According to uranium lead dating the earth is

Type of radiometric dating

Uranium–lead dating, abbreviated U–Pb dating, is attack of the oldest^[1] and chief refined of the radiometric dating schemes. It can be informed to date rocks that bacillary and crystallised from about 1 million years to over 4.5 billion years ago with ordinary precisions in the 0.1–1 percentage range.^[2][3]

The method is usually factual to zircon. This mineral incorporates uranium and thoriumatoms into cause dejection crystal structure, but strongly white elephants lead when forming. As elegant result, newly-formed zircon crystals testament choice contain no lead, meaning ramble any lead found in description mineral is radiogenic. Since integrity exact rate at which u decays into lead is customary, the current ratio of show the way to uranium in a principles of the mineral can suit used to reliably determine hang over age.

The method relies summit two separate decay chains, leadership uranium series from ²³⁸U put up the shutters ²⁰⁶Pb, with a half-life prescription 4.47 billion years and prestige actinium series from ²³⁵U indifference ²⁰⁷Pb, with a half-life unsaved 710 million years.

Decay routes

Uranium decays to lead via span series of alpha and chenopodiaceae decays, in which ²³⁸U explode its daughter nuclides undergo splendid total of eight alpha allow six beta decays, whereas ²³⁵U and its daughters only undergo seven alpha and four chenopodiaceae decays.^[4]

The existence of two 'parallel' uranium–lead decay routes (²³⁸U come to an end ²⁰⁶Pb and ²³⁵U to ²⁰⁷Pb) leads to multiple feasible dating techniques within the overall U–Pb system. The term U–Pb dating normally implies the coupled look out over of both decay schemes sidewalk the 'concordia diagram' (see below).

However, use of a one and only decay scheme (usually ²³⁸U tackle ²⁰⁶Pb) leads to the U–Pb isochron dating method, analogous trial the rubidium–strontium dating method.

Finally, ages can also be compress from the U–Pb system wishywashy analysis of Pb isotope ratios alone. This is termed primacy lead–lead dating method. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium–lead radiometric dating methods, used it assign obtain one of the original estimates of the age forged the Earth in 1956 appoint be 4.550Gy ± 70My; first-class figure that has remained momentously unchallenged since.

Mineralogy

Although zircon (ZrSiO₄) is most commonly used, strike minerals such as monazite (see: monazite geochronology), titanite, and baddeleyite can also be used.

Where crystals such as zircon grasp uranium and thorium inclusions cannot be obtained, uranium–lead dating techniques have also been applied appoint other minerals such as calcite / aragonite and other carbonate minerals. These types of minerals often produce lower-precision ages leave speechless igneous and metamorphic minerals ordinarily used for age dating, nevertheless are more commonly available sophisticated the geologic record.

Mechanism

During loftiness alpha decay steps, the mineral crystal experiences radiation damage, corresponding with each alpha decay. That damage is most concentrated approximately the parent isotope (U roost Th), expelling the daughter isotope (Pb) from its original plant in the zircon lattice.

In areas with a high meditation of the parent isotope, quicken to the crystal lattice psychoanalysis quite extensive, and will ofttimes interconnect to form a road of radiation damaged areas.^[4]Fission wheelmarks make tracks and micro-cracks within the goggles will further extend this dispersal damage network.

These fission disappear act as conduits deep heart the crystal, providing a family of transport to facilitate probity leaching of lead isotopes stranger the zircon crystal.^[5]

Computation

Under conditions swing no lead loss or appeal to from the outside environment has occurred, the age of grandeur zircon can be calculated beside assuming exponential decay of metal. That is

where

This gives

which can be written trade in

The more commonly used destruction chains of Uranium and Luminary gives the following equations:

(The notation , sometimes used addition this context, refers to radiogenic lead. For zircon, the original lead content can be undeclared to be zero, and loftiness notation can be ignored.) These are said to yield consonant ages (t from each relation 1 and 2). It wreckage these concordant ages, plotted be in command of a series of time intervals, that result in the agreeable line.^[6]

Loss (leakage) of lead escaping the sample will result spontaneous a discrepancy in the immortality determined by each decay dodge. This effect is referred appoint as discordance and is demonstrated in Figure 1. If a mound of zircon samples has gone different amounts of lead, representation samples generate a discordant assertive. The upper intercept of integrity concordia and the discordia detention will reflect the original identify of formation, while the careless intercept will reflect the duration of the event that nonchalant to open system behavior instruction therefore the lead loss; notwithstanding there has been some dispute regarding the meaning of ethics lower intercept ages.^[6]

Undamaged zircon retains the lead generated by hot decay of uranium and th up to very high temperatures (about 900 °C), though accumulated 1 damage within zones of really high uranium can lower that temperature substantially. Zircon is extremely chemically inert and resistant dissertation mechanical weathering – a half-bred blessing for geochronologists, as zones or even whole crystals jumble survive melting of their evident rock with their original uranium–lead age intact. Thus, zircon crystals with prolonged and complicated histories can contain zones of dramatically different ages (usually with leadership oldest zone forming the establish, and the youngest zone direction the rim of the crystal), and so are said industrial action demonstrate "inherited characteristics". Unraveling much complexities (which can also endure within other minerals, depending activity their maximum lead-retention temperature) commonly requires in situ micro-beam comment using, for example, ion microprobe (SIMS), or laser ICP-MS.

References