Zircon dating methods
Radiometric dating is a much misunderstood phenomenon. Evolutionists often misunderstand the method, assuming it gives a definite age for tested samples. Creationists also often misunderstand it, claiming that the process is inaccurate. Perhaps a good place to start this article would be to affirm that radiometric dating is not inaccurate. It is certainly incorrect, and it is certainly based on wrong assumptions, but it is not inaccurate. What do I mean?
Detrital zircon geochronology is the science of analyzing the age of zircons deposited within a specific sedimentary unit by examining their inherent radioisotopes , most commonly the uranium—lead ratio. The chemical name of zircon is zirconium silicate and its corresponding chemical formula is Zr SiO 4.
Zircon is a common accessory or trace mineral constituent of most granite and felsic igneous rocks. Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques.
It has become increasingly popular in geological studies from the s mainly due to the advancement in radiometric dating techniques. Detrital zircons are part of the sediment derived from weathering and erosion of pre-existing rocks. Since zircons are heavy and highly resistant at Earth's surface,  many zircons are transported, deposited and preserved as detrital zircon grains in sedimentary rocks.
Detrital zircon in reality can be products from all kind of rocks, not necessarily igneous rocks. Detrital zircons usually retain similar properties as their parent igneous rocks , such as age, rough size and mineral chemistry. In fact, many of them are modified by later processes in the sedimentary cycle. Depending on the degree of physical sorting , mechanical abrasion and dissolution, a detrital zircon grain may lose some of its inherent features and gain some over-printed properties like rounded shape and smaller size.
This give rise to a natural complexity of associating detrital zircon populations and their sources. Zircon is a strong tool for uranium-lead age determination because of its inherent properties: There are no set rules for sample selection in detrital zircon geochronology studies. The objective and scale of the research project govern the type and number of samples taken.
In some cases, the sedimentary rock type and depositional setting can significantly affect the final result. After rock samples are collected, they are cleaned, chipped, crushed and milled through standardized procedures. Then, detrital zircons are separated from the fine rock powder by three different ways, namely gravity separation using water, magnetic separation, and gravity separation using heavy liquid. There are two main types of detrital zircon analysis: The biggest advantage of qualitative analysis is being able to uncover all possible origin of the sedimentary unit, whereas quantitative analysis should allow meaningful comparison of proportions in the sample.
Qualitative approach examines all the available detrital zircons individually regardless of their abundance among all grains. Quantitative approach requires large number of grain analyses within a sample rock in order to represent the overall detrital zircon population  statistically i. In this case, BSE and CL imagery are applied to select the best spot on a zircon grain for acquiring reliable age. Different methods in detrital zircon analysis yield different results.
There are generally three categories, which are the instrument s used for zircon analysis, their calibration standards and instrument s used for zircon imagery. Details are listed in Table 1. Gives the general appearance of the zircon, cannot identify internal zircon texture properly, especially when the zircon is neither zoned nor metamictized. Difficult to identify zircon from other high-relief and high-birefringence minerals such as monazite.
Has implications on the amount and distribution of radioactive elements i. Almost like a reversed CL imagery, as the brightness correlates to atomic number. Depending on the detrital zircon study, there should be different variables included for analysis. Details are listed in Table 2. Different zircon shape corresponds to different crystallization medium chemistry and temperature.
A general crystal shape classification would be:. Different elongation defined by length-to-width ratio corresponds to the zircon crystallization rate. The higher the ratio, the higher the crystallization speed. In detrital zircons, however, zircon morphology may not be well-preserved because of the damage caused on zircon grains during weathering, erosion and transportation. Zircon growth zoning correlates magmatic melt condition, such as the crystal-melt interface, the melt's degree of saturation, the melt's ion diffusion rate and oxidation state.
All data acquired first-hand should be cleansed before using to avoid error, normally by computer. Before applying detrital zircon ages, they should be evaluated and screened accordingly. In most cases, data are compared with U-Pb Concordia graphically. The acceptable discordance level is often adjusted with the age of the detrital zircon since older population should experience higher chances of alteration and project higher discordance.
An overall consensus for age with higher accuracy is to adopt:. Given the possibility of concordant yet incorrect detrital zircon U-Pb ages associated with lead loss or inclusion of older components, some scientists apply data selection through clustering and comparing the ages. There are no set limit for age uncertainty and the cut-off value varies with different precision requirement.
Although excluding data with huge age uncertainty would enhance the overall zircon grain age accuracy, over elimination may lower overall research reliability decrease in size of the database. The best practice would be to filter accordingly, i. Depending on the required analytical accuracy, researchers may filter data via their analytical instruments. An older analytical technique, lead-lead evaporation,  is no longer used since it cannot determine the U-Pb concordance of the age data.
Apart from analytical methods, researchers would isolate core or rim ages for analysis. Normally, core ages would be used as crystallization age as they are first generated and least disturbed part in zircon grains. On the other hand, rim ages can be used to track peak metamorphism as they are first in contact with certain temperature and pressure condition. One of the most important information we can get from detrital zircon ages is the maximum depositional age of the referring sedimentary unit.
The sedimentary unit cannot be older than the youngest age of the analyzed detrital zircons because the zircon should have existed before the rock formation. This provides useful age information to rock strata where fossils are unavailable, such as the terrestrial successions during Precambrian or pre-Devonian times. In a global scale, detrital zircon age abundance can be used as a tool to infer significant tectonic events in the past.
Apart from the detrital zircon age abundance, difference between detrital zircons crystallisation ages CA and their corresponding maximum depositional age DA can be plotted in cumulative distribution function to correlate particular tectonic regime in the past. The effect of different tectonic settings on the difference between CA and DA is illustrated in Figure 7 and summarized in Table.
From Wikipedia, the free encyclopedia. Hanchar, J. Reviews in Mineralogy and Geochemistry. CS1 maint: Sedimentary Geology. June Constraints from igneous and detrital zircon age spectra". Gondwana Research. Journal of the Geological Society. UThPb analytical methods for Zircon. Arizona LaserChron Center. Retrieved 10 November , from https: Geological Society of America Bulletin.
Limitations of detrital zircon data for provenance analysis: Geoscience Australia, Canberra. Special Paper of the Geological Society of America. Annual Review of Earth and Planetary Sciences. Current methods and new opportunities. Tectonics of sedimentary basins: Am Mineral. Carnegie Inst Washington Yrbk Magnetic Deflection of Molecular Trajectory.
Phosphorogenic Properties of Molecular Discharge". Philosophical Transactions of the Royal Society of London. Implications for interpretation of complex crustal histories. Chem Geol The Journal of Geology. Retrieved 15 November , from http: U and Th. Physical Review C. Subcommission on Geochronology: Convention on the use of decay constants in geochronology and cosmochronology.
Contributions to Mineralogy and Petrology. Science in China Series B: Earth Processes: Reading the Isotopic Code , — Earth and Planetary Science Letters. Geochemistry, Geophysics, Geosystems, 9, Q Evaluation of the laser probe-inductively coupled plasma mass spectrometry technique". Geochimica et Cosmochimica Acta.
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the The dating method is usually performed on the mineral zircon. Detrital zircon geochronology is the science of analyzing the age of zircons deposited within a . Different methods in detrital zircon analysis yield different results. . calculated with the associated decay constants, (see Uranium–lead dating).
In this article we shall discuss the basis of the U-Pb and Pb-Pb methods, and also fission track dating. It has a half-life of 4. It is also useful to know of the existence of Pb lead , which is neither unstable nor radiogenic.
Detrital zircon geochronology is the science of analyzing the age of zircons deposited within a specific sedimentary unit by examining their inherent radioisotopes , most commonly the uranium—lead ratio.
Many radioactive dating methods are based on minute additions of daughter products to a rock or mineral in which a considerable amount of daughter-type isotopes already exists. These isotopes did not come from radioactive decay in the system but rather formed during the original creation of the elements.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
The nitty gritty on radioisotopic dating Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history. Radioactive decay Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms emit particles. This releases energy in the form of radiation and often transforms one element into another. For example, over time, uranium atoms lose alpha particles each made up of two protons and two neutrons and decay, via a chain of unstable daughters, into stable lead. Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms. In other words, the chance that a given atom will decay is constant over time.
Petrology Tulane University Prof. Stephen A.
How old are you? How old are your grandparents?
Radiometric Dating — Is It Accurate?
Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest  and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised  from about 1 million years to over 4. The dating method is usually performed on the mineral zircon. The mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead. 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 [ citation needed ]. 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.
There was a problem providing the content you requested
Detrital zircon geochronology
The mineral zircon adds three more fundamental advantages to uranium—lead dating. First, its crystal structure allows a small amount of tetravalent uranium to substitute for zirconium but excludes with great efficiency the incorporation of lead. It might be said that one begins with an empty box. Second, zircon, once formed, is highly resistant to change and has the highest blocking temperature ever observed. Finally, with few predictable exceptions, zircon grows or regrows only in liquid rock or in solid rock reheated to approach its melting point. Combining all of these attributes, it is often possible to measure both the time of crystallization and the time of second melting in different parts of the same grain or in different selected grains from the same rock. Of course, such a high blocking temperature can have its disadvantages.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava. Lava properly called magma before it erupts fills large underground chambers called magma chambers. Most people are not aware of the many processes that take place in lava before it erupts and as it solidifies, processes that can have a tremendous influence on daughter to parent ratios. Such processes can cause the daughter product to be enriched relative to the parent, which would make the rock look older, or cause the parent to be enriched relative to the daughter, which would make the rock look younger. This calls the whole radiometric dating scheme into serious question. Geologists assert that older dates are found deeper down in the geologic column, which they take as evidence that radiometric dating is giving true ages, since it is apparent that rocks that are deeper must be older.
.U-Pb Zircon Geochronology - for determining the age of a rock