A: Map showing the distribution of a selection of well-known microporous carbonates in Europe, North Africa, and the Middle East. Stars show. A modified Tera-Wasserburg U/Pb concordia plot which takes care of initial disequilibrium of radioactive daughter nuclides has been applied to date discordant. Pb-Pb Isochron Dating. After Patterson, There are a number of useful isotope systems which constitute clocks in the rocks and are useful for geologic.
Prior studies have shown the importance of absolute dating of diagenetic cements, which may lead to major reinterpretation of the thermal history and the potential timing of oil generation, migration, and accumulation Mark et al. More specifically, the determination of absolute ages of diagenetic events such as micrite stabilization or massive low-Mg calcite cementation in relation to burial history and sea-level fluctuations would greatly improve our ability to constrain the overall reservoir evolution and the key processes preserving or enhancing reservoir quality in microporous carbonates.
Although most of these processes are thought to occur during early diagenesis, recent studies have shown that they could also take place later. U-Pb radiometric dating is the only absolute geochronometer applicable to diagenetic carbonates.
However, the most robust and accurate technique based on acid dissolution followed by isotope dilution remains inapplicable in many cases because of either low uranium or high common lead content, or because of the impossibility of microsampling a single monogeneration diagenetic cement of interest.
Recent development of U-Pb dating of carbonates by laser ablation—inductively coupled plasma—mass spectrometry LA-ICP-MS applied directly on thin sections or slabs opened a wealth of possibilities with which to date calcite-cemented fossils Li et al.
However, this very promising method is currently hindered by the lack of appropriately calibrated carbonate standards and international reference material for data correction interelement and isotopic fractionation.
In our study we combined the advantages of the two approaches, the rapid identification of appropriate samples by use of the laser ablation technique and the accuracy of isotope dilution to obtain robust absolute ages on the formation of microporosity in a typical micritic carbonate formation.
Uranium–lead dating - Wikipedia
The giant oil resources discovered in such microporous reservoirs motivated significant efforts to understand their genesis and evolution Volery et al. Two main generations of calcite cement, S1 and S2, have been documented in the formation, based on petrographical and textural evidence showing a continuum from the S1 microsparite to the S2 blocky calcite.
Note that the S1 phase is too small and inclusion rich to be subsampled for U-Pb dating purposes. The UL is marked by several short-term syn-Urgonian exposures as well as a major regional post-Urgonian subaerial exposure ca. The dissolution of metastable minerals, mainly composed of rudist aragonitic shells and small-sized micrite particles, provided solutes to allow the development of low-Mg calcite overgrowth around more stable, large-size, calcite particles, according to a process called micrite stabilization Volery et al.
Lead–lead dating - Wikipedia
This process is critical in microporosity preservation, not only because it improves connectivity between micropores, eventually increasing permeability, but also because it enhances resistance to compaction during burial. Such cementation and stabilization processes are commonly invoked in formation of microporous reservoirs Volery et al.
Coarse crystalline blocky calcite interpreted as S2 occurs within the U2 stratigraphic unit Fig. Several samples of the low-Mg calcite S2 representative of this particular diagenetic phase along the platform were investigated for U-Pb dating.
The S2 calcite is therefore the best candidate to constrain the timing and duration of micrite stabilization responsible for the preservation and excellent reservoir quality of the studied formation.
Historical Geology/U-Pb, Pb-Pb, and fission track dating
All samples were collected in a rudist-rich unit U2 within the main reservoir facies. Seven samples of well-developed sparry crystals of vug-filling calcite were selected for U-Pb dating. The Simiane sample two fragments in the same sample displays a more complex pattern see Item DR1with data spread in two different sets, suggesting the existence of two distinct isochrons.
The first set shows 42 laser spots nicely aligned, forming an isochron at The second set 32 laser spots shows a significantly younger age at Several samples 11 spots are intermediate between the two sets.
- Lead–lead dating
- Uranium–lead dating
Ages for samples from La Nesque and Rustrel are Excluding Simiane, the four other sites can be considered as representative of a same calcite generation yielding a weighted mean age for S2 of This Turonian age is considered as representing the absolute time of the fluid circulation event responsible for S2 cementation at the regional scale.
This would be consistent with meteoric water percolation during the Durancian exposure period.
The method[ edit ] If there is no lead in the zircon originally, and if no lead or uranium has been added or subtracted to the zircon since its formation, then the following formula will hold: It is possible to refine this date still further. If we suspect that the zircon, despite its chemical properties, still managed to incorporate a little lead at or after its formation, then since all lead isotopes are chemically the same, we can measure the amount of Pb the zircon contains.
Since we know the ratios in which the various lead isotopes are usually found, we can then apply the same sort of correction we used to account for atmospheric argon in the K-Ar method.
While zircon has been the most popular mineral for U-Pb dating, other minerals have been employed, including apatite, monazite, titanite, allanite and, most interesting of all, xenotime. Xenotime[ edit ] There is a difficulty in using radiometric dating to put an age on sedimentary rock.
The problem is that sediment is made up of clasts of some parent rockand when we date these clastswe are in effect dating the parent rock rather than the sediment as such. If, for example, we apply U-Pb dating to a grain of zircon found in sandstonewe aren't dating the formation of the sandstone, we're dating the formation of the granite that the zircon came from; all we could say about the sandstone is that it must be younger than that.
However, it is possible to put a date on some sedimentary rocks using the mineral xenotime Y P O 4. Uranium can and often does substitute for the element yttrium, whereas lead cannot, making xenotime suitable for radiometric dating.
The key fact about xenotime is that since it has the same crystal structure as zircon, it can grow on zircon crystalsforming a crust; and this process, of course, cannot begin to take place while the zircon crystal is still locked inside its parent rock.
The zircon will only start acquiring its xenotime crust after weathering and erosion have freed it from its parent rock and it becomes sediment. So by dating the xenotime crust, we can find out when the zircon it's growing on became sediment ; dating the zircon itself would tell you the age of the parent rock.
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Speleothems[ edit ] A cave with speleothems. A speleothem, more colloquially known as a cave formation, is formed when minerals dissolved in water precipitate out of the water as it drips, seeps, or flows into a cave.
The reader will probably be familiar with stalagmites and stalagtites; more speleothems are shown in the photograph to the right. Now, compounds of uranium are often highly soluble in water this, indeed, is one of the major problems with U-Pb isochron dating whereas compounds of lead are stubbornly insoluble. As a result, we expect speleothems when they are first formed to contain some uranium but little or no lead — just like zircons.
So we can apply the same technique to speleothems as we do to zircons. It can be shown mathematically that if the rock has been undisturbed, so that the isotope ratios reflect nothing but the passage of time, then just as with the isochron diagrams we've already discussed though for a different reason the minerals so plotted will lie on a straight line on the graph; and the age of the rock can be calculated from the slope of the line.
Unlike the ordinary isochron methods such as Rb-Srthe Pb-Pb method does not allow us to deduce the original proportions of the various lead isotopes from the data acquired from the sample.
Instead, we need to find this out some other way. We can do this by finding minerals that contain lead but never contained any uranium, or only ever contained it in negligible quantities. Troilite Fe S from iron-rich meteorites fits the bill: You might perhaps doubt that meteorites would have the same initial lead isotope ratios as the Earth.
Planetary scientists maintain that they should, for reasons which are somewhat beyond the scope of this textbook. Another reason for believing it is that if we calculate Pb-Pb dates on this basis, the dates we get are in agreement with dates produced by other methods where they can be applied: So taking the figure derived from the troilite as an "anchor" for our calculations, we can then go ahead and apply the Pb-Pb method to rocks which do contain significant quantities of uranium.
Now, recalling that I began this article by explaining that the isochron method is no use for U-Pb, you may wonder why this Pb-Pb isochron should be any better.