Geochronometry is a branch ofstratigraphy aimed at the quantitative measurement of geologictime. It is considered a branch ofgeochronology.
The measurement of geologictime is a long-standing problem ofgeology.[1] When geology was at its beginnings, a major problem for stratigraphers was to find a reliable method for the measurement oftime. In the eighteenth century, and during most of the nineteenth century, the ideas on the geologic time were indeed so controversial that the estimates for theage of the Earth encompassed the whole range from ca. 6000 years to 300 million years. The longer estimate came fromCharles Darwin, who probably went closer to the truth because he had clear in mind that theevolution of life must have required a lot of time to take place. The current estimate of the age of the Earth is ca. 4500 million years.The solution of the dating problem arrived only with the discovery that some naturalelements undergo a continuous decay. This led to the firstradiometric datings by Boltwood[2] and Strutt.[3]Today, the determination of the age of the Earth is not a primary scope of geochronometry anymore, and most efforts are rather aimed at obtaining increasingly precise radiometric datings.At the same time, other methods for the measurement of time were developed, so the quantification of geologic time can now be endeavored with a variety of approaches.
All methods based on theradioactive decay belong to this category. The principle at the base ofradiometric dating is that natural unstableisotopes, called 'parent isotopes', decay to some isotope which is instead stable, called the 'daughter isotope'.
Under the assumptions that:
(1) the initial amount of parent and daughter isotopes can be estimated, and
(2) after the geologic material formed, parent and daughter isotopes did not escape the system, the age of the material can be obtained from the measurement of isotope concentrations, through the laws of radioactive decay.Methods of this kind are usually identified with the names of the parent/daughter elements. The radiometric methods under this category are:
Each of these methods perform better in different time ranges and has different limitations. However, uranium–lead dating onzircon[4] andArgon-argon dating onsanidine andhornblende are the two single methods that achieve today the best results.[5]
Other methods ofradiometric dating are also available, that are based on slightly or largely different principles, but always rely on the phenomenon ofradioactive decay. These alternative radiometric methods are:
These methods, especiallyradiocarbon, are particularly reliable for recent samples, but are much less accurate for deep geologic time.[5] More specifically, radiocarbon becomes unreliable already for samples >50000 years old.
These methods are based on the building of incremental chronologies from a point of known age, which is usually the present. When a chronology is not tied to such a known age point, it is called afloating chronology.Incremental dating methods include:
A major achievement of geochronometry is the documentation of geologictime, as represented in geologic time scales. Ageologic time scale is a scheme that integrates thegeochronologic subdivisions of geologic time and their absolute ages and durations. The latest version of the geologic time scale was published in 2004,[6] and includes a comparison of present and past time scales. The greater efforts of geochronometry today are aimed at retrieving accurate ages of major events in the Earth's history and ofstage/age boundaries.[5]
This article incorporates material from theCitizendium article "Geochronometry", which is licensed under theCreative Commons Attribution-ShareAlike 3.0 Unported License but not under theGFDL.