Chondrite - Origin and History

Origin and History

The parent bodies of chondrites are (or were) small to medium sized asteroids that were never part of any body large enough to undergo melting and planetary differentiation. These bodies accreted shortly after the beginning of the Solar System's history, with 206Pb/204Pb age of 4566.6 ± 1.0 Ma years ago and matching ages for other chronometers. Although chondritic asteroids never became hot enough to melt based upon internal textures, many of them reached high enough temperatures that they experienced significant thermal metamorphism in their interiors. The source of the heat was most likely energy coming from the decay of short-lived radioisotopes (half-lives less than a few million years) that were present in the newly formed solar system, especially 26Al and 60Fe, although heating may have been caused by impacts onto the asteroids as well. Many chondritic asteroids also contained significant amounts of water, possibly due to the accretion of ice along with rocky material. As a result, many chondrites contain hydrous minerals, such as clays, that formed when the water interacted with the rock on the asteroid in a process known as aqueous alteration. In addition, all chondritic asteroids were affected by impact and shock processes due to collisions with other asteroids. These events caused a variety of effects, ranging from simple compaction to brecciation, veining, localized melting, and formation of high-pressure minerals. The net result of these secondary thermal, aqueous, and shock processes is that only a few known chondrites preserve in pristine form the original dust, chondrules, and inclusions from which they formed.