Detrital zircon geochronology is a scientific technique for understanding the age and provenance of sedimentary deposits. To determine the age of zircons deposited within a specific sedimentary unit, mass spectrometry is used to measure the abundance of radioisotopes in the grains, most commonly the uranium–lead ratio. Zircon is a common accessory or trace mineral constituent of most granite and felsic igneous rocks. Due to its hardness, durability and chemical inertness, zircon persists in sedimentary deposits and is a common constituent of most sands. Zircons contain trace amounts of uranium and thorium and can be dated using several modern analytical techniques.
Detrital zircon geochronology has become increasingly popular in geological studies from the 2000s mainly due to the advancement in radiometric dating techniques. Detrital zircon age data can be used to constrain the maximum depositional age, determine provenance, and reconstruct the tectonic setting on a regional scale.
Detrital zircon
Origin 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.
Properties Detrital zircons usually retain similar properties as their parent igneous rocks, such as age, rough size and mineral chemistry. However, the composition of detrital zircons is not entirely controlled by the crystallization of the zircon mineral. 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. On a larger scale, two or more tribes of detrital zircons from different origins may deposit within the same sedimentary basin. This gives 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:Gehrels, G. (12 August 2010). UThPb analytical methods for Zircon. Arizona LaserChron Center. Retrieved 10 November 2016, from https://drive.google.com/file/d/0B9ezu34P5h8eMzkyMGFlNjgtMDU0Zi00MTQyLTliZDMtODU2NGE0MDQ2NGU2/view?hl=en. Zircon contains high amount of uranium for machine recognition, commonly 100–1000 ppm. Zircon has a low amount of lead during crystallization, in parts per trillion. Thus, lead found in zircon can be assumed as daughter nuclei from parent uranium. Zircon crystals grow between 600 and 1100 °C, while lead is retained within the crystal structure below 800 °C (see Closure temperature). So once zircon has cooled below 800 °C it retains all the lead from the radioactive decay. Therefore, U-Pb age can be treated as the age of crystallization, if the mineral/sample itself has not undergone high temperature metamorphism after formation. Zircon commonly crystallizes in felsic igneous rocks, with greater than 60% silica (SiO2) content. These rocks are generally less dense and more buoyant. They sit high in the Earth's (continental crust), and have good preservation potential. Zircon is physically and chemically resistant, so it is more likely to be preserved in the sedimentary cycle. Zircon contains other elements which gives supplementary information, such as hafnium (Hf), uranium/thorium (U/Th) ratio.
Sample collection 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 result. Examples include: Matured quartz arenite within Vlamy Formation yield older and more diverse ages given by well-rounded detrital zircons, which may correlate to multiple sedimentary reworking events. On the contrary, Harmony Formation in the same region has younger and homogenous ages given by euhedral detrital zircons. These two formations illustrate the possibility of relating sedimentary maturity with resulting zircon ages, meaning that rounded and well-sorted sedimentary rocks (e.g. siltstone and mudstone) may have older and more diverse ages. Turbidites in Harts Pass Formation contain homogenous detrital zircons ages. On the other hand, fluvial Winthrop Formation in another strata of the same basin has various detrital zircon age populations. Comparing the vertical detrital zircon distribution within these two formations, one can expect a narrower age population of detrital zircons from rocks which are rapidly deposited, such as turbidites. Rocks that are gradually deposited (e.g. marine mudstone), however, have a greater chance and time to incorporate zircon sediments from different localities.DeGraaff-Surpless, K., McWilliams, M. O., Wooden, J. L., & Ireland, T. R. (2000). Limitations of detrital zircon data for provenance analysis: an example from the Methow Basin, Washington and British Columbia. In Geol Soc Am Abstr Progr (Vol. 32, No. 9).
Detrital zircon extraction 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.Chisholm, E. I., Sircombe, K. N. and DiBugnara, D. L. 2014. Handbook of Geochronology Mineral Separation Laboratory Techniques. Record 2014/46. Geoscience Australia, Canberra. In the process, grains are also sieved according to their size. The commonly used grain size for detrital zircon provenance analysis is 63–125 μm, which is equivalent to fine sand grain size.
Type of detrital zircon analysis There are two main types of detrital zircon analysis: qualitative analysis and quantitative 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 analysis Qualitative approach examines all the available detrital zircons individually regardless of their abundance among all grains. This approach is usually conducted with high precision thermal ionization mass spectrometry (TIMS) and sometimes secondary ion mass spectrometry (SIMS). Optical examination and classification of detrital zircon grains are commonly included in qualitative studies through back-scatter electrons (BSE) or cathodoluminescence (CL) imagery, despite the relationship between the age and optical classification of detrital zircon grains is not always reliable.
Quantitative analysis Quantitative approach requires large number of grain analyses within a sample rock in order to represent the overall detrital zircon population statistically (i.e. the total number of analyses should achieve an appropriate level of confidence). Because of the large sample size, secondary ion mass spectrometry (SIMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) are used instead of thermal ionization mass spectrometry (TIMS). In this case, BSE and CL imagery are applied to select the best spot on a zircon grain for acquiring reliable age.
Methods Different methods in detrital zircon analysis yield different results. Generally, researchers would include the methods/ analytical instruments they used within their studies. 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. Table 1. Different types of analytical methods in detrital zircon studyType of instrument for zircon analysisIn modern research, common instruments for U-Pb analysis are sensitive high-resolution ion microprobe (SHRIMP), inductively coupled plasma mass spectrometry (LA-ICPMS) and thermal ionization mass spectrometry (TIMS). Ion microprobe (non-SHRIMP) and lead-lead evaporation techniques were more commonly used in older research.Zircon calibration standardsBasically analytical machines need to be calibrated before use. Scientists use age-similar (comparable to the sampled zircons) and accurate zircons as their machine calibration standards. Different calibration standards may give slight deviation of the resulting ages. For example, there are at least twelve different standards catering for different sample zircons in Arizona Laserchron Center, primarily using Sri Lanka zircon, followed by Oracle.Type of instrument for zircon imageryInstrumentsUsageFor macroscopic view (Gives the general appearance of the zircon, cannot identify internal zircon texture properly, especially when the zircon is neither zoned nor metamictized)Binocular microscope (BM)Can examine zircon grain as a whole: color, transparency, crystal morphology and form growth, inclusions, fractures and alterations.Transmitted light microscopy (TLM)Can examine zircon growth zoning and metamictization in cross-polarized light.
Challenging for small zircon grains due to limited resolution.
Difficult to identify zircon from other high-relief and high-birefringence minerals (such as monazite).Reflected light microscopy (RLM)Can examine zircon growth zoning, alteration and metamictization.Krogh TE, Davis GL (1975) Alteration in zircons and differential dissolution of altered and metamict zircon. Carnegie Inst Washington Yrbk74:619–623For zircon internal structureUranium Mapping (UM)Induce fission tracks within the zircon by neutron flux reactor and record the tracks into an image. Has implications on the amount and distribution of radioactive elements (i.e. uranium) within the zircon grain.Cathodoluminescence (CL)One of the best resolution instruments. Induced CL by bombarding zircon with electrons, where U4+ ions and radiation damages suppress CL and give darker bands.
Different colored CL emission may imply the presence of different element, such as blue (Y3+) and yellow (Ti4+ or U4+)Back-scattered electron microscopy (BSM)Also one of the best resolution instruments currently. Almost like a reversed CL imagery, as the brightness correlates to atomic number. The brightness/ color intensity in BSM is primarily due to hafnium (Hf), with uranium (U) being second.Secondary electron microscopy (SEM)See scanning electron microscope.
Detrital zircon data Depending on the detrital zircon study, there should be different variables included for analysis. There are two main types of data, analyzed zircon data (quantifiable data and imagery/descriptive data), and sample (where they extract the zircon grains) data. Details are listed in Table 2. Table 2. Different types of data in detrital zircon studyWelcome to Geochron | EarthChem. (n.d.). Retrieved 15 November 2016, from https://www.geochron.org/DataExplanationAnalyzed zircon dataQuantifiable dataGrain NumberGrain number is necessary for multiple detrital zircon grains yielded within the same sample rockU contentUranium content, usually in ppm.Th contentThorium content, usually in ppm.Th/U ratioThorium content divided by uranium content. Most of the detrital zircon grain origins can be identified through Th/U ratio, where Th/U < 0.01 implies possible metamorphic origin and Th/U > 0.5 implies igneous origin. Intermediate origin lies between 0.01 and 0.5.206Pb/238UIsotope ratios measured by instrument for further age calculation.207Pb/206Pb207Pb/235UObtained by calculation since 238U/235U is constant (137.82), i.e. 206Pb/204PbMeasured to correct the amount of lead incorporated into the zircon during initial crystallization.The three resulting ages and their uncertaintiesAges (Ma) are calculated with the associated decay constants, (see Uranium–lead dating)