![]() Dating to 4568.2 ☒2 Ma (Bouvier & Wadhwa 2010), they mark the official birth of the solar system and formed all about the same time, at most within 0.4 Ma of each other. The oldest constituents are, or seem to be, the CAIs. ![]() Commonly they include the decay products of several extinct, very short-lived radioactive elements such as iron-60 (written as 60Fe, a neutron-rich isotope of iron) and aluminium-26 ( 26Al), and these enable the determination of a remarkably precise chronology for the early solar system. The existence of CAIs and chondrules was not predicted by models of how the solar system originated (Connolly et al. That is, the chondrules accreted very quickly, before differences in their composition could be smoothed out. Since an originating nebula of dust and gas would be expected to have been homogeneous over medium-scale distances, one has to excuse the variability by supposing that, for example, ‘the separate classes of chondrules were derived from separate regions and that mixing subsequent to chondrule formation was not thorough’ (Taylor 2001). They must have come together – have accreted – from a variety of sources. Evidently they did not all form together in the rock where they are found. The variability of these constituents, not only from one chondrite to another but also within the same chondrite, is no trivial detail. The conditions in which the grains formed are unknown condensation from a high-temperature nebula is possible but conjectural. 2022), indicating that they had the same source. A matrix of mostly very fine (The diamonds have a solar-like isotopic composition the carborundum grains possibly suggest nucleosynthetic production in types of star more evolved than the Sun. Nano-sized grains of corundum (Al 2O 3), diamond, graphite carbon and carborundum (SiC).They are much less common than chondrules, ranging from 0 to 10% of the total. These formed in similarly high temperatures, if not higher, and also have very variable composition. Millimetre-sized inclusions rich in refractory calcium and aluminium, known as ‘CAIs’ (calcium-aluminium-rich inclusions).Their composition varies from silicate-rich to metal-sulfide-rich, and the proportion of chondrules to the total volume varies from 0% to an extraordinary 80%. Millimetre-sized melt droplets, or ‘chondrules’, particles that were flash-heated to 1,500–1,800 ☌, then cooled.
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