Magnetism on the Angrite Parent Body and the Early Differentiation of Planetesimals

Weiss, B. P.; Berdahl, James S.; Elkins-Tanton, L.; Stanley, Sabine; Lima, Eduardo A.; Carporzen, L.

doi:10.1126/science.1162459

Cited by 114 publications

(114 citation statements)

References 19 publications

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“…3) are in the range expected for core dynamos in early planetesimals (12) and other large bodies. Hf∕W chronometry indicates that metallic cores formed in planetesimals prior to the final assembly of chondrite parent bodies (33).…”

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confidence: 88%

“…Hf∕W chronometry indicates that metallic cores formed in planetesimals prior to the final assembly of chondrite parent bodies (33). Recent paleomagnetic analyses of angrites (12) indicate that dynamos were likely generated in convecting metallic cores lasting for ≥11 Ma after solar system formation. Because such bodies melt from the inside out, some may preserve an unmelted, relic chondritic surface that could be magnetized during metasomatism in the presence of a core dynamo.…”

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confidence: 99%

“…We thermally calibrated the latter paleointensities also using our measurements of TRM/ARM and TRM/IRM. Shown for comparison are the surface fields of the Earth, the solar wind field 1 astronomical unit (AU) from the Sun, the galactic field, the inferred paleofields of a T Tauri short-lived flare at 0.2 AU, and surface fields inferred for the angrite parent body (12).…”

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Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

Carporzen

Weiss

Elkins-Tanton

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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106

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The textures of chondritic meteorites demonstrate that they are not the products of planetary melting processes. This has long been interpreted as evidence that chondrite parent bodies never experienced large-scale melting. As a result, the paleomagnetism of the CV carbonaceous chondrite Allende, most of which was acquired after accretion of the parent body, has been a long-standing mystery. The possibility of a core dynamo like that known for achondrite parent bodies has been discounted because chondrite parent bodies are assumed to be undifferentiated. Resolution of this conundrum requires a determination of the age and timescale over which Allende acquired its magnetization. Here, we report that Allende's magnetization was acquired over several million years (Ma) during metasomatism on the parent planetesimal in a > ∼ 20 μT field up to approximately 9-10 Ma after solar system formation. This field was present too recently and directionally stable for too long to have been generated by the protoplanetary disk or young Sun. The field intensity is in the range expected for planetesimal core dynamos, suggesting that CV chondrites are derived from the outer, unmelted layer of a partially differentiated body with a convecting metallic core.differentiation | planetesimal | magnetic field | early solar system | paleointensity A llende is an accretionary breccia from near the surface of the CV parent planetesimal (1). Following accretion, Allende experienced minor aqueous alteration and moderate thermal metamorphism and metasomatism (2) but has remained essentially unshocked (<5 GPa) (3). Its major ferromagnetic minerals are pyrrhotite, magnetite, and awaruite, with an average pseudo single-domain crystal size (4-8). We conducted alternating-field (AF) and thermal demagnetization, rock magnetic, and paleointensity measurements on 71 mutually oriented bulk subsamples of Allende sample AMNH5056 (approximately 10-cm diameter and 8-mm thick slab surrounded by fusion crust). Of these, 51 subsamples were taken from the interior of the meteorite (>1 mm from fusion crust), whereas 20 contained some fusion crust.The differing magnetization directions of interior and fusioncrusted samples demonstrate that >95% of the natural remanent magnetization (NRM) in interior samples is preterrestrial (Figs. 1 and 2 and SI Appendix). AF demagnetization revealed that the interior samples have at least two components: a weak, low-coercivity, nonunidirectional component blocked up to 5 or 10 mT and a high-coercivity (HC) component blocked from approximately 10 to >290 mT (Fig. 1). In agreement with previous studies (4, 9, 10), the HC magnetization is unidirectionally oriented throughout the meteorite's interior (Fig. 2 and SI Appendix, Table S1). Thermal demagnetization (Figs. 1 and 2 and SI Appendix) indicates that interior samples have a low-temperature (LT) component blocked up to approximately 190°C, a dominant middle-temperature (MT) component blocked between approximately 190-300°C and oriented similarly to the HC component isolated by...

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Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

Carporzen

Weiss

Elkins-Tanton

et al. 2011

Proc. Natl. Acad. Sci. U.S.A.

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106

144

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“…Convection on these bodies is currently thought to have been thermally driven 7,8 , implying that magnetic activity would have been short-lived 9 . Here we present a time-series paleomagnetic record of the field recorded by the Imilac and Esquel pallasite meteorites, derived from nanomagnetic images 10 of their metallic matrices.…”

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confidence: 99%

Long-lived magnetism from solidification-driven convection on the pallasite parent body

Bryson

Nichols

Herrero‐Albillos

et al. 2015

Nature

151

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Paleomagnetic measurements of meteorites 1-5 suggest that, shortly after the birth of the solar system, the molten metallic cores of many small planetary bodies convected vigorously and were capable of generating magnetic fields 6 . Convection on these bodies is currently thought to have been thermally driven 7,8 , implying that magnetic activity would have been short-lived 9 . Here we present a time-series paleomagnetic record of the field recorded by the Imilac and Esquel pallasite meteorites, derived from nanomagnetic images 10 of their metallic matrices. The results reveal a history of long-lived magnetic activity on the pallasite parent body, capturing the decay and eventual shut down of the magnetic field. We demonstrate that magnetic activity driven by progressive solidification of an inner core 11-13 is consistent with our measured magnetic field characteristics and cooling rates 14 . Solidification-driven convection was likely common among small body cores 15 , and, in contrast to thermally driven convection, will have led to a relatively late (hundreds of millions of years after accretion), long-lasting, intense and widespread epoch of magnetic activity among these bodies in the early solar system.The pallasites are slowly cooled (2 -9 K Myr -1 ) 14 stony-iron meteorites 16 , which originated from the mid-to upper-mantle of a ~200-km-radius body 1 . The slow cooling rate of these meteorites allowed for characteristic microstructures to form in their metal matrix 17 , a key feature of which are regions of intergrown nanoscale islands of tetrataenite (ordered FeNi) 18,19 and an ordered Fe 3 Ni matrix 20 , collectively known as cloudy zones (CZ) 21 . During parent body cooling, these tetrataenite islands exsolved and subsequently coarsened over tens of millions of years 22 . The island diameter decreases systematically across the CZ, reflecting a decrease in the local formation age of the islands 20 . Each island adopted one of three orthogonal magnetic easy axes as it formed 18,23 , thus could display any one of six magnetisation directions. Variations in the intensity and direction of an external magnetic field led to measurable differences in the populations of each magnetisation direction 20 . Crucially, the temporal evolution of an external field is recorded by the variations in the relative proportions of these directions across the CZ 10 , which can be quantified using high-resolution nanomagnetic imaging. These images were captured for the Imilac and Esquel pallasites, utilising X-ray magnetic circular dichroism 24,25 at the X-ray photoemission electron microscope (XPEEM) 26 at the BESSY II synchrotron, Berlin, which provides the spatially resolved magnetisation of a sample surface with a resolution down to 40 nm over a 5 µm field-of-view 10 .Four and six non-overlapping, 450-nm-wide regions across the CZ (decreasing age) were extracted from the XPEEM images of Imilac and Esquel meteorites, respectively (Fig. 1). The field recorded by each region was deduced by comparing the experimental XPEEM sig...

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“…Palaeomagnetic analyses of angrites, which are among the oldest known basaltic meteorites, revealed remnant magnetisation that suggests a magnetic field of the parent body at the time of magnetisation of ∼10 microteslas between 4564 and at least 4558 Ma b.p. (Weiss et al 2008). These authors suggest that this palaeofield originated from an early internal dynamo in a rapidly formed iron-rich core.…”

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Differentiation and core formation in accreting planetesimals

Neumann

Breuer

Spohn

2012

A&A

110

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Aims. The compositions of meteorites and the morphologies of asteroid surfaces provide strong evidence that partial melting and differentiation were widespread among the planetesimals of the early solar system. However, it is not easily understood how planetesimals can be differentiated. To account for significantly smaller radii, masses, gravity and accretion energies early, intense heat sources are required, e.g. the short-lived nuclides 26 Al and 60 Fe. Here, we investigate the process of differentiation and core formation in accreting planetesimals taking into account the effects of sintering, melt heat transport via porous flow and redistribution of the radiogenic heat sources. Methods. We use a spherically symmetric one-dimensional model of a partially molten planetesimal consisting of iron and silicates, which considers the accretion by radial growth. The common heat conduction equation has been modified to consider also melt segregation. In the initial state, the planetesimals are assumed to be highly porous and consist of a mixture of Fe,Ni-FeS and silicates consistent to an H-chondritic composition. The porosity change due to the so called hot pressing is simulated by solving a corresponding differential equation. Magma segregation of iron and silicate melt is treated according to the flow in porous media theory by using the Darcy flow equation and allowing a maximal melt fraction of 50%. Results. We show that the differentiation in planetesimals depends strongly on the formation time, accretion duration, and accretion law and cannot be assumed as instantaneous. Iron melt segregation starts almost simultaneously with silicate segregation and lasts between 0.4 and 10 Ma. The degree of differentiation varies significantly and the most evolved structure consists of an iron core, a silicate mantle, which are covered by an undifferentiated but sintered layer and an undifferentiated and unsintered regolith -suggesting that chondrites and achondrites can originate from the same parent body.

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Magnetism on the Angrite Parent Body and the Early Differentiation of Planetesimals

Cited by 114 publications

References 19 publications

Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

Magnetic evidence for a partially differentiated carbonaceous chondrite parent body

Long-lived magnetism from solidification-driven convection on the pallasite parent body

Differentiation and core formation in accreting planetesimals

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