Detrital Zircons Can Give False Geological Ages CEH

However, deformation and fluid migration inducing mineral reactions and/or deformation accommodated by dissolution/reprecipitation can lead to recrystallization of white mica with new ages. Also, the influx of external fluids released from crystallizing magmas can cause local age resetting. These processes or combinations of them can induce considerable age heterogeneity at thin section and outcrop scale and additionally, if recrystallization or dissolution-reprecipitation of mica is incomplete, generate mixed age populations. Furthermore, the 40Ar/39Ar UV laser ablation method can also be used to date detrital white mica which is still preserved at very-low-grade and which can well be distinguished from metamorphic mica by its larger grain size and generally different chemical composition.

A Brief Introduction to Zircon Geochronology

A dense fluid is also employed to separate the zircons, amongst the heaviest of minerals, from moderately dense materials. The investigated rocks of Siljan and Järna type are reddish, medium to coarse-grained, with equigranular to porphyritic textures. Rocks inside the central plateau are significantly affected by hydrothermal alteration, with a decrease of alteration away from the centre.

The Th/U ratios obtained for the detrital zircon grains within the kaolins were predominantly within known values for rocks with magmatic origin. The age populations obtained for the detrital zircon grains were dominated by values from 529 to 978 Ma within the Neoproterozoic, followed by values from 1754 to 2497 Ma of the Paleoproterozoic. Detrital zircon ages obtained between 553.2 ± 6.2 and 583.5 ± 2.0 Ma represent part of the minimum provenance ages for the primary minerals that were kaolinised. The Cretaceous–Paleogene/Neogene kaolins were derived from parent rocks of Eburnean and Pan African ages within the Western and Northern Nigeria Basements. Geologists extract the appropriate minerals from the rock (in this case, zircon crystals) and use a technique called mass spectrometry to figure out the relative amounts of uranium and lead in the zircon. Thus, when a geologist dates a rock using uranium-lead dating, he or she is actually getting an estimate on the age of its zircon crystals, which formed “shortly” before the volcanic eruption.

However, this hypothesis has yet to be empirically tested on natural zircon. U–Pb ages of detrital zircon grains present within the Cretaceous–Paleogene/Neogene kaolins can be used to identify specific sediment sources. A detailed inspection of the probability density plots revealed that the ages in the range of 1000–541 Ma (Neoproterozoic Era) constituted 48% and 85% of the Cretaceous and Paleogene/Neogene kaolins, respectively. Ages within 2500–1600 Ma (Paleoproterozoic Era) constituted 52% and 13% of the Cretaceous and Paleogene/Neogene detrital zircons, respectively.

Hence, carbon-14 dating can only be used to estimate much younger ages, up to around 60,000 years. Slightly different dating techniques are used with different radioactive elements, but the same basic logic of estimating backwards based on radioactive decay remains the same. Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms emit particles.

Regional zircon U-Pb geochronology for the Maniitsoq region, southwest Greenland

The obtained results were used to deduce the possible sources of sediments that formed the kaolins. The type and size of the sedimentary basin, the scale of the hinterland, and the presence and extent of synsedimentary igneous activity are a response to tectonic setting. Basins lying along plate margins (e.g., rift basins, arc-flanking basins, foreland basins) are characterized by syn-depositional tectonic activity resulting in spatial and temporal variations in the nature and size of the hinterland and, hence, in sediment provenance. Such basins will be characterized by rapid lateral changes in lithofacies, incorporation of detritus from uplifted nearby sources, and a restricted distributary province. Synsedimentary magmatic activity is likely in such settings and hence the youngest detrital zircon grains may approximate the time of sediment accumulation (Figs. 1A and 1B ) (Dickinson and Gehrels, 2009). In contrast, basins situated in intraplate (trailing-edge) settings (e.g., passive margins) are tectonically stable.

This service may include material from Agence France-Presse (AFP), APTN, Reuters, AAP, CNN and the BBC World Service which is copyright and cannot be reproduced. “Earth’s early impact history has been erased, so we look to the moon as a proxy for when Earth was impacted,” Dr Cavosie said. Lead researcher Aaron Cavosie, also a visiting professor at Wisconsin University’s NASA Astrobiology Institute, said meteorite impacts were relevant to when habitable conditions existed on Earth, with cool surface conditions and water. Armed with this information Miller and his students and colleagues can estimate the age at any point in the section to about a five-fold higher precision.

If no fluids are present, white mica will still preserve their original metamorphic ages. However, there are many factors (e.g. cooling rate) that control the closure temperature for daughter isotopes in garnet and these are usually difficult to constrain on individual samples. Nonetheless, careful assessment of trace element zoning in garnet can elucidate if a garnet age is biased towards the early or late stages of garnet growth and if there was extensive loss of the radiogenic daughter isotope. The relative concentrations of the heavy rare earth elements between garnet and zircon can also be used to evaluate if garnet and zircon were in equilibrium (Taylor et al., 2017). Apart from analytical methods, researchers would isolate core or rim ages for analysis. Normally, core ages would be used as crystallization age as they are first generated and least disturbed part in zircon grains.

Invited Research ArticleZircon Raman dating: Age equation and calibration

For basins containing a high degree of igneous activity that approximates, or is only slightly older than the depositional age of the sediment, then depositional ages should be known within a range of some 10 Ma. In contrast, for basins with little or no syndepositional igneous activity, the uncertainty in depositional age can be hundreds of millions of years or greater. For example, the distinctive detrital zircon age spectra of Cambrian passive margin strata in Scotland (Cawood et al., 2007a) fall in the extensional basin setting if the depositional age is taken as 500 Ma or 1500 Ma or anywhere in between. Similarly, the zircon data from the Archean and Proterozoic sedimentary sequences in the Jack Hills of the Yilgarn craton in Australia also indicate accumulation in extensional basins, even though the depositional age is poorly constrained (Fig. 3; Eriksson and Wilde, 2010). Most studies on zircon geochronology in Nigeria have been directed towards the determination of the ages of the crystalline basement rocks6,7,8,9,10,11.

These slices are examined to see if they had consistent growth or if they had disturbances; if they are simple or complex, in granite or metamorphic rock. Scientists observe how light reflects from the grains of zircon, and in detail, how it is transmitted through them. This presentation discusses continental growth as measured by the earth’s detrital zircon record. His language does get a little technical at times but I think the basic idea and the scale of the project can be grasped. There’s a lot of information packed in this Wikipedia article about zircon age dating. The FE-SEM imaging was done on a Tescan Mira3 High Resolution Schottky FE-SEM equipped with an Oxford energy dispersive spectrometry (EDS) and a CL system.

As a results, the sputtered pit geometry typically has a diameter of ~15 5o 25 microns, and a depth of ~2 microns. To take advantage of the relatively slow sputter rate, we will mount mineral grains (e.g., zircons) in soft indium metal to expose the euhedral mineral face parallel to the sample surface (see Sample Prep. page for more details about mounting). Using this approach, analyses performed on the sample surface will provide information about the change in composition and/or age with depth below the initial surface. 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.