# Tag Info

25

@gerrit provided a formula, but without stating the reasoning behind it. Radioactive decay is an exponential function. After $n$ half-lives, the amount of the original material remaining is $$\textrm{amount remaining after}\ n\ \textrm{half-lives} = \left(\frac{1}{2}\right)^n$$ Therefore, you want to solve \begin{align*} \left(\frac{1}{2}\right)^n &... 20 This hypothesis has been studied here as a possible explanation of selenogenesis (formation of the Moon). (Forming the Moon from terrestrial silicate-rich material. R.J. de Meijer, V.F. Anisichkin, W. van Westrenen. 2013.) The only place cited as suitable for spontaneous criticality is the Core-Mantle Boundary (CMB). The calculations above show that without ... 16 The other answers are entirely correct. But I like graphical representations. From http://en.wikipedia.org/wiki/Radioactive_decay we see the decay formula is:N(t) = N_0e^\frac{-t}{τ}$$Where N0 is the starting number of nuclides and τ is the mean lifetime. We also see that the half-life is$$t_{1/2} = τ ln(2)$$Substituting for τ, we get:$$N(t) = ...

15

You can use simple logarithms to calculate the answer. The number of half-lives that have elapsed can be calculated with $$- \frac{\log{f}}{\log{2}}$$ where $f$ is the fraction that remains. So plugging in the numbers gives $$- \frac{\log(0.75)}{\log(2)} = 0.415 = 41.5\%$$

15

To understand why sedimentary phosphate rocks (hereafter referred to as phosphorites) have elevated uranium contents we first need to understand what are they made of and why do they form in the first place. Phosphorites are rocks that are made of apatite, a mineral with the formula $\ce{Ca5(PO4)3(F,Cl,OH)}$. This mineral (especially the OH variant) is one ...

13

The conventional explanation for the Earth's magnetic field is that some combination of differential rotation and/or convection occurs in the Earth's outer core, primarily in molten iron-nickel (+ sulphur, hydrogen etc.), which acts as a kind of dynamo. Whilst we can't prove it by direct observation, this seems an eminently plausible mechanism. If this is ...

10

Most of this 40Ar makes its way to the atmosphere in two primary ways. Firstly, since most of the K is concentrated in the continental crust, volcanic eruptions on the continents over geological time have concentrated Ar in the atmosphere. Since it is too heavy and inert to decrease in any appreciable method it has a long residence time. Secondly, volcanism ...

10

"Doctor! It hurts when I do this! «bonk»" "So don't do that then!" One way around the issue of an unknown initial distribution of isotopes is to not do that. One of the most reliable mechanisms for dating the Earth is uranium-lead dating of zircons. Uranium can take the place of zirconium in the zircon crystal because of the chemical similarity of uranium ...

7

First off, the purported variations were reported to be cyclical. So the net effect on archeological dating would be null even if these variations existed. Secondly, these purported variations are very dubious. Scientists are not immune from "crackpot syndrome". If anything, scientists are more susceptible to this than the average public. There's a bit of a ...

7

As the previous responder noted, the continental crust forms a large reservoir of potassium. Much of this is in the form of feldspar. My answer primarily addresses this continental crust source. To get to the atmosphere, argon produced from the decay of 40K in the crystal lattice first must either diffuse out of the structure or be released during mineral ...

6

First modern carbon dating does not assume constant Carbon isotope ratios, in fact it is calibrated against known changes in atmospheric concentration, and has been since the 90's. These baselines were created using annular tree rings which of course reflect the atmospheric carbon when each ring was grown as well as ice core data. This was an issue with the ...

6

The cited paper, and ones like it, are young earth creationist nonsense. The Geoscience Research Institute is an arm of the Seventh Day Adventists, whose official position is that "in a recent six-day creation the Lord made “the heavens and the earth, the sea, and all that is in them” and rested on the seventh day." This is a central tenet of Seventh Day ...

5

1) D* (radiogenic isotope) and Dref (stable reference isotope) are two isotopes of the same element. P is the radioactive parent of D*, and is a different element. Chemical fractionation between P and Dref occurs as the rock solidifies because different elements are preferentially incorporated in different minerals depending on how well their ionic radii and ...

5

No. The reason for Australia’s unique evolution is that it was relatively isolated from the rest of the world. Australia isn’t too radioactive either. The fact that it has more uranium deposits than other countries is that it’s huge.

4

There are other, mostly chemical processes which alter the isotope ratios. Isotope dating uses a combination of them. This is why it can not be used always, for any radioactive isotopes, only in special circumstances. It also requires very sensitive measurements (below in the second example, close to induvidually count lead atoms in microscopic zircon ...

3

I've never heard of this exact type of variations, but there are many other variables that are considered by the scientists making radiocarbon dating, such as: C-14/C-12 ratio - this leads to calibration of the dates isotopic fractionation - living organisms have slightly less C-13 and C-14 ratio in comparison to the atmosphere; this ratio my vary during ...

3

Are Helium 3 and 4 being produced by earth's core? tl;dr no. The paper you are referring to has major flaws in it. Claims that should not be in any respectable scientific journal such as: ...the demise of the georeactor is approaching. Within the present level of uncertainty, one cannot say precisely when georeactor demise will occur, whether in the ...

3

Suppose you purify a sample of uranium 238, removing all of the lead. Half of those U-238 atoms will have decayed into lead 4.468 billion years from now. If the lead is removed from that half-uranium/half-lead sample 4.468 billion years from now, half of the U-238 atoms in that purified sample will have decayed into lead in yet another 4.468 billion years. ...

3

I will first focus on the 214Bi decay. The 3.272 MeV is the total amount available for the β- decay, i.e. the difference between the ground states of 214Bi and 214Po. If you have a beta decay into the ground state of of 214Po, which happens in 19.9 % of cases (all data are from Firestone, R. B. et al., “Table of Isotopes”, 8th ed., 1998, p. 9211, in ...

3

It sounds like there are two primary ones: No, these are not the two "primary ones". The method used depends on what you are dating, and what age you expect it to be. Radiocarbon dating is relevant to things younger than a few tens of thousands of years, and it's only relevant for things that were living (or growing), and incorporated atmospheric carbon. ...

3

Ahhhh, probably not. If, for example, you navigate to Marsupial at Wikipedia and click on the wonderful (it really really is!) DyMaxion map at the bottom of the info panel, you wiil see that a large part of South America, Central and North America also have these animals. FWIW, the USA does indeed have Uranium deposits although I'm not sure about Central ...

3

Weren't all the natural radioactive isotopes created during the formation of the solar system? The half life of carbon 14 is 5730 years, orders of magnitude less than the age of the solar system. Carbon 14 is constantly being created in the upper atmosphere by neutron bombardment of nitrogen 14. On the other hand, the other isotopes used in radioactive ...

2

Many of your questions are answered in this question and associated answers: Why is Earth's age given by dating meteorites rather than its own rocks? To add to some of your other questions: Wouldn't radioactive decay have already been occurring in all of those meteorites long before they hit earth or our moon? Yes. We have an assumption that the ...

2

Radioactive dating works on specific isotopes we use for specific time frames. Rubidium–strontium dating methods (because this substance has a half life of 50 billion years) to date extremely old geological samples as well as space samples like lunar rocks. Another issue is the quantity of synthetic isotopes in varying samples. Alot of these isotopes do not ...

1

Any nuclear activity would be visible in the changes of the isotope ratios far longer than the existence of our Universe. Nothing was found until now, except one in Gabon. It was natural phenomenon: at the time, 2billion years ago, there was far more ${}^{235} \rm U$ and the ground water could moderate uranium ore, if it was enough rich. Except that, nothing ...

1

While a long half-life, hundreds or millions of years or more, favors a radioisotope being relatively common or even occurring in nature at all, it is not the only consideration. Whether and how effectively the isotope can be regenerated also figures in. Niobium. occurs as only one isotope, $_{41}^{93}\text{Nb}$. All other isotopes decay away much too ...

1

Two websites I've come across that might be of interest are: Radioactivity Environmental Monitoring Joint Research Centre, for Europe. Radmon.org which provides radiation information from number of global station.

1

Potassium 40 has a half life of 1,300,000,000 years, which means that within that time half of the 11 percent which you refer to will have decayed to form Argon 40. Potassium 40 has two modes of decay, which is unusual for a radioactive isotope. Only 11 percent of it becomes argon 40, the remaining 89 percent decays by a different mode and becomes Calcium ...

1

There are huge datasets of geochronology available in the published literature. Unfortunately there is not one single database because the task is daunting (there are several thousand geochronology labs in the world which typically produce hundreds of ages per year). Some papers discussing how data should be (and often are) reported are these, for example : ...

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