High Natural Remanent Magnetization Research Articles

Petrophysical target characterization with lithogeochemical clustering: the Metsämonttu Zn–Pb–Cu deposit, southern Finland

ABSTRACTIn mineral exploration, reliable and comprehensive petrophysical groundwork helps evaluate the success rate of applying geophysical methods to the target and thus improves the quality of decision‐making related to geophysical surveys. Petrophysical data for potential field exploration can be efficiently collected from drill cores together with portable XRF data that provide insight into rock compositions. We acquired and analysed a multi‐parameter portable‐instrument drill core data set (2036 samples) with portable XRF, magnetic susceptibility and density measurements from the Metsämonttu Zn–Pb–Cu deposit, one of the base metal deposits in the Orijärvi Belt, southern Finland, with little post‐mining exploration during the last 50 years. Our study applied a machine learning technique (Gaussian Mixture Model clustering) to the portable XRF data to provide a pseudo‐lithological reclassification of the drill cores and used this as a basis for characterization of the deposit in petrophysical terms. We also used thermomagnetic measurements to inspect the magnetic mineralogy of the drill cores. Finally, we estimated the success of using potential field methods (magnetic and gravity) for brownfield exploration of analogous deposits in the area. With the portable XRF data clustering we were able to distinguish between barren mafic and felsic volcanic rocks, altered rocks and three different ore mineral enriched rock types. The petrophysical measurement data and thermomagnetic results were combined with the clusters to define the petrophysical characteristics for future mineral exploration with potential field methods. At Metsämonttu, the highest densities are related to the ore and iron sulphide‐bearing rocks. High natural remanent magnetization intensities are related to the iron sulphide‐bearing ore, and remanent magnetization is a significant magnetic anomaly source. Based on synthetic modelling results, both magnetic and gravimetric data could be used in direct exploration in the Metsämonttu area.

The Rikord Submarine Volcanic Massif, Kuril Island Arc

This paper reports a study of the Rikord volcanic massif. The massif consists of four volcanic edifices that coalesce in their bases, and is most likely Quaternary. The massif discharged basaltic and basaltic andesite lavas during its earlier life. The observed high natural remanent magnetization that was found in dredged rocks is due to high concentrations of single-domain and pseudo-single-domain grains of titanomagnetite and magnetite. We have identified the directions of the conduits and the presence of peripheral magma chambers. A 3D model has been developed for the central part of the Rikord volcanic massif; the model includes ten large disturbing magnetic blocks that are most likely cooled, nearly vertical, conduits.

Magnetic anomalies associated with abundant production of pyrrhotite in a sulfide deposit in the Okinawa Trough, Japan

AbstractWe report here results from a deep‐sea magnetic survey using an autonomous underwater vehicle over the Hakurei hydrothermal site, in the middle Okinawa Trough. Magnetic inversion revealed that the Hakurei site is associated with well‐defined high‐magnetization zones distributed within a broad low‐magnetization zone. Results from rock magnetic measurements, performed on sulfide ore samples obtained by drilling, showed that some samples possessed extremely high natural remanent magnetization (NRM) (as much as 6.8–953.0 A/m), although most of the measured samples had much lower NRM. These high‐NRM samples were characterized by high Königsberger ratios (101−103), indicating much larger NRM than induced magnetization, and contained pyrrhotite as the only magnetic mineral. This suggests that NRM carried by pyrrhotite is the source of the observed magnetic anomalies. The wide range of NRM intensity was considered to be due to a highly heterogeneous distribution of pyrrhotite, because pyrrhotite was commonly identified in both the high‐NRM and low‐NRM samples. Pyrrhotite production may have been occasionally drastically increased, with highly magnetic ores formed as a result. Rapid burial of active vents may result in the creation of an extensive reducing environment under the seafloor, which is favorable to pyrrhotite production, and may also prevent oxidation of pyrrhotite by isolating it from seawater. Because the magnetization intensity of sulfide ores was highly variable, it would not be straightforward to estimate the quantity of ore deposits from the magnetic anomalies. Nevertheless, this study demonstrates the usefulness of magnetic surveys in detecting hydrothermal deposits.

Magnetic anomalies and rock magnetism of basalts from Reykjanes (SW-Iceland)

This study presents rock magnetic properties along with magnetic field measurements of different stratigraphic and lithologic basalt units from Reykjanes, the southwestern promontory of the Reykjanes peninsula, where the submarine Reykjanes Ridge passes over into the rift zone of southwestern Iceland. The basaltic fissure eruptions and shield lava of tholeiitic composition (less than 11500 a old) show a high natural remanent magnetization (NRM, Jr) up to 33.6 A/m and high Koenigsberger ratio (Q) up to 52.2 indicating a clear dominance of the NRM compared to the induced part of the magnetization. Pillow basalts and picritic shield lava show distinctly lower Jr values below 10 A/m. Magnetic susceptibility (κ) ranges for all lithologies from 2.5 to 26×10−3 SI.

Magnetic properties of serpentinized garnet peridotites from the CCSD main hole in the Sulu ultrahigh‐pressure metamorphic belt, eastern China

Magnetic properties of 14 serpentinized garnet peridotites from the interval of 603.20–683.53 m of the Chinese Continental Scientific Drilling (CCSD) hole in the Sulu ultrahigh pressure (UHP) metamorphic belt, eastern China, were investigated and compared with those of ultramafic rocks from other tectonic settings, to relate them to the UHP (post‐)metamorphic evolution. Serpentinized garnet peridotites are characterized by high susceptibility (χ), natural remanent magnetization (NRM), and Q (Köenigsberger ratio = (NRM/χ × H), where H is the intensity of the local geomagnetic field) values. The average χ, NRM, and Q values are 299.65 × 10−7 m3 kg−1, 19.15 × 10−3 Am2 kg−1, and 16.59, respectively, which are generally higher than those of other serpentinized peridotites in the world. The serpentine concentration for most samples ranges from 40% to 60%, suggesting that the peridotites are of obvious homogeneous serpentinization degree. Micro‐Raman and X‐ray diffraction spectra of serpentine for representative samples indicate that lizardite is the only serpentine mineral. Opaque minerals in samples are mainly nonstoichiometric magnetite and Cr‐spinel with or without minor ilmenite and Fe‐sulfide. Magnetite occurs as rims around olivine and garnet as well as in clusters. A chromian garnet further overgrew on the rims of the garnet. The grain size of magnetite in rims is in the pseudo‐single‐domain region and favorable for development of high NRM. High pressure‐UHP metamorphic processes possibly lead to enhanced concentrations of Fe‐Ti oxides in garnet peridotites and, thus, increase the magnetism of serpentinized garnet peridotites in the CCSD main hole. Integrated lizardite formation temperature (<450°C) and its relationships with the pressure‐temperature‐time evolution of the associated UHP rocks permit us to infer that serpentinization of garnet peridotites in the CCSD main hole occurred during late exhumation stages, ca. 200–215 Ma ago, under temperatures of 425°C. However, the exact relationship between magnetic properties of garnet peridotite and the UHP metamorphic process will require further studies.

The Stardalur magnetic anomaly revisited—New insights into a complex cooling and alteration history

This study provides new rock magnetic and magneto-mineralogical data including Mössbauer spectroscopy of basaltic drill cores from the Stardalur volcanic complex, Iceland, in order to better understand the strong magnetic anomaly, which is caused by an extraordinary high natural remanent magnetization (NRM). NRM and magnetic susceptibility ( χ) display a positive linear correlation ( R 2 = 0.81) and reach very high values up to 121 A/m and 148 × 10 −3 SI. Although a Curie temperature of 580 °C and a Verwey transition at about −160 °C is indicative of magnetite, χ– T heating experiments in argon and air atmosphere and thermal demagnetization measurements of NRM revealed a slight cation-deficiency. According to induced remanent magnetization experiments the remanence is carried solely by this low coercive phase. Minor titanomaghemite with a T C at about 340 °C only occurs in samples with larger oxide grains (20–80 μm). High vesicle abundances and the exsolution texture of Fe–Ti oxides suggest subaerial extrusion of the lava. A high oxygen fugacity (probably above the NNO buffer) and a low Ti/(Ti + Fe) ratio of the basaltic melt are suggested as a precondition for high concentration of magnetic minerals and therefore high primary TRM. During high temperature oxidation, ilmenite exsolution-lamellae, developed in titanomagnetite, and symplectic magnetite (+ pyroxene) formed by the breakdown of olivine. This secondary magnetite, grown at temperatures above the Curie temperature, increases the primary TRM. Early stage hydrothermal alteration (below about 375 °C) led to maghemitization of (titano)magnetite, clearly indicated by shrinkage cracks and irreversible χ– T curves. During later stage hydrothermal alteration, NRM intensity increased slightly due to the growth of secondary magnetite at lower temperatures (about 250–300 °C). This hydrothermally formed magnetite acquired only a low CRM but increased magnetic susceptibility significantly. According to our results it is suggested, that hydrothermal alteration does not necessarily lower remanent magnetization, but contributes to an increase in magnetization. The interplay of the three factors melt composition, small grain sizes of secondary magnetite due to decomposition of silicates and new formation under hydrothermal conditions caused the strong magnetic anomaly at the surface.

Electric currents along earthquake faults and the magnetization of pseudotachylite veins

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km 3 of stressed igneous rocks adjacent to the fault plane can deliver 10 3–10 5 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

Comment on a paper “The origin of high magnetic remanence in fault pseudotachylites: Theoretical considerations and implication for coseismic electrical currents” by E.C. Ferré, M.S. Zechmeister, J.W. Geissman, N. Mathana Sekaran, and K. Kocak

Pseudotachylites occur in the form of thin glassy veins quenched from frictional melts along the fault planes of major earthquakes. They contain finely grained magnetite and often exhibit a high natural remanent magnetization (NRM). High NRM values imply strong local electric currents. These currents must persist for some time, while the pseudotachylite veins cool through the Curie temperature of magnetite around 580 °C. There is no generally accepted theory explaining how such powerful, persistent currents may be generated along the fault plane. Data presented here suggest the activation of electronic charge carriers, which are present in igneous rocks in a dormant, inactive form. These charge carriers can be “awakened” by the application of stress. They are electrons and defect electrons, also known as positive holes or p-holes for short. While p-holes are capable of spreading out of the stressed rock volume into adjacent p-type conductive unstressed rocks, electrons require a connection to the hot, n-type conductive lower crust. However, as long as the (downward) electron flow is not connected, the circuit is not closed. Hence, with the outflow of p-holes impeded, no current can be sustained. This situation is comparable to that of a charged battery where one pole remains unconnected. The friction melt that forms coseismically during rupture, provides a conductive path downward, which closes the circuit. This allows a current to flow along the fault plane. Extrapolating from laboratory data, every km3 of stressed igneous rocks adjacent to the fault plane can deliver 103–105 A. Hence, the current along the fault plane will not be limited by the number of charge carriers but more likely by the (electronic) conductivity of the cooling pseudotachylite vein. The sheet current will produce a magnetic field, whose vectors will lie in the fault plane and perpendicular to the flow direction.

The origin of high magnetic remanence in fault pseudotachylites: Theoretical considerations and implication for coseismic electrical currents

Several examples of fault-related pseudotachylites display a significantly higher initial magnetic susceptibility than their granitic host rock (10:1 to 20:1). These higher values are attributed to the presence of fine magnetic particles formed during melt quenching. The hysteresis properties of the particles indicate a single domain (SD) to pseudo single domain (PSD) magnetic grain size. The Curie temperature (Tc) of the magnetic particles is close to 580 °C. The natural remanent magnetization (NRM) of these pseudotachylites is also significantly higher than that of the host rock (up to 300:1). Such anomalously high remanence cannot be explained by a magnetization acquired in the Earth's magnetic field, regardless of pseudotachylite age. Ground lightning and other strong electric pulses can cause anomalously high NRM intensities. A ground lightning explanation seems unlikely to explain the systematically high NRM intensities, particularly in the case of recently exposed samples that have been collected from active quarries. Alternatively, high NRM intensities could be explained by earthquake lightning (EQL), a seismic phenomenon occasionally reported in connection with large magnitude earthquakes ( M > 6.0). The coseismic electrical properties of the pseudotachylite vein–host rock system are characterized by (1) a core of molten material (high conductivity), (2) vapor-rich margins of thermally and mechanically fractured host rocks (low conductivity) and (3) moderately fractured to undeformed host rock (normal conductivity). Such a core conductor bordered by insulating margins is potentially responsible for the propagation of EQL pulses. The coseismic thermal history of pseudotachylite veins has been modeled in 2-D using conductive heat transfer equations. It shows that EQL can be recorded only during a brief time interval (less than 1 min) for a given vein thickness and host-rock temperatures. If the vein is too thick or if the host rock is too hot, the pseudotachylite remains above Tc after the electric pulse has lapsed.

Earth analog for Martian magnetic anomalies: remanence properties of hemo-ilmenite norites in the Bjerkreim-Sokndal intrusion, Rogaland, Norway

To explain the very large remanent magnetic anomalies on Mars, which no longer has a global magnetic field, it is important to evaluate rocks on Earth with the necessary properties of high natural remanent magnetization (NRM) and coercivity. Here, we describe a possible analog from the 230-km 2 930 Ma Bjerkreim-Sokndal layered intrusion (BKS) in Rogaland, Norway. In the layered series of the BKS, fractional crystallization of jotunitic magma was punctuated by influx and mixing of more primitive magmas, producing six megacyclic units, each typically with early plagioclase-rich norites, intermediate hemo-ilmenite-rich norites and late magnetite norites with subordinate near end-member ilmenite. Following each influx, the magma resumed normal crystallization and, following the last, near the base of Megacyclic Unit IV, crystallization continued until norites gave way to massive fayalite-magnetite mangerites and quartz mangerites in the upper part of the intrusion. The Megacycles are marked on a regional aeromagnetic map by remanent-controlled negative anomalies over ilmenite norites and induced positive anomalies over magnetite norites and mangerites. A prominent negative anomaly (with amplitude −13,000 nT in a high-resolution helicopter survey, down to −27,000 nT below background in ground magnetic profiles) occurs over the central part of Megacyclic Unit IV. The anomaly is centered on ilmenite norite Unit IVe and is most intense where cumulate layering is near vertical at the southeast edge of the Bjerkreim Lobe of the intrusion at Heskestad. Here, Unit IVe is flanked to the east by magnetite norite of Unit IVc and country-rock gneisses (group E) and to the west by Unit IVf magnetite norite and mangerites (group W). Magnetic properties were measured on 128 oriented samples. Susceptibilities are similar for all three sample groups at ∼8×10 −2, but Koenigsberger ratios are very different, with average values of 7.7 for IVe, and <1 for groups E and W. The IVe samples, with only a few percent of oxides, have the highest NRMs measured from the BKS, up to 74 A/m, with an average of 30.6 A/m, making them prime candidates for consideration as Mars analogs. The mean direction for IVe samples is D=17.6°, I=−79.9, a 95=10°, almost opposite the present field. Evidence on origin of the strong NRM in IVe as compared to groups E and W, include greater abundance of hemo-ilmenite and of orthopyroxene with hemo-ilmenite exsolution, and the strong lattice-preferred orientation of both in a relationship favorable for “lamellar magnetism”. Massive magnetite-free hemo-ilmenite ores in anorthosite from the same district also produce negative magnetic anomalies. They have a substantial but much lower NRM, suggesting that there are special oxide properties in the IVe rocks at Heskestad.

Effects of nanoscale exsolution in hematite–ilmenite on the acquisition of stable natural remanent magnetization

To investigate the acquisition mechanism of high and stable natural remanent magnetization (NRM) in rocks of the Russell Belt, Adirondack Mountains, New York, we examined the exsolution microstructures and compositions of magnetic minerals using three samples with different magnetic properties. The samples contain titanohematite with ilmenite lamellae, end-member hematite without lamellae and rare magnetite as potential carriers for the NRM. Transmission electron microscopy (TEM) observations and element mapping by energy-filtered TEM (EFTEM) of the titanohematite indicated that very fine ilmenite lamellae with a minimum thickness ∼2 nm are abundant between larger ilmenite lamellae a few hundreds of nanometers thick. The ilmenite lamellae and titanohematite hosts, with the compositions of Ilm 90–100Hem 10–0 and Ilm 7–16Hem 93–84, respectively, share (001) planes, and the abundant fine ilmenite lamellae have coherent, sharp structural and compositional interfaces with their titanohematite hosts. Comparison between samples shows that the magnetization is correlated with the amount of fine exsolution lamellae. These results are consistent with the lamellar magnetism hypothesis, suggesting that the acquisition of high and stable NRM is related to the interfacial area between fine ilmenite lamellae and their host titanohematite. End-member hematite with a multi-domain magnetic structure only contributes a minor amount to the NRM in these samples.

Paleomagnetic results from deep-sea sediment of the Korea Deep Ocean Study (KODOS) area (northern equatorial Pacific) and their paleodepositional implications

Paleomagnetic properties of sediment cores were examined to reconstruct paleodepositional conditions in the Korea Deep Ocean Study (KODOS) area, located in the northeastern equatorial Pacific. The studied KODOS sediments have a stable remanent magnetization with both normal and reversed polarities, which are well correlated with the geomagnetic polarity timescale for the late Pliocene and Pleistocene. Average sedimentation rates are 1.56 and 0.88 mm/kiloyear for the Pleistocene and late Pliocene, respectively. Clay mineralogy and scanning electron microscope analyses of the sediments indicate that terrestrial material was transported to the deep-sea floor during these times. The variations of sedimentation rates with age may be explained by the onset of the northern hemisphere glaciation and subsequent climatic deterioration during the Pliocene and Pleistocene. For the Pleistocene, an increasing sedimentation rate implies that input of terrestrial materials was high, and also a high input of biogenic materials was detected as a result of increased primary production in the surface water. The down-core variations in paleomagnetic and rock-magnetic properties of the KODOS sediments were affected by dissolution processes in an oxic depositional regime. As shown by magnetic intensity and hysteresis parameters, the high natural remanent magnetization (NRM) stability in the upper, yellowish brown layers indicates that the magnetic carrier was in pseudo-single domain states. In the lower, dark brown sediments, only coarse magnetic grains survived dissolution and the NRM was carried by more abundant, multi-domain grains of low magnetic stability. The down-core variation of magnetic properties suggests that the KODOS sediments were subjected to dissolution processes resulting in a loss of the more stable components of the magnetic fraction with increasing core depth.

The relationship between exsolution and magnetic properties in hemo‐ilmenite: Insights from Mössbauer spectroscopy with implications for planetary magnetic anomalies

Remanence properties of ilmenites with exsolved hematite have recently been the object of much study, because their high natural remanent magnetization (NRM) is greater than predicted for these two minerals. X‐ray diffraction (XRD) and Mössbauer spectroscopy in conjunction with electron microprobe and NRM measurements were used to explore the possibility that single domain (SD) hematite lamellae in the ilmenite host could explain the observed remanence, and to determine if magnetite is present. XRD results show the presence of ilmenite and hematite. Mössbauer data show that only species assigned to hematite, ilmenite, and a small amount of tetrahedral Fe3+ are present. Magnetic properties at high and low T also indicate that the only magnetic minerals are ilmenite and hematite. Magnetic data suggest that ultra‐fine hematite lamellae are magnetically ordered, and their resultant remanent magnetic anomalies may contribute significantly to magnetism on terrestrial planets, even those without present‐day magnetic fields.

On the abnormally high natural remanent magnetization of certain lavas

The rock specimens found to have natural remanent magnetization of abnormally high intensity, have been generally from hill sides or tops, or from ridges high up from the local surroundings. A field of several hundred oersteds has been found sufficient to produce in some of the artificially demagnetized specimens an isothermal magnetization of the same order as the abnormal natural ones, and this magnetization has shown a similar degree of stability as the natural one. Variations in the direction and magnitude of the natural magnetic vector have been found over a distance of a few centimetres. Laboratory tests indicate a normal chemical composition for the specimens. The lightning discharge seems to be the plausible cause of abnormally high magnetization of rocks, which is generally isothermal and might have originated in the recent past, but the magnetization is sometimes complicated probably by the thermal effect of the discharge.