Natural Remanent Magnetization Intensity Research Articles

Geomagnetic paleointensity dating of mid-ocean ridge basalts from the neo-volcanic zone of the Central Indian Ridge

Describing the evolution of the neo-volcanic zone in the spreading ridge is essential for understanding the dynamics and environments of abyssal basins. However, the absolute dating of ocean floor basalts is generally difficult. As a characteristic age indicator, absolute intensity of past geomagnetic field (absolute paleointensity, API) is useful to date ocean floor basalts. In this study, we adopted the Tsunakawa–Shaw method to measure APIs of whole-rock seafloor basalts collected from a conical cone on the Central Indian Ridge and performed rock magnetic experiments. We conducted the experiments on a total of 18 specimens (two or three specimens from each of eight lava sites). Six specimens from two lava sites with different morphologies (pillow and sheet), three for each, passed the acceptance criteria. API means at site level are 33.0 ± 1.0 and 35.8 ± 1.7 μT, respectively. The similarity of API site means suggests that they erupted within a short period. These site-level API means are approximately 0.7 to 0.8 times the present geomagnetic intensity of 46.0 μT at the sampling sites. The accepted specimens show higher Curie temperature, lower initial intensity of natural remanent magnetization, higher ratio of saturation remanence to saturation magnetization (Mrs/Ms), and signal of harder magnetic mineral than rejected ones. Our primary comparison between the two site-level API means and the 1590–present high-resolution IGRF-13 + gufm1 model constrains that the eruption timing of the conical cone to be < 1590 CE. When we compared the two site-level API means with the paleointensity curves calculated from the BIGMUDI4k.1 and ArchKalmag14 k.r, we found that they overlap in the period of − 7575 to −1675 CE or − 25 to 1590 CE, which may be the eruption timing of the conical cone. We concluded that timing of recent volcanic eruption in abyssal environment could be investigated by using appropriate rock magnetic selection and carefully examined API.Graphical

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

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

Impact of turbulence on magnetic alignment in sediments

Rapidly deposited layers (RDL) such as turbidites or hyperpycnites are mostly studied for their sedimentological properties, but are carefully avoided in paleomagnetic studies due to the disturbances caused by such sudden and rapid sediment accumulation. Therefore, these layers can also be seen as potential indicators of sediment parameters susceptible of affecting the alignment of magnetic grains and ultimately the acquisition of the natural remanent magnetization (NRM). We have compiled 13 Holocene rapidly deposited layers from core MD99-2222 in the Saguenay Fjord, eastern Canada (St-Onge and al., 2004) with varying thicknesses (from 7.1 cm to 1,510 cm) and 4 Quaternary turbidites of different origins, to document the influence of sedimentary and magnetic parameters on natural remanent magnetization acquisition. We found a logarithmic relationship between rapidly deposited layers thickness on the one hand, and the amplitude of inclination changes and magnetic grain sizes on the other. Inclination and magnetic grain sizes are themselves correlated to each other by a logarithmic law. As there is no relationship between inclination deviation and stratigraphic depth, compaction alone cannot account for such large effects on inclination. Flocculation is grain size sensitive, but it is expected to affect mainly the natural remanent magnetization intensity, rather than its direction. Turbulence that prevails during the rapid deposition of sediments during such events is most likely the dominant factor.

Paleomagnetism and Rock Magnetism of Permian sedimentary rocks and Early Jurassic Karoo (Large Igneous Province) sills intersected in the KWV-1 borecore, Eastern Cape Province, South Africa

The nearly 100 Ma record of almost continuous shallow marine to non-marine sedimentation from the Late Carboniferous to the Early Jurassic recorded in the Karoo Basin, South Africa, includes coal deposits and potentially significant shale gas occurrences. As such, numerous studies have addressed the thermal history of the Karoo Basin, using a range of approaches. We use paleomagnetic and rock magnetic data to evaluate the paleotemperature history of the host Permian strata (Ecca Group and lowermost Beaufort Group) intersected in the ∼2320 m deep KARIN KWV -1 drilling project core, obtained in 2015. Where cored, the Karoo sedimentary sequence has been intruded by several Early Jurassic mafic sills, belonging to the Karoo Large Igneous Province, of thickness ranging from less than 10 cm to ∼150 m. The continuous core provides a unique natural laboratory for studying the effects of Karoo magmatism on the basin's sedimentary sequence. The KWV-1 drilling project was spudded ∼10 km east of Willowvale town, Eastern Cape Province.The spatial variation in thermal effects in host sedimentary strata, relative to proximity to Karoo sill contacts, is well-defined on the basis of anisotropy of magnetic susceptibility (AMS) data, paleomagnetic demagnetization response, and thermal alteration index (A40) values based on temperature vs. magnetic susceptibility tests. A zone of pervasive remagnetization of host sedimentary strata characterizes the contact aureoles, the spatial extents of which are to a first order dictated by the thickness of each Karoo sill. A40 data are interpreted to show that ambient paleotemperature estimates increase from ∼150 °C in lower Beaufort/upper Ecca strata to ∼285° in Lower Ecca strata that are sufficiently removed from Karoo sills. However, lower Ecca Group rocks in close proximity to Karoo sills were likely heated to ∼500 °C. Natural remanent magnetization (NRM) intensities of rocks in proximity to sills range from ∼17 to ∼ 438 mA/m, and susceptibilities range from 1.3 to 3.2 × 10−3 SI volume. In contrast, for strata sampled at distances greater than the width of the nearest sill, NRM intensities range from 0.056 to ∼4 mA/m, and susceptibility values range between 193 and 488 X 10−6 SI volume. For host sedimentary rocks, remanence is completely unblocked at temperatures ranging from ∼350 °C to ∼630 °C and acquisition and backfield DC demagnetization of isothermal remanent magnetization (IRM) indicate that specimens are fully saturated in fields of ∼200–600 mT, with coercivity of remanence of ∼30–100 mT. All of the eleven Karoo sills examined in this study yield remanence inclinations that are moderate to steep negative, and are inferred to have recorded a normal polarity field, possibly during a single chron. Our results are interpreted to suggest that substantial thermal effects of Karoo sills on the host sedimentary strata are largely confined to the contact aureoles, with a width that is dependent on sill thickness. We hypothesize that parts of the Ecca sedimentary sequence have remained at ambient, basinal burial temperatures below about 150 °C, and thus that an early-acquired Permian remanence may possibly be preserved in these sedimentary rocks at sufficient distance from thermal contact aureoles. Alternatively, much of the sedimentary section may have been subject to a combination of thermoviscous/chemical remagnetization.

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.

Influence of Magnetofossils on Paleointensity Estimations Inferred From Principal Component Analyses of First‐Order Reversal Curve Diagrams for Sediments From the Western Equatorial Pacific

AbstractRelative abundance of magnetite originated from magnetotactic bacteria (magnetofossils) in sediments may influence relative paleointensity (RPI) estimations of the geomagnetic field, as some studies reported an inverse correlation between RPI and the ratio of anhysteretic remanent magnetization susceptibility to saturation isothermal remanent magnetization (kARM/SIRM), a proxy of the proportion of biogenic to terrigenous magnetic minerals as well as magnetic grain size. This study aims to evaluate the influence of magnetofossils on RPI estimations more selectively using first‐order reversal curve (FORC) diagrams. We studied three cores (KR0515‐PC4, MD982187, and MR1402‐PC1) from the western equatorial Pacific, among which large differences exist in the average natural remanent magnetization intensity normalized by ARM and kARM/SIRM. Principal component analyses (PCAs) were applied to FORC diagrams measured on bulk specimens from the three cores and silicate‐hosted magnetic inclusions extracted from Core MD982187, and three endmembers (EMs) were revealed (EM1: silicate‐hosted magnetic inclusions, EM2: other terrigenous, EM3: biogenic). EM3 proportions vary widely among the three cores. The average RPI decreases with increasing EM3 proportion, which is probably caused by higher ARM acquisition efficiency of magnetofossils due to small magnetostatic interactions. EM3 proportion correlates with kARM/SIRM, which confirms that kARM/SIRM represents the proportion of biogenic to terrigenous magnetic components. Core MR1402‐PC1 has the highest EM3 proportion, and its within‐core variation is small. From FORC‐PCA applied solely to this core, we infer that the configurations of biogenic magnetite chains such as bending and collapse may also influence kARM/SIRM and RPI estimations.

Dating the Hemudu Neolithic rice cultivation site, East China, by paleomagnetic chronostratigraphy

Hemudu is a famous archaeological site with the earliest rice remains unearthed in East China and represents a classic example of how Neolithic people managed coastal farmland. However, appreciation of human adaptation to Holocene sea-level changes has been hampered by low-resolution chronology due to lack of datable materials in coastal sediments. Correlation of paleomagnetic directions and intensity on a regional scale provides independent relative dating of such sediments. We did high-resolution paleomagnetic chronology work on core HMD13 (27 m in length) from Hemudu Neolithic site and discussed relevant early Holocene paleoclimate change of Southern China. Magnetic minerals identified by scanning electron microscopy are magnetite, iron-bearing silicates, and iron sulfide (pyrite or greigite). Rock magnetic measurements indicate that the primary remanence carrier is single-domain magnetite. Stepwise alternating field (AF) demagnetization experiments at 1 cm intervals on U-channel samples show that paleomagnetic inclinations fluctuate around 60°, with five major peaks and four troughs. A relative paleointensity (RPI) was estimated using normalizing natural remanent magnetization intensity to anhysteretic remanent magnetization over 20–40 mT AF range, which has four major peaks. Variations of inclination and RPI in core HMD13 resemble the data from the East China Sea and allowed the development of a refined age-depth model. The new adjusted age-depth model minimized dating errors of 14C ages among different datable materials. From the new age model, inclination and RPI of HMD13 are comparable to paleosecular variation (PSV) stack from East Asia and predicted results from geomagnetic models. Four cold events in the early Holocene recorded by geochemical proxies imply that sedimentary sequence of core HMD13 was influenced by global paleoclimate variation, while its long-term trend points to local paleoenvironmental change. Increases in sedimentation rates and the marine-derived sediment fractions suggest a massive transgression in the early Holocene (9–7.5 ka). Subsequently, ancient people developed and built pile dwellings since ~7 ka and cultivated wild rice from ~6.3 ka. Our study demonstrates that regional stratigraphic correlation with well-dated PSV and RPI records is a practical approach to establishing reliable age-depth models. A high-resolution chronology on continuous sedimentary sequence has excellent potential to elucidate prehistoric human-environment co-evolution.

Integrated Pliocene-Pleistocene magnetostratigraphy and tephrostratigraphy of deep-sea sediments at IODP Site U1424 (Yamato Basin, Japan Sea)

Sediments from the semi-enclosed Japan Sea are sensitive to paleoclimatic perturbations and they offer great opportunities for many regional and global paleoceanographic and paleoclimatic studies. These studies often require a robust chronology. However, due to rare preservation of calcareous microfossils and drastic changes in surface water salinity during glacial lowstands, the construction of a traditional oxygen isotope stratigraphy for Japan Sea sediments is often difficult. Here, we use sediments recovered at Integrated Ocean Drilling Program (IODP) Expedition 346 Site U1424 to build an integrated Pliocene-Pleistocene reference magnetostratigraphy and tephrostratigraphy for sedimentary sequences from the region. Rock magnetic experiments indicate that magnetic remanence of Site U1424 sediments are carried primarily by (titano)magnetite with small contributions from high coercivity minerals (e.g., hematite) and possibly iron sulphides (pyrrhotite and/or greigite). Dark-colored sediments appear to contain less (titano)magnetite probably due to reductive diagenesis under euxinic conditions. Natural remanent magnetization (NRM) of u-channel samples covering a continuous ~ 167.6 m sediment sequence at Site U1424 was repeatedly measured at 1 cm intervals before and after stepwise demagnetization. Despite lower NRM intensity in dark-colored sediments, NRM directional data from both dark- and light-colored sediments are considered suitable for the construction of magnetostratigraphy. Site U1424 sediments clearly recorded the majority of the polarity chrons and subchrons within the last ~ 4.89 Myr, with the Cobb Mountain subchron, the end of Kaena subchron, and the onset of Nunivak subchron less well preserved. Sixteen tephra layers from the site were sampled for chemical composition analyses and the results were correlated to reference tephrostratigraphy of the region. Ages of the identified tephras are consistent with and can be well integrated with the magnetostratigraphy. The resulting age model suggests that sedimentation rates at Site U1424 range between ~ 1.7 and 7.6 cm/kyr with an average of ~ 3.3 cm/kyr. The acquired magnetostratigraphy and tephrostratigraphy at Site U1424 provide a reference chronology that can be correlated with and transferred to other sediment sequences in the region to study paleoceanographic and paleoclimatic changes of the region as well as their links to other regional and global changes.Graphical abstract

Single Phase Oxidation of Ferrimagnetic Grains as a Cause of L-Shaped Arai–Nagata Diagrams

Abstract—The experiments on the determination of paleointensity Banc of the Earth’s magnetic field by Thellier method on basalts of the Berd (Jurassic) and Paravakar (Cretaceous) collections sampled in the northeastern Armenia in 2006 revealed an anomalous behavior of the Arai–Nagata diagrams. The anomaly manifests itself by a sharp drop in the intensity of natural remanent magnetization (NRM) under heating of samples to about 400°C, which is accompanied by a very weak acquisition of partial thermoremanent magnetizations (pTRMs) in this temperature interval. The further increase in temperature leads to the opposite phenomenon—an unexpectedly sharp rise in pTRMs intensity with almost no decrease in NRM, which creates the L-shape of the Arai–Nagata diagram. A similar phenomenon of a steep decline during thermal demagnetization was also established for remanent saturation magnetization Mrs(T). We carried out a number of magnetomineralogical experiments from which we conclude that, consistent with the hypothesis suggested by (Kosterov and Prevot, 1998), the sharp drop in Mrs(T) and NRM(T) curves is caused by the transitions of the domain state from a metastable configuration to a more stable one. These transitions are initiated by the processes of single- and heterophase oxidations of primary titanomagnetites under the laboratory heating of samples to moderate temperatures.

The magnetization of the ocean floor: stress and fracturing of titanomagnetite particles by low-temperature oxidation

SUMMARY The natural remanent magnetization (NRM) of the ocean floor is carried by titanomagnetite grains that undergo low-temperature oxidation after initial cooling. Progressing oxidation is known to generate shrinkage cracks in grains larger than approximately 5 μm, and is suspected to control the long wavelength variation of NRM-intensity across the ocean floor. Here we develop a quantitative theory of single-phase oxidation and crack formation by solving the vacancy-diffusion equation that describes the oxidation process for spherical titanomagnetite particles, where the diffusion coefficient strongly decreases with vacancy concentration. The latter dependence has been experimentally demonstrated and is essential to explain the peculiarities of the observed variations of oxidation-degree with ocean-floor age. The calculated diffusion profiles provide the exact stress distributions inside oxidized titanomagnetite spheres, and predict a size limit for shrinkage-crack formation that agrees with microscopic observations of crack appearance in ocean-floor basalt samples. The new diffusion model provides a unified explanation of long-known experimental facts that (1) temperatures for the onset of low-temperature oxidation during laboratory heating are theoretically estimated as 200–400 ○C, depending on grain size and (2) that heating to 400–500 ○C is required to obtain a sufficiently high degree of oxidation z ≈ 0.8 for the development of high-temperature exsolution lamellae. Calculations for ocean-floor conditions quantitatively suggest that a rapid decrease of NRM intensity during the first 40 ka results from a deflection of magnetization by strong stresses that emerge in titanomagnetite grains of subcritical sizes, and randomization of domain-state by crack formation in larger grains.

The Chemical Remagnetization of Ediacaran Dolomite in the Taishan Paleo‐Reservoir, South China

AbstractTo understand the origin of chemical remagnetization in carbonate reservoir rocks and associated chemical processes, we have conducted detailed petrologic, geochemical, and magnetic studies of the hydrocarbon‐impregnated Ediacaran dolomite in the Taishan Paleo‐reservoir, south China. Samples were collected across a continuous outcrop consisting of oil‐free dolomite, transition zone dolomite, and highly hydrocarbon‐impregnated reservoir dolomite. Paleomagnetic results show that dolomite from the reservoir and transition zone were remagnetized and carries a possible Cambrian‐Ordovician direction (Dec = 163.7°, Inc = 10.6°, and α95 = 1.9°), and the oil‐free dolomite shows unstable magnetizations. Hematite is the magnetic mineral in the oil‐free dolomite, whereas the hydrocarbon‐impregnated reservoir dolomite contains no hematite but mainly pyrrhotite nanoparticles. Transition‐zone dolomite contains both hematite and pyrrhotite. In addition, the natural remanent magnetization intensities decrease from the reservoir dolomite, through the transition zone, and into the oil‐free dolomite, and a similar trend was observed in the total organic carbon and bitumen contents. The results suggest that the magnetic mineral assemblage and magnetic intensity in dolomite appear to be closely related to the degree of hydrocarbon impregnation and that remagnetization of the Ediacaran dolomite was likely caused by hydrocarbon migration into the dolomite. Pyrrhotite nanoparticles, magnetite, and barite are suggested to be formed during hydrocarbon migration, by the reduction of detrital hematite in the dolomite and sulfur species in the hydrocarbons. The new findings into the origin of pervasive remagnetization have the potential to guide future hydrocarbon exploration within Ediacaran dolomite that is host to the largest single carbonate gas reservoir in China.

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

AbstractA strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) approximately opposite today's geomagnetic field, is observed above the MCU IVe' unit of the Bjerkreim Sokndal Layered Intrusion. The anomaly is strongest in the east, close to Heskestad, and decreases when following the layer toward the north and west. This study investigates how the NRM changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion. NRM, low‐field anisotropy of magnetic susceptibility, and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites. The orientation of both the magnetic fabrics and the NRM changes for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence. When NRM directions are corrected for magnetic fabrics, the angle between the NRM and mean paleofield direction generally decreases for specimens with a single‐component NRM. No correlation was found between the NRM intensity and the directional relationship between NRM, magnetic fabric, and mean paleofield.

Variation in magnetic properties of serpentinized peridotites exposed on the Yokoniwa Rise, Central Indian Ridge: Insights into the role of magnetite in serpentinization

AbstractMagnetic properties in serpentinized peridotites are of increasing interest in seafloor mapping and petrologic studies because such data can promote the understanding of serpentinization reactions and hydrogen creation in ultramafic rocks. In order to reveal the magnetic properties and magnetite growth in serpentinized peridotites, we analyzed 30 serpentinized peridotite samples from a nontransform offset massif called the Yokoniwa Rise in the Central Indian Ridge. The results from multiple rock magnetic analyses and petrological observations illustrate the details of the creation and growth of magnetite in serpentinized peridotites that have undergone 17–100% serpentinzation. The magnetic carrier of these samples is pure magnetite, which did not suffer from maghemitization (low‐temperature oxidation). The magnetic susceptibility ranged from 0.002 to 0.087 SI and increased nonlinearly with the progression of the serpentinization reaction. The natural remanent magnetization intensities of 0.2–8.4 A/m are comparable to those of basalts, which suggests that the remanence as well as induced magnetization of highly serpentinized peridotite can contribute to magnetization of the oceanic lithosphere. The amount of magnetite estimated from saturation magnetization increased nonlinearly from 0.1 to 5.5 wt % with the progression of the serpentinization. Highly serpentinized peridotites have a well‐developed serpentine mesh texture. Pseudo‐single‐domain (PSD) and multidomain (MD) grains were formed during igneous processes in the mantle and/or during the initial stages of serpentinization. Superparamagnetic (SP) particles were formed during the initial stages of serpentinization. Single‐domain (SD) magnetite was formed during the later stage of serpentinization, and it is assembled inside of mesh structures with strong magnetostatic interactions.

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.

Origin of magnetic highs at ultramafic hosted hydrothermal systems: Insights from the Yokoniwa site of Central Indian Ridge

High-resolution vector magnetic measurements were performed on an inactive ultramafic-hosted hydrothermal vent field, called Yokoniwa Hydrothermal Field (YHF), using a deep-sea manned submersible Shinkai6500 and an autonomous underwater vehicle r2D4. The YHF has developed at a non-transform offset massif of the Central Indian Ridge. Dead chimneys were widely observed around the YHF along with a very weak venting of low-temperature fluids so that hydrothermal activity of the YHF was almost finished. The distribution of crustal magnetization from the magnetic anomaly revealed that the YHF is associated with enhanced magnetization, as seen at the ultramafic-hosted Rainbow and Ashadze-1 hydrothermal sites of the Mid-Atlantic Ridge. The results of rock magnetic analysis on seafloor rock samples (including basalt, dolerite, gabbro, serpentinized peridotite, and hydrothermal sulfide) showed that only highly serpentinized peridotite carries high magnetic susceptibility and that the natural remanent magnetization intensity can explain the high magnetization of Yokoniwa. These observations reflect abundant and strongly magnetized magnetite grains within the highly serpentinized peridotite. Comparisons with the Rainbow and Ashadze-1 suggest that in ultramafic-hosted hydrothermal systems, strongly magnetized magnetite and pyrrhotite form during the progression of hydrothermal alteration of peridotite. After the completion of serpentinization and production of hydrogen, pyrrhotites convert into pyrite or nonmagnetic iron sulfides, which considerably reduces their levels of magnetization. Our results revealed origins of the magnetic high and the development of subsurface chemical processes in ultramafic-hosted hydrothermal systems. Furthermore, the results highlight the use of near-seafloor magnetic field measurements as a powerful tool for detecting and characterizing seafloor hydrothermal systems.

Environmental conditions for the presence of magnetofossils in the Last Glacial Maximum inferred from magnetic parameters of sediments from the Ulleung Basin, East Sea

Magnetic characterization of sediments from the Ulleung basin in the East Sea (Japan Sea) was carried out to decipher changes in environmental conditions over the last ~39,000years. Magnetic susceptibility (χ), intensities of natural remanent magnetization (NRM), anhysteretic remanent magnetization (ARM), partial ARM (pARM), and saturation isothermal remanent magnetization (SIRM) were measured. For selected samples, IRM acquisition, hysteresis and first-order reversal curve (FORC) measurements were made. The ratios of rock-magnetic parameters, such as χARM/χ, SIRM/ARM and S-ratio, were determined to identify magnetic grain size and mineralogical variations along with electron microscope observations of magnetic mineral extracts. Down-core variations in rock-magnetic parameters for the 5.21-m-long sediment core reveal distinct intervals with different grain size, concentration and composition of magnetic minerals. The uppermost portion and the high magnetic intensity zone (HIZ) in marine isotope stage (MIS) 2 are characterized by higher concentrations of fine-grained magnetite, whereas the MIS 1 and 3 intervals contain lower concentrations of coarser-grained magnetic minerals with increasing influence of hematite and/or iron sulphide minerals. The HIZ interval is inferred to contain high occurrences of fine-grained bacterial magnetosomes. Based on geochemical proxies, East Sea conditions for the HIZ interval, coincident with the last glacial maximum (LGM), can be summarized as having low total organic carbon (TOC), enhanced eolian contributions from terrestrial organic particles and iron-bearing minerals under the influence of an intensified East Asian winter monsoon, along with iron-reducing, but not sulphidic, conditions that favoured growth and preservation of magnetotactic bacteria. The delicate balance between reductive dissolution of magnetofossils under sulphidic conditions and production and preservation of magnetotactic bacteria in overlying iron-reducing sediments for the HIZ interval is responsible for the resultant abundance variations of magnetosomes in the studied sediments.

Rock-magnetic properties of single zircon crystals sampled from the Tanzawa tonalitic pluton, central Japan

This paper reports on the rock-magnetic properties of single zircon crystals, which are essential for future work establishing the reliable paleointensity method using single zircon crystals. Zircon crystals used in this study were sampled from the Nakagawa River, which crosses the Tanzawa tonalitic pluton in central Japan. Rock-magnetic measurements were conducted on 1037 grains of zircons, but many of these measurements are below the limits of the sensitivity of the magnetometers employed. Isothermal remanent magnetizations (IRMs) of 876 zircon crystal are below the practical resolution of this study; we infer that these crystals contain no or only minute quantities of ferromagnetic minerals. The other zircon crystals contain enough magnetic minerals to be measured in the DC SQUID magnetometer. For 81 zircon crystals, IRM intensities (M IRM) are larger than 4 × 10−12 Am2, while natural remanent magnetization (NRM) intensities (M NRM) are below 4 × 10−12 Am2, indicating that these crystals are inappropriate for the paleomagnetic study. For the samples that had values of M NRM ≥ 4 × 10−12 Am2 and M IRM ≥ 4 × 10−12 Am2 (80 zircons), combining the rock-magnetic parameter, we proposed the sample-selection criteria for future study of paleointensity experiments using single zircon crystals. In the case that the samples had high coercivity (B c) values (>10 mT) or high M NRM/M IRM values (>~0.1), main remanence carriers are probably pyrrhotite and these samples are inappropriate for the paleointensity study. In the case that the samples had low B c values (<10 mT) and low M NRM/M IRM values (<~0.1), main remanence carriers seem to be nearly pure magnetite with pseudo-single-domain grain sizes, and these samples are expected to appropriate for the paleointensity study. Total thermoremanent magnetization (TRM) acquisition experiments were also carried out for 12 samples satisfying the above criteria. The TRM intensity was comparable with that of NRM, and a rough estimation of the paleointensity using NRM/TRM ratios shows field intensities consistent with the average geomagnetic field intensity at the Tanzawa tonalitic pluton for last 5 Myr.

Strong clockwise block rotation of the Ali-Sabieh/Aïsha Block: evidence for opening of the Afar Depression by a ‘saloon-door’ mechanism

AbstractPalaeomagnetic investigations were carried out on tilted Jessoma sandstones in the Aïsha block. The average natural remanent magnetization intensity was 0.026A m−1 with good behaviour on demagnetization both by thermal and alternating fields. The normalized magnetization intensity decay curves indicated that fine-grained, Ti-poor titanomagnetites were the carriers of magnetization. A well-defined single-component magnetization with a poorly defined secondary component was found. The latter was removed at temperatures below 300°C or at 10 mT. After these steps, a straight-line segment directed towards the origin was identified, which was interpreted as the characteristic remanent magnetization. Site mean direction coordinates of Decg=227.8°, Incg=–33.9° (n=7, α95=2.8°, K=478) (in situ) and Decs=225.6°, Incs=0.4° (n=7, α95=2.8°, K=478) (tilt-corrected) were obtained. Corresponding mean virtual geomagnetic pole coordinates of Long=117.5° E, Latg=43.3° N (A95=3.2°, n=7) (in situ) and Lons=143.5° E, Lats=43.3° N (A95=2.8°, n=7) (tilt-corrected) were obtained. The corrected in situ mean pole position was compared with the African apparent polar wander path curve and a deposition age for the sandstone of 60±5 Ma was determined, which is consistent with previous reports. A vertical axis rotation (R±ΔR) of the Aïsha block of 29.1±4.06° was obtained, which is equivalent to an inferred 30° counterclockwise rotation of the Danakil block, in agreement with a ‘saloon-door’ mechanism for the opening of the Afar Depression.

Paleomagnetism of Lake Van sediments: chronology and paleoenvironment since 350 ka

In the framework of the PALEOVAN project, a high-resolution paleomagnetic and rock magnetic study was carried out on a 149 m sedimentary sequence recovered from Ahlat Ridge in the deepest part of the Lake Van (Eastern Turkey; ICDP Site 5034-2). High sedimentation rates (average ∼30 cm/ka) allow high-resolution measurements of natural remanent magnetization (NRM), magnetic susceptibility (κ), anhysteretic remanence (ARM) and hence of anhysteretic susceptibility (κARM) over the last 350 ka. The carrier of the remanence is detrital titanomagnetite largely from volcanic sources, based on the similarity of magnetite grain size of tephra layers and the other lacustrine lithologies observed in a plot of the κ versus κARM. Bulk magnetic parameters often covary with paleoclimatic signals in the Lake Van sediments. A correlation exists between variations of κ and ARM intensity and glacial-interglacial marine isotopic stages, as well as dust flux and temperatures observed in ice cores from Greenland and Antarctica. The quality of the paleomagnetic record is compromised by weak NRM intensities, as well as by the presence of tephra and turbidites throughout the sequence. Nonetheless, a correlation is observed between the relative paleointensity (RPI) record, based on NRM/ARM, and the calibrated PISO RPI stack, that supports the independently derived age model for the site.

Coseismic magnetization of fault pseudotachylytes: 1. Thermal demagnetization experiments

AbstractFault pseudotachylytes form by quenching of silicate liquids produced through coseismic frictional melting. Here we show that in natural pseudotachylytes the main carrier of magnetic remanence blocked in during cooling of the frictional melt is fine‐grained magnetite. This confirms previous studies on friction melt experiments. Stoichiometric magnetite, produced during earthquakes by the breakdown of ferromagnesian silicates, records the ambient magnetic field during seismic slip. We find that most fault pseudotachylytes exposed in the Santa Rosa Mountains, southern California, a classic pseudotachylyte locality, acquired their natural remanent magnetization (NRM) upon cooling of the frictional melt through the range of magnetization blocking temperatures of the magnetite grains and this primarily constitutes a thermal remanent magnetization. NRM intensities typical of most pseudotachylyte veins range from 1 to 60·10−4 Am2/kg. A few specimens, however, contain magnetizations significantly higher than that caused by the Earth's field as well as magnetization directions that are highly variable over short distances. Other magnetization processes, possibly related to coseismic electric currents, may be involved during the seismogenic process to control NRM acquisition.