Titanomagnetite Grains Research Articles

Potential Bias in Volcanic Paleomagnetic Records Due To Superimposed Chemical Remanent Magnetization

AbstractVolcanic rocks, preserving paleorecords of Earth's magnetic field, are essential to constrain the working of the geodynamo, provided their primary signal was not biased. Using a thermomagnetometer, we simulate a situation where a sample's primary record, carried by a thermoremanence (TRM, acquired by cooling in air from 600°C to room temperature), is partly overprinted by a chemical remanence (CRM, acquired by 200 hr of isothermal exposure at 400°C). This situation leads to two directional and intensity components (in the form of linear segments) in the Zijderveld and Arai‐Nagata diagrams. In the case of unstable titanomagnetite grains prior to CRM acquisition, we show that both components can be strongly biased by up to ∼50° for paleodirections and ∼50% for paleointensities. In such a worst‐case scenario, the secondary CRM strongly overprints the primary TRM, rendering the common interpretation of Zijderveld and Arai‐Nagata diagrams in terms of characteristic components invalid.

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 (>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.

Properties of Red Sea Pillow-Basalt Titanomagnetite at Different Distances from the Crust

The magnetic properties and mineralogy of titanomagnetite in the Red Sea pillow-basalt at different distances from the contact surface with water were studied. It was revealed that the composition of titanomagnetite in the pillow basalt changes from the crust deep into the pillow: the titanium content in titanomagnetite increases, while the magnesium and aluminum content decreases. Titanomagnetite in the surface layer of pillow basalt (0–2 cm) is single-phase oxidized, with a maximum oxidation degree of 0.13, and at more than 3 cm from the crust has stoichiometric composition. According to the increase in magnetic susceptibility from 0.2 up to 1.8 × 10–2 SI, saturation magnetization from 0.026 to 0.895 A m2/kg and residual saturation magnetization from 0.014 to 0.296 A m2/kg, decrease in coercivity (Hc from 90 to 15 mT, Hcr from 153 to 20 mT), as well as by observations of electron and magnetic force microscopy, it is shown that the concentration of titanomagnetite in basalt and the grain size from single-domain state to pseudo-single-domain state increase with distance from the crust. The magnetic hardness of titanomagnetite grains was found to correlate with the maximum cooling rate of basalt. The NRM of basalt varies non-monotonically with distance from the pillow surface: up to a depth of about 3 cm its growth caused by the growth of titanomagnetite concentration was revealed, then a decrease caused by a decrease in the magnetic hardness of the grains. In spite of a rather strong variation of the magnetic properties in the 0–7 cm layer of pillow basalt, the experiments by the Thellier-Coe method for all layers showed close values of the geomagnetic field paleointensity (62.5–66.0 μT) with a high value of quality coefficient q (11–45). No regularity in the value of paleointensity with distance from the cushion surface was detected.

Features of granulometric and material composition of road dust, bottom sediments and soils in the Rostov region

An expedition was carried out to take samples of road dust, soils, sediments at the water’s edge and bottom sediments of watercourses and reservoirs of Eastern Donbass and other districts of the Rostov region. The goal was to study the granulometric composition of the selected material. In all samples, particles with a dimension of less than 0.01 mm were dominant, while particles with a diameter of <0.002 played a subordinate role. Particles with a diameter of <0.01 prevailed in the composition of the studied samples, regardless of the place of their selection. It was found that the content of the sum of fractions with particle sizes < 0.01 mm and <0.002 are in the selected samples in the following sequence: bottom sediments > sediments at the water’s edge > soil > road dust. The composition of natural formations includes soil particles, aggregates of clay particles, fine sand-siltstone quartz grains, calcite, feldspar, gypsum, single grains of magnetite and titanomagnetite. Technogenic impurities in the dust were represented by aluminosilicate microspheres, metal particles of varying degrees of corrosion, carbon particles.

Assessing the Magnetic Mineralogy of the Pre-Variscan Manteigas Granodiorite: An Unexpected Case of a Magnetite-Series Granitoid in Portugal

The Early Ordovician Manteigas granodiorite has been characterized as having a magnetic susceptibility with high heterogeneity and values compatible with the presence of ferromagnetic phases, such as magnetite. However, granitoids with magnetite are rare in the Iberian Massif. Therefore, an in-depth study of mineralogical composition was carried out on the Manteigas granodiorite by evaluating its magnetic susceptibility, analyzing the isothermal remanent magnetization (IRM) and the IRM curve, identifying the magnetic coercivity components, evaluating the magnetic grain size and performing a petrographic study. Low concentrations of very small grains of magnetite and/or titanomagnetite have been found in areas with low magnetic susceptibility and with values not compatible with ferromagnetic phases. The petrographic study has explained this phenomenon as a result of the transformation of magnetite into hematite (martitization), indicating a redox reaction. A high concentration of magnetite and/or titanomagnetite, occurring as multidomain crystals, has been found in areas with high magnetic susceptibility. This study indicates that the Manteigas granodiorite has a deep magma origin, and formed under oxidizing conditions since it has magnetite instead of ilmenite, which is an uncommon phenomenon in the Iberian massif.

The Properties of Thermochemical Remanent Magnetization Acquired by Slow Laboratory Cooling of Titanomagnetite-Bearing Basalt Samples from Different Temperatures and the Results of the Thellier Method

—The experiments have been carried out on the acquisition of thermochemical remanent magnetization (TCRM) in basalt samples containing titanomagnetite (TM) with the Curie temperature Тс ~200°C by their rapid heating to maximum temperatures Т* from 450 to 530°C followed by slow cooling in the laboratory magnetic field Blab. At different stages of the preliminary thermal treatment of the initial samples, a set of magnetomineralogical studies including electron microscopy, X-ray diffraction and thermomagnetic analyzes, and measurements of magnetic hysteresis parameters were performed. It is shown that as early as the very beginning of the cooling process, all samples demonstrate explosive growth of TCRM corresponding to the stage of rapid single-phase oxidation of the initial titanomagnetite fraction of basalt, and that TCRM is acquired by the increase of Тс and volume of single-phase oxidized parts of TM grains as well as by the growth of the volume of Ti-depleted (relative to the initial TM) cells of microstructure of the subsequent oxidative exsolution. The Arai–Nagata diagrams for the samples carrying TCRM have a form of a broken line consisting of two linear segments. The low-temperature interval T < Т* corresponds to a mixture of thermochemical and thermoremanent (TRM) magnetizations and gives a slightly overestimated Blab because of the effect of a low cooling rate during the acquisition of TCRM and TRM. The high-temperature interval corresponds to pure TCRM and the Blab strength determined from this interval is underestimated by 20–27%. It is recommended to reject samples whose Araii–Nagata diagram has two or more linear segments against the background single-component NRM.

Controls of contrasting provenance and fractionation on the sediment magnetic records from the Bay of Bengal

We present the results of rock magnetic, granulometric, microscopy and mineralogical analyses of shelf sediments off Krishna, Godavari, and Mahanadi Rivers, Bay of Bengal. The aim of the study was to assess the influence of contrasting lithologies and hydrodynamic controlled grain size fractionation processes on the magnetic properties of shelf sediments. Sediment magnetic mineralogy mainly consist of ferrimagnetic titanomagnetite grains of detrital origin with variable concentrations and grain sizes. A clear trend of alongshore (SE - NW directed) decrease in magnetite concentration followed by fining of magnetic grain size can be attributed to the enhanced change in magnetic contribution from Deccan basaltic (mafic) to granitic (felsic) gneissic dominated sediment province coupled with differential sediment fluxes at each site. Bivariate plot of magnetic and grain size parameters clearly records such shift in sediment provenance and differential transport mechanism of magnetic particles at the studied sites. Magnetogranulometric ratio (ARM/IRM) positively correlates with the magnetite content in majority of the studied samples. Higher magnetic susceptibilities were found associated with coarser magnetic particles. Granulometry of the magnetic fraction showed that magnetic grain size is independent of physical grain size above >40 μm. The non-monotonous relationship between the magnetic and physical grain size can be explained by the fact that few individual finer particles occurred as magnetic inclusions protected within host (coarser silt and sand) fractions. Rockmagnetic and granulometry based approach presented in this study bears high potential for efficient tracking of sediment provenance and fractionation mechanism of magnetic particles in the shelf system, where contrasting sediment sources and complex transport dynamics create a large variability in magnetite content and grain size.

Low Geomagnetic Field Paleointensity on the Permian–Triassic Boundary from Study of the Kuznetsk Basin Traps (Southern Siberia)

Abstract ––Here we report the first data on the absolute intensity value of the geomagnetic field on the Permian–Triassic boundary from basalts of the Kuznetsk basin. The latter are considered as one of the manifestations of the initial stage of trap magmatism during the formation of the Siberian large igneous province. The good preservation of information on the ancient geomagnetic field in the Mal’tsevskaya Formation basalts is due to the presence of small single domain and pseudo-single domain grains of primary magmatic titanomagnetite in the groundmass. The paleointensity values obtained following the Thellier-Coe method correspond to the generally accepted criteria of reliability and indicate that the geomagnetic field intensity during the formation of the Kuznetsk basin traps on the Permian–Triassic boundary was almost an order of magnitude lower than the present-day one. Moreover, the mean values of the virtual dipole moment for the Kozhukhta and the Vlasov units in the lower and middle Mal’tsevskaya Formation ((1.9 ± 0.6) ⋅ 1022 A ⋅ m2 and (1.1 ± 0.7) ⋅ 1022 A ⋅ m2, respectively) are in good alignment with determinations of the paleointensity during the accumulation of the Ivakinsky Formation of the Norilsk Region in the Siberian province, which confirms the accuracy of traditional regional correlations.

Reliable Determination of Paleointensity in Igneous Rocks Bearing Thermochemical Remanent Magnetization

Oxidation decomposition of titanomagnetite grains to the magnetite–ilmenite lattice during the formation of igneous rock can lead to thermochemical remanent magnetization (TCRM). Computer simulation of TCRM in the system of single-domain interacting particles and numerical simulation of the Thellier experiments to determine the paleointensity have been carried out. Contrary to the concerns, the Thellier technique factually applied to the TCRM-bearing samples yields a result that is very close to the true value of the external magnetic field applied to the sample during formation of the TCRM. Together with the recently published laboratory experimental data, these calculations are indicative of a good perspective for obtaining reliable paleomagnetic data on TCRM-bearing samples and can expand the class of rocks suitable for determination of the paleointensity.

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.

Determination of the possibility of separation of titanomagnetite and ilmenite in the selective separation of titanomagnetite ores

Experience in the development of iron-titanium ores has shown that their successful processing is possible only with the use of complex combined processing schemes. The possibility of selective extraction of titanomagnetite and ilmenite products during magnetic (electromagnetic) separation is considered during processing of altered disseminated titanomagnetite ores of the Medvedevsky deposit. Purpose of the research is to determine the possibility of separation of microaggregates of titanomagnetite and ilmenite during selective magnetic (electromagnetic) separation of disseminated titanomagnetite ores. Materials and methods. Classification of crushed material with its subsequent separation by magnetic (electromagnetic) properties. Analysis of the distribution of iron and titanium dioxide and the identification of the nature of the disclosure of ore and non-metallic minerals from the standpoint of technological mineralogy. Results. Products of classified ore after magnetic (electromagnetic) separation are characterized by uneven distribution. Most of the material (45,01%) is concentrated in fractions separated at a magnetic field with strength of more than 250 mT. The yield of magnetic fraction is 2,89%. A high content of Femagnetic is characteristically for the products of magnetic separation of titanomagnetite ore obtained at the magnetic field with strength of 10 mT. Generally, titanium dioxide is concentrated in the products of electromagnetic separation separated at a magnetic field with strength of 140 mT. Studies have established that the products obtained at H = 10 mT consist of 37% titanomagnetite aggregates of varying degrees of martitization. With increasing of magnetic field strength, the number of titanomagnetite grains decreases and the content of ilmenite grains increases in the products of electromagnetic separation. In this case at H = 140 mT, free grains (55%) are mainly consist of ilmenite. Conclusions. Analysis of the magnetic separation products showed that with a magnetic field strength of 10 mT it is possible to obtain a product with mainly titanomagnetite composition, and it is possible to obtain a product with mainly ilmenite composition with a magnetic field strength of 140 mT.

КОМПЛЕКСНЫЕ ГЕОЛОГО-ГЕОФИЗИЧЕСКИЕ ИССЛЕДОВАНИЯ ПОДВОДНОГО ВУЛКАНИЧЕСКОГО МАССИВА РАТМАНОВА (КУРИЛЬСКАЯ ОСТРОВНАЯ ДУГА)

The article presents the results of integrated geologic-geophysical investigation of the Ratmanov submarine volcanic massif, located in the Kurile Island Arc, 15 km southeast of the Chirinkotan island. There is a small submarine volcano on the northwestern slope of the massif with a relative height of 400–450 m, dredging of which raised andesites and a small number of andesite basalts. New data have been obtained on the mineral and chemical compositions and structural features of the massif rock formation. High values of the natural remnant magnetization of dredged rocks are caused by the high content of pseudo-single-domain of titanomagnetite grains. The foot of the massif is overlain by a volcanic-sedimentary sequence 400–800 m thick, and its formation most probably took place during periods of global geomagnetic disturbances. The maximum effective magnetization of the massif reaches 0.8 A / m. In the deep of the massif, supply channels, as well as peripheral magma chambers in the depth intervals of 3.8–4.6 km and 6–7.1 km, are identified.

High‐Temperature Observation of Transdomain Transitions in Vortex States in Intermediate Titanomagnetite

AbstractNatural materials contain small grains of magnetic iron oxides that can record information about the magnetic field of the Earth when they form and can be used to document changes in the geomagnetic field through time. Thermoremanent magnetization is the most stable type of remanent magnetization in igneous rocks and can be carried by particle sizes above the upper size limit for single‐domain behavior. To better understand thermoremanent magnetization in particles larger than single domain, we imaged the thermal dependence of magnetic structures in ~1.5‐μm grains of titanomagnetite (Fe2.46Ti0.54O4) using variable‐temperature magnetic force microscopy. At room temperature, grains displayed single‐vortex and multivortex states. Upon heating, the single‐vortex state was found to be stable up to the Curie temperature (~215 °C), whereas multivortex states unblocked between 125 and 200 °C by transitioning into single‐vortex states. During cooling in a weak field (~0.1 mT), single‐vortex states nucleated just below the Curie temperature and remained unchanged to room temperature. The single‐vortex state was the only magnetic state observed at room temperature after weak field thermoremanent magnetization acquisition experiments. These observations indicate that single‐vortex states occur in titanomagnetite and, like single‐domain particles, have high thermal stability necessary for carrying stable paleomagnetic remanence.

Domain structure of volcanics’ ferromagnetic grains: A case study of the Saatly ultradeep well, Azerbaijan

Various magnetic properties of magnetic minerals show strong grain-size depen-dence. This dependency occurs because these parameters are influenced by the magnetic grain-size (domain state) of the samples. Natural rocks often contain a mixture of different magnetic minerals with different grain-size (domain states). Thus determination of the domain state is important. The Saatly ultradeep well’s volcanics were used as a research material. In the depth intervals of 3540—8126 m the volcanics had been revealed by the borehole. According to data of the petrographic and microprobe analysis the volcanogenics of the ultradeep borehole is conditionally divided into a two parts: 1) the upper part (an interval of depths of 3540—5500 m); 2) the bottom part (an interval of depths of 5500—8126 m). In the upper part of section according to microprobe analysis of volcanogenic rocks ore grains are presented by two generations: 1) large grain of titanomagnetites (Ti-poor), homogeneous or with a decay structures; 2) small-sized usually well faced grain of practically stoichiometric magnetite. So, homogeneous small-sized grains or fragments of decay large grains (which are strongly changed chemically) can have properties of single-domain particles. Here the ferromagnetic grains (especially in the depth interval 3540—5000 m) are often noted as a pseudo-single domain. In the bottom part of the section the presence of multidomain grains of the Ti-poor titano-magnetite is noted. And since depths of 7000 m the oxidized magnetite is also observed. The pseudo-single-domain grains are fixed much less than in the upper section. And, since a depth mark of 7300 m and deeper, they are absent wholly. In the result of rese-arch it turned out that the ferromagnetic is presented by basically multidomain grains. It should be noted that Lauri—Fuller’s Criterion is not applicable at identification of a domain structure of the complex ensemble of the ferromagnetic grains.

Solid-Phase Reduction of Iron from Suroyam Titanomagnetite Ore during Metallization in Rotary Kiln

In this work the solid-phase reduction of iron from the Suroyam titanomagnetite ore was studied during metallization in a rotary kiln. The technique of preparation of the ore and reducing agent for metallization and the process of continuous processing of materials in a rotary kiln were described in detail. For metallization the temperature was chosen 1150°C, due to low melting point of apatite from one of the components. The results of the electron microscope analysis of the initial ore and samples subjected to metallization for 1-hour reduction time were presented. The reduction of iron occurred despite absence of pores and contact with a reducing agent in the grains of titanomagnetite. Iron in the grains of titanomagnetite surrounded by apatite was reduced to wustite; whereas, iron surrounded by clinopyroxene was reduced to metallic iron. This indicated the effect of composition of the gangue materials on the reduction process.

КОМПЛЕКСНЫЕ ГЕОЛОГО-ГЕОФИЗИЧЕСКИЕ ИССЛЕДОВАНИЯ ПОДВОДНОГО ВУЛКАНИЧЕСКОГО МАССИВА АРХАНГЕЛЬСКОГО (КУРИЛЬСКАЯ ОСТРОВНАЯ ДУГА)

The article presents the results of integrated geologic-geophysical investigation of the Arkhangelsky submarine volcanic massif, which is located in the Bussol Strait in the central zone of the Kurile Island Arc. The massif consists of amalgamated and eroded volcanic edifices and is built up of endogenous basalts and exogenous tuffs. Rocks and minerals to the exclusion of quartz are hydrothermally altered; moreover they were altered after the formation of tuff. High values of the natural remnant magnetization of dredged rocks are caused by the high content of single-domain and pseudo-single-domain grains of titanomagnetite and magnetite. The Arkhangelsky massif most likely was formed during periods of global geomagnetic disturbances. The authors have revealed feeders of the near-vertical and southeastern trends and causative magnetic blocks with different geometric and magnetic characteristics in the edifice of the massif. A peripheral magma chamber has been revealed between depths of 1100–1200 m. At the southeast foot of the massif we found large landslide blocks.

Low absolute paleointensity during Late Miocene Noma excursion of the Earth’s magnetic field

Paleointensity variation is investigated for a sequence of andesitic lavas which records the Noma geomagnetic excursion at 6.66 ± 0.45 Ma. Sixty-six samples are collected from 11 lava flows at the Kamegaoka Mountain (31° 21′N, 130° 13′E) on the Noma Peninsula, Kyushu Island, Japan. Rock magnetic experiments confirm the presence of Ti-poor titanomagnetite grains with pseudo-single-domain or a mixture of single and multi-domains as the carrier of the remanent magnetization. The application of the Tsunakawa-Shaw method yields reliable geomagnetic paleointensity data set of 29 samples from 9 lavas that cover the Noma excursion. The mean paleointensities of lava flows vary from 1.9 to 10.3 µT. Paleointensity variation reveals a ‘V -shape’ of high–low–high paleointensities during the excursion. The highest value of 10.3 ± 3.5 µT appears prior to the excursion, the paleointensity decreases down to 1.9 ± 0.3 µT as the excursion progresses, and the paleointensity finally regenerates to 5.1 ± 3.0 µT at the end of the excursion. The virtual dipole moment (VDM) during the Noma excursion (0.3–1.1 × 1022 Am2) is less than 16% of the mean VDM during the period between 4 and 8 Ma in Miocene, suggesting that this weak geomagnetic field intensity is ascribed to the nature of excursions rather than the Miocene geomagnetic field properties. The weakest VDM (0.3 × 1022 Am2) in the Noma excursion is detected at the pole position that is located at fairly high latitude (58.9°N) in the opposite hemisphere. The Noma excursion is characterized by a large shift in virtual geomagnetic poles of more than 140° from the geographic pole and by a weak VDM of 1.0 × 1022 Am2 or less.

Seafloor hydrothermal alteration affecting magnetic properties of abyssal basaltic rocks: insights from back-arc lavas of the Okinawa Trough

Seafloor hydrothermal systems in the back-arc region of the Okinawa Trough have been viewed as a modern analogue to the Kuroko-type volcanogenic massive sulfide deposits. Detection of magnetic signatures is widely utilized and assumed to facilitate the understanding of geological controls on hydrothermal system genesis. However, the magnetic properties of seafloor volcanic rocks are still poorly understood because of the difficulties of sample acquisition. Here, we report rock magnetic data along with linked geochemical and petrological data of volcanic rock samples obtained from the Irabu knolls of the southern Okinawa Trough. Both fresh and hydrothermally altered basaltic andesites were successfully obtained from the seafloor via submersible. A fresh sample, with single-domain titanomagnetite grains, is strongly magnetized with NRM intensity of up to 100 A/m. Minute skeletal and dendritic titanomagnetite grains are also observed. A second fresh sample, with multi-domain titanomagnetite grains, contains a greater amount of titanomagnetite grains, but exhibits NRM intensity ~ 10 A/m at most. In contrast to the fresh samples, hydrothermally altered samples show extremely low NRM intensities along with low saturation magnetization and certain contribution of paramagnetic minerals. Grain assemblages of pyrite and chalcopyrite grains appear along cracks in the groundmass. Our results indicated that fine titanomagnetite grains in groundmass within back-arc lava flows are altered due to hydrothermal processes. The recorded primary remanent magnetization of the lava flows is thus partly removed by hydrothermal alteration. Magnetization reduction related to hydrothermal activity produces local crustal magnetization lows and thus enables us to detect hydrothermal alteration zones by utilizing magnetic field measurements in space. In particular, the lavas we examined (via their resultant basaltic andesites) have high Curie temperatures greater than 400 °C, which is significantly higher than those indicated by mid-ocean ridge basalts, suggesting that the thermal effect for crustal magnetization may be less in back-arc settings.

Magnetic Vortex States in Small Octahedral Particles of Intermediate Titanomagnetite

AbstractFine particles of titanomagnetites (Fe3‐xTixO4, x &gt; 0.5) in the pseudo‐single‐domain (PSD) size (0.5–20 μm) are important carriers of natural remanent magnetization in basalts. Understanding the mechanism of magnetic recording in these grains has important implications for paleomagnetic studies. This study reports first observations of magnetic vortex states in intermediate titanomagnetite. We imaged magnetic structures of 109 synthetic titanomagnetite grains with x = 0.54 (TM54) and 1–4‐μm size using magnetic force microscopy. For six grains, we explored local energy minimum states after alternating field demagnetization and saturation isothermal remanent magnetization. According to the magnetic force microscopy results, 80% of TM54 grains display in‐plane magnetization with one to four domains, vortex‐like or flux‐closure structures, and Néel‐like domain walls. Electron backscatter diffraction data on six grains showed that their surface orientations are cutting planes of octahedral crystals and those with approximately square cross sections are within 15° of a (100) crystallographic plane. Magnetic force microscopy observations of magnetic structures in ~1.5‐μm grains agree well with numerical micromagnetic modeling of a pyramidal shaped grain with a (100) square base and displayed four discrete local energy minimum states: a single vortex as a ground state and three multivortex states with higher energy. Our observations show that vortex states in titanomagnetite grains (1–5 μm) occur at the lower end of the PSD size range in this mineral and corresponding to a size range known to carry stable and reliable remanence in natural titanomagnetites.

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.