Geology ; 46 3 : — In such deposits, assessing the exact timing of reservoir property stabilization is critical to better understand the postdepositional processes favorable to the creation or preservation of porosity. However, placing reliable and accurate chronological constraints on the formation of microporosity in these reservoirs is a major challenge. In this study we performed absolute U-Pb dating of calcite cements occurring in the Urgonian microporous limestone northern Tethys margin of southeastern France. U-Pb ages ranging between Our results show that 1 the mineralogical stabilization process responsible for the formation of an excellent pervasive microporous network took place relatively early, and 2 the so-acquired reservoir quality was preserved for more than 90 m. These observations emphasize the importance of long exposure periods and associated meteoric influx for the formation and preservation of good microporous reservoirs. Establishing the relative chronology of diagenetic transformation paragenesis from thin section petrography is of outmost importance but it is not sufficient to link the evolution of petrophysical properties in reservoirs with basin-scale structural and burial events in a proper temporal framework. 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.
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We think you have liked this presentation. If you wish to download it, please recommend it to your friends in any social system. Share buttons are a little bit lower. Thank you! Published by Austen Alexander Modified over 5 years ago. Therefore igneous rocks of granitic composition are more enriched in U and Th than basaltic or ultramafic rocks Consequently, the continental crust has more U and Th than the upper mantle.
U-Pb dating of Detrital Zircon from the Fond du Lac and Hinckley and Morey () explained the variability in paleocurrent indicators.
Lead—lead dating is a method for dating geological samples, normally based on ‘whole-rock’ samples of material such as granite. For most dating requirements it has been superseded by uranium—lead dating U—Pb dating , but in certain specialized situations such as dating meteorites and the age of the Earth it is more important than U—Pb dating. There are three stable “daughter” Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb.
These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U and Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This can be expressed by the following decay equations:. The concept of common Pb—Pb dating also referred to as whole rock lead isotope dating was deduced through mathematical manipulation of the above equations. This rearranged equation formed:.
As evident by the equation, initial Pb isotope ratios, as well as the age of the system are the two factors which determine the present day Pb isotope compositions. This was first established by Nier et al. The Pb ratios of three stony and two iron meteorites were measured.
At present, Chemostrat can determine U-Pb ages for zircon and apatite crystals. Zircon is a robust mineral and so the crystals preserve the age at which they formed or underwent high grade metamorphism. Consequently, U-Pb zircon geochronology can be employed to constrain the age of the basement rocks and in turn can help to identify sediment dispersal patterns and to correlate sandstones.
If the analysed zircon crystal has not suffered either Pb loss or U gain, it will plot on the concordia line from which its age can be deduced. Sandstones frequently contain detrital zircon grains and if these grains are undisturbed and concordant, their ages provide some clue as to their provenance.
The Huajian gold deposit is one of the largest hydrothermal intrusion-related gold deposits in eastern Hebei Province, located in the northern margin of the North China Craton NCC. The mineralization in this district displays a close spatial association with the shoshonitic Niuxinshan intrusive complex NIC , which contributes to the characterization of the metallogeny associated with convergent margin magmatism. The new geochronological data constrain the timing of the tectonic transformation between and Ma.
The other group exhibits flat REE patterns with obvious negative Eu anomalies, higher Yb, lower Sr, and weak NTT anomalies, which indicate an intra-plate extensional environment with a thinning crust. This is interpreted to be genetically related to the crystallization of the shallow crustal-sourced portions of this complex. Additionally, a tectonic model is presented that provides a plausible explanation for the abundant polymetallic mineralization that occurs in the northern margin of the NCC after Ma.
The association of mineralization with shoshonitic magmatism is a characteristic feature of the northern part of the North China Craton NCC [ 1 — 4 ]; however, this has not been previously described in detail for Huajian gold deposit Fig 1. Recently, dates for some deposits of the Middle Triassic-aged intrusive rocks, located in the northern part of the NCC, were obtained [ 5 — 12 ].
Historical Geology/U-Pb, Pb-Pb, and fission track dating
Passarelli; Miguel A. Basei; Oswaldo Siga Jr. Sproesser; Vasco A. It provides reliable and accurate results in age determination of superposed events. However, the open-system behavior such as Pb-loss, the inheritance problem and metamictization processes allow and impel us to a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology. Since , the Interdepartmental Laboratory of Isotopic Geology focus the study of the Earth’s geologic processes, dealing with themes such as plate tectonics, plutonism, volcanism, sedimentary rocks, tectono-thermal evolution, and more recently environmental studies.
SHRIMP U–Pb zircon dating has shown that – Ma has established the following framework that must be explained by any tectonic.
Precambrian Research, DOI However, their tectonic setting and evolution are still a matter of debate. The precise depositional and metamorphic data of the studied metasedimentary rocks from the South Liaohe Group provide important information on the controversial discussion of the tectonic setting and evolution. Based on cathodoluminescence CL imaging and connecting LA-ICP-MS U-Pb geochronology, zircon grains separated from metasedimentary rocks of different formations of the South Liaohe Group were subdivided into detrital and metamorphic zircons or zircon domains.
These new geochronological data can be well explained by 1 a continent-arc collision model which suggests that the South Liaohe Group formed an intervening island arc and foreland basin sandwiched between the Longgang and Nangrim Complexes, and 2 a northward subduction of the Nangrim Complex source rocks of the South Liaohe Group; these source rocks were successively deposited along the margin of the two Complexes.
In situ zircon U-Pb dating and whole-rock geochemistry of metasedimentary rocks from South Liaohe Group, Jiao-Liao-Ji orogenic belt: Constraints on the depositional and metamorphic ages, and implications for tectonic setting. Text wang et al. Supplementary data:.
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reference material used for zircon U-Pb dating in this study was the of clear overgrowths in the imaging can be explained by the magni-.
To browse Academia. Skip to main content. Log In Sign Up. Papers People. New U—Pb dating on zircon yielded ca. These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions These new ages do not support the previous interpretation of these granitoids as syn-tectonic intrusions emplaced during the Late Devonian-Early Carboniferous thrusting.
The geochemical and isotopic nature of this magmatism is linked to a major magmatic Ordovician event recorded throughout the European Variscan belt and related to extreme thinning of continental margins during a rifting event or a back-arc extension. Save to Library. Pavel Pitra. Two-stage cooling history of pelitic and semi-pelitic mylonite sensu lato from the Dongjiu—Milin shear zone, northwest flank of the eastern Himalayan syntaxis.
It is characterized by a thick sequence of highly strained and ductilely
Paula Andrea Duran Sierra, MD
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Other rare gas components in the atmosphere are not easily explained, rendering Thus, U-Pb zircon dating has become the ‘gold standard’ Principles of.
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.
But just how good is this dating method? It must be assumed that when the zircon grains crystallized, no radiogenic Pb was in them, and that all the radiogenic Pb now measured was derived by radioactive decay from U and Th. However, there are several lines of evidence that indicate radiogenic Pb can be inherited during crystallization of the mineral grains, and that open-system behavior is common, with radiogenic Pb lost by diffusion due to the way the Pb is held in the crystal lattice.
Wetherill 4 and Wasserburg 5 subsequently derived mathematical equations to describe this steady loss and demonstrated its consistency with published U-Pb age data. Wasserburg 5 also proposed that Pb loss by diffusion resulted from radiation damage to crystal lattices. In fact, it has now been confirmed that radiation damage can drastically increase the rate of Pb diffusion.
This is dramatically illustrated by the contact metamorphic effects of a Tertiary granite stock on zircon crystals in surrounding regionally metamorphosed Precambrian sediments and volcanics. Zircon crystals are often chemically and physically inhomogeneous, 7 reflecting growth during crystallization from magma.
Both zoned and unzoned zircon crystals may be found in the same rock.
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 ingrowth equations for the three radiogenic Pb isotopes are given by: 5.
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