Rock Magnetic Analyses Research Articles

Palaeomagnetic field intensity measurements from the 2.6 Ga Yandinilling dyke swarm (Western Australia)

SUMMARY Precambrian palaeointensity measurements provide fundamental constraints on the evolution of the deep Earth. Core evolution models predict trends in dipole moment on billion-year timescales that can be tested by palaeomagnetic records. Here, we report new palaeointensity results from the recently identified ∼2.62 Ga Yandinilling dyke swarm of the Yilgarn Craton, Western Australia, and consider them alongside published measurements spanning 500 Myr across the late Archaean to earliest Proterozoic. Rock magnetic and scanning electron microscopy analysis confirm that the magnetic mineralogy is fine-grained magnetite, appearing mostly as exsolved lamellae with ilmenite. Six sites produced acceptable palaeointensity estimates from thermal and microwave IZZI protocol Thellier experiments and from double-heating technique Shaw experiments. These site mean values of 9–26 µT translate to virtual dipole moments of 11–44 ZAm2 that are considerably lower than today's dipole moment of ∼80 ZAm2 and the value predicted for this time period by some thermal evolution models. Their average (median = 41 ZAm2) is, however, similar to the long-term average during both of the intervals 2300–2800 Ma (median = 44 ZAm2; N = 103) and 10–500 Ma (median 41 ZAm2; N = 997). While there is little evidence for a substantial net change in average dipole moment between the late Archaean and Phanerozoic, there is preliminary evidence that its variance has increased between the two intervals. This lower variance more than two billion years ago supports the idea that the geodynamo, even while not producing a stronger magnetic field, was more stable on average at the Archaean–Proterozoic transition than it is today.

Glacial‐Interglacial Circulation and Climatic Changes in the South Indian Ocean (Kerguelen Plateau Region) Recorded by Detrital and Biogenic Magnetic Minerals

AbstractThe Southern Ocean (SO) plays a fundamental role in global climate due to the presence of the intense eastward‐flowing Antarctic Circumpolar Current (ACC), one of the most important ocean current systems. The configuration of the frontal systems of the ACC is controlled by the orbital‐ and millennial‐scale variations of the Southern Hemisphere westerly winds (SHWW) and the SO upwelling. However, the reconstruction of paleoclimate and paleocurrent in the SO remains controversial possibly because of the complex interpretation of paleo‐proxies and the regional variations. Here, we present results from rock magnetic measurements and electron microscopic analyses on marine core MD11‐3353 over the past 150 Kyr, which is located in the Polar Front Zone, west of the Kerguelen Plateau. Our data sets indicate that magnetic mineral assemblages in the core are dominated by detrital magnetic minerals mixed with biogenic magnetic minerals (i.e., magnetofossils formed by magnetotactic bacteria) during the interglacial periods. Changes in the detrital magnetic mineral content indicate a strong modulation of the ACC intensity and the SHWW stress at the orbital scale over the last climatic cycle with an enhanced activity during the glacial periods. The increase in magnetofossils content during the interglacial periods indicates an enhanced nutrient supply from the SO upwelling resulting from the southward migration of the Antarctic Polar Front. Our results suggest that the variations of concentration and assemblage of magnetic minerals can be used as faithful proxies for reconstructing ocean circulation and upwelling changes in the South Indian Ocean over the past 150 Kyr.

Archaeomagnetic analyses on fumiers burned under controlled experimental conditions

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Qn) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples’ data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest.

No Late Cretaceous true polar wander oscillation and implications for stability of Earth relative to the rotation axis

Knowledge of Earth's stability with respect to the spin axis sets boundary conditions for understanding the deep mantle and interpreting records of past climate. Recently it has been argued that the solid Earth rotated by 12∘, and then rotated back, 86 to 78 million years ago. Herein we reanalyze the paleomagnetic data and report new rock magnetic analyses, from the Scaglia Rossa limestone of Italy on which this true polar wander (TPW) was based. We find that these data record an unrecognized secondary magnetization carried by authigenic hematite. This overprint has a differential angular effect on the normal and reversed polarity primary magnetizations, creating biased directions. This bias, together with probable tectonic and sedimentary slide rotations, creates false polar wander signals. The recognition of these artifacts serves as a cautionary tale and indicates that the null hypothesis of no large TPW since the Late Cretaceous cannot be rejected. This stability is fundamentally different from that of other planetary bodies which have undergone large polar wander, and provides the framework for Earth's climate history. Because of uncertainties in paleolongitude and geomagnetic field morphology, the unambiguous definition of terrestrial TPW deeper in time remains elusive.

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.

High‐Resolution Magnetic‐Geochemical Mapping of the Serpentinized and Carbonated Atlin Ophiolite, British Columbia: Toward Establishing Magnetometry as a Monitoring Tool for In Situ Mineral Carbonation

AbstractWe address in situ serpentinization and mineral carbonation processes in oceanic lithosphere using integrated field magnetic measurements, rock magnetic analyses, superconducting quantum interference device (SQUID) microscopy, microtextural observations, and energy dispersive spectroscopy phase mapping. A representative suite of ultramafic rock samples were collected, within the Atlin ophiolite, along a 100‐m long transect across a continuous outcrop of mantle harzburgite with several alteration fronts: serpentinite, soapstone (magnesite + talc), and listvenite (magnesite + quartz). Strong correlations between changes in magnetic signal strengths and amount of alteration are shown with distinctive contrasts between serpentinite, transitional soapstone, and listvenite that are linked to the formation and breakdown of magnetite. While previous observations of the Linnajavri ultramafic complex indicated that the breakdown of magnetite occurred during listvenite formation from the precursor soapstone (Tominaga et al., 2017, https://doi.org/10.1038/s41467-017-01610-4), results from our study suggest that magnetite destabilization already occurred during the replacement of serpentinite by soapstone (i.e., at lower fluid CO2 concentrations). This difference is attributed to fracture‐controlled flow of sulfur‐bearing alteration fluid at Atlin, causing reductive magnetite dissolution in thin soapstone zones separating serpentinite from sulfide‐mineralized listvenite. We argue that magnetite growth or breakdown in soapstone provides insight into the mode of fluid flow and the composition, which control the scale and extent of carbonation. This conclusion enables us to use magnetometry as a viable tool for monitoring the reaction progress from serpentinite to carbonate‐bearing assemblages in space and time with a caution that the three‐dimensionality of magnetic sources impacts the scalability of measurements.

Submarine paleoseismology in the Japan Trench of northeastern Japan: turbidite stratigraphy and sedimentology using paleomagnetic and rock magnetic analyses

Previous studies of sediments recovered from the Japan Trench between 37° 25′ N and 38° 30′ N document distinctive turbidite beds induced by huge earthquakes. We studied two sediment cores at 39°N to investigate the depositional record further north along the Japan Trench. These investigations spatially extend our knowledge of the depositional record of earthquakes in the Japan Trench. We examined specifically the precise stratigraphy of turbidite beds using paleomagnetic secular variation, and a tephra correlation. Additionally, anisotropy of magnetic susceptibility (AMS) was investigated to understand the depositional conditions of each turbidite bed. The inferred ages of turbidite beds in this study closely approximate their previously reported ages, which are correlated with the historical and prehistorical huge earthquakes off Tohoku, northeastern Japan. The paleo current directions during deposition of turbidite are inferred from their grain alignment based on AMS data. The directions of basal part reveal northeastward in the slope-side basin and north-northeast in oceanward basin. The directions of basal and upper thick muddy part of a turbidite bed are not always consistent, which suggests the hydraulic condition in the narrow elongated deep-sea basin. This fact could be essential information to elucidate a unique hydraulic condition during the turbidite deposition in the confined basin in the Japan Trench.

Rotation of a Ferromanganese Nodule in the Penrhyn Basin, South Pacific, Tracked by the Earth's Magnetic Field

AbstractFerromanganese nodules are cm‐sized, authigenic, abyssal manganese‐iron‐hydroxide concretions. They grow very slowly at rates of a few mm per million years. Although their ages are older than millions of years, they are often found half buried on the modern sediment surface. The mechanisms for the nodules' persistence at the surface without complete burial could be linked to their occasional motion or agitation. Here, we report evidence for the rotation of a nodule from the Penrhyn Basin, South Pacific detected by paleomagnetism. The paleomagnetic inclinations of specimens from the nodules' surface are consistent with the recent geomagnetic fields. The paleomagnetic directions from the surface to the core show successive changes and form a great circle with a pole at (azimuth = 53.9°, dip = 32.1°). This suggests that the nodule rotated along its pole while successively recording magnetizations. As the nodule was found on a gentle slope at the foot of an abyssal hill, it may have moved downslope due to bottom current underwashing. Rock magnetic analyses of the nodule suggest the presence of magnetite in single domain and vortex states. Low temperature magnetometry revealed that magnetite grains were heavily oxidized to maghemite, especially close to the core of the nodule. The rotation may have exposed the rising part of the nodule to oxidative pore water. Oxygenated Antarctic Bottom Water might have caused remagnetization due to low temperature oxidation of magnetite. The rotation would also facilitate the omnidirectional growth of the nodules' mixed layer of diagenetic buserite and hydrogenetic vernadite.

Sedimentary modulation of magnetic mineral records in the Central Bengal Fan

Submarine fan sediments are essential archives for understanding the interaction between continental weathering and climate change, but decoding multiple geological and environmental factors in such highly dynamic sedimentary environments is challenging. Here we present a high-resolution magnetic mineral record spanning the past ∼40 ka from the central Bengal Fan. Comprehensive rock magnetic and electron microscopic analyses are conducted to understand the origins of different magnetic mineral components in response to sedimentary and paleoclimate processes. Detrital magnetite and titanomagnetite are the dominant magnetic minerals in Bengal Fan sediments, whose abundance and grain size are primarily controlled by eustatic sea-level change. Increased Himalaya terrestrial input during Heinrich events is documented during sea-level low stand, but magnetic mineral variations and climate events are decoupled due to enhanced Indian hemipelagic sediment input since the last deglaciation sea-level rise. In addition, we identify an enriched biogenic magnetite layer near the Fe-redox boundary in the studied Bengal Fan sediment core, possibly related to relatively mild post-depositional diagenesis. Massive detrital inputs in the submarine fan sediments dilute the contribution of biogenic magnetite to the bulk magnetic record. Our results contribute to an integrated source-to-sink understanding of magnetic mineral components in continental margin submarine fans, which helps to construct multi-proxy Quaternary paleoclimate and paleogeography records in typical submarine fan deposits.

Late Miocene to Present Paleoclimatic and Paleoenvironmental Evolution of the South China Sea Recorded in the Magneto‐Cyclostratigraphy of IODP Site U1505

AbstractThe continuous sedimentary cores recovered at the International Ocean Discovery Program (IODP) Site U1505, Expedition 368, provide an opportunity for paleoceanography and paleoclimate reconstruction in the continental margin of the northern South China Sea (SCS). In this study, we conducted detailed rock‐ and paleomagnetic studies on 420 discrete samples from the top ∼200 m of the synthetic records of Holes U1505C and U1505D. Rock magnetic analyses indicate that low‐coercivity pseudosingle domain magnetite dominates as the primary ferromagnetic mineral of Site U1505. The magnetostratigraphic age model was constructed by correlating the interpreted polarity sequence with the Geomagnetic Polarity Time Scale 2020 with the constraints of the biostratigraphic data and the distribution probability of the age of each polarity zone provided by a Dynamic Time Warping algorithm. The Milankovitch cycles of the short eccentricity, obliquity, and precession cycles were identified in the magnetic susceptibility (MS) and natural gamma radiation (NGR) series based on paleomagnetic results. We established an ∼9 Myr high‐resolution astronomical time scale by tuning the MS and NGR records to the global oxygen isotope curves, the obliquity, and the eccentricity curves of the La2004 astronomical solution. Our new age model reveals detailed sedimentation rate variations and a ∼500 kyr hiatus across the Brunhes‐Matuyama boundary related to local tectonic activity. These results lay the foundation for understanding the paleoceanography and paleoclimate evolution of the SCS.

First magnetostratigraphic results in the Aïn Beni Mathar-Guefaït Basin, Northern High Plateaus (Morocco): The Pliocene-Pleistocene Dhar Iroumyane composite section

The Aïn Beni Mathar-Guefaït Basin, in the High Plateau Region (Morocco), is dissected by the Za River, the main eastern tributary of the Moulouya, which incises more than 150 m into Plio-Pleistocene sediments. The main goal of the present study is to provide an initial geochronologic framework for such basin infill based on a combination of magnetostratigraphy and electron spin resonance (ESR). The combined results have implications on the age of the paleontological record, the evolution of the Moulouya River, and the activity of the faults that delimit the basin. We have studied sedimentary rocks that are essentially flat-lying and of an alluvial and lacustrine/palustrine origin. An approximately 140 m-thick section has been sampled at an average of 2.5 m per site, allowing to build a local magnetic polarity stratigraphy that includes nine geomagnetic reversals. Although no fold test is available, the presence of dual polarities and rockmagnetic analysis give us confidence that magnetization directions are primary. We then anchored the obtained magnetozones to the Geomagnetic Polarity Timescale (GPTS) using the biostratigraphic data as well as local geological observations. Our proposed magnetostratigraphy-based chronology reveals a Plio-Pleistocene sedimentary infill spanning from Gauss to Olduvai Chrons. The lower, detrital formations mostly fall within the normal Gauss Chron, whereas the upper lacustrine and palustrine carbonates, which are almost widespread to the top of the sedimentary fill, are Olduvai in age. These results provide the first chronological constraints for the basin fill in one of the largest intermontane basins of the High Plateaus. The new magnetostratigraphy also reveals that the major environmental change that triggered a switch from alluvial to lacustrine-palustrine conditions in Northern Maghreb occurred near the Gauss-Matuyama reversal, ca. 2.6 Ma. In addition, it shows that the age of the paleontological site Guefaït-4 is approximately 1 myr older than the ESR ages obtained from the quartz grains. The possible sources for this underestimation are discussed. However, we also acknowledge and discuss other possible chronostratigraphic interpretations of the current data, although less likely.

Magnetic signatures of a creosote oil contaminated site: case study in São Paulo, Brazil

Soils and groundwater contamination modifies the physical–chemical conditions of the environment, altering natural biogeochemical processes of the ground. As a result, several mineral transformations occur, in which iron plays a decisive role. The presence of iron enables the study of magnetic properties, improving the understanding of the geophysical signatures of highly dynamic environments (e.g., biogeochemical hotspots and contamination plumes). In this work, we seek to identify creosote biodegradation related to the precipitation of magnetic minerals on sediments at a contaminated site in São Paulo, Brazil. Several rock magnetism analyses were carried out to provide the magnetic mineralogy of the samples in terms of their composition, size, and abundance. We conducted high-temperature thermomagnetic curves, frequency-dependent magnetic susceptibility, anesthetic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) data, superparamagnetic concentration and dipole moment (SPCDM), and scanning electron microscopy (SEM) analyses. The magnetic signatures of the contaminated samples suggest an increase of superparamagnetic grains in the water table fluctuation zone if compared to the magnetic signatures of the uncontaminated samples. Thermomagnetic curves of contaminated samples showed a lower heterogeneity of the magnetic mineral phases than the uncontaminated ones. This work contributes to the advancement of the understanding of how natural biogeochemical processes are impacted by human actions, such as soil contamination, and even by climate change, which should affect soil redox conditions in periods of drought and flooding.

Factors affecting thickness and frequency of turbidites triggered by earthquakes in Kumburgaz Basin, Sea of Marmara

Submerged fault ruptures generate earthquake-triggered mass flow deposits, which are extensively used as a tool in subaqueous paleoseismology. In tectonically active deep sedimentary basins, such as the Sea of Marmara (SoM), these mass flow deposits are defined as turbidite-homogenite units (THUs), consisting essentially of a coarse basal part and an overlying homogeneous mud (homogenite). Detailed characterization of THUs is crucial in order to establish meaningful criteria to link these units with earthquakes events and to identify their transport routes and depositional mechanisms. Here, we combine μ-X-ray Fluorescence (μ-XRF), Anisotropy of Magnetic Susceptibility (AMS) and additional rock magnetism analyses of a 21-m long piston core from the Kumburgaz Basin of SoM to define the upper stratigraphic boundary of THUs with hemipelagic sediments and investigate the controls of hydrological changes on turbidite frequency and thickness over the last 15 kyrs BP. The sedimentary succession of this period includes a lower lacustrine and an upper marine unit with two Holocene sapropel intervals. The sequence is interrupted by a total of 70 THUs, characterized by a significant magnetic foliation related to the depositional setting rather than the magnetic signature. Magnetic mineralogy of the coarse basal parts of THUs have more ferromagnetic particles than the overlying homogenites and background sediments. While the homogenite parts have a more constant mineralogy than the basal parts, they do not differ the background sediments.Based on an event-free chronostratigraphic model derived from radiocarbon ages and the published age of lacustrine-marine (L-M) transition, the average THU occurrence intervals in the lacustrine (14.8–12.6 kyrs BP), lower sapropel (11.2–5.7 kyrs BP), upper sapropel (5.4–2.7 kyrs BP) and non-sapropelic part of the marine unit (2.7 kyrs BP-present) are 235 yrs., 287 yrs., 114 yrs. and 160 yrs. respectively. The average thickness of the THUs in the same units are 20.8 cm, 15.7 cm, 6.1 cm and 6.1 cm. The variability of average THU occurrence intervals and THU thicknesses are controlled by the sea level rise and salinity increase following the full marine connection of the SoM at 12.6 kyrs BP, which caused changes in slope stability, sediment composition and sediment deposition in different parts of the basin. Geomechanical properties of the lower sapropel appears to have been important in resulting long THU recurrence intervals and relatively high THU thicknesses.

Paleomagnetic Study of the Late Miocene Volcanic Sequence in Sonora, Mexico: Clockwise Vertical Axis Rotation Related to the Oblique Rifting in the Pacific‐North American Plate Boundary

AbstractThe evolution of oblique divergence along the Pacific‐North American plate boundary during the late Miocene is well‐preserved along the eastern margin of the Gulf of California. The early phase of this deformation occurred between ∼12.5 and ∼6 Ma in Sonora, NW Mexico, related to oblique rifting of the Gulf of California. The magmatic sequence for this period is exposed in the Cerro El Vigía, Guaymas region. This sequence is characterized by intrusive rocks, lava flows, and pyroclastic flows ranging in age from 12 to 10 Ma. The Cerro El Vigia has a predominant semicircular geomorphology traditionally ascribed to a volcanic caldera structure, such as large ring faults. Rock magnetic analyses of these magmatic products identify the magnetic minerals and their reliability for tectonic interpretations. The mean paleomagnetic direction from 14 sites from the Cerro El Vigia reveals up to 90° clockwise rotation about an inclined axis (65–85° axis plunge) with respect to stable North America. When interpreted in the context of the semicircular structure, these observations suggest that the structure has a tectonic, not volcanic origin. Paleomagnetic results and tectonics during the late Miocene suggest that this crustal block rotation corresponds to the development of large tension gashes deformed within a dextral transtensional system associated with continental deformation of the proto‐Gulf of California rift. Based on the paleomagnetic data, reported ages, and magmatic stratigraphy, we conclude that the magmatism and associated deformation at Cerro El Vigia was synchronous with transtensional tectonics between ∼11 Ma to ∼8 Ma.

Depositional rate, grain size and magnetic mineral sulfidization in turbidite sequences, Hikurangi Margin, New Zealand

ABSTRACT Sedimentological and rock magnetic analysis was performed on 67 turbidite samples recovered from Lithostratigraphic units I–III at IODP Site U1520 with the aim to characterise the sedimentary processes and post-depositional diagenesis within a Quaternary sequence in the Hikurangi Trough, New Zealand. Lithostratigraphic Unit I was rapidly emplaced with frequent turbidite deposition with high sedimentation rate (∼1.9 m/kyr). In contrast, in units II–III, the turbidites were finer grained and more thinly bedded, and were deposited at less than half the deposition rate of Unit I. The turbidites were classified into three clusters based on the magnetic properties. Clusters 1 and 2 are characterised by a high coercivity and SIRM/χ, but low to moderate Bcr/Bc. The recognition of gyroremanent magnetisation acquisition in their clusters indicates the diagenetic sulfidization. Cluster 3 was represented by low coercivity and SIRM/χ, but high Bcr/Bc, indicating lack of sulfidization. Restricted occurrence of Cluster 3 in Unit I may be ascribed to the high sedimentation rate, which prevents sulfidization at the sulfate–methane transition zone. Degree of sulfidization might be related with sedimentary features of turbidites in the trough, such as grain size, bed thickness and depositional rate.

Paleomagnetism and 40Ar/39Ar geochronology of Meso-Neoproterozoic rocks from southwest Norway. Implications for magnetic remanence ages and the paleogeography of Baltica in a Rodinia supercontinent context

Here we present new 40Ar/39Ar geochronology, rock magnetic, and paleomagnetic data obtained from rocks from three distinct domains of the Precambrian Sveconorwegian orogen (Southern Norway) that record progressively younger orogenic events from east to west. The first locality is in the Bamble lithotectonic unit and the other two areas are located within the Telemark and Rogaland–Vest Agder sectors in the central and southwestern parts of the Telemark lithotectonic unit, respectively. New and existing geochronology data on metamorphism and cooling, combined with detailed rock magnetic analyses indicate that studied rocks cooled and blocked (acquired) the natural remanent magnetization at ∼ 1090 Ma (Bamble Lithotectonic unit), 1000 Ma (Central Telemark), and at around 900 Ma in the westernmost part of the Telemark Lithotectonic Unit. Accordingly, the new paleomagnetic poles for the Bamble (Plat = -6.6°N, Plong = 218.4°E, N = 38, A95 = 6.1°, K = 15.6), central Telemark (Plat = -17.4° N, Plong = 229.8° E, N = 23. A95 = 6.5 °, K = 22.6), and western Telemark Lithotetonic Unit (Plat = -46.2° N, Plong = 209.4° E, N = 9, A95 = 12.0, K = 19.4) were assigned 1090 Ma, 1000 Ma, and 900 Ma ages, respectively. The new Bamble paleomagnetic pole reconstructs Baltica at ∼ 1090 Ma with noticeable latitudinal gap from the eastern Laurentian margin and thus does not support classical Baltica-Laurentia configuration within the Rodinia supercontinent. The other two younger poles, when used together with the coeval paleomagnetic data from Laurentia do not support the traditionally envisaged Meso- to Neoproterozoic Baltica-Laurentia relationship either. Overall, the new data do not lend any support to the close connection between the two continents during the proposed Rodinia tenure but instead emphasize the problems with the traditional paleogeographic reconstructions of Rodinia. The new data appear more in line with the geological record within the Sveconorwegian orogen, which indicates that the orogeny developed at or behind active continental margins. While Baltica and Laurentia may have resided at broadly similar latitudes between 1000 and 900 Ma, there is no geologic and paleomagnetic evidence for placing them next to each other.

New Rock Magnetic Analysis of Ultramafic Cores From the Oman Drilling Project and Its Implications for Alteration of Lower Crust and Upper Mantle

AbstractDetailed magnetic analyses of ultramafic rock samples recovered from the Wadi Tayin massif as part of the Oman Drilling Project provide a unique opportunity to understand the magnetic properties of oceanic lithosphere and its alteration since obduction. We examined 300‐ to 400‐m‐long cores from 3 sites (BA1B, BA3A, and BA4A) that transected the crust‐mantle boundary. Serpentinization that produces a small amount of magnetite from olivine alteration plays an important role in shaping the magnetic signature of the rocks recovered from these holes. Our investigation reveals new features, which have not been reported in previous studies of ophiolite largely based on surface rock samples. It appears that when serpentinization reaches a certain point (65%), a sharp increase in magnetic susceptibility occurs. Such pattern opens the possibility of additional production of magnetic minerals induced by various reactions (e.g., further hydration of Fe‐rich serpentine minerals), which may have occurred, while the rocks were being serpentinized. In general, the uppermost section (0–40 m) shows higher susceptibility than the deeper part. Also, higher susceptibility is found in dunite‐dominated sections than those composed of harzburgite. Overall, our observations suggest that mantle rocks of Semail ophiolite have undergone multiple stages of serpentinization over different time periods.

Jurassic true polar wander recorded by the Lhasa terrane on its northward journey from Gondwana to Eurasia

Paleomagnetic data constrain paleogeographic motion of rocks relative to the Earth's spin axis, which is a sum of plate motion relative to the mantle and true polar wander. Discerning between these effects is challenging for studies aiming to reconstruct paleo-plate motions from deformed orogenic terranes. Here, we study the paleolatitudinal drift history of the Lhasa terrane of southern Tibet that migrated from the northern Gondwana to the southern Eurasian margin between late Triassic and early Cretaceous time. Previous work identified a 180 Ma near-equatorial Lhasa latitude and assumed near-constant paleolatitudinal drift. Large-scale true polar wander at this time, however, which has been argued for in previous work, requires highly variable Lhasa plate motion rates relative to Gondwana. Here, we test whether the alternative interpretation of constant plate motion rates provides a better prediction of paleomagnetic data. To this end, we present a new paleomagnetic pole from ∼155 Ma volcanics (here dated by U/Pb zircon) of the Lhasa terrane. Our pole comprises site mean directions from 46 lavas, passes a fold test, and is supported by an extensive rock magnetic and microscopic analysis that reveals no evidence of remagnetization. Our results give an average direction of D±ΔDx=337.1°±2.6°, I±ΔIx=−13.8°±4.9°, and a corresponding paleopole position at λp=45.3°N, φp=295.3°E with K=69.8 and A95=2.5°, predicting a similar near-equatorial, paleolatitude as at 180 Ma. This paleolatitudinal standstill is consistent with predicted paleolatitudes from global APWPs that account for Jurassic TPW and assume a constant Lhasa-Gondwana (India) oceanic spreading rate of ∼8 cm/a during the 215-130 Ma opening of the Neotethys and closure of the Mesotethys oceans. Contemporaneous arc magmatism on the Lhasa terrane was previously interpreted to indicate that the Lhasa terrane was located above a subduction zone during its northward journey to Tibet: our results show that if our reconstruction is accurate, then trench migration rates also must have been ∼8 cm/a. Our results are consistent with the range of rates of global long-lived trench migration that have been reconstructed in other studies, but they are much higher than measured in most modern systems. Our results more completely document the Lhasa terrane's separation from Gondwana and provide a straightforward methodology of using paleomagnetic data to test plate kinematic scenarios, overcoming the problem of the unknown effect of true polar wander on paleomagnetic data from deformed terranes.

Matuyama–Brunhes geomagnetic reversal record and associated key tephra layers in Boso Peninsula: extraction of primary magnetization of geomagnetic fields from mixed magnetic minerals of depositional, diagenesis, and weathering processes

We report paleomagnetic records of Matuyama–Brunhes geomagnetic polarity reversal and associated key tephra layers from the Early–Middle Pleistocene marine sedimentary succession in the Boso Peninsula. The outcrop is in Terasaki, Chiba, Japan and ~ 25 km northeast of the Chiba section. The sediment succession consists of a massive siltstone layer of the Kokumoto Formation, Kazusa Group. A tephra layer was identified in the middle of the outcrop with chemical composition comparable to that of the Byk-E tephra layer from the Chiba section defining the base of the Chibanian Stage. Oriented paleomagnetic samples were collected at intervals of 1–10 cm from the siltstone. To identify the primary remanent magnetization, progressive alternating field demagnetization (PAFD) and progressive thermal demagnetization (PThD) were conducted on pilot samples. Identification of primary magnetization with PAFD was not successful, especially for reversely magnetized samples. In addition, magnetization during PThD showed sharp drops around 175 °C, which decreased gradually between 175 °C and ~ 300 °C, and became unstable above ~ 350 °C. To extract the primary remanent magnetization while avoiding laboratory alteration by heating, a PThD up to 175 °C followed by PAFD was conducted. Combined analysis of remagnetization circles enables extraction of primary magnetization with improved reliability. Rock magnetic experiments were conducted during stepwise heating to understand the magnetic minerals involved and to evaluate the influence of laboratory heating. During heating, FORC-PCA revealed significant changes of magnetic minerals at 200 °C, 400 °C, 450 °C and 550 °C. Rock magnetic analyses and electron microscopy indicate that titanomagnetite/magnetite are magnetic minerals contributing to primary remanent magnetization. Greigite was also identified preserving secondary magnetizations during sub-seafloor diagenesis. The presence of feroxyhyte is suggested as secondary magnetization through the weathering of pyrite by exposure to the air after the Boso Peninsula uplift. The correlation of relative paleointensity with the Chiba section provides an age model with sedimentation rates of 30 cm/kyr and 18 cm/kr for the intervals above and below the Byk-E tephra. VGP latitudes are highly consistent with those from the Chiba section based on the age model, which assigns the main directional swing from reversed to normal polarities as 772.8 ± 0.5 ka.Graphical

Middle Miocene-Pleistocene Magneto-Cyclostratigraphy from IODP Site U1501 in the Northern South China Sea

Site U1501 of International Ocean Discovery Program (IODP) Expedition 368 locates on a broad regional basement high in the northern margin of the South China Sea (SCS). This study refines the chronostratigraphy of the upper 160 m sedimentary succession from Hole U1501C using paleomagnetic measurements and cyclostratigraphic analysis on the Natural Gamma Radiation (NGR) data. Rock magnetic analysis displays that the magnetic signal of the sediments is mainly carried by single-domain (SD) and multi-domain (MD) magnetite. A total of 12 geomagnetic reversals are identified and correlated to the geomagnetic polarity time scale (GPTS) in Geologic Time Scale 2020 (GTS 2020), combining biostratigraphic data and planktonic foraminiferal oxygen records. The Milankovitch cycles of 405-kyr long orbital eccentricity, ∼100-kyr short orbital eccentricity, and obliquity cycles are identified in the NGR profile. A 15.54 Myr astronomical time scale is constructed by tuning the short eccentricity cycles filtered from the NGR profiles to the La2010 astronomical solution with the constraints of the magnetostratigraphic results, biostratigraphic age datum and planktonic foraminiferal oxygen records. This new high-resolution age model provides a new temporal constraint on the tectonic and paleoenvironmental evolution in the South China Sea.