Calcium–aluminium-rich inclusion
Open-knowledge reference entry
A calcium–aluminium-rich inclusion or Ca–Al-rich inclusion (CAI) is a submillimeter- to centimeter-sized light-colored calcium- and aluminium-rich inclusion found in carbonaceous chondrite meteorites. The first high-precision radiometric datings of CAIs involved four samples examined through the Pb–Pb chronometer, yielding a weighted mean age of 4567.30 ± 0.16 Ma. Subsequent studies including additional samples suggest a slightly older age of 4568.3 ± 0.7 Ma to rectify inconsistencies regarding Hf–W and Al–Mg chronometry methods. As CAIs are the oldest dated solids, this age is commonly used to define the age of the Solar System.
Description
CAIs consist of minerals that are among the first solids condensed from the cooling protoplanetary disk. They are thought to have formed as fine-grained condensates from a high temperature (>1300 K) gas that existed in the protoplanetary disk at early stages of Solar System formations. Some of them were probably remelted later resulting in distinct coarser textures. The most common and characteristic minerals in CAIs include anorthite, melilite, perovskite, aluminous spinel, hibonite, calcic pyroxene, and forsterite-rich olivine.
Using the lead-lead isotope chronometer ('Pb–Pb dating'), the absolute age of four CAIs have been calculated. They yield a weighted mean age of 4567.30 ± 0.16 Myr, which is often interpreted as representing the beginning of the formation of the planetary system (so-called 'CAI time-zero).
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