Zircon dating metamorphic rocks
This requires that the low-grade metasedimentary rocks were overthrust for several tens of kilometres onto the South China Craton, explaining why source materials from the North China Craton are common in Triassic to Jurassic sedimentary basins adjacent to the Dabie—Sulu orogenic belt. Sign In or Create an Account. Close mobile search navigation Article navigation.
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Librarian Administrator Sign In. Email alerts New issue alert. 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. 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.duebinmiputhe.tk
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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.
Another sample, this one from sandstone deposited by a large river in northern Scotland, must have been derived from continental rocks whose ages are represented by those determined for the individually dated sand grains. In this case, the continent from which the sand was derived has moved away as a result of continental drift , but it can be identified by the ages measured.
We welcome suggested improvements to any of our articles. You can make it easier for us to review and, hopefully, publish your contribution by keeping a few points in mind. The material that holds the greatest insight into these fundamental questions, because it can contain a record of some of the earliest history of the Earth, is a mineral named zircon. For example, a few grains of zircon found in the early s in a sandstone from western Australia dates back 4.
Geology professors Darrell Henry of Louisiana State University and Paul Mueller of the University of Florida are expert practitioners of several techniques that can extract precise age information from zircons. Originally formed by crystallization from a magma or in metamorphic rocks, zircons are so durable and resistant to chemical attack that they rarely go away.
They may survive many geologic events, which can be recorded in rings of additional zircon that grow around the original crystal like tree rings.
Zircon Chronology: Dating the Oldest Material on Earth | AMNH
Like a tiny time capsule, the zircon records these events, each one of which may last hundreds of millions of years. Meanwhile, the core of the zircon itself remains unchanged, and preserves the chemical characteristics of the rock in which it originally crystallized. 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.