Chemical Remanent Magnetization Research Articles

Paleomagnetic dating of fracturing using breccia veins in Durness group carbonates, NW Scotland

A paleomagnetic study of red fault-related breccia veins in the Cambro-Ordovician Durness Group in NW Scotland was conducted to determine the time of brecciation, the origin of the veins, and the nature and timing of associated fluid-related diagenetic alteration. The veins contain brecciated fragments of the host Durness Group and strike either east–west or north–south. Clasts of breccia cemented by calcite suggest multiple brecciation events. The host Durness Limestone is a gray dolomite and contains a Devonian chemical remanent magnetization (CRM) that resides in magnetite. The veins contain magnetizations that reside in hematite and are interpreted as CRMs. The breccias in north–south veins contain a Triassic CRM whereas the veins with east–west strikes contain a Jurassic CRM. Authigenic hematite is common in the breccias along growth planes in the calcite cements. The two CRMs within the veins are interpreted as dating two separate brecciation and fluid flow events that precipitated authigenic hematite. The brecciation and fluid flow events are interpreted to be related to extension in the Mesozoic which is consistent with the extensional history of the northern Atlantic margins.

Toward a new < 250 °C pyrrhotite–magnetite geothermometer for claystones

We investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation: immature (burial temperature ∼ 40 °C) of Bure Callovo–Oxfordian claystones in the Basin of Paris (France); early mature (burial temperature ∼ 85 °C) of Opalinus Lower Dogger claystones from the Mont Terri anticline in front of the Jura fold belt (Switzerland); and mature to overmature (burial temperature < 170 °C) of Chartreuse Callovian–Oxfordian claystones from Chartreuse Sub-Alpine chains. To have information about the nature of the magnetic assemblage, we perform low-temperature (10 K–300 K) investigation of an isothermal remanent magnetization. In a first set of laboratory heating experiments, we aim to impart a chemical remanent magnetization (CRM) at 95 °C for several weeks in Bure and Opalinus claystones. Thermal demagnetization of the CRM reveals that magnetite is formed by heating the Opalinus claystones while an assemblage of magnetite and iron sulphide is formed in Bure claystones. Further, we document the appearance of a magnetic transition at ∼ 35 K in Bure claystones after heating. We name this transition the P-transition and we propose that it is related to the formation of fine-grained pyrrhotite (Fe 7S 8). The P-transition is also detected in early mature to mature Opalinus and Chartreuse claystones. We conduct additional experimental heating of natural Opalinus claystones. One set of experiments is referred to as short-term heating (1 h) from 100 °C to 200 °C. It is dedicated to an investigation of the effect of short-lived heating processes in geology. A second set of heating experiments is designed to approach burial conditions using a gold capsule. In burial-like experiments, we heated Opalinus claystones from 150 °C to 250 °C for several weeks under a pressure of 100 MPa. In both experiments, we observe a correlative diminution of the pyrrhotite signature at 35 K with increasing temperature. We interpret this trend as the appearance of magnetite. We derive a parameter PM from the warming curve of a saturated isothermal remanent magnetization acquired at 10 K (ZFC). We report on a consistent evolution of PM with temperature in the range of 40 °C to 250 °C, including natural samples, heated samples at 95 °C, and burial-like heated samples. PM first increases between ∼ 40 °C up to ∼ 85 °C, implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature. For temperatures above 85 °C, PM decreases up to 250 °C, implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite. PM values obtained from mature to overmature claystones from the Chartreuse are lower than the PM values obtained from the burial-like heated Opalinus claystones, suggesting that the formation of magnetite is driven by kinetics. The continuous trend of the PM parameter suggests that the magnetic properties of pyrrhotite–magnetite claystones can be used to infer paleo-temperatures and we propose to name this geothermometer MagEval.

Assessment of the usefulness of lithic clasts from pyroclastic deposits for paleointensity determination

Paleomagnetic and rock magnetic measurements were carried out on lithic clasts found within pyroclastic deposits to assess their potential for paleointensity determinations. The use of multiple lithologies in a single paleointensity determination would provide confidence that the result is not biased by alteration within one lithology. Lithic clasts were sampled from three historically active volcanoes: Láscar in the Chilean Andes, Mt. St. Helens, United States, and Vesuvius, Italy. At Láscar, triple heating paleointensity experiments allow development of new selection criteria for lithic clasts found within pyroclastic deposits. Using these criteria, the Láscar data yield a mean paleointensity of 24.3 ± 1.3 μT (1σ, N = 26), which agrees well with the expected value of 24.0 μT. This indicates that pyroclastic rocks have promise for paleointensity determinations. Pyroclastics, however, still suffer from the range of problems associated with conventional paleointensity experiments on lava flows. Samples from Mt. St. Helens are strongly affected by multidomain (MD) behavior, which results in all samples failing to pass the paleointensity selection criteria. At Vesuvius, MD grains, magnetic interactions, and chemical remanent magnetizations contributed to failure of all paleointensity experiments. Rock magnetic analyses allow identification of the causes of failure of the paleointensity experiments. However, in this study, they have not provided adequate preselection criteria for identifying pyroclastics that are suitable for paleointensity determination.

How does Chinese loess become magnetized?

Despite nearly three decades of paleomagnetic research on the extensive Chinese loess deposits, a convincing explanation has yet to be developed for how Chinese loess becomes magnetized. To address this problem, we conducted re-deposition experiments using weakly consolidated Holocene Chinese loess that was disaggregated in the laboratory, and compare our results with published paleomagnetic data. We simulated a depositional remanent magnetization (DRM) associated with dry deposition of eolian sediments, and a post-depositional remanent magnetization (PDRM) in which the sediment was water-saturated after deposition. The simulated DRM faithfully records the declination of the applied field, but with systematic inclination flattening. Addition of minor water slightly improves recording of the applied field, but inclination flattening persists. Reliable recording of the applied field occurs for PDRM simulation in water-saturated sediment. Our synthesis of paleomagnetic data from Chinese loess indicates that time-averaged paleomagnetic directions are often indistinguishable from the expected geocentric axial dipole (GAD) field, but in many cases inclinations are shallower than for a GAD field. We conclude that the Chinese loess is magnetized by a combination of DRM and PDRM mechanisms, with water content providing the dominant control on which mechanism aligns the detrital mineral fraction. Where pedogenesis causes neoformation of magnetic minerals, an additional chemical remanent magnetization (CRM) will occur. The magnetization of Chinese loess therefore appears to be controlled by a complex time-varying combination of DRM, PDRM and CRM mechanisms.

Origin of apparent magnetic excursions in deep‐sea sediments from Mendeleev‐Alpha Ridge, Arctic Ocean

Arctic deep‐sea sediments often record intervals of negative inclination of natural remanence that are tens of centimeters thick, implying magnetic excursions with durations of tens of thousand years that far exceed excursion durations estimated elsewhere, and the lack of tight age control usually provides excessive freedom in the labeling of Arctic excursions. Fortuitous variations in sedimentation rate have been invoked to explain the “amplified” excursions. Alternating field demagnetization of natural remanent magnetization (NRM) of sediment cores 08JPC, 10JPC, 11JPC, and 13JPC recovered by the Healy Oden Trans‐Arctic Expedition in August 2005 (HOTRAX05) to the Mendeleev‐Alpha Ridge yields apparent magnetic “excursions” in sediments deposited in the Brunhes Chron. Thermal demagnetization of the NRM, however, implies multiple magnetization components with negative inclination components usually “unblocked” below ∼350°C. Analysis of isothermal remanent magnetization acquisition curves from magnetic extracts indicates two magnetic coercivity components superimposed on one another. Magnetic experiments conducted under high and low temperatures show features that are characteristic for (titano)magnetite and titanomaghemite. Presence of the two magnetic phases is further confirmed by elemental mapping on a scanning electron microscope equipped for X‐ray energy dispersive spectroscopy (EDS) and by high‐resolution X‐ray diffraction (XRD). It is unlikely that anomalously thick intervals of negative inclination in these Brunhes‐aged sediments are caused by unusual behavior of the magnetic field in the Arctic area. We therefore attribute low and negative NRM inclinations in these cores to partially self‐reversed chemical remanent magnetizations, apparently carried by titanomaghemite and acquired during the oxidation of detrital (titano)magnetite grains. The high Ti contents and high oxidation states indicated by EDS and XRD data provide the conditions required for partial self‐reversal by ionic reordering during diagenetic maghemitization, and this process appears to have affected all HOTRAX05 cores collected from the Mendeleev‐Alpha Ridge.

Thermochemical remanence acquisition in single‐domain particle ensembles: A case for possible overestimation of the geomagnetic paleointensity

Single‐domain (SD) particle ensembles are the best studied systems in rock magnetism. Yet even for this simple system, a complete theoretical treatment of the acquisition of thermochemical remanent magnetization (TCRM) is missing. On the basis of the SD theory of chemical remanent magnetization, it is generally taken for granted that TCRM acquisition must be less efficient than acquisition of a thermoremanent magnetization (TRM) and that consequently, the paleofield intensity is inevitably underestimated when a TCRM is interpreted as a TRM. Here it is shown that this conclusion holds only when a continuous chemical change (e.g., grain growth) takes place during remanence blocking. In cases where an initial TRM, at a later time and at lower temperature, is modified by chemical processes, the final TCRM can result in considerable paleofield overestimation when interpreted as a TRM. This is demonstrated for two different scenarios. First, partial grain dissolution after TRM acquisition and second, low‐temperature oxidation. These mechanisms are not unlikely to occur in nature, and numerical experiments indicate that both would lead to perfectly straight Arai plots, which makes such SD TCRMs indistinguishable from true TRMs in the Thellier experiment.

Self-reversal and apparent magnetic excursions in Arctic sediments

The Arctic oceans have been fertile ground for the recording of apparent excursions of the geomagnetic field, implying that the high latitude field had unusual characteristics at least over the last 1–2 Myrs. Alternating field demagnetization of the natural remanent magnetization (NRM) of Core HLY0503-6JPC from the Mendeleev Ridge (Arctic Ocean) implies the presence of primary magnetizations with negative inclination apparently recording excursions in sediments deposited during the Brunhes Chron. Thermal demagnetization, on the other hand, indicates the presence of multiple (often anti-parallel) magnetization components with negative inclination components having blocking temperatures predominantly, but not entirely, below ~ 350 °C. Thermo-magnetic tests, X-ray diffraction (XRD) and scanning electron microscopy (SEM) indicate that the negative inclination components are carried by titanomaghemite, presumably formed by seafloor oxidation of titanomagnetite. The titanomaghemite apparently carries a chemical remanent magnetization (CRM) that is partially self-reversed relative to the detrital remanent magnetization (DRM) carried by the host titanomagnetite. The partial self-reversal could have been accomplished by ionic ordering during oxidation, thereby changing the balance of the magnetic moments in the ferrimagnetic sublattices.

Paleomagnetism and U–Pb geochronology of the Clarence Head dykes, Arctic Canada: orthogonal emplacement of mafic dykes in a large igneous province

The north–south-trending Clarence Head dyke swarm, located on Devon and Ellesmere Islands in the Canadian High Arctic, has a trend orthogonal to that of the Neoproterozoic Franklin swarm that surrounds it. The Clarence Head dykes are dated by the U–Pb method on baddeleyite to between 716 ± 1 and 713 ± 1 Ma, ages apparently younger than, but within the published age range of, the Franklin dykes. Alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages, but a comparison of the U–Pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the Clarence Head dykes are indeed a distinctly younger event within the period of Franklin magmatism. The Clarence Head dykes represent a large swarm tangential to, and cogenetic with, a giant radiating dyke swarm ∼800 km from the indicated source. The preferred mechanism for the emplacement of the Clarence Head dykes is the exploitation of concentric zones of extension around a depleting and collapsing plume source. While the paleomagnetism of most Clarence Head dykes agrees with that of the Franklin dykes, two dykes have anomalous remanence directions, interpreted to be a chemical remanent magnetization carried by pyrrhotite. The pyrrhotite was likely deposited from fluids mobilized southward from the Devonian Ellesmerian Orogeny to the north that used the interiors of the dykes as conduits and precipitated pyrrhotite en route.

Rock magnetism and microscopy of the Jacupiranga alkaline-carbonatitic complex, southern Brazil

Abstract This study of the Cajati deposit provides evidence that the ore was neither purely hydrothermal, nor volcanic in origin, as previous workers have proposed. The ores were formed from magnetite-rich magmas, hydrothermally altered and intruded at an indicated crustal depth in excess of 500 m. The mineralogical and textural association between magnetite and magnesioferrite in the carbonatite, and between the titanomagnetite and magnesioferrite-Ti mineralization in the pyroxenite of hedenbergite, seems to be analog mineralizations strongly related to the ionic substitution of Fe2+ by Mg. Relatively high Q ratios (≥5) for Jacupirangite-pyroxenite may indicate a thermo remanent magnetization (TRM) by the ore during post-metamorphic cooling, however it can also be developed from chemical remanent magnetization (CRM). Vector plots for the pyroxenite samples show reasonably linear and stable magnetic components. The intensity decay curves show that only two components of magnetizations are likely present. Continuous susceptibility measurements with increasing temperature show that the main magnetic phase seems to be magnetite. Maghemite is probably produced during the cooling process. Susceptibility recorded from low temperature (liquid nitrogen (-196°C)) to room temperature produces typical curves, indicating Verwey transition of magnetite. Hysteresis parameters point out that nearly all values fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.

Serpentinization of the martian crust during Noachian

This paper proposes a model of serpentinization of the Southern martian crust that may explain the topographic dichotomy, the absence of an associated free-air gravity anomaly and the presence of strong magnetic anomalies in the Southern Hemisphere. The thermodynamical conditions for serpentinization were likely met in the lithosphere during the Noachian period. This process may have decreased the density in the Southern crust and created the topographic dichotomy. Different reactions of serpentinization that can form magnetite have been considered. Assuming an intense magnetic field (core dynamo), we obtain chemical remanent magnetizations that are in the order of the estimates deduced from martian magnetic anomaly studies. The pertinence and the implications of our model concerning the early thermal evolution of Mars are discussed, with emphasis on the intensity of the paleo-magnetic field.

Self-reversal of chemical remanent magnetization during multiphase oxidation of synthetic titanomagnetites

[1] The possibility of self-reversal of chemical remanent magnetization (CRM) is studied on synthetic samples of titanomagnetites (Fe3−xTixO4, where x = 0.2, 0.4, and 0.6) that experienced multiphase oxidation in air at 500◦C in the presence of the constant magnetic field H = 0.1 mT. The self-reversal was fixed in accordance with anomalous behavior of CRM curves of cooling (in H = 0) and heating (in H = 0). Complete self-reversal of CRM is discovered experimentally in samples of oxidized titanomagnetites with initial x = 0.6, while partial self-reversal is revealed in other titanomagnetite compositions. Superposition of a partial thermoremanent magnetization (pTRM) on CRM showed that heating curves of CRM+pTRM are also anomalous, but negative values of the total magnetization are not observed.

An assessment of the reliability of palaeointensity results obtained from the Cretaceous aged Suhongtu section, Inner Mongolia, China

Here we present microwave palaeointensity results from 89 sister samples from the study of Zhu et al. [Zhu, R., Pan, Y., He, H., Qin, H., Ren, S., 2008. Palaeomagnetism and 40Ar/ 39Ar age from a Cretaceous volcanic sequence, Inner Mongolia, China: implications for the field variation during the Cretaceous normal superchron. Phys. Earth Planet. Int., 169, 59–75] who carried out Thellier palaeointensity analysis as part of their integrated palaeomagnetic and 40Ar/ 39Ar dating study of Cretaceous lava from the Suhungtu section, Inner Mongolia, China. Additionally, a comprehensive rock magnetic investigation has been carried out in order to determine the mineralogy and hence the validity of assuming that the remanence is a thermal remanent magnetisation (TRM). The microwave results are of apparent high quality and give flow mean palaeointensity estimates ranging from 13 to 49 μT corresponding to virtual dipole moment (VDM) estimates ranging from 2.5 to 8.9 × 10 22 Am 2, and an overall mean VDM of 5.5 ± 1.9 × 10 22 Am 2 for the 24 flows (aged 110.6 ± 0.1 Ma). When the microwave results (using the perpendicular applied field method with partial microwave thermal remanence (pT MRM) and pT MRM tail checks) are compared to those obtained with the Thellier method (Coe version with pTRM but not tail checks, and heating in argon atmosphere) differences are seen at the sample, flow and palaeomagnetic unit level however, the overall means and spread in palaeointensity estimates are consistent. Some discrepancy is due to the differing sized sample sets and sample inhomogeneity but discrepancy is also interpreted to be due to the differing protocols, methodology, plus the subjectivity in interpretation. Considering only those results that are consistent to within 20% the spread in palaeointensity estimates remains. There is substantial rock magnetic evidence from progressive heating in air and argon experiments (both showing irreversible thermomagnetic behaviour) as well as looking at samples under the scanning electron microscope to suggest that maghaemite is present (albeit to varying degrees) in many of the samples. Alteration therefore occurred in nature and it is likely that the remanence will have been affected to differing degrees potentially causing underestimates in palaeointensity. Maghaemite is interpreted to be a remanence carrier where a component of remanence remains after heating to 580 °C and loss of pTRM acquisition capacity is found on heating. No correlation was found between the estimated palaeointensity (microwave or Thellier datasets) and amount of high temperature remanence or any rock magnetic parameter. This could suggest that the palaeointensity estimates are reliable or as seems likely more than one factor (such as methodology, protocol, interpretation, chemical remanent magnetisation (CRM) contamination and the geomagnetic field) are influencing the palaeointensity estimates. In an attempt to remove the influence of palaeointensity protocol and methodology the palaeointensity datasets were reduced to consist only of those results mutually consistent to 20% ( N = 22). No obvious correlation was again seen between the inferred maghaemite contribution and palaeointensity estimate. There is therefore no clear evidence for the biasing of results due to CRM contamination (assuming a simple relationship between biasing and maghaemite contribution) and it can thus be inferred that the geomagnetic field was a major influencing factor. Further studies are however needed to fully elucidate the extent and result of each possible influencing factor and ultimately the reliability of palaeointensity estimates from rocks of this type.

Imprinting chemical remanent magnetization in claystones at 95 °C

Thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization (CRM). We present an experimental study of CRM production by heating claystones at 95 °C. A vertical magnetic field of 2 mT was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months. Solid fragments (9 to 26 g) of claystones are included in a Teflon holder that is placed in the oven under a controlled atmosphere. Newly formed grains acquire a CRM and a thermoviscous magnetization (TVRM), both being parallel to the applied magnetic field. CRM is related to the amount of newly formed grains that pass the critical volume during the reaction. To measure the acquired remanence, the claystones are first cooled in a zero magnetic field and then measured using a 2G SQUID magnetometer. In the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation, we investigate the magnetic transformation of Mont Terri Lower Dogger claystones (Switzerland) due to thermal imprinting at 95 °C. We simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones. During dehydration, the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0.23 ± 0.07 mA m − 1 /day between 3 and 14 days. The magnetic susceptibility increases by a few percent. The increase of the remanence and of the magnetic susceptibility stops after 15 days. Mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes. During rehydration, the remanence increases again whilst magnetic susceptibility drops by a few percent. After 20 days, the remanence during rehydration follows a rate of 0.42 ± 0.15 mA m − 1 /day. By contrast, when rehydration takes place later, after 66 days, the rate is much lower (0.09 ± 0.04 mA m − 1 /day). Low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite. Newly formed magnetite and hematite carry the remanence. We propose that magnetite is formed at the expense of pyrite. Hematite results from the progressive oxidation of newly formed magnetite. Our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature.

Chemical overprint on the natural remanent magnetization of a subtropical red soil sequence in the Bose Basin, southern China

We present a high‐resolution paleomagnetic investigation of the subtropical red soil sequence at the Damei section, Bose Basin, southern China. Maghemite with low coercivities and fine‐grained hematite with high coercivities but relatively low unblocking temperatures were identified as main carriers of the natural remanent magnetization (NRM). Strong chemical weathering occurring under subtropical climatic conditions in southern China led to a chemical remanent magnetization (CRM) overprint that is sufficiently strong to mask the primary NRM. Analysis of the Bose Basin soil sequence indicates that the CRM has a large lock‐in depth (&gt;4 m). This example shows that magnetostratigraphic studies on red soil sequences in subtropical‐tropical southern China should be interpreted with caution.

Origin of orogenic remagnetizations in Mississippian carbonates, Sawtooth Range, Montana

Paleomagnetic results are presented from Mississippian Madison Group carbonates in the Sawtooth Range, northwestern Montana. Samples were collected from sites along two east‐west trending transects perpendicular to the thrust faults in the Sun River Canyon and in the North and South Forks of the Teton River and from three folds. The Madison Group contains a widespread characteristic remanent magnetization (ChRM) that resides in magnetite with southerly declinations and moderately steep up inclinations. Tilt test results suggest that the ChRM is pretilting in the thrust sheets and Teton anticline but syntilting in the Clary Coulee and Swift Dam folds. The ChRMs all have the same characteristics and were probably caused by the same remagnetization event, yet the tilt test results are different. One explanation involves the difference in fold types between the Teton anticline (fault bend fold geometry) and the Clary Coulee and Swift Dam folds (fault propagation fold geometries). The deformation that produced the two geometries could have caused variations in strain/stress, which may have altered an original pretilting into a syntilting ChRM. A mean paleopole for the three pretilting tilt test results (67.2°N, 177.9°E; A95 = 13.1°) suggests remanence acquisition in the late Jurassic–early Tertiary. The ChRM is interpreted as a chemical remanent magnetization (CRM). Geochemical studies indicate that the Mississippian carbonates were altered by evolved fluids with radiogenic 87Sr/86Sr values, and petrographic analysis indicates that hydrocarbons migrated through the carbonates. The CRM is interpreted to be related to alteration by one of these fluids.

Greigite detected as dominating remanence carrier in Late Pleistocene sediments, Lisan formation, from Lake Kinneret (Sea of Galilee), Israel

SUMMARY A rock magnetic investigation of three sedimentary cores of Lisan formation of late Pleistocene age from Lake Kinneret (Sea of Galilee) northern Israel demonstrates that the magnetization of these sediments is controlled by various degrees of a secondary chemical remanent magnetization (CRM) carried by greigite (Fe3S4). This CRM is superimposed on a primary detrital remanent magnetization (DRM) that resides in Ti-magnetite. This finding is independently confirmed by X-ray diffraction (XRD) measurements performed on magnetic extracts of the sediments. The domain state of the greigite is largely single domain behaviour (SD), thus dominating the magnetization. Therefore, the magnetic record retrieve from of these sediments is not reflecting geomagnetic variations but rather chemical rock magnetic properties, resulting from diagenetic processes. The results of our study suggest that paleomagnetic record of greigite bearing sediments should be interpreted with caution because of the following reasons: 1. Geomagnetic secular variations can be biased, due to large coercivity overlap between magnetite and greigite. 2. Alternating field (AF) demagnetization can produce erroneous directions due to vector distortion by acquisition of a gyro-remnant magnetization (GRM). 3. Estimation of relative paleointensity can be hampered by large-scale variations in natural remnant magnetization (NRM) intensity, caused by the acquisition of a secondary CRM of unknown age and unknown extent superimposed on the DRM. The precipitation of greigite requires reducing conditions at the sediment–water interface and/or interstitial water, associated with the presence of sulphur and iron oxides. Reducing conditions are typical of stratified lakes and other stratified water bodies, such as the Dead Sea, paleo-Lake Lisan, temporarily the Eastern Mediterranean Sea, the Caspian Sea, the Black Sea and many other marine basins. According to our findings, paleomagnetic records from such environments carried dominantly by greigite therefore should be treated with care.

The Early Tertiary chemical remagnetization in the Bakjisan Syncline, Korea: Its geotectonic implications

The Bakjisan Syncline is located in the northwestern part of the Taebaeksan Basin, Korea. New paleomagnetic data for the Upper Carboniferous–Lower Triassic Pyeongan Supergroup from the Pyeongchang area on the west limb of the Bakjisan Syncline have been obtained, and synthesized and compared with previous data from the Jeongseon area on the east limb of the syncline. A total of 350 specimens were collected from 21 sites to clarify the relationship between the spatial distribution of remagnetized areas and the thrust system in the Taebaeksan Basin. The characteristic remanent magnetization (ChRM) isolated from all samples was a remagnetized component acquired after tilting of the strata and carried by various magnetic minerals (magnetite, hematite and pyrrhotite). From rock magnetic studies, electron microscope observations and XRD analyses, the pervasive remagnetization is interpreted to be associated mainly with a fluid-mediated chemical remanent magnetization (CRM). This is consistent with the results of previous work in adjacent areas. The paleomagnetic pole position (88.3°E, 83.9°N, A 95 = 4.9°) from the Pyeongan Supergroup in the Bakjisan Syncline indicates that the timing of the remagnetization event is Early Tertiary times (i.e. Paleocene to Eocene) by comparison with reliable paleopoles from the Korean Peninsula. Early Tertiary CRMs are also reported from previous studies of an adjacent region within the northwestern part of the Taebaeksan Basin. In contrast, a primary remanent magnetization was reported in the southeastern part of the Taebaeksan Basin. This implies that the major thrust system (the Gakdong thrust) which separates the two regions has caused them to experience substantially different geologic histories since deposition of the strata. Since many thrusts with NS trend are observed in the northwestern part of the Taebaeksan Basin compared with the southeastern region, it appears that the remagnetizing fluids pervasively penetrated the northwestern part of the basin by utilizing the already well-developed thrust system.

Paleomagnetic and mineral magnetic constraints on Zn–Pb ore genesis in the Pend Oreille Mine, Metaline district, Washington, USA

Zinc–lead mineralization in the Metaline mining district of northeastern Washington, USA, is hosted by the Cambrian Metaline Formation and is classified into Yellowhead-type (YO) and Josephine-type (JO) ore based on texture and mineralogy. Paleomagnetic results are reported for four Cambrian Metaline Formation sites, one Ordovician Ledbetter slate site, 12 YO and 13 JO (including two breccia sites) mineralization sites in the Pend Oreille Mine, and eight sites from the nearby Cretaceous Kaniksu granite batholith. Thermal and alternating field step demagnetization, saturation isothermal remanence analysis, and synthetic specimen tests show that the remanence in the host carbonates and Zn–Pb mineralization is carried mostly by pseudosingle (PSD) to single domain (SD) pyrrhotite and mostly by PSD to SD magnetite in the Kaniksu granite. Based on thermomagnetic measurements, sphalerite and galena concentrates and tailings from the mine’s mill contain hexagonal and monoclinic pyrrhotite. The postfolding characteristic remanent magnetization (ChRM), known thermal data, and paleoarc method of dating suggest that the Zn–Pb mineralization carries a primary chemical remanent magnetization (CRM), and Metaline Formation carbonates a secondary CRM that were acquired during the Middle Jurassic (166 ± 6 Ma) during the waning stages of the Nevadan orogeny. A paleomagnetic breccia test favours a solution-collapse origin for the Josephine breccia. Finally, the Kaniksu paleopole is concordant with the North American Cretaceous reference paleopole, suggesting the Kootenay terrane has not been rotated since emplacement of the batholith at ~94 Ma.

Remagnetization of the Tonoloway Formation and the Helderberg Group in the Central Appalachians: testing the origin of syntilting magnetizations

SUMMARY Palaeomagnetic, rock magnetic, geochemical and strain studies of the Helderberg Group and the Tonoloway Formation in the Valley and Ridge province of West Virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization (CRM) acquisition. The test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse-grained, thickly bedded Helderberg and the thinly bedded and finer-grained Tonoloway. Standard tilt tests as well as optimal differential untilting from two transects on the Wills Mountain anticline indicate that the Helderberg contains a pervasive late syntilting (∼30 per cent untilting) characteristic remanent magnetization (ChRM) whereas the Tonoloway contains a well defined early syntilting ChRM-1 (∼76 per cent untilting) as well as a ChRM-2, which is similar to the ChRM in the Helderberg. The ChRMs reside in magnetite. The rock magnetic results from the two units are similar although the Helderberg samples have a finer apparent grain size, perhaps as a result of magnetic hardening. Strain data reveal an apparent correlation between the syntilting magnetizations and types/degrees of pressure solution strain. Pressure solution strain is higher in the Helderberg and lower in the Tonoloway. Strain may have caused rotation of magnetic minerals, but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results. Several other viable hypotheses were also investigated. One hypothesis is a strain-enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding. A preliminary test of this hypothesis produced inconclusive results. Another hypothesis, consistent with some of the characteristics of the magnetizations in the Helderberg and Tonoloway units, is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process. Geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process.

Microstructure and magnetism in the ilmenite-hematite solid solution: A Monte Carlo simulation study

The energetics of magnetic and cation ordering have been modeled using an atomistic approach based on empirical exchange interaction parameters. The model has been applied to the study of magnetic and cation ordering in the ilmenite-hematite solid solution via Monte Carlo simulations, providing new insight into the effect of nanoscale microstructures on the magnetic properties of this system. The lowest energy state for intermediate compositions is an intergrowth of cation-disordered antiferromagnetic (AF) hematite and cation-ordered paramagnetic (PM) ilmenite, separated by mixed Fe2+/Fe3+ “contact layers”. The intergrowth carries a stable defect moment (“lamellar magnetism”) due to the presence of uncompensated spins. The net magnetization is parallel to the spin alignment direction, i.e., perpendicular to the spin-canted magnetization of the AF hematite. Contact-layer spins are coordinated to fewer magnetic neighbors than bulk spins and thus disorder more rapidly on heating. This leads to a decrease of net moment with increasing temperature. A small net moment is present, however, up to the Néel temperature of the AF hematite phase. The maximum temperature for acquisition of a chemical remanent magnetization due to lamellar magnetism is 800 ± 25 K, corresponding to the temperature for the eutectoid reaction PM hematite → AF hematite + PM ilmenite. If only cation interactions are considered, Fe3+ and Fe2+ in the contact layers become ordered so that Fe3+ shares an octahedral face with Fe3+ in the neighboring hematite layer and Fe2+ shares an octahedral face with Ti4+ in the neighboring ilmenite layer. If both cation and magnetic interactions are considered, however, an alternative ordering scheme is stabilized, whereby Fe3+ shares an octahedral face with Ti4+ in the neighboring ilmenite layer and Fe2+ shares an octahedral face with Fe3+ in the neighboring hematite layer.