Pallasite paleomagnetism: Quiescence of a core dynamo

Nichols, Claire; Bryson, Joanna J.; Herrero‐Albillos, Julia; Kronast, Florian; Nimmo, F.; Harrison, R. J.

doi:10.1016/j.epsl.2016.02.037

Cited by 32 publications

(77 citation statements)

References 38 publications

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“…• Mesosiderites (cooling rate ≲ 0.5-3.3 • C Myr −1 ): Cloudy zone is multidomain and does not acquire a stable paleomagnetic remanence, possibly due to large island size or the composition and mineralogy of the matrix. • Pallasites and IAB iron meteorites (cooling rate ∼ 1-100 • C Myr −1 ): Cloudy zone is a reliable paleomagnetic recorder (Bryson et al, 2015;Nichols et al, 2016Nichols et al, , 2018, and remanence is acquired during tetrataenite formation. Ordering occurs simultaneously across the cloudy zone, and therefore no time-resolved paleomagnetic signals are preserved.…”

Section: Discussionmentioning

confidence: 99%

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

Nichols¹,

Bryson

Blukis

et al. 2020

Geochem Geophys Geosyst

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Iron and stony‐iron meteorites form the Widmanstätten pattern during slow cooling. This pattern is composed of several microstructures whose length‐scale, composition and magnetic properties are dependent upon cooling rate. Here we focus on the cloudy zone: a region containing nanoscale tetrataenite islands with exceptional paleomagnetic recording properties. We present a systematic review of how cloudy zone properties vary with cooling rate and proximity to the adjacent tetrataenite rim. X‐ray photoemission electron microscopy is used to compare compositional and magnetization maps of the cloudy zone in the mesosiderites (slow cooling rates), the IAB iron meteorites and the pallasites (intermediate cooling rates), and the IVA iron meteorites (fast cooling rates). The proportions of magnetic phases within the cloudy zone are also characterized using Mössbauer spectroscopy. We present the first observations of the magnetic state of the cloudy zone in the mesosiderites, showing that, for such slow cooling rates, tetrataenite islands grow larger than the multidomain threshold, creating large‐scale regions of uniform magnetization across the cloudy zone that render it unsuitable for paleomagnetic analysis. For the most rapidly cooled IVA meteorites, the time available for Fe‐Ni ordering is insufficient to allow tetrataenite formation, again leading to behavior that is unsuitable for paleomagnetic analysis. The most reliable paleomagnetic remanence is recorded by meteorites with intermediate cooling rates ( ∼ 2–500 °C Myr −1) which produces islands that are “just right” in both size and degree of Fe‐Ni order.

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Section: Discussionmentioning

confidence: 99%

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

Nichols¹,

Bryson

Blukis

et al. 2020

Geochem Geophys Geosyst

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“…Nichols et al (2016) used XPEEM to study the Brenham and Marjalahti pallasites (~123nm and ~118-nm islands, respectively;Yang et al, 2010). The authors analyzed twelve and nine 4500 × 450-nm regions resulting in 1800 islands and 1480 for Brenham and Marjalahti, respectively.…”

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confidence: 99%

Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

Maurel

Weiss

Bryson

2019

Earth and Planetary Science Letters

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Metallic microstructures in slowly-cooled iron-rich meteorites reflect the thermal and magnetic histories of their parent planetesimals. Of particular interest is the cloudy zone, a nanoscale intergrowth of Ni-rich islands within a Ni-poor matrix that forms below ~350°C by spinodal decomposition. The sizes of the islands have long been recognized as reflecting the lowtemperature cooling rates of meteorite parent bodies. However, a model capable of providing quantitative cooling rate estimates from island sizes has been lacking. Moreover, these islands are also capable of preserving a record of the ambient magnetic field as they grew, but some of the key physical parameters required for recovering reliable paleointensity estimates from magnetic measurements of these islands have been poorly constrained. To address both of these issues, we present a numerical model of the structural and compositional evolution of the cloudy zone as a function of cooling rate and local composition. Our model produces island sizes that are consistent with present-day measured sizes. This model enables a substantial improvement in the calibration of paleointensity estimates and associated uncertainties. In particular, we can now accurately quantify the statistical uncertainty associated with the finite number of islands acquiring the magnetization and the uncertainty on their size at the time of the record. We use this new understanding to revisit paleointensities from previous pioneering paleomagnetic studies of cloudy zones. We show that these could have been overestimated by up to one order of magnitude but nevertheless still require substantial magnetic fields to have been present on their parent bodies.Our model also allows us to estimate absolute cooling rates for meteorites that cooled slower than < 10,000°C My -1 . We demonstrate how these cooling rate estimates can uniquely constrain the low-temperature thermal history of meteorite parent bodies. Using the main-group pallasites as an example, we show that our results are consistent with the previously-proposed unperturbed, conductive cooling at low temperature of a ~200-km radius main-group pallasite parent body.

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“…; Nichols et al. ) and electron holography (Bryson et al. ) imaging studies of the CZ, which all point toward the presence of soft ferromagnetic matrix.…”

Section: Discussionmentioning

confidence: 99%

“…; Nichols et al. ). Meteoritic metal alloys have traditionally been assumed to be rather poor paleomagnetic recorders.…”

Section: Introductionmentioning

confidence: 99%

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A high spatial resolution synchrotron Mössbauer study of the Tazewell IIICD and Esquel pallasite meteorites

Blukis

Rüffer

Chumakov

et al. 2017

Meteorit & Planetary Scien

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Metallic phases in the Tazewell IIICD iron and Esquel pallasite meteorites were examined using 57Fe synchrotron Mössbauer spectroscopy. Spatial resolution of ~10–20 μm was achieved, together with high throughput, enabling individual spectra to be recorded in less than 1 h. Spectra were recorded every 5–10 μm, allowing phase fractions and hyperfine parameters to be traced along transects of key microstructural features. The main focus of the study was the transitional region between kamacite and plessite, known as the “cloudy zone.” Results confirm the presence of tetrataenite and antitaenite in the cloudy zone as its only components. However, both phases were also found in plessite, indicating that antitaenite is not restricted exclusively to the cloudy zone, as previously thought. The confirmation of paramagnetic antitaenite as the matrix phase of the cloudy zone contrasts with recent observations of a ferromagnetic matrix phase using X‐ray photoemission electron spectroscopy. Possible explanations for the different results seen using these techniques are proposed.

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Pallasite paleomagnetism: Quiescence of a core dynamo

Cited by 32 publications

References 38 publications

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

Variations in the Magnetic Properties of Meteoritic Cloudy Zone

Meteorite cloudy zone formation as a quantitative indicator of paleomagnetic field intensities and cooling rates on planetesimals

A high spatial resolution synchrotron Mössbauer study of the Tazewell IIICD and Esquel pallasite meteorites

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