UNB Laser Ablation ICP-MS facility | UNB
Pb/Pb or U/Pb dating can be done on carbonates. Please see the . The is a small Book called "The Dating Game" that traces the history. of dating methods. Based on the new and previous U-Pb zircon ages, the bulk of the NGU Permian magmatic activity . Evidence from electron-microprobe dating of monazite. Geochronology is the science of determining the age of rocks, fossils, and sediments using Monazite geochronology is another example of U–Pb dating, employed for dating This technique is used to date speleothems, corals, carbonates, and fossil bones. .. Create a book · Download as PDF · Printable version.
Cortini says geologists discovered that ten times more Ra than the equilibrium value was present in rocks from Vesuvius. They found similar excess radium at Mount St. Helens, Vulcanello, and Lipari and other volcanic sites.
The only place where radioactive equilibrium of the U series exists in zero age lavas is in Hawiian rocks. We need to consider the implications of this for radiometric dating. How is this excess of radium being produced? This radium cannot be the result of decay of uranium, since there is far too much of it.
Either it is the result of an unknown decay process, or it is the result of fractionation which is greatly increasing the concentration of radium or greatly decreasing the concentration of uranium.
Thus only a small fraction of the radium present in the lava at most 10 percent is the result of decay of the uranium in the lava. This is interesting because both radium and lead are daughter products of uranium. If similar fractionation processes are operating for lead, this would mean that only a small fraction of the lead is the result of decay from the parent uranium, implying that the U-Pb radiometric dates are much, much too old.
Cortini, in an article appearing in the Journal of Volcanology and Geothermal Research also suggests this possibility. By analogy with the behaviour of Ra, Th and U it can be suggested that Pb, owing to its large mobility, was also fed to the magma by fluids. This can and must be tested. The open-system behaviour of Pb, if true, would have dramatic consequences On the other hand, even if such a process is not operating for lead, the extra radium will decay rapidly to lead, and so in either case we have much too much lead in the lava and radiometric dates that are much, much too ancient!
It is also a convincing proof that some kind of drastic fractionation is taking place, or else an unknown process is responsible. He says this is inexplicable in a closed-system framework and certainly invalidates the Th dating method.
And it is also possible that something similar is happening in the U decay chain, invalidating U based radiometric dates as well. In fact, U and Th both have isotopes of radium in their decay chains with half lives of a week or two, and 6. Any process that is concentrating one isotope of radium will probably concentrate the others as well and invalidate these dating methods, too. Radium has a low melting point degrees K which may account for its concentration at the top of magma chambers.
What radiometric dating needs to do to show its reliability is to demonstrate that no such fractionation could take place. Can this be done? With so many unknowns I don't think so.
How Uranium and Thorium are preferentially incorporated in various minerals I now give evidences that uranium and thorium are incorporated into some minerals more than others. This is not necessarily a problem for radiometric dating, because it can be taken into account.
But as we saw above, processes that take place within magma chambers involving crystallization could result in a different concentration of uranium and thorium at the top of a magma chamber than at the bottom.
This can happen because different minerals incorporate different amounts of uranium and thorium, and these different minerals also have different melting points and different densities. If minerals that crystallize at the top of a magma chamber and fall, tend to incorporate a lot of uranium, this will tend to deplete uranium at the top of the magma chamber, and make the magma there look older. Concerning the distribution of parent and daughter isotopes in various substances, there are appreciable differences.
Faure shows that in granite U is 4. Some process is causing the differences in the ratios of these magmatic rocks. Depending on their oxidation state, according to Faure, uranium minerals can be very soluble in water while thorium compounds are, generally, very insoluble. These elements also show preferences for the minerals in which they are incorporated, so that they will tend to be "dissolved" in certain mineral "solutions" preferentially to one another.
More U is found in carbonate rocks, while Th has a very strong preference for granites in comparison. I saw a reference that uranium reacts strongly, and is never found pure in nature. So the question is what the melting points of its oxides or salts would be, I suppose.
I also saw a statement that uranium is abundant in the crust, but never found in high concentrations. To me this indicates a high melting point for its minerals, as those with a low melting point might be expected to concentrate in the magma remaining after others crystallized out.
Such a high melting point would imply fractionation in the magma. Thorium is close to uranium in the periodic table, so it may have similar properties, and similar remarks may apply to it. It turns out that uranium in magma is typically found in the form of uranium dioxide, with a melting point of degrees centrigrade. This high melting point suggests that uranium would crystallize and fall to the bottom of magma chambers. Geologists are aware of the problem of initial concentration of daughter elements, and attempt to take it into account.
U-Pb dating attempts to get around the lack of information about initial daughter concentrations by the choice of minerals that are dated. For example, zircons are thought to accept little lead but much uranium.
Thus geologists assume that the lead in zircons resulted from radioactive decay. But I don't know how they can be sure how much lead zircons accept, and even they admit that zircons accept some lead. Lead could easily reside in impurities and imperfections in the crystal structure. Also, John Woodmorappe's paper has some examples of anomalies involving zircons. It is known that the crystal structure of zircons does not accept much lead.
However, it is unrealistic to expect a pure crystal to form in nature. Perfect crystals are very rare. In reality, I would expect that crystal growth would be blocked locally by various things, possibly particles in the way. Then the surrounding crystal surface would continue to grow and close up the gap, incorporating a tiny amount of magma.
I even read something about geologists trying to choose crystals without impurities by visual examination when doing radiometric dating. Thus we can assume that zircons would incorporate some lead in their impurities, potentially invalidating uranium-lead dates obtained from zircons.U-Pb zircon geochronology by ID-TIMS: chemical abrasion & dissolution
Chemical fractionation, as we have seen, calls radiometric dates into question. But this cannot explain the distribution of lead isotopes.
There are actually several isotopes of lead that are produced by different parent substances uraniumuraniumand thorium.
More Bad News for Radiometric Dating
One would not expect there to be much difference in the concentration of lead isotopes due to fractionation, since isotopes have properties that are very similar. So one could argue that any variations in Pb ratios would have to result from radioactive decay. However, the composition of lead isotopes between magma chambers could still differ, and lead could be incorporated into lava as it traveled to the surface from surrounding materials.
I also recall reading that geologists assume the initial Pb isotope ratios vary from place to place anyway. Later we will see that mixing of two kinds of magma, with different proportions of lead isotopes, could also lead to differences in concentrations.
Mechanism of uranium crystallization and falling through the magma We now consider in more detail the process of fractionation that can cause uranium to be depleted at the top of magma chambers.
Uranium and thorium have high melting points and as magma cools, these elements crystallize out of solution and fall to the magma chamber's depths and remelt. This process is known as fractional crystallization. What this does is deplete the upper parts of the chamber of uranium and thorium, leaving the radiogenic lead. As this material leaves, that which is first out will be high in lead and low in parent isotopes. This will date oldest. Magma escaping later will date younger because it is enriched in U and Th.
There will be a concordance or agreement in dates obtained by these seemingly very different dating methods. This mechanism was suggested by Jon Covey. Tarbuck and Lutgens carefully explain the process of fractional crystallization in The Earth: An Introduction to Physical Geology.
They show clear drawings of crystallized minerals falling through the magma and explain that the crystallized minerals do indeed fall through the magma chamber. Further, most minerals of uranium and thorium are denser than other minerals, especially when those minerals are in the liquid phase.
Crystalline solids tend to be denser than liquids from which they came. But the degree to which they are incorporated in other minerals with high melting points might have a greater influence, since the concentrations of uranium and thorium are so low. Now another issue is simply the atomic weight of uranium and thorium, which is high. Any compound containing them is also likely to be heavy and sink to the bottom relative to others, even in a liquid form.
If there is significant convection in the magma, this would be minimized, however. At any rate, there will be some effects of this nature that will produce some kinds of changes in concentration of uranium and thorium relative to lead from the top to the bottom of a magma chamber. Some of the patterns that are produced may appear to give valid radiometric dates.
The latter may be explained away due to various mechanisms. Let us consider processes that could cause uranium and thorium to be incorporated into minerals with a high melting point.
Directly dating geologic events: U-Pb dating of carbonates
I read that zircons absorb uranium, but not much lead. Thus they are used for U-Pb dating. But many minerals take in a lot of uranium. It is also known that uranium is highly reactive. To me this suggests that it is eager to give up its 2 outer electrons. This would tend to produce compounds with a high dipole moment, with a positive charge on uranium and a negative charge on the other elements.
This would in turn tend to produce a high melting point, since the atoms would attract one another electrostatically. I'm guessing a little bit here. There are a number of uranium compounds with different melting points, and in general it seems that the ones with the highest melting points are more stable. I would suppose that in magma, due to reactions, most of the uranium would end up in the most stable compounds with the highest melting points. These would also tend to have high dipole moments.
Now, this would also help the uranium to be incorporated into other minerals. The electric charge distribution would create an attraction between the uranium compound and a crystallizing mineral, enabling uranium to be incorporated. But this would be less so for lead, which reacts less strongly, and probably is not incorporated so easily into minerals.
So in the minerals crystallizing at the top of the magma, uranium would be taken in more than lead. These minerals would then fall to the bottom of the magma chamber and thus uranium at the top would be depleted. It doesn't matter if these minerals are relatively lighter than others. The point is that they are heavier than the magma.
Two kinds of magma and implications for radiometric dating It turns out that magma has two sources, ocean plates and material from the continents crustal rock. This fact has profound implications for radiometric dating.
Mantle material is very low in uranium and thorium, having only 0. The source of magma for volcanic activity is subducted oceanic plates. Subduction means that these plates are pushed under the continents by motions of the earth's crust.
While oceanic plates are basaltic mafic originating from the mid-oceanic ridges due to partial melting of mantle rock, the material that is magma is a combination of oceanic plate material and continental sediments. Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p. In other words, mantle is not the direct source of magma.
Further, Faure explains that uraninite UO sub2 is a component of igneous rocks Faure, p. Uraninite is also known as pitchblende. According to plate tectonic theory, continental crust overrides oceanic crust when these plates collide because the continental crust is less dense than the ocean floor. As the ocean floor sinks, it encounters increasing pressures and temperatures within the crust. Ultimately, the pressures and temperatures are so high that the rocks in the subducted oceanic crust melt.
Once the rocks melt, a plume of molten material begins to rise in the crust. As the plume rises it melts and incorporates other crustal rocks. This rising body of magma is an open system with respect to the surrounding crustal rocks. It is possible that these physical processes have an impact on the determined radiometric age of the rock as it cools and crystallizes.
Time is not a direct measurement.
The actual data are the ratios of parent and daughter isotopes present in the sample. Time is one of the values that can be determined from the slope of the line representing the distribution of the isotopes.
Isotope distributions are determined by the chemical and physical factors governing a given magma chamber. Uranium is believed to be able to incorporate itself as a trace material in many other minerals of low density, and so be relatively highly concentrated in the crust. A lower mantle concentration of uranium is inferred because if the mantle contained the same uranium concentration as the crust, then the uranium's heat of radiactive decay would keep the crust molten.
Rhyolites in Yellowstone N. Most genetic models for uranium deposits in sandstones in the U. Most of the uranium deposits in Wyoming are formed from uraniferous groundwaters derived from Precambrian granitic terranes. Uranium in the major uranium deposits in the San Juan basin of New Mexico is believed to have been derived from silicic volcanic ash from Jurassic island arcs at the edge of the continent. From the above sources, we see that another factor influencing radiometric dates is the proportion of the magma that comes from subducted oceanic plates and the proportion that comes from crustal rock.
Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. Later, more of the crustal rock would be incorporated by melting into the magma, and thus the magma would be richer in uranium and thorium and poorer in lead.
So this factor would also make the age appear to become younger with time. There are two kinds of magma, and the crustal material which is enriched in uranium also tends to be lighter. For our topic on radiometric dating and fractional crystallization, there is nothing that would prevent uranium and thorium ores from crystallizing within the upper, lighter portion of the magma chamber and descending to the lower boundaries of the sialic portion.
The upper portion of the sialic magma would be cooler since its in contact with continental rock, and the high melting point of UO sub 2 uranium dioxide, the common form in granite: The same kind of fractional crystallization would be true of non-granitic melts. I think we can build a strong case for fictitious ages in magmatic rocks as a result of fractional cystallization and geochemical processes.
As we have seen, we cannot ignore geochemical effects while we consider geophysical effects. Sialic granitic and mafic basaltic magma are separated from each other, with uranium and thorium chemically predestined to reside mainly in sialic magma and less in mafic rock. Here is yet another mechanism that can cause trouble for radiometric dating: As lava rises through the crust, it will heat up surrounding rock. Lead has a low melting point, so it will melt early and enter the magma. This will cause an apparent large age.
Geochronology - Wikipedia
Uranium has a much higher melting point. It will enter later, probably due to melting of materials in which it is embedded. This will tend to lower the ages. Mechanisms that can create isochrons giving meaningless ages: Geologists attempt to estimate the initial concentration of daughter product by a clever device called an isochron. Let me make some general comments about isochrons. The idea of isochrons is that one has a parent element, P, a daughter element, D, and another isotope, N, of the daughter that is not generated by decay.
One would assume that initially, the concentration of N and D in different locations are proportional, since their chemical properties are very similar. Note that this assumption implies a thorough mixing and melting of the magma, which would also mix in the parent substances as well. Then we require some process to preferentially concentrate the parent substances in certain places. Radioactive decay would generate a concentration of D proportional to P.
By taking enough measurements of the concentrations of P, D, and N, we can solve for c1 and c2, and from c1 we can determine the radiometric age of the sample.
Otherwise, the system is degenerate. Thus we need to have an uneven distribution of D relative to N at the start. If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them.
The bigger c1 is, the older the rock is. That is, the more daughter product relative to parent product, the greater the age. Thus we have the same general situation as with simiple parent-to-daughter computations, more daughter product implies an older age.
This is a very clever idea. However, there are some problems with it. First, in order to have a meaningful isochron, it is necessary to have an unusual chain of events.
Initially, one has to have a uniform ratio of lead isotopes in the magma. Usually the concentration of uranium and thorium varies in different places in rock.
This will, over the assumed millions of years, produce uneven concentrations of lead isotopes. Upper Cretaceous-Paleocene series consists mainly of marls, which in turn are overlain by Eocene limestones. The overlying Oligocene-Miocene strata are made of sandstones. Five successive extensional and compressional episodes have characterized the tectonic evolution of Jebel Hallouf [ 17 ]: Geological map of Jebel Hallouf adapted from . The Jebel Hallouf deposit was mined by the SOTEMI mining company between and and has produced since its exploitation in the equivalent oftons of Pb and 14, tons of Zn metals.
Mineralization is hosted by Campanian-Maastrichtian limestones. The style of mineralization consists of cavity-filling of karstic features that cut the mentioned host rocks on the north side of the Jebel Hallouf anticline. The mineral association consists of calcite, sphalerite, galena, jordanite and pyrite. Oued Maden The lithostratigraphic units consist of a series of stacked nappe structures produced by regional westward intra-Miocene tangential over-thrusting and Upper Miocene to Pliocene-Quaternary post-nappe tectonic phases [ 18 ].
Two main nappes are recognized [ 4 ]: The autochthon beneath these nappes displays a normal stratigraphic sequence consisting of Santonian-Maastrichtian Late Cretaceous rocks Figure 5. The Upper Santonian-Lower Campanian consists of thinly bedded gray marls with rare intercalations of marly limestones.
These series are in fault contact with the underlying Triassic rocks and are overlain by thick succession of marls and limestones of Upper Campanian-Lower Maastrichtian age, followed by thick gray sulfide-rich limestones of Middle Maastrichtian age.
Geological map of Oued Maden. Adapted from Slim-Shimi . From toabout 11, tons of Pb and 89 tons of Cu pure metals with ca. Mineralization occurs as open-space fillings of veinlets and stockwork structures superimposed on the NE-SW-trending Groura and Ferza faults cutting across the Triassic dolostone and the Campanian-Maastrichtian limestone country rocks. The ore mineralogy is dominated by galena, sphalerite, pyrite and sulfosalts mainly tetrahedrite.
The Cretaceous ends with thick Coniacian-Santonian marls followed by a succession of Campanian-Maastrichtian marls and limestones. Geological map of Fedj Hassene. Description of instrumentation Some key benefits of the Resonetics S laser ablation system: Aerosol washout is rapid even with in-line signal smoothing 5 orders of magnitude in about 5 seconds.
This allows us to employ short background counting times typically 30 to 40 seconds between each ablation. For high-spatial resolution techniques such as linescans or 2D elemental mapping, the smoothing device is removed to obtain even faster washout times. Large format sample holders capable of accommodating: This allows us to load several samples along with all the standards we need to carry out an analyses: The design of the S results in identical ablation anywhere within the cell.
As a result, standards do not need to be mounted together with unknowns. We simply drive back and forth between unknowns and standards. This allows us to analyze minerals at high spatial resolution using shallow craters to minimize down-hole elemental fractionation. Laser control software Resonetics Geostar for online and offline sequence definition and automated acquisition. Options for unsupervised and supervised ablation. Between and craters per 8-hour day for routine analyses. Carbon coat should be removed.
The locations of targets must be well-defined and identified. For obvious targets it is acceptable to simply circle the areas of interest. For small targets, detailed reflected light images e. Fredericton offers a variety of low-cost accommodation including the nearby Rosary Hall Hostel that provides a quiet, convenient, and accessible stay for visiting students.
Off-campus hotels are also located near the UNB campus e. Crowne Plaza 'Lord Beaverbrook' Hotel.
A typical analytical day 8: These tuning conditions help minimize the most common isotope interferences. The ICP-MS method is checked and isotopes and dwell times adjusted based on the likely abundance of each analyte plus consideration of isobaric interferences or tailing from high abundance peaks such as 40Ar under 39K.
The ablation sequence is then defined. This will typically comprise a large number of unknowns interspersed with external calibration standards and consistency standards.
The total time required to carry out the ablation sequence is calculated. The intensity data for each ablation is, therefore, collected in a signal ICP-MS file that may be up to s long about 2.
This is typically enough time to collect between and ablations per run. For U-Pb geochronology we use a minimum of 15 calibration standards interspersed with unknowns as well as 5 consistency standards. At the end of the ablation sequence, the laser control software Geostar generates a laser log file which is meshed to the ICP-MS output file using Iolite Paton et al. This data reduction can be done quickly enough e. Because preliminary data reduction can be carried out at the end of each ablation sequence, users typically leave the lab at the end of the day with a large dataset in hand.
Offline sequence definition The laser ablation system is designed to import ablation sequences defined offline. This requires a reflected light image that can be used to identify targets.
These mosaics are used as a basemap to identify targets and define offline ablation sequences.