Rock Magnetic Properties Research Articles

Mineral fabrics unravel syn-magmatic deformation and constrain Permian and Jurassic tectonic regimes in arc plutons from the southern Colombian Andes

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

Clay Minerals and Continental‐Scale Remagnetization: A Case Study of South American Neoproterozoic Carbonates

AbstractThe Neoproterozoic carbonate rocks of the Araras Group (Amazon Craton) and the Sete‐Lagoas and Salitre Formations (São Francisco Craton) share a statistically indistinguishable single‐polarity (reversed) characteristic direction. This direction is associated with paleomagnetic poles that do not align with the expected directions for primary detrital remanence. We employ a combination of classical rock magnetic properties and micro imaging/chemical analysis (in thin sections) using synchrotron radiation to examine these remagnetized carbonate rocks. Magnetic data indicate that most samples lack the anomalous hysteresis properties typically associated with carbonate remagnetization (except for distorted loops). Through a combination of Scanning Electron Microscopy with Energy Dispersive X‐ray Spectroscopy (SEM‐EDS), X‐ray Fluorescence (XRF), and X‐ray Absorption Spectroscopy (XAS), we identified subhedral/anhedral magnetite, or spherical grains with a core‐shell structure of magnetite surrounded by maghemite. These grains are within the pseudo‐single domain size range, as do most of the iron sulfides, and are spatially associated with potassium‐bearing aluminosilicates. While fluid percolation and organic matter maturation play a role, smectite‐illitization appears to be crucial for the growth of these phases. X‐ray diffraction analysis, in addition, identifies these silicates as predominantly highly crystalline illite, suggesting exposure to epizone temperatures. These temperatures were likely reached during the final stages of the Gondwana assembly (Cambrian), but remanence was only locked in afterward, in successive cooling events during the Early Middle Ordovician. This is supported by the carbonates' paleomagnetic pole positions compared to Gondwana's apparent polar wander path, and the absence of reversals, contrasting with the high reversal frequency of the Late Ediacaran/Cambrian.

Thermal rock magnetic cycling (TRMC): a method to track thermal alteration details for palaeointensity interpretations

SUMMARY Accurate absolute palaeointensity is essential for understanding dynamo processes on the Earth and other planetary bodies. Although great efforts have been made to propose techniques to obtain magnetic field strength from rock samples, such as Thellier-series methods, the amount of high-fidelity palaeointensities remains limited. One primary reason for this is the thermal alteration of samples that pervasively occurred during palaeointensity experiments. In this study, we developed a comprehensive rock magnetic experiment, termed thermal rock magnetic cycling (TRMC), that can utilize measurements of critical rock magnetic properties at elevated temperatures during multiple heating-cooling cycles to track thermal changes in bulk samples and individual magnetic components with different Curie temperatures in samples for palaeointensity interpretations. We demonstrate this method on a Galapagos lava sample, GA 84.6. The results for this specimen revealed that GA 84.6v underwent thermophysical alteration throughout the TRMC experiment, resulting in changes in its remanence carrying capacity. These findings were then used to interpret the palaeointensity results of specimen GA 84.6c, which revealed that the two-slope Arai plot yielded two linear segments with distinct palaeointensity values that were both biased by thermophysical alteration. To further test the TRMC method, we selected another historical lava sample (HS 2) from Mt Lassen, detecting slight thermal-physical changes after heating the specimen HS 2–8C to a target temperature of 400 °C. We also isolated a stable magnetic component with a Curie temperature below 400 °C using the TRMC method, which may provide a more reliable palaeointensity estimate of 51 μT. By providing a method for tracking thermal alteration independent of palaeointensity experiments, the TRMC method can explore subtle, unrecognizable thermal alteration processes in less detailed palaeointensity measurements, which can help to assess the thermal stability of the measured samples and interpret the changes in the TRM unblocking spectrum and palaeointensity estimates, facilitating the acquisition of more reliable records for constrain the formation of the inner core and the evolution of Earth's magnetic field.

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

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

Hydrothermal alteration of the surface volcanic rocks at the Acoculco geothermal field, Mexico: a multi-parametric approach

AbstractThe studies on hydrothermal alteration-induced effects in surface and subsurface rocks provide useful information in the characterization and exploitation of a geothermal reservoir. Generally, these studies are based on traditional, and reliable methods like petrography (primary and secondary minerals, and grade of alteration), and geochemistry (mobility of elements, changes in mass and concentration of elements, and fluid inclusions). Recently, apart from these established methods, some methods based on the geochemical (Chemical Index of Alteration, CIA; Weathering Index of Parkar, WIP; Loss on Ignition, LOI; and Sulfur, S) and rock magnetic properties (magnetic susceptibility, χlf; and percentage frequency-dependent susceptibility, χfd%) are also being applied in the identification of whether a rock is an altered or a fresh one. The Acoculco Geothermal Field (AGF), Mexico, is characterized by high temperature and very low permeability, and it is considered a promissory Enhanced Geothermal System. The following changes are observed in the rocks as a result of an increase in hydrothermal alteration: (1) an increase in CIA, LOI, and S values, and a decrease in WIP; (2) an increase in quartz and quartz polymorph minerals (silicification), and clay minerals (argillization); and (3) decrease in χlf values. At AGF, the most altered surface acid rocks are characterized by entirely quartz and its polymorphs, and clay minerals. The present study also indicates the applicability of the binary plots of major elements (felsic vs mafic component) and rock magnetic parameters (χlf vs. χfd%). The rock with χfd% value of 2–10 and χlf value < 0.5 × 10–6 m3 kg−1 indicate the presence of single domain and stable single domain grains, which in turn suggests that it is an altered rock. These methods are simple to apply, rapid, reliable, and have the potential to become effective tools for the identification of hydrothermally altered rocks during the initial stage of geothermal exploration.

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

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

Weathering trends in the Norian through geochemical and rock magnetic analyses from the Pignola–Abriola section (Lagonegro Basin, Italy)

Abstract. We investigated the geochemical and rock magnetic properties of the magnetostratigraphically calibrated Pignola–Abriola section (Italy) in order to understand the climatic perturbations that characterize the late Norian–early Rhaetian interval (Late Triassic). We performed experiments on anhysteretic and isothermal remanence (ARM and IRM) and on magnetic susceptibility (χ) to obtain the rock magnetic parameters necessary for our paleoclimatic investigation. An episode of increase in the relative quantity of hematite, suggesting the enhanced subaerial oxidation of iron minerals, was identified in the Norian from ∼ 217 Ma in the Alaunian up to ∼ 211 Ma in the early Sevatian, followed by a decline up to 207–206 Ma at the end of the Norian (late Sevatian). The results of geochemical and multivariate statistical analyses support a long-term increase and reduction in rock weathering, confirming and extending previous 87Sr / 86Sr data from the Pizzo Mondello section (Italy). Possible causes of these long-term weathering trends are the multiphase uplifting of the Cimmerian orogen, occurring at mid-northern latitudes along the southern margin of Asia in the Late Triassic, and/or the northward motion of Pangea across the equatorial humid belt. Rapid excursions in oxidized iron minerals have also been observed across the Norian–Rhaetian boundary, the origin of which still has to be determined.

Modern sedimentation in lakes Chukcha and Bezymyanka, Okhotsk coast

Lakes Chukcha (59°31’56” N; 147° 23’07” E) and Bezymyanka (59°29’56” N; 147°21’02” E) located in the Kava-Taui depression, in the norther cost of Sea of Okhotsk. The thickness of the sediments, rock magnetic properties, sedimentation rates in both lakes are different. The surface bottom sediments of Chukcha lake are heterogeneous in grain composition. The rock magnetic properties of surface sediments are controlled by the grain size. The shallow water sandy sediments show the low of MS, Jrs, Js, and high values of Jp while silt have opposite characteristics. Magnetic minerals from silt are single domain with low content of paramagnetic component. The magnetic properties of fine-grained sediments may be associated with the authigenic (hemogenic) iron sulfides and oxides.

Late Pleistocene and Holocene Sedimentation in the glacial Lake Sosednee, Magadan region

Lake Sosednee is located in the Yagodninsky district of the Magadan region. It was formed in the Late Pleistocene around 22,500 cal. years ago. The rock magnetic, geochemical, palynological, and mineralogical characteristics of the sediments of four cores, as well as moraine, and various grain size fractions of the hillslope sediments were studied. It has been established that the geochemical and rock magnetic properties of sediments from lake Sosednee reflect different sedimentation conditions, changing from the cold climate of the Late Pleistocene to the warm climate of the Holocene. In the lower part of the core, authigenic (hemogenic) formation of iron sulfides, including magnetic ones, was revealed. According to the rock magnetic and geochemical data, the Younger Dryas was identified, the end of which corresponds to the Pleistocene–Holocene boundary.

Changes in rock magnetic properties of sediments along the Carlsberg Ridge: Inferences on active/extinct vent fields

Hydrothermal vents at Mid Ocean Ridges (MOR) are known to leave metalliferous, bacterial, temperature-induced signatures in sediments. Knowledge of active/extinct vents along the Carlsberg Ridge (CR) is sparse; further conventional methods to distinguish relevant areas are tedious/expensive. We show that basic rock magnetic measurements, including magnetic susceptibility and laboratory-induced remanent magnetizations, can help to characterize sediment samples near and away from hydrothermally activity/extinct areas at the CR. Eight spade cores from the Carlsberg ridge (of which three are in the vicinity of the Tianxiu hydrothermal field and one from the ridge flank) collected on board R.V. Sindhu Sadhana during 77th and 83rd cruises are investigated for the present study. Two spade cores consist of top 2 cm and six core lengths ranged between 22 and 38 cm. Coarse fraction and magnetic minerals were separated and studied under a binocular microscope and scanning electron microscope (SEM). Magnetic susceptibility (χlf) of sediments ranges between 1.3 and 37.1 × 10−8 m3 kg−1 with higher values for ridge valley and lower values for ridge flank. Ridge flank sediments are associated with low χlf, Saturation Isothermal Remanent Magnetization (SIRM), Hard Isothermal Remanent Magnetization (HIRM) and high χARM/SIRM, which are taken as base values. Sediments in the vicinity of the Tianxiu hydrothermal vent field are associated with high χlf, SIRM, HIRM, and low Anhysteretic remanent magnetic susceptibility (χARM)/SIRM, coarse Stable Single Domain (SSD) and fine SSD & Mixture grains with magnetite as the dominant mineral. Similar signatures are observed in two more spade cores ∼250 km south of the Tianxiu vent field, suggesting new active/relict site/sites in the vicinity. Signatures like that of flank sediments are also observed in two cores from the ridge valley indicating, the absence of hydrothermal activity in the vicinity. In the marine environment, identifying/locating metalliferous hydrothermal sediment/sites along extensive MOR is expensive and time-consuming; rock magnetic could be a non-destructive method to shortlist the areas for detailed studies.

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

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

Coseismic Frictional Heating With Concomitant Hydrothermal Fluid Circulation Revealed by Rock Magnetic Properties of Fault Rocks From the Rupture of the 2008 Wenchuan Earthquake, China

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

Magnetic field map of the Wilhelm Archipelago shelf zone, West Antarctica

The Antarctic Digital Magnetic Anomaly Project is an international research effort to construct a magnetic map of the continent based on ground, satellite, marine, and aeromagnetic surveys. This paper reports the magnetic mapping of the shelf zone in the SE part of the Wilhelm Archipelago, West Antarctica, based on magnetic surveys conducted with Zodiac boats. A spectacular feature of this area is the strong magnetic anomaly of the Antarctic Peninsula (AP) batholith, which was the product of subduction-related Mesozoic–Cenozoic arc magmatism on the former margin of Western Gondwana. We constructed and analyzed a detailed magnetic map of magnetic field anomalies using field observations of rock exposures on the islands and magnetic properties of rocks from laboratory data. The oldest volcanic rocks of Jurassic to Lower Cretaceous age relate to N-NE trending bands of negative magnetic field. The largest feature in the study area is an Upper Cretaceous/Paleogene granodiorite complex that produces a positive magnetic anomaly. Many smaller anomalies are also present over gabbroid bodies of Cretaceous age. Two-dimensional magnetic modeling shows that heterogeneities in the upper crust may have magnetic susceptibilities in the range of 0.005–0.13 SI. Magnetic field anomalies also delineate an orthogonal system of tectonic faults, including the main NE fault along the Penola Strait (sub-parallel to the AP coastline) and four intersecting faults. These fault systems may be associated with different stages of continental margin evolution along the Antarctic Peninsula.

Rock Magnetic and Mineralogical Properties of Lateritic Soil Profiles Developed on Two Different Parent Rocks in Northern Kerala, South-Western India

The rock magnetic and mineralogical properties of two lateritic soil profiles developed on two different parent rocks (khondalite and granite) in northern Kerala, south-western India were investigated to better understand pedogenic processes under tropical climate. Field investigations reveal the presence of various horizons in the lateritic profiles, such as saprolite, mottled zone, pebble horizon and top soil. The particle size, rock magnetic, Vibrating Sample Magnetometer (VSM), Scanning Electron Microscopy (SEM)-Energy-Dispersive X-ray Spectrometer (EDS) and X-Ray Diffraction (XRD) analyses were performed on samples collected from different horizons of the two profiles. Magnetic susceptibility (χlf) values of the lateritic profiles vary from 9.97 to 1717.04 x 10-8 m3 kg-1, with the granitic profile exhibiting overall higher values. The percentage frequency-dependent susceptibility (χ %) ranges between 0 % and 13.9 %. The XRD data reveal the presence of kaolinite, gibbsite (except for top soil layer) and quartz along with magnetic minerals like magnetite, hematite and goethite. The upper horizons (top-soil and pebble horizon) exhibit high values for concentration dependent magnetic parameters like χlf and χfd%, whereas, the lower horizons (saprock, saprolite, pallid zone horizons) exhibit low values. The tropical soils in the region have undergone a higher degree of pedogenesis with increased magnetic mineral concentration compared to temperate soils. The magnetic enhancement in the topsoil may be due to the neoformation of ultra fine-grained SP magnetite/maghemite (with minor anti-ferro magnetic component), aided by sufficient Fe supply, alternate wetting and drying cycles, dehydration, oxidation and redox conditions. Keywords: Lateritic Soils; Soil Magnetism; XRD; SEM; Magnetic Enhancement; Pedogenesis; Southern India

HIGH PRECISION AEROMAGNETOMETRIC SURVEY ALONG THE GEOTRAVERSE IN THE SHU-SARYSU GAS BASIN

Magnetic survey is a geophysical method of solving geological problems based on the study of the Earth's magnetic response and the magnetic properties of rocks and minerals. The main task of magnetic survey is to determine the depth of the basement, trace major fault zones, types of fault tectonics, mapping in the section of intrusive massifs of different composition to determine the morphology and depth of objects.

Relationships between Abyssal Redox Conditions and Rock Magnetic Properties of Surficial Sediments in the Western Pacific

Reconstructing changes in deep/bottom-water redox conditions are critical for understanding the role of the deep ocean in global carbon and metals cycling; nevertheless, the quantitative relationships between redox proxies and abyssal dissolved oxygen are poorly investigated. In this work, we studied the rock magnetic properties of surficial sediments in the western Pacific to investigate their relationship with regional redox conditions. Our results reveal a consistent sedimentary magnetic mineral assemblage in the western Pacific, dominated by pseudo-single-domain magnetite (Fe3O4), while the ratio of detrital and biogenic magnetite particles in different sites varies substantially. Detailed analyses identified two major magnetic-coercivity components, with modal coercivity values of 13.1 ± 1.6 mT and 54.7 ± 5.3 mT, respectively. All the magnetic parameters we measured, including both concentration-dependent and grainsize-dependent parameters, and the magnetic coercivities, are generally correlated to sedimentary redox conditions; however, the coercivities obtained by mathematical unmixing exhibit a stronger linkage, explaining about a quarter of variance of redox changes. Our findings confirm the potential of magnetic properties for tracing abyssal redox changes in the western Pacific, while the observed magnetic-redox relationships are complex and need further investigation.

Mapping the structural and tectonic objects of the Upper-Kama potassium salt deposit in the magnetic field

Introduction. Petrophysical properties of rocks and strata reflect the long history of the Upper Kama potassium salt deposit (VKMKS) formation and development (about 300 million years) as well as its various endogenous and exogenous geological and geochemical processes. VKMKS geological structure as well as past and present processes at the field are effectively studied by geophysical methods. Experience has shown that, considering the gigantic area of the salt strata development and multi-layer potassium-magnesium salt deposits, high-precision and detailed geophysical studies performed on most of the VKMKS territory are most effective from a geological viewpoint, namely, gravity survey at a scale of 1 : 25,000 and aeromagnetic survey at a scale of 1 : 10,000 together with other geological and geophysical data. VKMKS structural-tectonic objects occurrence conditions and magnetic properties of their rocks and strata, especially magnetic susceptibility, form the petrophysical basis for the aeromagnetic survey applied to map the objects. The task of mapping structural-tectonic objects on the VKMKS territory in a magnetic field is a specific task of using aeromagnetic survey to map structural-tectonic objects in a weakly magnetic sedimentary cover of the entire Perm Cis-Urals. Methods of research includes studying and analyzing the research data of the Perm Cis-Urals sedimentary cover rock magnetic properties (magnetic susceptibility). The morphology of structural-tectonic objects magnetic field anomalies in the Perm Cis-Urals sedimentary cover and the morphology of VKMKS structural-tectonic objects magnetic field anomalies are studied and analyzed. Research results. The analysis of sedimentary cover rocks magnetic susceptibility and aeromagnetic survey data interpretation showed that the structural-tectonic objects of the cover create anomalies in the magnetic field. The anomalies exceed the intensity of three times the root-mean-square measurement error, and therefore are mapped with confidence. Conclusions. It is reasonable to use the data of areal high-precision aeromagnetic survey at a scale of 1 : 10,000 together with other geological and geophysical methods to improve the reliability of mapping such important structural and tectonic objects as tectonic faults of various ranks, from deep faults to fracture zones, at VKMKS.

Impact of temperature increase on the formation of magnetic minerals in shales. The example of Tournemire, France

Remagnetization is a pervasive mechanism affecting all kinds of rocks. Sedimentary rocks are particularly prone to such a mechanism because they can experience a wide range of physical and biogeochemical processes which can lead to partial or total loss of the original paleomagnetic record. Temperature increase is one of key factors affecting the rock magnetic properties in sedimentary rocks. Laboratory experiments on shales in the literature have shown that the magnetic mineralogy changed with temperature as low as 50 °C, and that new magnetic phase(s) are formed. The latter was identified to be mostly iron oxides (e.g., magnetite) and iron sulfides (e.g., pyrrhotite). In this study similar to what previously conducted on samples from the Paris Basin, a laboratory heating experiment is conducted on shale samples from the Tournemire tunnel (SE France) in a temperature range of 50 to 130 °C. This aims to simulate a new exposition of the samples to temperature and to identify the changes in magnetic phases, especially on FeIII-bearing minerals, with temperature. The remanence of the analyzed samples increases stepwise, suggesting an increasing concentration of magnetic minerals above their blocking volume with increasing temperature. Magnetite is the main magnetic carrier present and formed in the heated samples, with possibly pyrrhotite forming at high temperature. A conceptual model of magnetic mineralogy changes with temperature is presented and compared to previous results on claystones from the Paris Basin. The experimental observations support that the paleomagnetic record in sedimentary rocks affected by a later temperature increase during burial should be interpreted carefully. Indeed, remagnetization will likely occur when magnetic minerals record a secondary remanence either growing above their blocking volume or when exposed again to a temperature higher than the maximum burial temperature formerly experienced by the samples.

Paleomagnetism, rock magnetism and age determination of effusive and explosive Holocene volcanism in the Momotombo-Managua-Masaya region, Nicaragua

The Central America Volcanic Arc shows an intense and continuous volcanic activity spanning the last 10 ky because of the subduction processes parallel to the Middle America Trench. Several volcanic events have occurred during the Plio-Holocene, along the coastal Nicaraguan region, characterized either by explosive or effusive activity. Volcanic deposits were emplaced at high temperatures and have similar basaltic-andesitic composition suggesting a long-lived magma chamber but emplaced and cooled by different mechanisms. We present paleodirectional and paleointensity results of four lava flows, and the paleointensity recorded on different pyroclastic deposits in the Managua-Masaya region, western Nicaragua. For each, we characterize the rock magnetic properties as a function of temperature and magnetic field. Absolute paleointensity experiments estimated a mean range from 38.6 to 52.9 μT; and a mean virtual axial dipole moment from 9.4 × 1022 Am2 to 12.8 × 1022 Am2. Two paleomagnetic ages were determined for two cooling units: (1) the Masaya Caldera Walls with an age range of 8580–8535 BP, and (2) a dating for a lava flow from Momotombo Volcanic Complex estimated of 1500–1600 CE. These results are consistent with the expected values according to the actual paleosecular variation global models, and with the regional stratigraphy and historical ages. The high reliability of paleointensity determination from these pyroclastic deposits and lava flows suggest that both are suitable for paleomagnetic dating. Finally, an accurate chronology of the volcanic events, will improve the understanding of the evolution of the volcanism and their implications for volcanic risk on the region. This knowledge will help to decrease the considerable hazard for the population on the surrounding areas.

Pinpointing the Mechanism of Magnetic Enhancement in Modern Soils Using High‐Resolution Magnetic Field Imaging

AbstractIn well‐buffered modern soils, higher annual rainfall is associated with enhanced soil ferrimagnetic mineral content, especially of ultrafine particles that result in distinctive rock magnetic properties. Hence, paleosol magnetism has been widely used as a paleoprecipitation proxy. Identifying the dominant mechanism(s) of magnetic enhancement in a given sample is critical for reliable inference of paleoprecipitation. Here, we use high‐resolution magnetic field and electron microscopy to identify the grain‐scale setting and formation pathway of magnetic enhancement in two modern soils developed in higher (∼580 mm/y) and lower (∼190 mm/y) precipitation settings from the Qilianshan Range, China. We found that both soils contain 1–30 μm aeolian Fe‐oxide grains with indistinguishable rock magnetic properties, while the higher‐precipitation soil contains an additional population of ultrafine (&lt;150 nm) magnetically distinct magnetite grains. We show that the in situ precipitation of these ultrafine particles, likely during wet‐dry cycling, is the only significant magnetic enhancement mechanism in this soil. These results demonstrate the potential of quantum diamond microscope magnetic microscopy to extract magnetic information from distinct, even intimately mixed, grain populations. This information can be used to evaluate the contribution of distinct enhancement mechanisms to the total magnetization.