Interpretation Of Discordant U‐Pb Zircon Ages: An Evaluation, Journal Of Metamorphic Geology 10 1111 J.1525-1314.1997.00008.x

Interpretation Of Discordant U‐Pb Zircon Ages: An Evaluation, Journal Of Metamorphic Geology 10 1111 J.1525-1314.1997.00008.x

These represent a kind of weighted mean between the
206Pb/238U and 207Pb/206Pb methods, which
offers better precision than either of the latter two methods. Zircon is commonly found as the primary mineral in igneous rocks. Since igneous rocks have no fossils, this makes zircon valuable in dating them. Zircon also concentrates Uranium (You) and (although less so) Thorium (Th) and excludes lead (Pb), which means it has a very high U/Pb ratio. This means that any lead found in zircon minerals was made by radioactive decay, after the formation of the mineral.

346 has a good description of Concordia plots in cross-correlating dates between multiple decay results. • They have a very high melting temperature and hardness, which means that they are very durable.

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Impact-generated hydrothermal activity that might enhance Pb-loss, especially in radiation-damaged zircon, was more prevalent inside the central plateau than outside. It could thus be expected that resetting of zircon within the central plateau should be more effective than outside, and that the lower intercept of these crystals could approximate the age of the impact event. However, there is no significant age difference of the lower intercept dates between the two subsets (see Supplementary Fig. S6). In addition, a majority of the lower intercepts are within the same 150–500 Ma age range as obtained from Paleoproterozoic rocks elsewhere in the Baltic Shield56, rendering the lower intercept dates meaningless with respect to the age of the impact.

Geologists then break up the rock into mineral grains and set them in a very thick liquid. Because zircon is one of the densest minerals, the rest of the minerals raise to the top and the zircon sinks. Also, zircons are magnetic so scientists can separate the finest pieces with magnets. The evidence from textural-petrologic imaging (CL, BSE) and element mapping by electron probe microanalyzer (EPMA) or high spatial resolution SIMS or LA-ICP-MS provides the decisive constraints. All microcline and mica geochronometers that have been characterized in detail document patchy textures and evidence for mineral replacement reactions.

Thanks to all authors for creating a page that has been read 3,400 times. Portland State University Department of Geology, our gracious host since 1971, offers undergraduate and graduate programs in geology, as well as free auditing for seniors. Here’s a news-flash style video from SciShow entitled “How the Oldest Rocks on Earth Changed History”that lays out the basics of what we know about the oldest zircon found from earth and more. Blair Schoene, Princeton University, “U–Th–Pb Geochronology,” Elsevier, 2014.

The graph will show not only the age of the rocks but also when important geological events occurred in the past. Detrital zircons are those that have weathered out of the parent igneous or metamorphic rock and these can be found in sediments just about everywhere. For example, a bucket of sand is extracted from a river or ancient lake bed and the results of all the ages of the zircons therein are plotted on the same graph. The peaks showing the most numerously computed ages are compared with possible igneous or metamorphic sources for the sediment.


It is important not to confuse causes and effects; heterochemical microstructures are not the cause of Ar and Sr loss; rather, they follow it. Ar and Sr loss by dissolution of the older mineral generation occurs first, heterochemical textures form later, when the replacive assemblage recrystallizes. Heterochemical mineral generations are identified and dated by their Ca/Cl/K systematics in 39Ar-40Ar.

Radiation damage allows identification of truly inherited zircon

The zircons are studied with an electron microprobe; they hit the zircons with an electron beam to see the cathodoluminescent light that results after it. All of the atoms in the sample give off X-rays with different wavelengths after being hit with the electron beam, according to their atomic makeup. That way, scientists know the exact composition of that tiny sample. A single grain of zircon may contain different compositions and ages.

The U-Pb concordia-discordia method is especially resistant to heating and metamorphism and thus is extremely useful in rocks with complex histories. Quite often this method is used in conjunction with the K-Ar and Rb-Sr isochron methods to unravel the history of metamorphic rocks, because each of these methods responds differently to metamorphism and heating. For example, the U-Pb discordia age might give the age of initial formation of the rock, whereas the K-Ar method, which is especially sensitive to heating, might give the age of the latest heating event. There are examples of concordia-discordia diagrams in succeeding chapters. The minimum provenance ages of the rocks from which the studied kaolins were derived must be the age of the youngest detrital zircon, provided no disturbance occurred in the U–Pb isotopic system39,40. In obtaining the youngest age41,42, the weighted average (WA) age of the youngest zircon population group was used.

The central plateau is surrounded by an annular depression that is partly filled by lakes and down-faulted Paleozoic sediments24,25,26,27 (Fig. 1). The example below is from a high-U (2000 ppm) metamorphic zircon rim, showing a well-resolved 22.5 Ma rim, that is approximately 450 nanometer thick. After 450 nanometers, the age begins to steadily increase because the pit is sputtering into an older interior of the grain. However, because the pit geometry is bowl-shaped, the increase in age after 450 nanometers defines a mixing trend between the 22.5 Ma rim and ~500 Ma core. If the sputtered spot was better focused, with steep sides and a flat bottom, this transition could be better resolved.

12 13 Observations of discordant ages, meaning that an age given by one mineral geochronometer is 14 different from the age given by another geochronometer from the same rock, began in the early days of 15 geochronology. In the late 1950s and 1960s, discordant U-Pb zircon ages were unquestioningly 16 attributed to Pb diffusion at high temperature. Later, the mineralogical properties and the petrogenesis 17 of the zircon crystals being dated was recognized as a key factor in obtaining concordant U-Pb ages.

Second, the p value
cutoffs that are equivalent to any given relative age difference are
highly laboratory dependent, with precise equipment requiring different dp cutoffs than imprecise instruments. The other five discordance filters are more universally applicable. So using a different set of test data should only make a modest difference to the values in Table 1. Each discordia definition that we have studied thus far is expressed
in different units. For the absolute age definition, degrees of discordance are expressed in units of time (ranging from 0 to 4.5 Ga). The relative age definition uses fractions of time (ranging from −∞ to 1).

However this bias can be removed by applying a common-Pb correction after the data have been filtered. As you know, radioisotopes do not decay directly into a stable state; rather they go through stages of radioactive decay until reaching a stable isotope. The two decay chains used on zircon dating are the uranium series and the actinium series. Zircon grains were mounted in an epoxy mould that was ground down and then polished using a 3-micron pad followed by 1-micron pad to finish and a 1-micron diamond paste on a Struers Rotopol-35 equipment to expose grain interior. The mould was gold coated using an Edwards S150A sputter coater.

Scientists observe how light reflects from the grains of zircon, and in detail, how it is transmitted through them. Scientists have many ways to discover how old rocks and fossils are. The oldest and most reliable method they use is called Uranium-lead (U-Pb) dating. Scientists use this method to date rocks that formed from between 1 million to 4.5 billion years ago, and they estimate the Earth is 4.543 billion years old. Scientists can use monazite, titanite, baddeleyite and zirconolite for uranium dating. The graphing of concordant U-Pb data on a concordia diagram does not provide any information that is not obvious from the individual U-Pb ages themselves.

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