact hydrothermalism that affected the porous impact breccia layer. During this process, the breccia acquired a CRM. The paleomagnetic direction is compatible with a Cenozoic age for the impact event, but cannot bring more precise constraint on the age because of the stable position of the Eurasian plate over the last 60 Myr. However, the presence of both polarities indicates that mild hydrothermalism took place over a period of time long enough to span at least one reversal of the geomagnetic field, that is, over a time scale of the order of 100 kyr. This confirms that protracted hydrothermal systems associated with impact craters are long lived, even in relatively small craters such as Karla, and are key features of the geologic and environmental effects of impacts on Earth.

Paleomagnetism of the Grum Zn–Pb–Ag deposit, Selwyn Basin, Yukon, Canada

The Grum deposit is one of five large clastic-dominated Zn–Pb–Ag deposits in the Anvil district of the Selwyn Basin, Yukon. Grum's mineralization occurs at the contact between the late Proterozoic to Cambrian Mount Mye Formation and the Cambrian to Early Ordovician Vangorda Formation. Paleomagnetic analyses of 113 specimens from 15 mineralized sites in the Grum open pit were undertaken to date Grum's mineralization. The analyses isolated a stable characteristic remanent magnetization (ChRM), mostly by thermal and then alternating field demagnetization. The main ChRM carrier is single- or pseudosingle-domain pyrrhotite with minor magnetite that give a paleopole at 67.8°N latitude and 317.4°E longitude (radius of the cone of 95% confidence, A95 = 7.2°). The paleopole yields an age of 176 ± 12 Ma for the Grum deposit after a clockwise rotation and northward translation of the basin to best fit the North American apparent polar wander path. Thus, the Zn–Pb–Ag Grum mineralization significantly predates intrusion of the nearby mid-Cretaceous Anvil Batholith, but is coeval with widespread Early Jurassic metamorphism in the Selwyn Basin that correlates with the Early Jurassic collision of the Intermontane Belt terranes with the North America craton.

Late Holocene Paleomagnetic Secular Variation in the Chukchi Sea, Arctic Ocean

AbstractThe geomagnetic field behavior in polar regions remains poorly understood and documented. Although a number of Late Holocene paleomagnetic secular variation (PSV) records exist from marginal settings of the Amerasian Basin in the Arctic Ocean, their age control often relies on a handful of radiocarbon dates to constrain ages over the past 4,200 years. Here we present well‐dated Late Holocene PSV records from two sediment cores recovered from the Chukchi Sea, Arctic Ocean. The records are dated using 26 14C measurements, with local marine reservoir corrections calibrated using tephra layers from the 3.6 cal ka BP Aniakchak eruption in Northern Alaska. These 14C‐based chronologies are extended into the post‐bomb era using caesium‐137 dating, and mercury isochrons. Paleomagnetic measurements and rock magnetic analyses reveal stable characteristic remanent magnetization directions, and a magnetic mineralogy dominated by low‐coercivity minerals. The PSV records conform well to global spherical harmonic field model outputs. Centennial to millennial scale directional features are synchronous between the cores and other Western Arctic records from the area. Due to the robust chronology, these new high‐resolution PSV records provide a valuable contribution to the characterization of geomagnetic field behavior in the Arctic over the past few thousand years, and can aid in developing age models for suitable sediments found in this region.

Enhancement of critical current density and strong vortex pinning in high entropy alloy superconductor Ta[formula omitted]Nb[formula omitted]Hf[formula omitted]Zr[formula omitted]Ti[formula omitted] synthesized by spark plasma sintering

We synthesized the high entropy alloy (HEA) Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound by planetary ball milling and the spark plasma sintering (SPS) method. The sintered HEA sample had a BCC crystal structure with small amount of secondary phases. The superconducting phase transition is observed in the electrical resistivity (Tc∼ 7.8 K) and magnetic susceptibility (Tc∼ 7.6 K) measurements, and the Tc was similar to that of an arc-melted sample. The zero-temperature limit of the upper critical magnetic fields Hc2(0)= 10.5 T and coherence length ξ = 5.66 nm were slightly decreased or comparable to the arc-melted HEA sample (Hc2(0)= 12.05 T and ξ= 5.23 nm). The field-dependent isothermal magnetization hysteresis M(H) exhibited the typical type- 2 superconducting behavior with magnetic flux avalanches in the low magnetic field region. The calculated critical current densities were significantly increased by 286 % (Jc= 32,606 A cm−2 at 2 K) ∼ 687 % (Jc= 73,200 A cm−2 at 4 K) compared to the arc-melted HEA sample (Jc= 11,401 A cm−2 at T= 2 K). The enhanced Jc of the sintered HEA sample is caused by enhancement of the pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional sueprconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced critical current density Jc with stable remanent magnetization Mrem(t).

Oriented Magnetite Inclusions in Plagioclase: Implications for the Anisotropy of Magnetic Remanence

AbstractMicron to sub‐micron sized ferromagnetic inclusions in rock forming silicate minerals may give rise to particularly stable remanent magnetizations. When a population of inclusions have a preferred crystallographic or shape orientation in a rock, the recorded paleomagnetic direction and intensity may be biased by magnetic anisotropy. To better understand this effect, we investigated plagioclase grains from oceanic gabbro dredged from the Mid‐Atlantic Ridge at 11°–17°N. The plagioclase grains contain abundant needle and lath shaped magnetite inclusions aligned along specific directions of the plagioclase lattice. Electron back scatter diffraction and anisotropy of magnetic remanence measurements are used to correlate the orientation distribution of the magnetite inclusions in the host plagioclase that contains multiple twin types (Manebach, Carlsbad, Albite, and Pericline) with the bulk magnetic anisotropy of the inclusion‐host assembly. In non‐modified plagioclase, the anisotropy ellipsoid of magnetic remanence has oblate shapes that parallels the plagioclase (010) plane. It is suggested that recrystallization of magnetite inclusions during hydrothermal overprint shifts the relative abundance of the inclusions pertaining to the different orientation classes. We show that the maximum axis of the anisotropy ellipsoid of magnetic remanence parallels the plagioclase [001] direction, which in turn controls the recorded remanent magnetization direction. Our results are relevant for paleointensity and paleodirection determinations and for the interpretation of magnetic fabrics.

Demagnetization Energy and Internal Stress in Magnetite From Temperature‐Dependent Hysteresis Measurements

AbstractThe magnetization state of magnetite controls acquisition and stability of remanent magnetization in Earth and planetary rocks. Although commonly interpreted in terms of grain size, also stress, grain shape, and magnetostatic interactions can substantially modify magnetic stability. Here, we show that scaled reversible work (SRW) in the approach‐to‐saturation (ATS) of hysteresis curves is temperature independent if anisotropy is due to demagnetizing energy. Stress anisotropy vanishes with increasing temperature. With a new measurement and evaluation procedure stress‐induced anisotropy is separated from demagnetization effects. We calibrated the new method using theoretical ATS curves for different stress and demagnetization regimes. Experimental results for synthetic magnetite samples underpin the validity of the method and provide insight into the relationship between magnetostatic, magnetocrystalline, and stress energies. The SRW method provides a new tool to study the reliability of paleomagnetic recording mechanisms, and enables quantitative investigation of stresses due to tectonics, meteorite impacts, oxidation, exsolution, or quenching.

Micro‐Blocks in NE Asia Amalgamated Into the Unified Amuria Block by ∼300 Ma: First Paleomagnetic Evidence From the Songliao Block, NE China

AbstractAmuria formed in the late Paleozoic through amalgamation of the Mongolia and Songliao‐Jiamusi blocks in the eastern Central Asian Orogenic Belt (CAOB). The exact timing of the assembly is critical towards unraveling the Phanerozoic evolution of the Paleo‐Asian Ocean (PAO) and assembly of the Asian blocks in Pangea. Here, we report the first Permo‐Carboniferous paleomagnetic data from the Songliao‐Jiamusi block, aiming to constrain its relative paleoposition with respect to Mongolia. A stable characteristic remanent magnetization (ChRM) was isolated from 313–292 Ma volcanic‐sedimentary sequences of the Dashizhai Formation. The ChRM yields a primary record of the geomagnetic field with positive fold, reversal and paleosecular variation tests. Along with published paleomagnetic data and geological constraints, we proposed a new reconstruction for the CAOB at 300 Ma. Our model posits that the Songliao‐Jiamusi and Mongolia blocks were welded into the unified Amuria block situated along the northern side of the PAO (∼34°N). On the southern side of the PAO, the Xilinhot and North China blocks were situated at equatorial paleolatitudes and were not amalgamated with Amuria until 250 Ma. This finding provides new insights into Asian paleogeography and the reconstruction of the Pangea supercontinent.

High-resolution late Pliocene-quaternary magnetostratigraphy of the Yinchuan Basin, NE Tibetan Plateau

The Yinchuan Basin, one of the most important basins in the area adjacent to the northeastern Tibetan Plateau, contains a continuous sequence of fluvio-lacustrine deposits, which makes it an ideal area to study the interactions between the Tibetan Plateau uplift, climate change, and geomorphological evolution. Here, a 1200-m-deep drill core (YNZK01) is described with an average recovery rate of 96% in the depo-center of the Yinchuan Basin to establish a high-resolution palaeoenvironmental and tectonic reconstruction. Stepwise alternating-field demagnetization results showed a stable and high-qualified remanent magnetization of the samples. The polarity sequences consisted of 8 normal and 8 reverse zones that correlate well with the geomagnetic polarity timescale. Magnetostratigraphic results date the entire YNZK01 core from 3.927 Ma to the present. The B/M boundary (0.781 Ma) is at a depth of 246.4 m while the M/G boundary (2.581 Ma) is at a depth of 863.4 m. The G/Gi boundary (3.596 Ma) is identified at 1117.8 m and the bottom of the core was extrapolated to 3.927 Ma (1201 m) with an average sedimentation rate of 863.4–1117.8 m. This high-resolution magnetostratigraphy record provides a baseline chronological framework for further research and publications on the tectonic, paleoclimatic, and paleoenvironmental proxy data of the YNZK01 core in the Yinchuan Basin, NE Tibetan Plateau.

Paleolatitude and tectonic rotations of the Early Carboniferous Fountain Lake Group, Cobequid Highlands, Nova Scotia, Canada

The ca. 355 Ma Fountain Lake Group in the Cobequid Highlands of Nova Scotia, is part of the transtensional basin fill that formed during dextral strike-slip motion between Avalonia and the Meguma terranes following the Acadian Orogeny. Paleomagnetic analysis of the Fountain Lake Group offers a paleolatitude estimate for the Laurentian accretionary margin in the Early Carboniferous as well as locality-specific paleomagnetic directions, which indicate clockwise-sense block rotations during dextral strike-slip motion along the Cobequid Fault zone. Stepwise demagnetization of 142 specimens from 20 sites in three Fountain Lake Group localities across the Cobequid Highlands (Squally Point, West Moose River, and Wentworth exposures) reveals remanence consisting of an easily removed component of probable recent origin, and more persistent components carried by magnetite and hematite, which in petrographic and electron beam analysis appear to be of primary igneous and volcanic oxidation origins, respectively. Sites from all three localities carry stable characteristic remanent magnetization (ChRM) directions that assume similar moderate downward inclinations when tilt-corrected. A Block Rotation Fisher analysis inclination-only fold test demonstrated best agreement at 90% unfolding, showing that remanence acquisition pre-dates Alleghenian deformation in the Late Carboniferous and is most likely of primary 355 Ma age. Paleomagnetic results for the Squally Point, West Moose River, and Wentworth localities show relative rotations between the blocks that are variously clockwise-rotated compared with a Laurentia cratonic reference frame. Inclinations at all three localities imply a subtropics paleolatitude for the margin (at Squally Point, 27.2° ± 9.4°; N = 7 sites), directly supporting the depicted location of Laurentia and its Appalachian accretionary margin in most Devonian to Early Carboniferous reconstructions.

Mapping and Modeling Sources of Natural Remanent Magnetization in the Microcline–Sillimanite Gneiss, Northwest Adirondack Mountains: Implications for Crustal Magnetism

AbstractDistinct crustal remanent magnetic anomalies are a strong indicator of rocks with stable natural remanent magnetization (NRM) carriers. The latter are able to store information on the history of a rock over long geological periods, and can therefore be used for a variety of applications in the field of paleomagnetism and rock magnetism. Typically, paleomagnetic and rock‐magnetic studies rely on rocks bulk magnetic properties. With the advent of high‐resolution magnetometric scanning techniques, it is now possible to map magnetic sources at the mineral scale, identify the different magnetic carriers, and analyze the effect of geometry, microstructures, and composition on their magnetic response. We investigate the stability of discrete remanent magnetization sources of a microcline–sillimanite gneiss sample from Russell Belt with a strong NRM, by scanning the sample before, and after alternating field demagnetization to 100 mT. We quantified changes in the magnetization intensity and direction by inverting the magnetic scan data. Here, we confirm that the exsolved titanohematite with ilmenite lamellae is the major source of magnetization, and the coexisting multidomain hematite grains contribute little to the NRM, or the magnetic anomalies. The microstructures in the titanohematite control local magnetic properties at the mineral scale. Magnetic modeling results suggest a consistent average magnetization direction before and after demagnetization at both the grain and thin section scale, with a decrease in the magnetization intensity of ≈30%. Results are consistent with previous bulk magnetic measurements and the likelihood to use high‐resolution magnetometric techniques in future magnetic studies is high.

Hysteresis of Natural Magnetite Ensembles: Micromagnetics of Silicate‐Hosted Magnetite Inclusions Based on Focused‐Ion‐Beam Nanotomography

AbstractThree‐dimensional geometries of silicate‐hosted magnetic inclusions from the Harcus intrusion, South Australia have been determined using focused‐ion‐beam nanotomography (FIB‐nt). By developing an effective workflow, the geometries were reconstructed for magnetic particles in a plagioclase (162) and a pyroxene (282), respectively. For each inclusion, micromagnetic modeling using Micromagnetic Earth Related Rapid Interpreted Language Laboratory provided averaged hysteresis loops and backfield remanence curves of 20 equidistributed field directions together with average Ms, Mrs, Hc, and Hcr. The micromagnetic structures within each silicate are single‐domain (SD), single‐vortex (SV), multivortex (MV) and multidomain states. They have been analyzed using domain‐state diagnostic plots, such as the Day plot and the Néel plot. SD particles can be subdivided into groups with dominant uniaxial anisotropy (Mrs/Ms ∼0.5 and 10 &lt; Hc &lt; 100 mT) and mixed uniaxial/multiaxial anisotropy (Mrs/Ms ∼0.7 and 10 &lt; Hc &lt; 30 mT). Most SV particles lie on a trend with 0 &lt; Mrs/Ms &lt; 0.1 and 0 &lt; Hc &lt; 10 mT, while others display a broad range of intermediate Mrs/Ms and Hc values. SV and MV states do not plot on systematic grain‐size trends. Instead, the multicomponent mixture of domain states within each silicate spans the entire range of natural variability seen in bulk samples. This questions the interpretation of bulk average hysteresis parameters in terms of grain size alone. FIB‐nt combined with large‐scale micromagnetic simulations provides a more complete characterization of silicate‐hosted carriers of stable magnetic remanence. This approach will improve the understanding of single‐crystal paleomagnetism and enable primary paleomagnetic data to be extracted from ancient rocks.