Radiometric dating or radioactive dating is any technique used to date organic and also inorganic materials from a process involving radioactive decay. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. The radioactive decay law states that the probability per unit time that a nucleus will decay is a constant, independent of time. This constant probability may vary greatly between different types of nuclei, leading to the many different observed decay rates. The radioactive decay of certain number of atoms mass is exponential in time. One of the oldest radiometric dating methods is uranium-lead dating. The long half-life of the isotope uranium 4.
However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating.
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But even the best geologic methods are imperfect. Dating a rock involves uranium-lead measurements on many zirconsthen assessing the quality of the data. Some zircons are obviously disturbed and can be ignored, while other cases are harder to judge. In these cases, the concordia diagram is a valuable tool. Consider the concordia: as zircons age, they move outward along the curve.
But now imagine that some geologic event disturbs things to make the lead escape. That would take the zircons on a straight line back to zero on the concordia diagram. The straight line takes the zircons off the concordia. This is where data from many zircons is important. The disturbing event affects the zircons unequally, stripping all the lead from some, only part of it from others and leaving some untouched. The results from these zircons therefore plot along that straight line, establishing what is called a discordia.
Now consider the discordia.
If a million-year-old rock is disturbed to create a discordia, then is undisturbed for another billion years, the whole discordia line will migrate along the curve of the concordia, always pointing to the age of the disturbance.
This means that zircon data can tell us not only when a rock formed, but also when significant events occurred during its life. The oldest zircon yet found dates from 4.
With this background in the uranium-lead method, you may have a deeper appreciation of the research presented on the University of Wisconsin's " Earliest Piece of the Earth " page, including the paper in Nature that announced the record-setting date. Share Flipboard Email. Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. Age patterns within single grains of zircon and monazite, in particular, can be quite complicated because of these factors and require careful analysis in the petrographic context, especially in metamorphic rocks Cottle et al.
A method of visually displaying measured isotope ratios to better interpret potential complexities resulting from open-system behavior can be attributed to Wetherillwho devised the Concordia diagram. The use of two parent U isotopes and two daughter Pb isotopes is a very useful property; it allows a correction for disturbed systems and recovery of the initial age of a mineral, a feature unique to the U-Pb dating method among all isotope geochronometers.
The theory of this is well-developed in many geochemistry or geochronology textbooks and is not detailed herein. The discovery of radioactivity of uranium was a pivotal event, with recognition that transformation of uranium into other elements was inevitable. Early on, lead was suspected of being the ultimate stable end product of this decay Boltwood Even before the discovery of isotopes by Frederick Soddy in SoddyU-Pb ages of geological materials were calculated, using initial estimates of the half-life or decay rate of uranium.
Although Boltwood was the first to calculate the age of a mineral based on its U-Pb ratio, Arthur Holmes Holmesunder the guidance of his supervisor R J Strutt, determined dates of a suite of igneous-related Devonian minerals of variable U-Pb ratio in Norway by measuring the atomic U-Pb ratio in minerals with improved methods. This study used a combination of appropriate geological samples with clever methods of classical wet chemistry and radiochemistry.
These were of real importance because this began the first numerical calibration of the geological timescale as well as proving that the Earth was much older than 1 Ga. The discovery of isotopes by Soddy had little impact initially to U-Pb dating because the development of mass spectrometry capable of measuring U or Pb isotopes was only achieved decades later.
Alfred Nier, among the most brilliant of all mass spectrometrists of the twentieth century, designed instruments and ion signal amplification devices that could measure accurately and relatively precisely the isotope composition of U and Pb; indeed, his measurement of U isotopes were pivotal in demonstrating that U was the isotope capable of fission by thermal neutron irradiation Nier One of the most important collateral outcomes of atomic energy research was the accurate and precise method of isotope quantification known as isotope dilution.
In this method a known quantity of an enriched isotope of an element i. This method is an extension of the original method using radioactive tracers invented by George de Hevesy infor which he was awarded the Nobel Prize in This procedure is simple and superbly reliable.
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To illustrate how this works, imagine a bucket of unknown quantity of colored balls that are black and white in a ratio of black-white representing the natural isotopes in a sample.
If one adds red balls representing the enriched tracer and mixes the balls up and then measures a ratio of red-black of 0. One does not have to count all the balls to know this; one only needs to know the number of tracer balls added and the ratio i.
This methodology was developed and applied to a wide range of elements and sample materials. It still represents the most accurate and precise manner of isotope quantification in geological samples.
One of the finest early papers to date accessory minerals by these methods is the tour de force of Tilton et al. This was the standard method of U-Pb geochronology until the mids and is still widely used today.
In the isotope dilution method of U-Pb dating, decomposition of samples into a homogeneous solution is a requirement so that tracer and sample isotopes can fully mix prior to chemical separation and isotope analysis by mass spectrometry.
Uranium-lead dating formula
This was a hindrance to widespread application of the U-Pb dating method partly because of the time and expense involved in the procedure but also because zircon commonly loses Pb due to radiation damage of its crystal lattice. When large quantities of zircon were analyzed by this flux method, this inevitably resulted in many discordant dates being produced because of the prevalence of Pb loss in some of the grains; analysis of tiny amounts of high quality zircon was simply not generally possible.
This frustrating reality proved to be a strong motivation to find a better way - to be able to analyze much smaller quantities of zircon and ultimately single grains that were of very high quality not subject to significant Pb loss.
In the late s and early s, two young geologists - Tom Krogh and James Mattinson - devised a solution to this conundrum while doing work early in their careers at the Carnegie Institute of Washington. Together with the silica gel Pb ionization method on a re-thermal ionization filament, it became possible to analyze single grains of zircon or other minerals; a practice that is routine today. Despite these improvements, the issue of Pb loss in zircon still hindered high-precision age determination.
Krogh addressed the problem of zircon discordance by inventing the air abrasion technique to remove the discordant outer part of zircon crystals, which resulted in production of vastly improved concordance i. All of these analytical advances conspired to facilitate a very significant growth in the number of U-Pb laboratories and the quantity of high-accuracy, high-precision zircon dates.
A great advance in Precambrian continental orogenic evolution then proceeded, especially in the Canadian Shield. At the same time, when these methods were applied to complicated igneous and metamorphic zircon, the complexity in zircon U-Pb systematics became apparent, including the phenomenon of zircon inheritance and multiple metamorphic zircon growth periods during high-grade metamorphism.
In such cases, single-grain analysis produced consistently discordant data, and the determination of precise ages of metamorphism or igneous events proved to be a challenge in many samples. Complex zircon growth and alteration were imaged using electron microscopy, suggesting that the complexities in U-Pb data were the result of a combination of lead loss and multiple zircon age domains within single grains.
The paper by Corfu et al. The analysis of zircon on the micron scale became an obvious tantalizing goal, but it presented major technical challenges and was impossible at the time for multi- or single grain zircon analysis by ID-TIMS.
One way forward using ID-TIMS came from accessory minerals like monazite with less inheritance and Pb loss to determine igneous and metamorphic ages Parrish In this article, in situ U-Pb dating refers to the measurement of age of a mineral in a solid state using microbeam sampling techniques, and it includes first SIMS and later LA-ICP-MS methods, as well as the less-used electron microprobe chemical dating method, all of which are described briefly below.
The SIMS method consists of firing a primary highly focused ion beam oxygen or cesium ions onto the surface of polished zircon in order to produce secondary sample ions which are then accelerated and focused into a beam which could be analyzed by a mass spectrometer.
However, in SIMS zircon analysis, a wide range of molecular ions are produced during the primary sputtering process where the beam interacts with the sample. Many of these molecular ions have very similar masses in relation to the Pb isotopes that are of interest for dating, and which occur in very small quantities.
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This development heralded a new period of exploration of zircon as a geochronometer, although the application of the SIMS zircon work to terrestrial rocks did not see wide application until the s. The search for, and characterization of, suitable reference materials for isotopic and elemental analysis is an ongoing effort for in situ analysis, and not just for zircon. Building on earlier work with quadrupole mass spectrometry and ionization using plasma sources, inductively coupled plasma ionization ICP was combined in the early s with multiple-collector mass spectrometry MC-MS using an additional electrostatic analyzer to make the first ICP multi-collector mass spectrometer ICP-MC-MS.
One of the more comprehensive early papers using this method was that of Horstwood et al. These were the precursors to more widespread LA-ICP-MS instruments on the market now, which offer sufficient sensitivity and precision to compete directly with SIMS in situ analysis, but at less cost and with greater sample throughput, though with higher sample consumption. SIMS U-Pb methods are very sensitive to surface polish quality and surface-charging characteristics and require a careful matching of the composition matrix of sample to the reference material.
LA-ICP-MS methods, on the other hand, are more tolerant of surface irregularities, are less sensitive than SIMS methods to sample matrix matching, consume a larger quantity of sample, and have limited ability to measure the common Pb reference isotope Pb due to the interference with Hg in the carrier gas.
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Depth profiling by both methods is possible but is more controlled via SIMS analysis. The exact reason for this limitation is likely some combination of plasma chemical fractionation and laser ablation processes, perhaps compounded by matrix matching issues. Unfortunately, a misunderstanding of the strengths and weaknesses of both isotope dilution methods and in situ SIMS or LA-ICP-MS methods traditionally led to tension between the two methodological communities.
On the other hand, many papers contain published isotope dilution U-Pb zircon data that are discordant and yield non-unique and ambiguous interpretations of age on complex multiple growth-zoned zircons. More often than not, these complex and ambiguous interpretations are from metamorphosed rocks where multiple high-temperature events have produced complicated zircons.
The primary lesson from these examples is that both methods have their strengths, and when used appropriately, rocks and events can be dated in the best possible way using one or both methods. All of these methods have their place in U-Pb geochronology.
U-Pb Zircon Geochronology - for determining the age of a rock
Two further methodological cts deserve mention. Electron microprobe U-Th-Pb analysis of monazite has been done since Suzuki et al. Monazite U-Th-Pb isotope dating in thin sections is the only method able to reveal the ages of very young monazites of Mesozoic-Neogene age, however.
When the residual zircon is analyzed, concordant analyses often result, allowing very precise concordant ages to be determined.
This is such a profoundly important procedure that it has been adopted worldwide as the routine zircon procedure for ID-TIMS methods. Finally, any U-Pb geochronologist will understand clearly the value in accurate and sophisticated data reduction, linear regression, and graphical plotting software to diversely illustrate and determine uncertainties in ages of U-Pb data, the latter being its most important attribute. The most commonly used software to plot data, termed Isoplot, was conceived, written and refined over several decades primarily by Ken Ludwig of the Berkeley Geochronology Center Ludwig It is one of the most important underpinning tools that U-Pb geochronologists use.
Geologists and geochronologists, indeed humans in general, have a tendency to take for granted the technological advances that make our lives easier; it is important to remember the intellectual breakthroughs as well as the sheer effort of scientists who pioneered these advances.
Sep 04, Uranium-Lead dating is the geological age-determination method that uses the radioactive decay of uranium (U) isotopes ( U, U, and also in this entry Th) into stable isotopes of lead (Pb) ( Pb, Pb, and Pb, respectively). U-Pb geochronology is the science of both the methodology but also the application of these methods to geological problems.
These breakthroughs have made the science of U-Pb geochronology as apparently routine as we know it today. The age of igneous events throughout Earth history, plutonic and volcanic, mafic, alkaline, and felsic.
The behavior of intermediate daughter radioactive isotopes and their impact on high-precision dating.
Continental reconstructions over time involving amalgamation and dispersal of continents. Quantification of time in pressure-temperature paths of metamorphic rocks P-T- t paths. Chronostratigraphy, in particular in Precambrian where biostratigraphy is absent. Rates of cooling of orogens and rocks by using U-Pb dates as thermochronometers in the context of closure temperatures.
Identifying the age of sources of igneous rocks by zircon and monazite inheritance. Dating multiple growth zones in complex minerals even at micron-scale dimension. Precise dating of extinction events facilitating the search for causality. Dating of calcite and phosphate of Quaternary age for environmental reconstructions and human evolution. Provenance of sediments, sedimentary rocks, dust, river sand, and ice-rafted glacial debris.
Some of these topics are elaborated in chapters within this volume, but others are less well described. The following section highlights cts of applications that are considered particularly interesting additions to complement the other chapters. The early quest in U-Pb geochronology was mainly about determining the age of the Earth.
Most work in U-Pb geochronology of the s-s concerned dating igneous minerals - zircon and titanite - to constrain igneous events and populate the geological time scale and using the punctuated emplacement of felsic volcanic or plutonic rocks to constrain tectonic and orogenic events.
These topics are dealt with in other chapters. Zircon became the mineral of choice mainly because it was realized early on that it was very refractory, resisted resetting, and could reveal igneous events in spite of subsequent alteration and metamorphism. Indeed, the work of Gulson and Krogh and Copeland et al. Beginning aroun the mineral baddeleyite ZrO 2 began to be dated in diabase and gabbroic rocks by U-Pb methods Heaman and LeCheminantallowing reliable and precise ages of mafic magmas, including ophiolites to be dated.
This development was paired quickly with palaeomagnetism of such rocks dyke swarms, major mafic igneous provinces to more robustly reconstruct the whereabouts of continental fragments dispersed by rifting and to determine the positions of continents on the Earth more accurately throughout geological time. Kimberlite dating by U-Pb is dealt with in a related chapter using the minerals perovskite and baddeleyite.
In dating: Principles of isotopic dating. In uranium-lead dating, minerals virtually free of initial lead can be isolated and corrections made for the trivial amounts present. In whole-rock isochron methods that make use of the rubidium-strontium or samarium-neodymium decay schemes, a series of rocks or minerals are chosen that can be assumed. Uranium-lead dating formula - Men looking for a woman - Women looking for a man. Register and search over 40 million singles: matches and more. Is the number one destination for online dating with more marriages than any other dating or personals site. Radioactive decay law: N = N 0.e-?t. One of the oldest radiometric dating methods is uranium-lead truthexchange-sow.com age of the earth's crust can be estimated from the ratio between the amounts of uranium and lead found in geological specimens.
U-Pb geochronology of igneous zircon and its application to geological problems are addressed in other chapters of this volume. In terms of the oldest materials identifiable on the Earth, the detrital zircons of quartzose metasedimentary rocks Mt Narryer quartzite; Jack Hills conglomerate of Western Australia have been a gold mine of information. The geochemical characteristics of these old grains Hf isotopes, rare earth elements REE spectra, oxygen isotopes, Ti thermometry, inclusion mineral characteristics have been researched at length Amelin and Ireland ; these are mentioned in a related chapter.
Hf isotope studies by many authors on these old zircons reveal that most if not all are derived from mantle reservoirs that are depleted to chondritic in composition, indicating that significant silicate mantle differentiation took place very early in Earth history.
A related chapter describes the application of U-Th-Pb dating of monazite, xenotime, allanite, rutile, and titanite to the chronology of metamorphism. The chemistry of these accessory minerals makes it a challenge to relate their growth directly to the pressure and temperature conditions of metamorphism, since they do not form directly from reactions of P- and T-sensitive minerals like garnet, aluminosilicates, micas, etc.
Nevertheless, studies of Smith and Barreiro showed how monazite grows in amphibolite facies conditions, appearing usually in the staurolite zone of Barrovian metamorphism, a generalization that still appears to hold. Other minerals such as allanite preserve reaction relationships to apatite and monazite and at times other more abundant rock-forming minerals.
This allows monazite chemistry to be used to relate its growth to conditions of metamorphism defined by other pelitic minerals. Clearly, the textural relationships of these accessory minerals in relation to the fabric of other rock-forming minerals are crucial pieces of evidence concerning the relative age of accessory minerals.
For example, inclusions of accessory minerals within other P-T-sensitive phases garnet, coesite, diamond, etc. In these geological situations, the use of in situ methods is usually a requirement, along with data on mineral chemistry, phase equilibria, textural data, geological setting, and field relations. In situ methods need to measure wherever possible all U-Th-Pb isotopes, not just U-Pb, for monazite, due to the problem of excess Pb in Th-rich monazite Parrishin order to determine accurate ages in young metamorphic rocks.
Often, multiple minerals dated in the same sample will reveal a richer and more complete metamorphic history.
A particularly common characteristic, which is increasingly documented in detailed studies of metamorphic rocks, is that monazite growth takes place over a long period of time, including prograde, peak, and in part retrograde conditions. One of the challenges in doing this in situ work is that reference minerals of well-determined age suitable for age calibration may not exist, prompting the search for better mineral standards, for example, for rutile and allanite Bracciali et al.
Although many minerals datable by U-Pb methods were initially regarded as having high closure temperatures, many of these can be effectively used as thermochronometers. In contrast to zircon and monazite, it has long been documented that the retention of Pb by titanite, allanite, rutile, and apatite is not complete during amphibolite facies conditions of metamorphism or reheating. Early work in case studies showed clearly that U-Pb dates on titanite, rutile, and apatite were generally younger than the peak of metamorphism, often much younger.
More recent experimental diffusion studies have generally confirmed these estimates. The identification of the source of clastic sedimentary rocks, or provenance analysis, enables a reconstruction of part of the geological history. The first lines of evidence are clast composition in conglomerate and the composition of grains lithic, quartz, feldspar, heavy minerals in sandstone.
However, without further information, few rocks can be proven to be derived from a specific source. U-Pb dating of detrital minerals provides probably the most important information allowing the discrimination of source areas for clastic rocks. Detrital mineral dating began to be tractable once single minerals could be dated.
This work began in the mid-late s using single-grain ID-TIMS dating, but the tedious ct of the dating meant that few grains could be dated in a given study. The appeal of this type of data captured the imagination of geologists, and a new field of research increasingly gained momentum through the s and in recent years. Some of these studies emphasized the desirability of systematically examining the detrital zircon signatures of formations in continents and in orogens, whereas other studies applied this single-grain dating technique to specific problems, for example, the determination of the maximum age of sedimentation of a Precambrian sedimentary rock for which few depositional age constraints existed.
This trend involves generating a huge amount of data often illustrated in probability distribution plots that show both uncertainties and abundance of data in only semiquantified form. Although the data is usually published in supplementary tables online, the interpretation of detailed data is a challenge to obtain as an interested reader. This can compromise the robustness of such datasets. A great variety of quality of data from such studies exists, and this diversity in quality continues to be published.
Readers are cautioned to examine data thoroughly and question whether interpretations made are sufficiently rigorous! Many geological questions pertaining to provenance of sediments simply cannot be answered by zircon single-grain dating alone. Excellent science demands a broad approach that is tailored to the question being addressed and which uses the most appropriate isotope methodology.
A chapter on U-Pb applications to diagenetic events is part of this volume and presents methods for addressing this topic. A key part of this field is the application of U-Pb methods both ID-TIMS and in situ methods to carbonate calcite, aragonite and phosphate that occur as cement or as primary sedimentary materials deposited usually from marine or freshwater fossils, cements, speleothems, lake carbonate, carbonate veins, hominin or other animal phosphate.
While there are currently few extensive studies published, it is clear that this is a field that will grow rapidly in the near future. U is partitioned into aragonite in marine fossils, and calcite includes sufficient U to also be dated, when the initial incorporation of lead into the mineral is low, as is often the case with precipitation of calcite from groundwater. Indeed it is now tractable to apply LA-ICP-MS using high-sensitivity instrumentation to date calcite in situ even when the U content is not particularly high, via the use of a calcite standard.
This field of research will likely explode in popularity and demand over the next 10 years. The field of U-Pb dating has come a long way since its inception by the work of Boltwood and Holmes about years ago in the period immediately following the discovery of radioactivity. With the development of sufficiently sensitive mass spectrometry by Nier in the -s and the availability of isotope tracers of U and Pb in the s and s, the field of ID-TIMS exploded with refinement of methods, use of Teflon acid distillation and mineral dissolution vessels and multi-collector high-sensitivity mass spectrometers.
Calculates the dating in the past from the ratio of Uranium in natural uranium. Current ratio of U in natural uranium is the same anywhere in the solar system. Because the half-life is different with U and U, the higher the percentage of U so retroactively. ratio at the dating . Feb 10, Of all the isotopic dating methods in use today, the uranium-lead method is the oldest and, when done carefully, the most reliable. Unlike any other method, uranium-lead has a natural cross-check built into it that shows when nature has tampered with the evidence. The long half-life of the isotope uranium (?10 9 years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating, including uranium-thorium dating and uranium-uranium dating. Uranium-lead dating is based on the measurement of the first and the last member of the.
Recognition of complex zircon growth histories demanded the development of better ways to analyze concordant zircons and develop in situ methods, which in the past 20 years have exploded in availability and popularity.
This has brought U-Pb geochronology within reach to geologists, some of whom never worked in a laboratory previously. Nevertheless, the advancement in methods is very impressive, both intellectually and technologically. Innovative applications of U-Pb dating to geological problems have generally kept pace with technology, because geological questions usually precede the availability of efficient and accurate instrumentation and methods.
For example, widespread detrital single-grain dating had to wait until efficient in situ methods of analysis of large numbers of grains became available. The use of single grain rutile U-Pb dating in provenance studies will become more common following the recent development of rutile reference materials and sensitive LA-ICP-MS methods Bracciali et al.
Similarly, the availability of calcite U-Pb reference materials and sensitive mass spectrometry capable of measuring young ages precisely in minerals with low U contents is just now tractable; this will cause a rapid increase in demand and application of U-Pb dating to calcite and Quaternary materials, hardly addressed at the present time.
Collaboration among U-Pb laboratories has been essential because of the inherent ability of ID-TIMS methods to measure isotope ratios apparently more precisely than one can confidently calibrate isotope tracer solutions.
For example, age solutions and mixed U- U- Pb isotope tracers are available to world laboratories for the purpose of improving the precision and accuracy of data produced. This effort has been extended, for example, to U-series dating and to in situ methods of U-Pb dating, so that numerous world laboratories are able to compare data, assess their own performance against international standards, and share the best practice.
This type of collaborative philosophy, combined with continued technological and method innovation, is the way forward for this important area of geochronology.
Geological Time Scale. Mass Spectrometry. Radiation and Radioactivity. Tectonic Processes Thermochronology.