Zircon age dating

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: Fossil Microbes on Mars?

Neutrino Observatories Case Study: Ernst Chladni and Rocks from the Sky Profile: Cecilia Payne and the Composition of the Stars Profile: Ole Roemer and the Speed of Light Profile: Vera Rubin and Dark Matter Profile: Lyman Spitzer and the Space Telescope Profile: Fritz Zwicky's Extraordinary Vision Profile: Dating the Oldest Material on Earth Earth: Inside and Out Glossary Harry Hess: How Big Were Dinosaurs? 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.

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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.

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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. 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 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. 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. The disintegration products of uranium: American Journal of Science Retrieved 7 January Alpha-recoil in U-Pb geochronology: Effective sample size matters. Contributions to Mineralogy and Petrology , 4: