The eclogite sample contains large garnet porphyroblasts up to several centimetres in diameter and a second major population of smaller, millimetre-sized garnet grains. Elemental compositions and mineral inclusions in garnet suggest that the large garnet crystal formed during an early episode of metamorphism, with the abundant small-sized garnets in the matrix growing during a late metamorphic period contemporaneous with the overgrowth of the rim of the large garnet crystal. Lu—Hf dates from each section are consistently older than the corresponding Sm—Nd dates. Petrographic and chemical observations of the eclogite show that three generations of garnet growth occurred, with the latter two generations coincident with growth of the rim of the large garnet, and that of the smaller matrix garnets. Five micro-sawed growth zones were analysed for Lu—Hf and Sm—Nd isotopes. The calculated Lu—Hf dates from the mega garnet record garnet growth initiation at about Ma, during prograde to peak metamorphic conditions of the first orogenic episode, lasting about 11 Ma.
Dr. Audrey Bouvier
Slowly and painstakingly, geologists have assembled this record into the generalized geologic time scale shown in Figure 1. This was done by observing the relative age sequence of rock units in a given area and determining, from stratigraphic relations, which rock units are younger, which are older, and what assemblages of fossils are contained in each unit. Using fossils to correlate from area to area, geologists have been able to work out a relative worldwide order of rock formations and to divide the rock record and geologic time into the eras, periods, and epochs shown in Figure 1.
Therefore, it’s not surprising that Lu-Hf dating is yet another target for the wrath of young-Earth creationists. Nevertheless, the only response that Woodmorappe (, p. 68) has for the Lu-Hf method is a brief and inadequate statement, which claims that the method has “severe problems.
Papers Application of Lu—Hf garnet dating to unravel the relationships between deformation, metamorphism and plutonism: An example from the Prince Rupert area, British Columbia The tectonic history of the Prince Rupert area is marked by profound crustal thickening from thru Syn-tectonic garnet in the Prince Rupert area has Lu—Hf ages of Porphyroblast—matrix relationships in these samples, and samples from the same outcrops, indicate syn-tectonic garnet growth. These relationships imply that the garnet ages directly date the development of the metamorphic foliations.
A third sample of migmatitic garnet amphibolite from the contact aureole of the 94—90 Ma Ecstall pluton had complex isotope systematics interpreted to indicate a garnet growth episode not, vert, similar Ma, similar to the ages obtained from the other samples and a growth or equilibration event at not, vert, similar 94 Ma during pluton emplacement. The data show that the older Lu—Hf garnet ages date prograde metamorphism during foliation development and modification during pluton emplacement.
The Ecstall pluton was emplaced 10 to 15 m. In order to place our samples in a regional tectonic context we compare our results to patterns of regional deformation, metamorphism and plutonism throughout the North American Cordillera in the time period between and 85 Ma. Contractional and transpressional deformation occurred throughout much of the North American Cordillera at this time, from southeastern Alaska to the Baja Peninsula in Mexico.
Left-lateral transpression dominated the Canadian Cordillera, whereas, right-lateral transpression affected areas south of the Idaho—Salmon River suture zone, including the Sierra Nevada batholith. This reversal in kinematics in the northern and southern cordillera within coeval magmatic belts appears to be a first-order feature of the geology of the North American Cordillera during the Cretaceous.
How Old is the Earth
Il decadimento radioattivo[ modifica modifica wikitesto ] Esempio di un decadimento radioattivo , dal Piombo Pb al Piombo Pb. Alcuni nuclidi sono instabili: Molte sostanze radioattive decadono da un isotopo fino ad un isotopo stabile figlio attraverso una serie di passaggi conosciuta come catena di decadimento. Gli isotopi utili per la datazione radiometrica sono quelli con vita media da poche migliaia di anni fino a miliardi di anni.
Lu – Hf and Sm Nd dating of garnets has been – undertaken on twenty one samples of pelites, granulites, amphibolites and eclogites from the Scottish Northern.
Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. The final decay product, lead Pb , is stable and can no longer undergo spontaneous radioactive decay. All ordinary matter is made up of combinations of chemical elements , each with its own atomic number , indicating the number of protons in the atomic nucleus.
Additionally, elements may exist in different isotopes , with each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide.
This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture. Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-life , usually given in units of years when discussing dating techniques.
After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a “daughter” nuclide or decay product.
The Age of the Earth
Full citation Abstract The Lu— Hf isotopic system is widely used for dating and tracing cosmochemical and geological processes, but still suffers from two uncertainties. First, Lu—Hf isochrons for some early Solar System materials have excess slope of unknown origin that should not be expected for meteorites with ages precisely determined with other isotopic chronometers.
This observation translates to an apparent Lu decay constant higher than the one calculated by comparing ages obtained with various dating methods on terrestrial samples. In order to better understand the Lu—Hf systematics of chondrites, we analyzed mineral fractions from the Richardton H5 chondrite to construct an internal Lu—Hf isochron, and set up a numerical model to investigate the effect of preferential diffusion of Lu compared to Hf from phosphate, the phase with the highest Lu—Hf ratio in chondrites, to other minerals.
Combining this study with the phosphate fractions measured in a previous study yields a slope of 0. This is critical as apatite has substantially higher diffusion rates of rare earth elements than most silicate minerals that comprise stony meteorites.
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This article has been cited by other articles in PMC. Abstract New Rb-Sr, , Sm , Nd and Lu-Hf isotopic analyses of Mg-suite lunar crustal rocks , , and , including an internal isochron for norite , were undertaken to better define the time and duration of lunar crust formation and the history of the source materials of the Mg-suite. Isochron ages determined in this study for are: The initial Nd and Hf isotopic compositions of all samples indicate a source region with slight enrichment in the incompatible elements in accord with previous suggestions that the Mg-suite crustal rocks contain a component of KREEP.
These data, along with similar ages for various early Earth differentiation events, are in accord with the model of lunar formation via giant impact into Earth at ca 4. Introduction The time and duration of lunar highlands crust formation plays an unusually important role in models for lunar origin because of the suggestion that much of the highlands crust grew by plagioclase flotation from a crystallizing magma ocean [ 1 ].
In the magma ocean model for the Moon, an initially extensively molten Moon first crystallized mafic silicates that sank into the mantle to form the source regions of much later mare basalt magmatism [ 2 , 3 ]. The extraction of plagioclase from the magma ocean imprinted the later mafic cumulates in the lunar interior with a deficiency in Eu relative to neighbouring rare earth elements REEs that is reflected in the negative Eu anomalies of some mare basalts [ 5 ]. Further crystallization resulted in a residual liquid strongly enriched in incompatible elements that was given the name KREEP for its enrichment in potassium, REE and phosphorus, among many other incompatible elements [ 6 ].
Reference to a case where the given method did not work This is perhaps the most common objection of all. Creationists point to instances where a given method produced a result that is clearly wrong, and then argue that therefore all such dates may be ignored. Such an argument fails on two counts:
To these data sets, we add Sm‐Nd and Lu‐Hf dating of garnets from the base of the arc, 40Ar/39Ar hornblende and mica ages from all levels of the arc plutonic section, and U‐Th/He ages of the arc plutonic section (Table 1 and auxiliary material).1 3. Analytical Techniques.
However, the increase in geochronological constraints over the past five years indicates that progress is possible. Techniques The determination of ‘absolute’ age constraints for Neoproterozoic successions can be achieved using a variety of geochronological techniques. These include whole rock approaches using Re-Os, Pb-Pb and Lu-Hf decay schemes as well as U-Pb dating of zircon from volcanic rocks to directly date the horizon sampled, and detrital zircon to constrain the maximum depositional age.
While the database of geochronological constraints for the Neoproterozoic is growing the data are of variable precision and accuracy and typically subject to multiple assumptions and interpretations. These assumptions are difficult to evaluate in many cases and it is possible to produce statistically significant linear arrays on isotope correlation diagrams that are mixing lines and have no geological significance e.
However, new developments in Re-Os dating of black shales show considerable promise. While it is tempting to use all available geochronological data in compilations irrespective of decay scheme and precision, caution is urged. In contrast, ion-probe single spot analyses have typical analytical uncertainties of about percent and are reliant upon the pooling of datasets with lots of samples in order to calculate dates with uncertainties of about 1 percent.
What do we know?
Canadian Journal of Earth Sciences
Physical characteristics[ edit ] Pieces of hafnium Hafnium is a shiny, silvery, ductile metal that is corrosion -resistant and chemically similar to zirconium  due to its having the same number of valence electrons , being in the same group, but also to relativistic effects ; the expected expansion of atomic radii from period 5 to 6 is almost exactly cancelled out by the lanthanide contraction. The physical properties of hafnium metal samples are markedly affected by zirconium impurities, especially the nuclear properties, as these two elements are among the most difficult to separate because of their chemical similarity.
The most notable nuclear properties of hafnium are its high thermal neutron-capture cross-section and that the nuclei of several different hafnium isotopes readily absorb two or more neutrons apiece. Hafnium dioxide Hafnium reacts in air to form a protective film that inhibits further corrosion. The metal is not readily attacked by acids but can be oxidized with halogens or it can be burnt in air.
Early (≥ Ga) formation of terrestrial crust: Lu–Hf, δ18O, and Ti thermometry results for Hadean zircons Sample details and U–Pb dating The zircon-bearing rocks in the Jack Hills form part of a thick (N2 km) series of fan delta sequences deposited in a fault-.
Full citation Abstract The Neoproterozoic Doushantuo Formation, South China, preserves a unique assemblage of early multicellular fossils and overlies rocks, which are thought to have formed during an ice age of global extent. The age of this formation is thus critical for understanding the important biological and climatic events that occurred towards the end of the Proterozoic Eon. Until now, direct dating of sedimentary formations such as the Doushantuo has been difficult and associated with large uncertainties.
These ages are in agreement with bio- and chemostratigraphical observations and show that the Doushantuo animal remains predate diverse Ediacaran fossil assemblages, making them the oldest unambiguous remains of metazoans currently known. Furthermore, the Pb-Pb age for the post-glacial Doushantuo rocks suggests that the Neoproterozoic glaciation in China might predate glacial rocks in Eastern North America commonly associated with the younger Marinoan of two major Neoproterozoic glaciations.
The combination of Lu-Hf and Pb-Pb dating shows considerable potential for dating other phosphorite successions and future application of these methods could therefore provide further constraints on Proterozoic biological and environmental history. Organismic and Evolutionary Biolog Topics:
The Age of the Earth
Providing customized analytical solutions at the highest standards of quality assurance and quality control. Geochronology U-Pb Dating Ar-Ar Dating Lu-Hf Dating U-Pb Dating Samples for U-Pb dating are processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator.
Four binocular microscope work stations are available for sample picking. The external morphology of mineral grains for analysis can be documented by SEM, and internal structure can be examined in polished grain mounts by cathodoluminescence imaging. We have dated rocks from Pliocene to Archean in age, for clients from universities, government and industry.
Because of the low concentrations of Lu and Hf and the very long half-life of Lu 35, million years , the accurate use of the Lu-Hf method has been limited until recent technological advances Dalrymple, , p. The Lu-Hf method is now being effectively used to date certain rocks, especially meteorites Dalrymple, , p. As with any radiometric method, the ancient dates from the Lu-Hf method utterly refute the claims of young-Earth creationism. Therefore, it’s not surprising that Lu-Hf dating is yet another target for the wrath of young-Earth creationists.
Nevertheless, the only response that Woodmorappe , p. The Lu-Hf system is demonstrably sensitive to the mineralogy of magma sources in the mantle and crust. However, does this statement by Scherer et al. The answer is no. As with many of Woodmorappe’s quotations, when they are taken in context, they actually fail to support his antiquated claims. Specifically, the statements of Scherer et al.
The Lu-Hf system is demonstrably sensitive to the mineralogy of magma sources in the mantle and crust As a result, the Lu-Hf system provides a view of crust-mantle evolution that is unique as compared to more commonly used isotopic systems such as Rb-Sr, Sm-Nd, and U-Pb. Rather than undermining the reliability of the Lu-Hf method, the sensitivity of the method in identifying and dating magma sources is actually an asset!
Whereas the Lu-Hf system commonly follows the same sense of fractionation as Sm-Nd during differentiation processes i.
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Oxygen isotope composition of North American bobcat and puma bone phosphate: Implications for provenance and climate reconstruction. Isotopes in Environmental Science and Health Studies, accepted. Preservation of successive diagenetic events in Middle Triassic bonebeds:
Description Edited by two very well-known and respected scientists in the field, this excellent practical guide is the first to cover the fundamentals and a wide range of applications, as well as showing readers how to efficiently use this increasingly important technique.
One of my main research fields is evolution of continental collision zones such as the Himalaya or the Carpathians. Geochronology is of my particular interest, especially means of linking isotopic ages with specific geologic events. Recently I focus on application of isotopes to environmental and archaeological problems. Isotopic systematics and trace element partitioning under extreme conditions. Migration of mammals in Ice Age Reconstruction of migration paths of mammals during ice age by studying Sr isotopic signature and trace element record in teeth of mammoths and other animals.
Methodology Improvement of chemical procedures and mass spectrometry protocols for isotopic ratios determination in solutions and in situ with the use of laser ablation MC IPMS. Three potential pollution sources of the Arctic region: Revised Middle-Upper Jurassic strontium isotope stratigraphy. Chemical Geology w druku Chakraborty, S.