- Zircon Dating
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- Dating - Importance of zircon in uranium-lead dating | soulledlife.com
Zircon contains the radioactive element uranium, which Dr. From the types of minerals and their distributions in the rocks he reconstructs a relative sequence of events that reflects the change over time of parameters like pressure, temperature, and deformation. How precise are those actual numbers? That translates, for example, to plus or minus a million years out of three billion.
Carbon dating can go no further back than about 70, years, because the half-life of carbon is only 5, years. The half-life is the time it takes for half of the original radioactive isotope to change to another element.
In comparison, the half-life of the radioactive uranium isotope is 4. Zircon chronology begins in the field. Because of their position, we know that the rocks enclosed in the granite have to be older. Then they collect samples, which weigh from two to more than one hundred pounds, depending on the rock type. This makes separating out the zircons a painstaking process. The rock is ground up to break it into individual mineral grains.
We also use the magnetic qualities of the zircons to separate the most pristine ones from the rest.
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Then the detailed geochronology work begins. Is it in a granite? Or is it in a metamorphic rock that has had a more complex history? Or is it a metamorphosed sedimentary rock? By knowing its history, we can interpret the age of the rock much better. Quantitative microanalysis of the elements in zircon is done with an electron microprobe. Each of the atoms of the different elements in the sample gives off X-rays with characteristic wavelengths.
You can then compare these to a standard with a known concentration of the element, and come up with an exact composition of that small spot. An individual zircon grain may be composed of many zones of different compositions and ages. Isotopic compositions can be determined with an ion probe. Mueller sums it up: Mathez, a publication of the New Press. Amy Vedder The Biodiversity Crisis: Therefore, one can assume that the entire lead content of the zircon is radiogenic , i.
Thus the current ratio of lead to uranium in the mineral can be used to determine its age. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4. The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays.
The existence of two 'parallel' uranium—lead decay routes U to Pb and U to Pb leads to multiple dating techniques within the overall U—Pb system. The term U—Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' see below.
Dating - Importance of zircon in uranium-lead dating | soulledlife.com
However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method. Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. This is termed the lead—lead dating method. Clair Cameron Patterson , an American geochemist who pioneered studies of uranium—lead radiometric dating methods, is famous for having used it to obtain one of the earliest estimates of the age of the Earth.
Although zircon ZrSiO 4 is most commonly used, other minerals such as monazite see: Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied. These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record.
During the alpha decay steps, the zircon crystal experiences radiation damage, associated with each alpha decay. This damage is most concentrated around the parent isotope U and Th , expelling the daughter isotope Pb from its original position in the zircon lattice. In areas with a high concentration of the parent isotope, damage to the crystal lattice is quite extensive, and will often interconnect to form a network of radiation damaged areas. These fission tracks inevitably act as conduits deep within the crystal, thereby providing a method of transport to facilitate the leaching of lead isotopes from the zircon crystal.
Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium.
- Zircon Chronology: Dating the Oldest Material on Earth.
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- Uranium–lead dating - Wikipedia.
- Zircon Chronology: Dating the Oldest Material on Earth | AMNH!
These are said to yield concordant ages. Loss leakage of lead from the sample will result in a discrepancy in the ages determined by each decay scheme. This effect is referred to as discordance and is demonstrated in Figure 1. If a series of zircon samples has lost different amounts of lead, the samples generate a discordant line. The upper intercept of the concordia and the discordia line will reflect the original age of formation, while the lower intercept will reflect the age of the event that led to open system behavior and therefore the lead loss; although there has been some disagreement regarding the meaning of the lower intercept ages.
Zircon is very chemically inert and resistant to mechanical weathering—a mixed blessing for geochronologists, as zones or even whole crystals can survive melting of their parent rock with their original uranium-lead age intact. Zircon crystals with prolonged and complex histories can thus contain zones of dramatically different ages usually, with the oldest and youngest zones forming the core and rim, respectively, of the crystal , and thus are said to demonstrate inherited characteristics.
Unraveling such complications which, depending on their maximum lead-retention temperature, can also exist within other minerals generally requires in situ micro-beam analysis via, say, ion microprobe SIMS or laser ICP-MS. From Wikipedia, the free encyclopedia.