Uranium-lead dating explained
This gives geologists great confidence that the method correctly determines when that rock formed. Hope that helps, and please ask if you'd like more details! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer. Radiometric dating is the use of radioactive and radiogenic those formed from the decay of radioactive parents isotopes isotopes are atoms of the same element that have different numbers of neutrons in their nuclei to determine the age of something.
It is commonly used in earth science to determine the age of rock formations or features or to figure out how fast geologic processes take place for example, how fast marine terraces on Santa Cruz island are being uplifted. Radiometric dating relies on the principle of radioactive decay. All radioactive isotopes have a characteristic half-life the amount of time that it takes for one half of the original number of atoms of that isotope to decay.
By measuring the parent isotope radioactive and the daughter isotope radiogenic in a system for example, a rock , we can tell how long the system has been closed in our example, when the rock formed. The process of radiogenic dating is usually done using some sort of mass spectrometer. A mass spectrometer is an instrument that separates atoms based on their mass. Because geochronologists want to measure isotopes with different masses, a mass spectrometer works really well for dating things. I do think that radiometric dating is an accurate way to date the earth, although I am a geochronologist so I have my biases.
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Most estimates of the age of the earth come from dating meteorites that have fallen to Earth because we think that they formed in our solar nebula very close to the time that the earth formed. The fact that the age we calculate is reproducible for these different systems is significant. We have also obtained a very similar age by measuring Pb isotopes in materials from earth. I should mention that the decay constants basically a value that indicates how fast a certain radioactive isotope will decay for some of these isotope systems were calculated by assuming that the age of the earth is 4.
The decay constants for most of these systems have been confirmed in other ways, adding strength to our argument for the age of the earth. Radiometric dating depends on the chemistry and ratios of different elements. It works like this:. Take, for example, zircon, which is a mineral; its chemical formula is ZiSiO 4 , so there is one zirconium Zi for one silicon Si for four oxygen O.
One of the elements that can stand in chemically for zircon is uranium. Uranium eventually decays into lead, and lead does not normally occur in zircon, except as the radioactive decay product of uranium.
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Therefore, by measuring the ratio of lead to uranium in a crystal of zircon, you can tell how much uranium there originally was in the crystal, which, combined with knowing the radioactive half-life of uranium, tells you how old the crystal is. Obviously, if the substance you are measuring is contaminated, then all you know is the age since contamination, or worse, you don't know anything, because the contamination might be in the opposite direction - suppose, for example, you're looking at radio carbon carbon 14, which is produced in the atmosphere by cosmic rays, and which decays into nitrogen.
Since you are exposed to the atmosphere and contain carbon, if you get oils from your skin onto an archeological artifact, then attempting to date it using radio carbon will fail because you are measuring the age of the oils on your skin, not the age of the artifact. This is why crystals are good for radiometric dating: The oldest crystals on Earth that were formed on Earth are zircon crystals, and are approximately 4.
Asteroids in the solar system have been clocked at 4. We assume that the Earth is probably as old as the asteroids, because we believe the solar system to have formed from a collapsing nebula, and that the Earth, being geologically active, has simply destroyed any older zircon crystals that would be its true age, but we can't really be certain. The building blocks that the Earth is made of, the asteroids are 4.
Based on astronomical models of how stars work, we also believe the Sun to be about 4.
Radiometric dating is a widely accepted technique that measures the rate of decay of naturally occurring elements that have been incorporated into rocks and fossils. Every element is defined by the particular number of protons, neutrons, and electrons that make up it's atoms. Sometimes, the number of neutrons within the atom is off. These atoms, with an odd number of neutrons, are called isotopes.
Because they do not have the ideal number of neutrons, the isotopes are unstable and over time they will convert into more stable atoms. Scientists can measure the ratio of the parent isotopes compared to the converted isotopes. It wasn't explained that different isotopes of lead are produced by the decay of uranium. And, given that they are, i don't see why they couldn't separate the different isotope levels like you suggest. And this lead to it being banned as a fuel additive. I think I'll need to sit down with some paper and sketch this out to better understand, but you've gone a long way to explaining it.
Also to be kept in mind that Pb-Pb is also a thing, much more complex and finnicky, and sometimes produces paradoxical results such as "future ages" in some settings such as Mississippi-Vally-type deposits This method is not used on zircons, but on lead-rich minerals such as galena and can be usefull in provenance studies.
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It is quite sensitive to environmental contamination such as fuel-born lead. Look up Clair Patterson. He's the guy who figured this all out. It's a great story. They should make a movie about him some day. His story is really interesting. And well told in Cosmos. I'm still not sure I understand how the dating process works.
I still can't get my head around how he knew his original measurements of lead had to be too high. Surely that would just tell him that there was more uranium to begin with, and make his estimate for the age of the earth wrong. Possibly because there are multiple methods of age determination giving you a window of the correct answer and because it wasn't just the radiogenic forms of lead when he started to really look into why his answer was so far off.
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