Geochronology and thermochronology applied to detrital minerals such as zircons, monazites, white micas, and apatites have received increasing attention in the past decade for their unique power to determine the timing of crystallization and multiple tectono-thermal events, with relevance for sediment provenance, tectonic processes, and erosion. Recent breakthroughs in multi-dating applying different geochronologic and thermochronologic systems to the same detrital grains allow for unprecedented levels of detail in provenance and tectonic studies of detrital sediments.
The common pre-conditions for application of these methods are: Whereas zircons occur in most magmatic, metamorphic, and sedimentary rocks, other minerals, such as apatite, monazite, and white mica, are less abundant. This is why zircon geochronology and thermochronology is a particularly useful approach to detrital studies.
It is also important to note that different mineral geochronometers and thermochronometers can only answer specific questions. For example, if we want to determine the provenance of detrital minerals by studying the long history of crystallization of a tectonically complex source region, then U-Pb zircon geochronology is the ideal approach. The main strength of zircons resides in the fact that they are capable of surviving multiple phases of physical and chemical weathering, erosion, and deposition.
The increased use of multicollector-laser ablation-inductively coupled plasma—mass spectrometry MC-LA-ICPMS in recent years is a significant advancement in the application of U-Pb geochronology to provenance and tectonic problems, because the technique can efficiently generate a large number of analyses Gehrels et al. The method has become a common approach for determining sediment provenance, dispersal patterns, and recycling Dickinson and Gehrels, , a , b , timing of tectonic processes such as the onset and kinematic history of mountain building White et al.
The first scenario can be better addressed by zircon secondary ion mass spectrometry SIMS analysis Trail et al. In particular, whereas monazite dating by a variety of techniques isotope dilution mass spectrometry, SIMS, electron microprobe dating, and LA-ICPMS has been commonly applied to metamorphic and igneous rocks Harrison et al. Monazite is a common phosphate mineral in different rocks such as granite, pegmatite, felsic volcanic ash, low- to high-grade metamorphic rocks, and as a detrital mineral in sedimentary rocks, and can be used to date the ages of crystallization and of metamorphism in igneous and metamorphic rocks Parrish, Its relatively stability under a variety of geological conditions, and resilience to radiation damage, make monazite a reliable geochronometer Harrison et al.
- example of paleomagnetic dating;
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Early work on monazite showed large discrepancies between U-Pb and Th-Pb ages Tilton and Nicolaysen, , which were explained by Pb diffusion over significant T and time Shestakov, , or by Pb loss Michot and Deutsch, Since then, much progress has been made to understand diffusion properties of monazite Cherniak et al. In this issue of Geology , Hieptas et al. In this example, the authors collected sand samples from the French Broad River western North Carolina—eastern Tennessee, United States and six tributaries in the Appalachian Blue Ridge and analyzed them with both zircon and monazite U-Pb dating in order to investigate the geochronologial signature of the source area and its tectonic significance.
Detrital monazites from the same sediments record the complete Paleozoic collisional history of the Appalachian orogen including the Alleghanian event as well as the main events for the Grenville basement. The authors point out that the younger signals are only partially recorded in rims of detrital zircons, and may be missed without careful imaging and domain sampling in a strictly detrital zircon approach.
This study highlights the utility of a multi-dating geochronological approach to provenance analysis for resolving multiple orogenic phases spanning a range of temperatures. Overall, the advancement of new approches and analytical techniques such as the one described by Hietpas et al.
Closure T -windows characteristic of different geochronometers and thermochronometers. Note that for most chronometers, the temperature T at which the system became fully retentive closed depends on various parameters such as compositions, thermal history, and pressure, and on the details of diffusion. The two possible initial settings, backbulge or distal foredeep, require either normal cratonward migration of a forebulge or cratonward migration interrupted by a Callovian phase of orogenward retreat, respectively.
Besides filling a long-standing knowledge gap, our data allow direct comparison between the southern Canada and northern United States segments of the Cordilleran foreland basin. Sign In or Create an Account. Close mobile search navigation Article navigation.
Volume , Number Previous Article Next Article. Research Article August 17, U-Pb detrital zircon dating supports Early Jurassic initiation of the Cordilleran foreland basin in southwestern Canada D. 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 geochronology
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.
Detrital zircon geochronology - Wikipedia
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.
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An older analytical technique, lead-lead evaporation,  is no longer used since it cannot determine the U-Pb concordance of the age data.