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3 - Thermal Histories of Chondrules
- from Part I - Observations of Chondrules
- Edited by Sara S. Russell, Natural History Museum, London, Harold C. Connolly Jr., Rowan University, New Jersey, Alexander N. Krot, University of Hawaii, Manoa
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- Book:
- Chondrules
- Published online:
- 30 June 2018
- Print publication:
- 19 July 2018, pp 57-90
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Summary
Thermal histories of chondrules can be deduced by studying the petrology and mineral chemistry of natural chondrules and their experimental analogs. Dynamic crystallization experiments have successfully reproduced chondrule textures, and provide general but broad constraints on peak temperatures and cooling rates. Porphyritic textures result when a chondrule is heated to a maximum temperature close to, but below, its liquidus, and cooled at initial rates between about 10 and 1,000 °C/h. Typical liquidus temperatures for chondrules range from about 1,400–1,700 °C. Nonporphyritic chondrules are produced when peak temperatures exceed the liquidus slightly (for barred/dendritic textures) and significantly (radiating textures) and chondrules cool at rates around 500–3,000 °C/h. More quantitative constraints on cooling rates can be determined by considering growth and diffusion-related zoning in chondrule minerals. Results of such modeling are consistent with dynamic crystallization experiments. Rapid dissolution rates for relict olivine grains also indicate a limited time at high temperatures, and indicate fast cooling rates of hundreds to thousands of °C/h, close to peak temperatures. Other cooling rate indicators include disequilibrium partition coefficients between minerals and chondrule glass, and consideration of chemical and isotopic diffusion between relict grains and their overgrowths. Interpretation of both these features is currently ambiguous. Several lines of evidence suggest that cooling rates decreased at lower temperatures, as the chondrule approached the solidus, to <50 °C/h. These include slow cooling required to nucleate plagioclase, cooling rates inferred from trace element diffusion profiles in metal grains, and exsolution microstructures in clinopyroxene. In contrast, clinoenstatite microstructures, the presence of chondrule glass, and dislocation densities in chondrule olivine appear to argue for rapid cooling (103–104 °C/h) through the lower temperature regime, and textures in opaque (metal/sulfide) assemblages indicate cooling rates of hundreds of degrees per hour at subsolidus temperatures. Overall, thermal histories of chondrules can provide fundamental constraints for chondrule formation models. While high-temperature thermal histories are reasonably well constrained, there are currently some open questions about the nature of the cooling curve at lower temperatures. A better understanding of chondrule cooling rates at lower temperatures would help to discriminate between chondrule formation models that make quantitative predictions for thermal histories. Within a single chondrite, cooling rates may vary widely. It is also possible that the nature of cooling histories varies within a given population of chondrules. A statistical treatment of chondrule populations in which individual chondrules show distinct thermal histories would help to make predictions about chondrule formation environments, and the diversity of processes that might be represented in a single chondrule-forming region.
2 - Multiple Mechanisms of Transient Heating Events in the Protoplanetary Disk
- from Part I - Observations of Chondrules
- Edited by Sara S. Russell, Natural History Museum, London, Harold C. Connolly Jr., Rowan University, New Jersey, Alexander N. Krot, University of Hawaii, Manoa
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- Book:
- Chondrules
- Published online:
- 30 June 2018
- Print publication:
- 19 July 2018, pp 11-56
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Summary
In this chapter, we summarize our current knowledge of the mineralogy, petrography, oxygen-isotope compositions, and trace element abundances of precursors of chondrules and igneous Ca,Al-rich inclusions (CAIs), which provide important constraints on the mechanisms of transient heating events in the protoplanetary disk. We infer that porphyritic chondrules, the dominant textural type of chondrules in most chondrite groups, largely formed by incomplete melting of isotopically diverse solid precursors, including refractory inclusions (CAIs and amoeboid olivine aggregates (AOAs)), fragments of chondrules from earlier generations, and fine-grained matrix-like material during highly-localized transient heating events in dust-rich disk regions characterized by 16O-poor average compositions of dust (Δ17O ~ ‒5‰ to +3‰). These observations preclude formation of the majority of porphyritic chondrules by splashing of differentiated planetesimals; instead, they are consistent with melting of dustballs during localized transient heating events, such as bow shocks and magnetized turbulence in the protoplanetary disk, and, possibly, by collisions between chondritic planetesimals. Like porphyritic chondrules, igneous CAIs formed by incomplete melting of isotopically diverse solid precursors during localized transient heating events. These precursors, however, consisted exclusively of refractory inclusions, and the melting occurred in an 16O-rich gas (Δ17O ~ ‒24‰) of approximately solar composition, most likely near the protosun. The U-corrected Pb–Pb absolute and Al–Mg relative chronologies of igneous CAIs in CV chondrites indicate that these melting events started contemporaneously with condensation of CAI precursors (4567.3 ± 0.16 Ma) and lasted up to 0.3 Ma, providing evidence for the earliest transient heating events capable of melting refractory dustballs in the innermost part of the disk. There is no evidence that chondrules were among the precursors of igneous CAIs, which is consistent with an age gap between CAIs and chondrules. In contrast to typical (non–metal-rich) chondrites, the CB metal-rich carbonaceous chondrites contain exclusively magnesian nonporphyritic chondrules formed during a single-stage event ~5 Ma after CV CAIs, most likely in an impact-generated gas–melt plume. Bulk chemical compositions of CB chondrules and equilibrium thermodynamic calculations suggest that at least one of the colliding bodies was differentiated. The uniformly 16O-depleted igneous CAIs in CB chondrites most likely formed by complete melting of preexisting refractory inclusions that was accompanied by gas–melt interaction in the plume. CH metal-rich carbonaceous chondrites represent a mixture of the CB-like materials (magnesian skeletal olivine and cryptocrystalline chondrules and uniformly 16O-depleted igneous CAIs) formed in an impact plume and the typical chondritic materials (magnesian, ferroan, and Al-rich porphyritic chondrules, uniformly 16O-rich CAIs, and chondritic lithic clasts) that appear to have largely predated the impact plume event. We conclude that there are multiple mechanisms of transient heating events that operated in the protoplanetary disk during its entire lifetime and resulted in formation of chondrules and igneous CAIs.
6 - Vapor–Melt Exchange
- from Part I - Observations of Chondrules
- Edited by Sara S. Russell, Natural History Museum, London, Harold C. Connolly Jr., Rowan University, New Jersey, Alexander N. Krot, University of Hawaii, Manoa
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- Book:
- Chondrules
- Published online:
- 30 June 2018
- Print publication:
- 19 July 2018, pp 151-174
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The bulk volatile contents of chondritic meteorites provide clues to their origins. Matrix and chondrules carry differing abundances of moderately volatile elements, with chondrules carrying a refractory signature. At the high temperatures of chondrule formation and the low pressures of the solar nebula, many elements, including Na and Fe, should have been volatile. Yet the evidence is that even at peak temperatures, at or near the liquidus, Na and Fe (as FeO and Fe-metal) were present in about their current abundances in molten chondrules. This seems to require very high solid densities during chondrule formation to prevent significant evaporation. Evaporation should also be accompanied by isotopic mass fractionation. Evidence from a wide range of isotopic systems indicates only slight isotopic mass fractionations of moderately volatile elements, further supporting high solid densities. However, olivine-rich, FeO-poor chondrules commonly have pyroxene-dominated outer zones that have been interpreted as the products of late condensation of SiO2 into chondrule melts. Late condensation of more refractory SiO2 is inconsistent with the apparent abundances of more volatile Na, FeO and Fe-metal in many chondrules. Despite significant recent experimental work bearing on this problem, the conditions under which chondrules behaved as open systems remain enigmatic.
Temperature shocks at the origin of regolith on asteroids
- Patrick Michel, Marco Delbo, Guy Libourel, Clément Ganino, Chryst'le Verati, Benjamin Rémy
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- Journal:
- Proceedings of the International Astronomical Union / Volume 10 / Issue H16 / August 2012
- Published online by Cambridge University Press:
- 05 March 2015, p. 162
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- August 2012
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Space-based and remote sensing observations reveal that regolith – a layer of loose unconsolidated material – is present on all asteroids, including very small, subkm-sized near- Earth asteroids (NEAs) such as (25143) Itokawa. Classically, regolith is believed to be produced by the ejecta of impact craters produced by small particles hitting asteroid surfaces. Such an explanation works for bodies whose gravity field is strong enough for substantial reaccretion of impact debris, but it fails to account for the ubiquitous presence of regolith also on small asteroids with weaker gravity. Several works have proposed that the thermal fatigue due to a huge number of day/night temperature cycles is a process that contributes to the formation of regolith on the Moon, Mercury, and on the NEA (433) Eros by fracturing boulders and rocks on their surfaces. However, this process lacks a demonstration: in order to study under which conditions rock cracking on NEAs occurs, we calculated typical temperature cycles for NEAs and we performed laboratory experiments of similar thermal cycling on meteorites taken as analogue of asteroid surface material. We will present results of these experiments and discuss their implications regarding regolith formation on asteroids.