Conodont Alteration Index
Encyclopedia
The Conodont Alteration Index (CAI) is used to estimate the maximum temperature reached by a sedimentary rock
using thermal alteration
of conodont
fossils. Conodonts in fossiliferous carbonates are prepared by dissolving the matrix with acid, since the conodonts are composed of apatite
and thus do not dissolve. The fossils are then compared to the index under a microscope.
The CAI ranges from 1 to 6, as follows:
The CAI is commonly used by paleontologists due to its ease of measurement and the abundance of Conodonta throughout marine carbonates of the Paleozoic
. However, the organism disappears from the fossil record after the Triassic
period, so the CAI is not available to analyze rocks younger than . Additionally, the index can be positively skewed in regions of hydrothermal alteration.
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
using thermal alteration
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
of conodont
Conodont
Conodonts are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation. Knowledge about soft tissues remains relatively sparse to this day...
fossils. Conodonts in fossiliferous carbonates are prepared by dissolving the matrix with acid, since the conodonts are composed of apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
and thus do not dissolve. The fossils are then compared to the index under a microscope.
The CAI ranges from 1 to 6, as follows:
| CAI | Approximate conodont color | Temperature range (Celsius) |
|---|---|---|
| 1 | |
<50°-80° |
| 2 | |
60°-140° |
| 3 | |
110°-200° |
| 4 | |
190°-300° |
| 5 | |
300°-480° |
| 6 | |
360°-550° |
The CAI is commonly used by paleontologists due to its ease of measurement and the abundance of Conodonta throughout marine carbonates of the Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
. However, the organism disappears from the fossil record after the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...
period, so the CAI is not available to analyze rocks younger than . Additionally, the index can be positively skewed in regions of hydrothermal alteration.