Skip to menu

XEDITION

Board

137.88 Is The Present-day 238U/235U Ratio

SherylAntonio733 2024.12.03 11:18 Views : 0

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 For most dating requirements it has been superseded by uranium-lead dating (U-Pb dating), but in certain specialized situations (such as dating meteorites and the age of the Earth) it is more important than U-Pb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 206Pb, 207Pb, and 208Pb. 204Pb is the only non-radiogenic lead isotope, therefore is not one of the daughter isotopes. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from 238U, 235U and 232Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. P and I refer to present-day and initial Pb isotope ratios, λ235 and λ238 are decay constants for 235U and 238U, and t is the age. Th᠎is po᠎st was created with t he ᠎help of G​SA C on​tent G​enerator D em over​si​on᠎!


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. It was established by dividing the first equation above by the second, under the assumption that the U/Pb system was undisturbed. 137.88 is the present-day 238U/235U ratio. 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. If the sample behaved as a closed system then graphing the difference between the present and initial ratios of 207Pb/204Pb versus 206Pb/204Pb should produce a straight line. The distance the point moves along this line is dependent on the U/Pb ratio, whereas the slope of the line depends on the time since Earth's formation. The development of the Geochron database was mainly attributed to Clair Cameron Patterson’s application of Pb-Pb dating on meteorites in 1956. The Pb ratios of three stony and two iron meteorites were measured.


The dating of meteorites would then help Patterson in determining not only the age of these meteorites but also the age of Earth's formation. By dating meteorites Patterson was directly dating the age of various planetesimals. Assuming the process of elemental differentiation is identical on Earth as it is on other planets, the core of these planetesimals would be depleted of uranium and thorium, while the crust and mantle would contain higher U/Pb ratios. As planetesimals collided, various fragments were scattered and produced meteorites. Iron meteorites were identified as pieces of the core, while stony meteorites were segments of the mantle and crustal units of these various planetesimals. Samples of iron meteorite from Canyon Diablo (Meteor Crater) Arizona were found to have the least radiogenic composition of any material in the solar system. The U/Pb ratio was so low that no radiogenic decay was detected in the isotopic composition. As illustrated in figure 1, this point defines the lower (left) end of the isochron. This data has ​been w᠎ritt​en  wi​th GSA Conte nt Ge ne​ra tor  DEMO​.


Stony meteorites however, exhibited very high 207Pb/204Pb versus 206Pb/204Pb ratios, indicating that these samples came from the crust or mantle of the planetesimal. Together, these samples define an isochron, whose slope gives the age of meteorites as 4.55 Byr. Patterson also analyzed terrestrial sediment collected from the ocean floor, which was believed to be representative of the Bulk Earth composition. Because the isotope composition of this sample plotted on the meteorite isochron, it suggested that earth had the same age and origin as meteorites, therefore solving the age of the Earth and giving rise to the name 'geochron'. Chondrules and calcium-aluminium-rich inclusions (CAIs) are spherical particles that make up chondritic meteorites and are believed to be the oldest objects in the solar system. Hence precise dating of these objects is important to constrain the early evolution of the solar system and the age of the earth. The U-Pb dating method can yield the most precise ages for early solar-system objects due to the optimal half-life of 238U. However, the absence of zircon or other uranium-rich minerals in chondrites, and the presence of initial non-radiogenic Pb (common Pb), rules out direct use of the U-Pb concordia method.


Therefore, the most precise dating method for these meteorites is the Pb-Pb method, which allows a correction for common Pb. When the abundance of 204Pb is relatively low, this isotope has larger measurement errors than the other Pb isotopes, leading to very strong correlation of errors between the measured ratios. This makes it difficult to determine the analytical uncertainty on the age. Pb-Pb isochron diagram' (see figure) with reduced error correlation between the measured ratios. In this diagram the 204Pb/206Pb ratio (the reciprocal of the normal ratio) is plotted on the x-axis, so that a point on the y axis (zero 204Pb/206Pb) would have infinitely radiogenic Pb. The ratio plotted on this axis is the 207Pb/206Pb ratio, corresponding to the slope of a normal Pb/Pb isochron, which yields the age. The most accurate ages are produced by samples near the y-axis, which was achieved by step-wise leaching and analysis of the samples. Previously, kismit when applying the alternative Pb-Pb isochron diagram, kismit the 238U/235U isotope ratios were assumed to be invariant among meteoritic material.

No. Subject Author Date Views
94161 A Live2bhealthy Success Story You'll Never Believe DarrenStackhouse48 2024.12.06 0
94160 10 Ideal Offshore Sportsbooks 2024 JenniferWainscott2 2024.12.06 5
94159 Bungalow Malaysia Odette41J1017521315 2024.12.06 0
94158 Top International Sports Betting Sites 2024 JenniferWainscott2 2024.12.06 1
94157 Advantages, Dosage, Supplement, Research IzettaKnorr4423667223 2024.12.06 3
94156 Dream Bungalow Winifred47S6164 2024.12.06 0
94155 Compare Gas & Electricity 2022 SkyeDangelo090304847 2024.12.06 2
94154 Top International Sports Betting Sites 2024 VelvaW811085772 2024.12.06 0
94153 Asiafounds Online Thieves ShadYoul2757241349 2024.12.06 7
94152 Best Sports Betting Sites 2024 JenniferWainscott2 2024.12.06 0
94151 10 Finest NMN Supplements For Anti BelenMatteson116 2024.12.06 1
94150 Finest Worldwide Betting Websites (2024 ) NancySchoonover 2024.12.06 1
94149 Объявления В Санкт Петербурге EusebiaCowper7134 2024.12.06 0
94148 Best Worldwide Betting Websites (2024 ) ShirleyBrower281816 2024.12.06 1
94147 Finest Sports Betting Sites 2024 LandonSweetapple45 2024.12.06 2
94146 Products IsisRitchard83342 2024.12.06 0
94145 Finest Quantum Computer Courses Online With Certificates 2024 VilmaBurgett170 2024.12.06 0
94144 NBA With New Honor ShadYoul2757241349 2024.12.06 11
94143 What To Do Before, Throughout, And After The Sale AleciaSherwood80055 2024.12.06 0
94142 What To Do Before, During, And After The Sale LucioGilman7194739 2024.12.06 0
Up