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9 - 26Al–26Mg Systematics 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 247-275
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Summary
The 26Al–26Mg systematics of chondrules from ordinary and carbonaceous chondrites and their implications are reviewed. The initial 26Al/27Al ratios [(26Al/27Al)0] based on in situ analyses of chondrules from the least metamorphosed chondrites range from unresolved from zero to ~1.2 × 10‒5 and thus no chondrules have ratios corresponding to the canonical level (~5.2 × 10‒5) recorded by CAIs. Assuming homogeneous distribution of 26Al in the protoplanetary disk at the canonical level, these observations suggest chondrule formation started ~1.5 million years after CAIs and lasted over a few million years. The 26Al–26Mg systematics of bulk chondrules could have recorded ratios of chondrule precursors and may suggest that Al–Mg fractionation recorded by chondrule precursors started contemporaneously with CAIs and lasted over ~1.5 million years. The comparisons of formation ages of different meteorites and their components have been made with 26Al–26Mg, 182Hf–182W, and 206Pb–207Pb systematics. While the ages determined by 26Al–26Mg and 182Hf–182W systematics are generally consistent, those determined by 26Al–26Mg and 206Pb–207Pb systematics are largely inconsistent. The homogeneous versus heterogeneous distribution of 26Al in the protoplanetary disk remains controversial.
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
-
- Book:
- Chondrules
- Published online:
- 30 June 2018
- Print publication:
- 19 July 2018, pp 11-56
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- Chapter
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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.