Zircon age dating
Contents:
Implications for interpretation of complex crustal histories.
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The Journal of Geology. Retrieved 15 November , from http: Convention on the use of decay constants in geochronology and cosmochronology. Contributions to Mineralogy and Petrology. Science in China Series B: 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. Diversity of burial rates in convergent settings decreased as Earth aged.
Scientific reports , 6. Retrieved from " https: Sediments Geochronological dating methods Zircon. Uses editors parameter Use dmy dates from January Views Read Edit View history. This page was last edited on 27 May , at By using this site, you agree to the Terms of Use and Privacy Policy. Basically analytical machines need to be calibrated before use. Scientists use age-similar comparable to the sampled zircons and accurate zircons as their machine calibration standards.
Different calibration standards may give slight deviation of the resulting ages. For example, there are at least twelve different standards catering for different sample zircons in Arizona Laserchron Center, primarily using Sri Lanka zircon, followed by Oracle.
Type of instrument for zircon imagery [18]. Instruments Usage For macroscopic view Gives the general appearance of the zircon, cannot identify internal zircon texture properly, especially when the zircon is neither zoned nor metamictized.
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Can examine zircon grain as a whole: Transmitted light microscopy TLM. Can examine zircon growth zoning and metamictization in cross-polarized light. Can examine zircon growth zoning, alteration and metamictization.
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Induce fission tracks within the zircon by neutron flux reactor and record the tracks into an image. One of the best resolution instruments. Also one of the best resolution instruments currently. Secondary electron microscopy SEM. See scanning electron microscope. Grain number is necessary for multiple detrital zircon grains yielded within the same sample rock.
Thorium content divided by uranium content. Intermediate origin lies between 0. Also measured to correct the amount of lead incorporated into the zircon during initial crystallization.
Zircon Chronology: Dating the Oldest Material on Earth
Obtained by either comparing with the standard U-Pb Concordia or calculation: Generally, researchers analyze detrital zircon core for its oldest crystallization age because zircon grows outwards in rims. There may be rim analysis, which can correlate the late stage of zircon crystallization or metamorphism if any.
A general crystal shape classification would be: Zircon texture generally refers to the outlook of zircon, specifically its oscillatory zoning pattern under BSE or CL imagery. Zircon with good zoning would have alternating dark and light rim growth. Dark rim is associated with zircon-rich but trace-element poor geochemistry and vice versa. 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. Inherited cores may give a mixed false age when the age of crystallization is sought. For this reason, three or more grain types or parts of a grain are analyzed to establish that material of only one age is present.
Experience with the results of the uranium—lead method for zircons has demonstrated an interesting paradox. If left at low surface temperatures for a geologically long time, the radioactivity within the crystal can destroy the crystal lattice structure, whereas at higher temperatures this process is self-annealing. In fact, when examined by X-ray methods, some zircons have no detectable structure, indicating that at least 25 percent of the initial atoms have been displaced by radiation damage.
Under these conditions a low-temperature event insufficient to even reset the potassium—argon system see below Potassium—argon methods in biotite can cause lead to be lost in some grains. It is no coincidence that, when criteria were finally found to locate concordant grains, these grains were also found to be those with the lowest uranium content and the lowest related radiation damage. Given the two related uranium—lead parent—daughter systems, it is possible to determine both the time of the initial, or primary, rock-forming event and the time of a major reheating, or secondary, event.
The uranium—lead isotopes in the mineral titanite CaTiSiO 5 from a series of rocks that have a common geologic history can be plotted on a concordia diagram.
New titanite, distinguishable on the basis of colour, may form in the same rock, while older, partly reset titanite is still present. Geochronologists can separate recent lead loss due to some disturbance event, such as the reheating of the rock, from the normal rate of lead loss by plotting the ratio of lead to uranium in the sample. A new line, the discordia, will plot along a different trajectory, but it will intercept the concordia in two places. The upper intercept will denote the timing of the primary rock-forming event, while the lower intercept will denote the timing of the reheating event.
Uranium—lead dating relies on the isolation of very high-quality grains or parts of mineral grains that are extremely rare but nevertheless present in most igneous, metamorphic, and sedimentary rock units. Samples weighing 10 to 50 kg 22 to pounds are collected, crushed, and ground into a fine sand, and the various minerals are isolated on the basis of specific gravity , grain size, and magnetic properties.
The minerals used are not visible in the field, but their presence can be inferred from the easily identified major minerals present.
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One of the most interesting applications of the improved uranium—lead zircon technique has to do with its ability to achieve nearly concordant results from single grains extracted from sandstone. This is possible because zircon is chemically inert and is not disturbed during weathering and because single grains with a diameter about the thickness of a human hair contain sufficient uranium and lead for analysis in the most advanced laboratories.
In one sample it was determined that a sandstone that underlies most of the province of Nova Scotia in Canada was probably originally deposited off the coast of North Africa and thrust over the continent before the opening of the Atlantic Ocean. This follows because the ages observed occur in North Africa, whereas those common in North America are absent.
Uranium–lead dating
If all radiogenic lead accumulated up until that time is lost, then the coordinates of the point representing the system respond in such a manner that it returns to the origin. Such a uranium-bearing mineral contains no radiogenic lead when it crystallizes, total Pb loss returns the concordia point to the position which it occupied originally. The inference is that the U-Pb geochronometer recommences and thus all trace of the earlier history of the system disappears.
On the other hand, if only some of the radiogenic lead is lost, the point moves some way long the chord, as noted above. Such systems falling on to the chord have discordant dates and, as a result, the chord is termed 'discordia'.
Correction for incorporated common lead in zircon and monazite was effected with the isotopic compositions of Pb in the potassium feldspars from the same sample. Importance of zircon in uranium-lead dating The mineral zircon adds three more fundamental advantages to uranium—lead dating. With the passing of time, the decay of the radioactive elements in such crystals not only generates a correlated zonation in the concentration of radiogenic Pb but also, if the temperature remains below that at 'which zircon anneals, creates a zonation in the extent to which the crystal structure becomes damaged by radiation. In other instances, absurd results were obtained and these led to negative ages so that common lead ores now in existence would form in the far future, say up to 2 X 10 9 a from then. In some cases, the sedimentary rock type and depositional setting can significantly affect the final result. For single zircon evaporation only the highest-quality zircons idiomorphic, absolutely clear, no visible inclusions, no visible overgrowth or dissolution phenomena are used. The term U—Pb dating normally implies the coupled use of both decay schemes in the 'concordia diagram' see below.
Usually, minerals lose only a small proportion of their radiogenic Pb. Occasionally, varied fractions may be lost from zircons in the same sample of igneous rock despite the fact that they must have gone through the same conditions. This may relate to the actual sizes of the crystals concerned and their uranium concentrations as well as to radiation damage in them. Smaller grains, as well as those which have a high uranium concentration, can undergo larger lead losses than others of greater size and uranium concentration. Thus, several zircon fractions may be obtained from one rock sample and these plot as a set of points along the discordia line.
Hence, it is possible to ascertain the, position of the discordia by fitting a straight line to data points which represent zircons which lost varying amounts of their radiogenic Pb. Extrapolation of discordia gives two points of intersection with concordia. The first is at the time which has passed since the original crystallization of the minerals and the second is at the time elapsed since closure of the system after an episode either of lead loss or gain of uranium.
As noted above, lead loss may alternatively be continuous. They derive from pegmatites in the White Sea area of Karelia Belomoria containing minerals which probably all have the same age - not that the age of zircons is always identical to the age of their host rocks, since this mineral is very refractory and therefore can survive melting of sedimentary rocks to form magma.
Zircon Dating - ScienceAid
In metamorphic rocks, zircons usually contain rounded cores representing detrital grains originating from older rocks. Concordia diagrams allow interpretation of the geological histories of U-Pb systems as well as giving information regarding past disturbances of these systems. Unless the isotope composition of added lead is known, the effect of lead gain cannot be considered in the model.