Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Cloete, M.; Hart, R. J.; Schmid, H.; Drury, M. R.; Demanet, Chris M.; Sankar, K. Vijaya

doi:10.1007/s004100050548

Cited by 34 publications

(20 citation statements)

References 15 publications

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“…Petrographic observations including electron and magnetic force microscopy revealed additionally to the large magnetite grains <5 μm size magnetite particles with high aspect ratios (15:1) along PDFs of quartz. This magnetite crystallized from locally formed micromelts that intruded along zones of weakness such as microfractures and PDFs shortly after the shock event (Cloete et al 1999). High remanences in the Vredefort basement is related to high frequencies of microdeformation phenomena, in particular PDFs in quartz.…”

Section: Magnetization Mechanism For Pyrrhotitementioning

confidence: 99%

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Kontny

Elbra

Just

et al. 2007

Meteorit & Planetary Scien

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Abstract-Rock magnetic and magnetic mineralogy data are presented from the International Continental Scientific Drilling Program (ICDP) drill cores LB-07A and LB-08A of the Bosumtwi impact structure in order to understand the magnetic behavior of impact and target lithologies and their impact-related remagnetization mechanism. Basic data for the interpretation of the magnetic anomaly patterns and the magnetic borehole measurements as well as for new magnetic modeling are provided. Magnetic susceptibility (150-500 × 10 −6 SI) and natural remanent magnetization (10 −3 -10 −1 A/m) are generally weak, but locally higher values up to 10.6 × 10 −3 SI and 43 A/m occur. Sixty-three percent of the investigated rock specimens show Q values above 1 indicating that remanence clearly dominates over induced magnetization, which is a typical feature of impact structures. Ferrimagnetic pyrrhotite is the main magnetite phase, which occurs besides minor magnetite and a magnetic phase with a Curie temperature between 330 and 350 °C, interpreted as anomalous pyrrhotite. Coercive forces are between 20 and 40 mT. Brecciation and fracturing of pyrrhotite is a common feature confirming its pre-impact origin. Grain sizes of pyrrhotite show a large variation but the numerous stress-induced nanostructures observable by transmission electron microscopy (TEM) are assumed to behave as single-domain grains. We suggest that the drilled rocks lost their pre-shock remanence memory during the shock event and acquired a new, stable remanence during shock-induced grain size reduction. The observed brittle microstructures indicate temperatures not higher than 250 °C, which is below the Curie temperature of ferrimagnetic pyrrhotite (310 °C).

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Section: Magnetization Mechanism For Pyrrhotitementioning

confidence: 99%

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Kontny

Elbra

Just

et al. 2007

Meteorit & Planetary Scien

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“…Magnetic force microscopy (MFM) permits imaging of the magnetic structure of materials, using much of the same equipment as AFM (Cloete et al, 1998(Cloete et al, , 1999. In this technique, the AFM tip interacts with the stray magnetic field emanating from the sample.…”

Section: Physical Propertiesmentioning

confidence: 99%

Small is beautiful: The analysis of nanogram‐sized astromaterials

Zolensky

Pieters

Clark

et al. 2000

Meteorit & Planetary Scien

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Abstract-The capability of modem methods to characterize ultra-small samples is well established from analysis of interplanetary dust particles (IDPs), interstellar grains recovered from meteorites, and other materials requiring ultra-sensitive analytical capabilities. Powerful analytical techniques are available that require, under favorable circumstances, single particles of only a few nanograms for entire suites of fairly comprehensive characterizations. A returned sample of >lo00 particles with total mass of just 1 pg permits comprehensive quantitative geochemical measurements that are impractical to carry out in situ by flight instruments. The main goal of this paper is to describe the state-of-the-art in microanalysis of astrornaterials.Given that we can analyze fantastically small quantities of asteroids and comets, etc., we have to ask ourselves, how representative are microscopic samples of bodies that measure a few to many kilometers across? With the Galileo flybys of Gaspra and Ida, it is now recognized that even very small airless bodies have indeed developed a particulate regolith. Acquiring a sample of the bulk regolith, a simple sampling strategy, provides two critical pieces of information about the body. Regolith samples are excellent bulk samples because they normally contain all the key components of the local environment, albeit in particulate form, Furthermore, because this fine fraction dominates remote measurements, regolith samples also provide information about surface alteration processes and are a key link to remote sensing of other bodies. Studies indicate that a statistically significant number of nanogram-sized particles should be able to characterize the regolith of a primitive asteroid, although the presence of larger components (e.g., chondrules, calciumaluminum-rich inclusions, large crystal fragments, etc.) within even primitive meteorites (e.g., Murchison)points out the limitations of using data obtained from nanogram-sized samples to characterize entire primitive asteroids. However, the most important asteroidal geological processes have lefi their mark on the matrix, because this is the finest-grained portion and therefore most sensitive to chemical and physical changes. Thus, the following information can be learned from this fine grain size fraction alone: (1) mineral paragenesis; (2) regolith processes; (3) bulk composition; (4) conditions of thermal and aqueous alteration (if any); (5) relationships to planets, comets, meteorites (via isotopic analyses, including 0); ( 6 ) abundance of water and hydrated material; (7) abundance of organics; (8) history of volatile mobility; (9) presence and origin of presolar and/or interstellar material. Most of this information can be obtained even from dust samples from bodies for which nanogram-sized samples are not truly representative. Future advances in sensitivity and accuracy of laboratory analytical techniques can be expected to enhance the science value of nano-to microgram-sized samples even further. This highlights a key adv...

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“…Shocked Archean granitic rocks in the crystalline core of the Vredefort crater exhibit anomalously strong natural remanent magnetization (NRM), but with random centimeterscale orientation (Hart et al, 1995;Carporzen et al, 2005;Salminen et al, 2009). Previous researchers have hypothesized that the origin of these unique magnetic remanence properties is associated with very intense impact-generated small-wavelength magnetic fields that not only produce strong remanence, but also randomize the directions of the remanence (e.g., Hart et al, 1995;Cloete et al, 1999;Carporzen et al, 2005). This hypothesis has been rejected based on the observation of terrestrial lightning remagnetization in studies that utilized 10-m borehole paleomagnetic measurements (Carporzen et al, 2012) and artificial lightning experiments (Salminen et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

“…Vredefort granitic rocks contain magnetite with two different grain sizes. The first size is often on the order of microns to millimeters and grains of this size are formed at about 3.0 Ga (e.g., Cloete et al, 1999;Nakamura et al, 2010). The second size is less than 1 µm and grains of such size are formed within the interstices of the planar deformation features (PDFs) of quartz formed by impacts of 2.02 Ga (e.g., Cloete et al, 1999).…”

Section: Introductionmentioning

confidence: 99%

“…The first size is often on the order of microns to millimeters and grains of this size are formed at about 3.0 Ga (e.g., Cloete et al, 1999;Nakamura et al, 2010). The second size is less than 1 µm and grains of such size are formed within the interstices of the planar deformation features (PDFs) of quartz formed by impacts of 2.02 Ga (e.g., Cloete et al, 1999). Although it is generally believed that coarse-grained magnetites contain soft secondary components that are of "little geological interest" (Butler, 1992;Nakamura et al, 2010) showed that such magnetites embedded in biotite produce highly coercive and strong remanence, as determined by surface magnetic imaging using a scanning magneto-impedance magnetic microscope ( Figure 1A).…”

Section: Introductionmentioning

confidence: 99%

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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

et al. 2015

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The characteristics of a coarse-grained high-remanence magnetite obtained from shocked Vredefort granite were investigated by X-ray magnetic circular dichroism (XMCD) analysis and X-ray absorption spectroscopy (XAS). The study utilized a spectroscopic photoelectron low-energy electron emission microscope (SPELEEM) and was conducted in the SPring-8 large-synchrotron radiation facility. It is generally believed that the strong and stable bulk remanence of Vredefort granites is due to the presence of minerals that have been strongly magnetized by either an impact-generated magnetic field or terrestrial lightning strikes. Although coarse-grained magnetite is traditionally characterized by weak coercivity and remanence, the specimen used in the present study exhibited high coercivity and an intense remanent magnetization. The presence of hematite lamellae observed on the partially oxidized magnetite specimen indicated an array of striped domains, intensifying a remanence and coercivity. We also conducted XAS and XMCD analyses on a natural lodestone permanent magnet produced by lightning strikes; while maghemite was found to be present, no magnetic domain structures were observed. Considering that the nucleation of hematite lamellae on magnetite/maghemite grains is due to oxidation, we attribute the intense remanent magnetization and magnetic hardening of Vredefort granites to post-impact hydrothermal activity.

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Characterization of magnetite particles in shocked quartz by means of electron- and magnetic force microscopy: Vredefort, South Africa

Cited by 34 publications

References 15 publications

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Petrography and shock‐related remagnetization of pyrrhotite in drill cores from the Bosumtwi Impact Crater Drilling Project, Ghana

Small is beautiful: The analysis of nanogram‐sized astromaterials

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

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