Heavy Metal Clocks, U-Pb and Th-Pb Dating Models: Radioactive Dating, Part 7
However, it is important to remember that all radiometric dating methods are based on three main assumptions: The highly speculative nature of all radiometric dating methods becomes apparent when one realizes that none of the above assumptions is either valid or provable. Put simply, none of these assumptions can have been observed to have always been true throughout the supposed millions of years the radioactive elements have presumed to have been decaying.
Of the various radiometric methods, uranium-thorium- lead U-Th-Pb was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated. Pb is also prone to diffusion from minerals. In the conclusion to a recent paper exposing shortcomings and criticising the validity of the popular rubidium-strontium Rb-Sr isochron method, Zheng wrote:. This problem cannot be overlooked, especially in evaluating the numerical time scale.
Zheng documented the copious reporting of this problem in the literature where various names had been given to these anomalous isochrons, such as apparent isochron, mantle isochron and pseudoisochron; secondary isochron, inherited isochron, source isochron, erupted isochron, mixing line, and mixing isochron.
Similar anomalous or false isochrons are commonly obtained from U- Th-Pb data, which is hardly surprising given the common open system behaviour of the U- Th-Pb system. The Koongarra uranium deposit occurs in a metamorphic terrain that has an Archaean basement consisting of domes of granitoids and granitic gneisses the Nanambu Complex , the nearest outcrop being 5 km to the north see Figure 1.
Multiple isoclinal recumbent folding accompanied metamorphism. The lower member is dominated by a thick basal dolomite and passes transitionally upwards into the psammitic upper member, which is largely feldspathic schist and quartzite. A Ma period of weathering and erosion followed metamorphism. A thick sequence of essentially flat-lying sandstones the Middle Proterozoic Kombolgie Formation was then deposited unconformably on the Archaean-Lower Proterozoic basement and metasediments.
At Koongarra subsequent reverse faulting has juxtaposed the lower Cahill Formation schists and Kombolgie Formation sandstone. Owing to the isoclinal recumbent folding of metasedimentary units of the Cahill Formation, the typical rock sequence encountered at Koongarra is probably a tectono-stratigraphy see Figure 3: Polyphase deformation accompanied metamorphism of the original sediments, that were probably dolomite, shales and siltstones.
There are two discrete uranium orebodies at Koongarra, separated by a m wide barren zone see Figure 2. The main No. Secondary uranium mineralisation is present in the weathered schists, from below the surficial sand cover to the base of weathering at depths varying between 25 and 30 m see Figure 3. This secondary mineralisation has been derived from decomposition and leaching of the primary mineralised zone, and forms a tongue-like fan of ore-grade material dispersed down-slope for about 80 m to the southeast.
True widths average 30 m at the top of the primary mineralised zone but taper out at about m below the surface and along strike. Superimposed on the primary prograde metamorphic mineral assemblages of the host schist units is a distinct and extensive primary alteration halo associated, and cogenetic, with the uranium mineralisation see Figure 3. This alteration extends for up to 1. The outer zone of the alteration halo is most extensively developed in the semi-pelitic schists, and is manifested by the pseudomorphous replacement of biotite by chlorite, rutile and quartz, and feldspar by sericite.
Silicification has also occurred in fault planes and within the Kombolgie Formation sandstone beneath the mineralisation, particularly adjacent to the reverse fault. Association of this outer halo alteration with the mineralisation is demonstrated by the apparent symmetrical distribution of this alteration about the orebody. In the inner alteration zone, less than 50 m from ore; the metamorphic rock fabric is disrupted, and quartz is replaced by pervasive chlorite and phengitic mica, and garnet by chlorite.
Uranium mineralisation is only present where this alteration has taken place. The primary ore consists of uraninite veins and veinlets mm thick that cross-cut the S 2 foliation of the brecciated and hydrothermally altered quartz-chlorite schist host. Groups of uraninite veinlets are intimately intergrown with chlorite, which forms the matrix to the host breccias. Small m m euhedral and subhedral uraninite grains are finely disseminated in the chloritic alteration adjacent to veins, but these grains may coalesce to form clusters, strings and massive uraninite.
Galena is the most abundant, commonly occurring as cubes mm wide disseminated in uraninite or gangue, and as stringers and veinlets particularly filling thin fractures within uraninite. Galena may also overgrow clausthalite, and replace pyrite and chalcopyrite. Chlorite, predominantly magnesium chlorite, is the principal gangue, and its intimate association with the uraninite indicates that the two minerals formed together.
Oxidation and alteration of uraninite within the primary ore zone has produced a variety of secondary uranium minerals, principally uranyl silicates. Within the primary ore zone this in situ replacement of uraninite is most pronounced immediately above the reverse fault breccia, and this alteration and oxidation diminish upwards stratigraphically. It is accompanied by hematite staining of the schists, the more intense hematite alteration in and near the reverse fault breccia being due to hematite replacement of chlorite.
The secondary mineralisation of the dispersion fan in the weathered schist above the No. Away from the tail uranium is dispersed in the weathered schists and adsorbed onto clays and iron oxides. The mineralisation, however, must post-date both the Kombolgie Formation sandstone and the Koongarra reverse fault, since it occupies the breccia zones generated by the post Kombolgie reverse faulting. The pattern of alteration which is intimately associated with the ore also crosses the reverse fault into the Kombolgie sandstone beneath the ore zone, so this again implies that the ore was formed after the reverse fault and therefore is younger than both the Kombolgie sandstone and the reverse fault.
Because of these geological constraints, Page et al. Sm-Nd isotopic data obtained on Koongarra uraninites 9 , 10 appears to narrow down the timing of mineralisation to Ma. It is unclear as to when deep groundwater circulation began to cause oxidation and alteration of the primary uraninite ore at depth, but Airey et al.
There are five main lines of independent evidence that the mineral-rock systems at Koongarra have been open to diffusion and migration of U, Th and daughter isotopes including Pb. Mineralogical and textural studies of the ore under both optical and scanning electron microscopes 12 , 13 indicate that there have been as many as three remobilisations of the uranium during the history of the ore. Pb has likewise been mobile. This is shown diagrammatically in Figure 4 as several generations of uraninite and galena.
Figures illustrate examples of the ore textures under the microscopes, the accompanying descriptions indicating how the textures have been interpreted. Table 1. Analyses of some representative Koongarra uraninites. Uraninite compositions in the ore are never uniform. Electron microprobe analyses of uraninite grains and veins, 13 that is, micro-analyses of volumes of uraninite between 5 and 10 m m in diameter see Table 1 , reveal that uraninite compositions, particularly U, Pb and Ca contents, vary not only from grain to grain within anyone sample regardless of which generation of uraninite it is, but even at the microscopic level within uraninite grains themselves.
Figure 11 illustrates how Pb and Ca have both substituted for U in the UO 2 cubic lattice in varying amounts across the uraninite veins and grains. As has already been briefly noted, supergene alteration principally oxidation of uraninite has not only occurred where the zone of surficial weathering has intersected the top of the No. The net result has been the complete destruction of the uraninite in what was the top of the No. Additionally, at the same time there has been yet another remobilisation of both U and Pb in the primary ore zones, with in situ replacement of uraninite see Figures and deposition of supergene uraninite see Figure 16 and the uranyl silicate minerals sklodowskite and uranophane see Figures 17 and 18 from the U in solution from circulating ground waters see Figure 3 again.
Table 3. Analyses of alteration sequences of uraninites to uranyl silicates at Koongarra. Table 4. Analyses of iron and manganese oxides in fractures in the Koongarra primary ore. However, because the radioactivity measured is actually the gamma radiation given off by the daughter element bismuth Bi far down the U decay chain, any addition or removal of daughter elements between U and Bi will result in a discrepancy between the above two measurements of the U content of the ore sample.
To assess this possibility the two measurements are compared: Table 5. Summary of disequilibrium patterns in the Koongarra orebodies. Measurements on ore samples from Koongarra indicate that the ore is in overall disequilibrium Table 5 and Figure Figure 20 schematically illustrates these movements of isotopes caused by the present day circulation of groundwaters. Because of the tropical, monsoonal climate, the ground waters in the Koongarra area are fast moving, annually recharged and low in salinity, the water table rising and falling by as much as 10 m between the wet and the dry seasons.
However, U is dissolved by the ground waters from the mineralised aquifer rocks, the level of dissolved U depending on the prevailing pH, Eh, salinity and degree of adsorption. Furthermore, the ground waters are also dispersing U- Th decay products such as helium He from the ore zone, with measured levels up to It is hardly surprising, therefore, that the soils overlying the ore zones and the immediate areas of host rocks carry anomalous U concentrations compared to background levels.
Furthermore, Dickson et al. Table 6. U-Th-Pb concentrations and isotopic compositions of Koongarra uraninites. Hills and Richards 21 , 22 isotipically analysed individual grains of uraninite and galena that had been hand-picked from drill core see Table 6 and 7. The other four uraninite samples all lay well below concordia and did not conform to any regular linear array.
Hills and Richards were left with two possible interpretations. On the one hand, preferential loss of the intermediate daughter products of U that is, escape of radon, a gas would cause vertical displacement of points below an episodic-loss line, but this would only produce a significant Pb isotopic effect if the loss had persisted for a very long proportion of the life of the uraninite which is incidentally not only feasible but likely.
Alternatively, they suggested that contamination by small amounts of an older pre Ma Pb could cause such a pattern as on their concordia plot, to which they added mixing lines that they postulated arose from the restoration to each uraninite sample of the galena which separated from it see Figure 21 again. This of course assumes that the Pb in the galenas was also derived predominantly from U decay.
In a separate study Carr and Dean 23 isotopically analysed unweathered whole- rock samples from the Koongarra primary ore zone see Table 8. These were samples of drill core that had been crushed. Their isotopic data on four samples were plotted on a U-Pb isochron diagram and indicated a non-systematic relationship between the U parent and the Pb daughter. In other words, the quantities of Pb could not simply be accounted for by radioactive decay of U, implying open system behaviour.
Table 8. Results of Pb isotopic, U concentration and Pb concentration analyses for Koongarra whole-rock samples. Carr and Dean 23 also isotopically analysed a further nine whole-rock samples from the weathered schist zone at Koongarra see Table 8. Some of these samples were again crushed drill core, but the majority were crushed percussion drill chips.
When their isotopic data were plotted on a U-Pb isochron diagram, six of the nine samples plotted close to the reference Ma isochron, while the other three were widely scattered see Figure In unrelated investigations, Dickson et al. The technique did in fact work, Pb isotopic traces of the deeply buried No. This mineralisation, 40 m below the surface, is blind to other detection techniques. Dickson et al. However, most of the soil samples consisted of detritus eroded from the Middle Proterozoic Kombolgie sandstone, so because the samples from near the mineralisation gave a radiogenic Pb signature Dickson et al.
Snelling 24 has already highlighted a telling omission by Hills and Richards. The standard used to correct the data in Table 6 was the Mt Isa Pb standard with an isotopic composition: Ludwig et al. Similarly, Cunningham et al.
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the This damage is most concentrated around the parent isotope (U and Th), expelling the daughter isotope (Pb) from its original position in the zircon . General. Three independent ages may be obtained in the U-Th-Pb system: Pb/U, Pb/U or Pb/Pb, and Pb/Th. Emphasis has been .
In the laboratory, rock samples are crushed and the zircon grains are separated from the other minerals by heavy liquid and other mineral separation techniques. After being mounted, the crystals can be analyzed using an instrument such as a SHRIMP Sensitive High mass Resolution Ion MicroProbe which focuses a very narrow ion beam onto the grains so that mass spectrometers can measure the ratios of the isotopes vaporized from the targeted spot. In this way, even different growth zones in individual crystals can be analyzed and thus "dated. An alternative procedure is to take all the zircon grains liberated from a rock sample, and if they are of uniform composition, chemically digest them into solution for standard mass spectrometer analysis. This dating method has become very popular for dealing with Precambrian terranes where it can often be difficult to resolve relationships between rock units and the geological history.
Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda , was constructed at the Australian National University. Since its installation, our focus has been on the in-situ U—Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years.
Dubious Radiogenic Pb Places U-Th-Pb Mineral Dating in Doubt
However, it is important to remember that all radiometric dating methods are based on three main assumptions: The highly speculative nature of all radiometric dating methods becomes apparent when one realizes that none of the above assumptions is either valid or provable. Put simply, none of these assumptions can have been observed to have always been true throughout the supposed millions of years the radioactive elements have presumed to have been decaying. Of the various radiometric methods, uranium-thorium- lead U-Th-Pb was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated. Pb is also prone to diffusion from minerals.
Skip to main content. Log In Sign Up. David Chew. Chemical Geology — Contents lists available at ScienceDirect Chemical Geology j o u r n a l h o m e p a g e: Sylvester b, Mike N. Apatite is a common U- and Th-bearing accessory mineral in igneous and metamorphic rocks, and a minor but Received 6 July widespread detrital component in clastic sedimentary rocks. Rudnick consequently in developing the chronometer as a provenance tool. These raster conditions minimized laser-induced Geochronology inter-element fractionation, which was corrected for using the back-calculated intercept of the time-resolved Apatite signal.
Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest  and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised  from about 1 million years to over 4.
The heavens declare the glory of God; and the firmament shows His handiwork. Day unto day utters speech, and night unto night reveals knowledge. There is no speech nor language where their voice is not heard.
U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from the U and the Pb from the Th. The U-Th-Pb method Section 5. The ingrowth equations for the three radiogenic Pb isotopes are given by: The corresponding age equations are: This assumption cannot be made for other minerals, young ages, and high precision geochronology. The corresponding age equations then become: This built-in redundancy provides a powerful internal quality check which makes the method arguably the most robust and reliable dating technique in the geological toolbox. The initial Pb composition can either be determined by analysing the Pb composition of a U-poor mineral e. Note that isotopic closure is required for all intermediary isotopes as well. Initially, the U-Pb method was applied to U-ores, but nowadays it is predominantly applied to accessory minerals such zircon and, to a lesser extent, apatite, monazite and allanite.
Mark Harrison, Elizabeth J. Reviews in Mineralogy and Geochemistry ; 48 1: The dominant occurrence of phosphate minerals in crystalline rocks is as accessory phases, most notably apatite, monazite, and xenotime. Because these minerals tend, to varying degrees, to partition U and Th into their structures they can often contain the majority of those elements in a rock. These three phases, again to varying degrees, tend not to incorporate significant amounts of Pb during crystallization and thus were early candidates for utilization as U-Th-Pb geochronometers.
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