Striped domains of coarse-grained magnetite observed by X-ray photoemission electron microscopy as a source of the high remanence of granites in the Vredefort dome

Kubo, Hiroto; Nakamura, Norihiro; Kotsugi, Masato; Ohkochi, Takuo; Terada, Kentaro; Fukuda, Kohei

doi:10.3389/feart.2015.00031

Cited by 6 publications

(4 citation statements)

References 39 publications

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“…A natural lodestone sample (Nichika Inc., Japan) mainly consists of the mineral magnetite. A loadstone sample also contains maghemite and hematite, which have been transformed from magnetite through natural weathering (oxidation) processes (Kubo et al, 2015; Wasilewski & Kletetschka, 1999). We use the loadstone data as a reference for a mixture of magnetite, maghemite, and hematite, even though the actual mixing ratios of the three FeO x species in the sample were not analyzed.…”

Section: Resultsmentioning

confidence: 99%

Abundances and Microphysical Properties of Light‐Absorbing Iron Oxide and Black Carbon Aerosols Over East Asia and the Arctic

et al. 2020

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Black carbon (BC), brown carbon, and light‐absorbing iron oxides (FeOx) aerosols affect Earth's energy budget through their strong absorption of solar radiation. FeOx aerosols can also affect global biogeochemical cycles through their role as a nutrient for oceanic phytoplankton. However, observational data for these aerosols required for evaluating their effects using global models are scarce, especially for FeOx. Here, we summarize and compare the data sets of BC and FeOx from five ground‐based and three aircraft observation campaigns conducted in the East Asian and Arctic regions during the 2009–2018 period acquired using a modified single‐particle soot photometer. In these campaigns, >80% of FeOx‐containing aerosols in the 170–270 nm FeOx core size range had microphysical features indicating an anthropogenic origin. The particle size distribution for each of the BC and FeOx was similar in all of the data sets except for those dominated by fresh urban pollution or pristine Arctic air. The campaign‐averaged mass concentrations of FeOx and BC were ~60–360 ng/m3 and ~240–1,300 ng/m3, respectively, in East Asia, and ~6 ng/m3 and ~20–30 ng/m3, respectively, in the Arctic. The campaign‐averaged FeOx/BC mass concentration ratio varied within a narrow range of 0.2–0.6 in both East Asian and Arctic regions. In every campaign, FeOx, BC, and carbon monoxide were tightly correlated with each other with similar slope to the urban campaigns (around Tokyo), implying the spatiotemporal variation of anthropogenic FeOx emission around northern middle‐to‐high latitudes is similar to those of anthropogenic BC and CO emissions.

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Section: Resultsmentioning

confidence: 99%

Abundances and Microphysical Properties of Light‐Absorbing Iron Oxide and Black Carbon Aerosols Over East Asia and the Arctic

et al. 2020

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“…Although the remanence of multi‐domain and pseudo single‐domain grains of unpinned or loosely pinned domain walls can be removed through the low‐temperature demagnetization, the remanence strongly pinned domain walls are excluded [ Özdemir and Dunlop , ]. The pinning may explain the remanence observed in multi‐domain and single‐domain‐like remanence in pseudo single‐domain grains [e.g., Kubo et al ., ]. Our estimated unblocking temperature of 145°C translates to an age of 14 kyr B.P.…”

Section: Resultsmentioning

confidence: 99%

Stretched exponential relaxation of viscous remanence and magnetic dating of erratic boulders

et al. 2016

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Viscous remanence continuously increases with the duration of reorientation of rocks, and the remanence gets partially overprinted in rocks parallel to the Earth's magnetic field. This overprinted viscous remanence is unblocked at a certain temperature that enables the estimation of the time required for the rock to acquire the magnetism, by assuming the exponential law of Néel's single‐domain theory. However, previous results of dating the rocks by the exponential law have shown older ages than radiometric or cosmogenic exposure ages. Néel's exponential decay law is applicable to a system whose magnetic grains have an identical relaxation time. However, in real systems, the expected behavior is not usually observed because relaxation times vary for individual grains. Moreover, the variation of viscous remanence with the logarithmic law for a distribution of relaxation times is predicted to be concave downward. Here we found that the stretched exponential law, exp{−(t/τ)1 − n} with 0 ≤ n < 1, explains previously published data on viscous decay. The observed stretched exponential relaxation can be interpreted in terms of the global relaxation of a system containing many relaxing species, each of which decays exponentially in time with a specific fixed relaxation rate. Using this law, we derived an extended time‐temperature relationship of magnetite involving the Néel's exponential decay law with n = 0 and a system containing many relaxing times with variable n. The extended time‐temperature relationship shows that the age of a coral tsunami boulder with high anomalous unblocking temperatures can be fitted with an assigned radiometric age.

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“…Kubo et al. () used X‐ray magnetic circular dichroism analysis and X‐ray absorption spectroscopy to study the characteristics of coarse‐grained high‐remanence magnetite from the shocked Vredefort basement rocks. Based on their results the authors attributed the high remanent magnetism to postimpact hydrothermal activity but conceded that terrestrial lightning strikes could have intensified the remanence.…”

Section: Geomagnetism Of the Vredefort Impact Structurementioning

confidence: 99%

“…Salminen et al (2013) followed these results by conducting laboratory experiments on rock samples subjected to lightning strikes and concluded that lightning strikes could explain the observed high Q values. Kubo et al (2015) used X-ray magnetic circular dichroism analysis and X-ray absorption spectroscopy to study the characteristics of coarse-grained high-remanence magnetite from the shocked Vredefort basement rocks. Based on their results the authors attributed the high remanent magnetism to postimpact hydrothermal activity but conceded that terrestrial lightning strikes could have intensified the remanence.…”

Section: Magnetization Of the Archean Rocks In The Core Of The Structurementioning

confidence: 99%

Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

Fourie

Huber

Kovaleva

2019

Meteorit & Planetary Scien

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Granophyre dykes in the central part of the Vredefort impact structure are believed to be the remnants of the impact melt sheet, which intruded downwards along the fractures in the crater floor. Little is known about their original penetration depth, dip, structural relationships with the host rocks, and their general geophysical characteristics. This information is critical to understand the emplacement history of the granophyre dykes, as it relates to the formation and modification of large impact structures. We conducted magnetic and resistivity surveys across the Daskop granophyre dyke (DGD), one of the impact melt dykes in the structure's core. The magnetic survey revealed that the DGD gives a strong magnetic response at positions where the dyke outcrop exceeds the surface topography, but a very weak response where the outcrop is nearly at the same elevation as the surrounding topography. The magnetic anomaly is thus predominantly due to the outcrop protruding above ground level, suggesting a limited volume of dyke material in the subsurface and a small penetration depth. The resistivity survey performed on two profiles, set perpendicularly across the DGD, indicated a shallow penetration depth (<3 m), consistent with the magnetic interpretation. Thus, our geophysical study demonstrates that the DGD is currently at the very bottom of its original emplacement. This may either be an erosional coincidence, or it may be controlled by a fundamental process of impact cratering. Further studies are warranted to determine if other granophyre dykes at Vredefort are similarly at their lowermost terminations.

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Striped domains of coarse-grained magnetite observed by X-ray photoemission electron microscopy as a source of the high remanence of granites in the Vredefort dome

Cited by 6 publications

References 39 publications

Abundances and Microphysical Properties of Light‐Absorbing Iron Oxide and Black Carbon Aerosols Over East Asia and the Arctic

Abundances and Microphysical Properties of Light‐Absorbing Iron Oxide and Black Carbon Aerosols Over East Asia and the Arctic

Stretched exponential relaxation of viscous remanence and magnetic dating of erratic boulders

Geophysical characterization of the Daskop granophyre dyke and surrounding host rocks, Vredefort impact structure, South Africa

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