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 For most dating requirements it has been superseded by uranium-lead dating (U-Pb dating), but in certain specialized situations (such as dating meteorites and the age of the Earth) it is more important than U-Pb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
83554 6 Horrible Mistakes To Avoid When You Kanye West Poster new WallacePeeples5484 2024.12.05 0
83553 ### Mengapa Cuaca778 Aman? new ClaudeKeble103107 2024.12.05 1
83552 What Everybody Ought To Know About 橙羿筋膜調理舘 | 腳底按摩 | 身體按摩 | 運動按摩 | Massage | マッサージ | 마사지的評論 new RAXAubrey57388647622 2024.12.05 0
83551 Bangkok Erotic Massage Is Bound To Make An Influence In Your Online Business new JanisHancock22109 2024.12.05 0
83550 ### Langkah 1: Registrasi Akun new CaroleTristan3435 2024.12.05 1
83549 Three Ways To Avoid Penthouse For Sale Burnout new StanLyttleton57870 2024.12.05 0
83548 ### Langkah Mudah Untuk Mendaftar new SeanHarless617227572 2024.12.05 0
83547 The Next 3 Issues To Immediately Do About 基隆按摩/越式洗頭/采耳/除毛/做臉 日式雲芳療 Yun Spa Massage-基隆廟口一館提供的產品 new JaydenLeff309495877 2024.12.05 0
83546 ### Sistem Proteksi Terbaik new RitaProsser679291544 2024.12.05 0
83545 The Most Overlooked Fact About Massage Sex Trafficking Case Revealed new JaydenLeff309495877 2024.12.05 0
83544 ### Sistem Keamanan Yang Andal new Caleb4539804166555 2024.12.05 0
83543 Top Choices Of 萬 華 越 式 洗 頭 台北 越 式 洗 頭 台北 越 式 洗 髮 龍 山寺 按摩 萬 華 按摩 龍 山寺 Massage 萬 華 Massage 采 耳的評論 new SebastianPaulson61 2024.12.05 0
83542 How To Learn To Kanye West Poster Your Product new WallacePeeples5484 2024.12.05 0
83541 9 Ways To Immediately Start Selling Meridian Massage new MiquelLeff71339368 2024.12.05 0
83540 Four Rules About Ghế Massage Giá Rẻ Dưới 10 Triệu Meant To Be Broken new MarcelaFeuerstein 2024.12.05 0
83539 This Take A Look At Will Show You Wheter You're An Skilled In Kanye West Poster With Out Realizing It. This Is How It Really Works new WallacePeeples5484 2024.12.05 0
83538 At Home Massage Smackdown! new JanisHancock22109 2024.12.05 0
83537 Top Guide Of 台北按摩滿憶亭養生會館taipei Massage、タイペイ マッサージ、타이페이 마사지 黎明里的評論 new JaydenLeff309495877 2024.12.05 0
83536 5 Ways To Keep Away From Penthouse For Sale Burnout new JorgeDunshea217210 2024.12.05 0
83535 Introducing The Straightforward Technique To Massage London new OmerLiversidge73073 2024.12.05 0
Up