K-Ar dating calculation (video) | Khan Academy
Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. Although we now recognize lots of problems with that calculation, the age of 25 .. Radiocarbon dating is different than the other methods of dating because it with the atmosphere by breathing, feeding, and photosynthesis. Chronological Methods 9 - Potassium-Argon Dating. Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating.
Uranium—thorium dating method[ edit ] Main article: Uranium—thorium dating A relatively short-range dating technique is based on the decay of uranium into thorium, a substance with a half-life of about 80, years. It is accompanied by a sister process, in which uranium decays into protactinium, which has a half-life of 32, years.
While uranium is water-soluble, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sedimentsfrom which their ratios are measured. The scheme has a range of several hundred thousand years. A related method is ionium—thorium datingwhich measures the ratio of ionium thorium to thorium in ocean sediment. Radiocarbon dating method[ edit ] Main article: Carbon is a radioactive isotope of carbon, with a half-life of 5, years,   which is very short compared with the above isotopes and decays into nitrogen.
Carbon, though, is continuously created through collisions of neutrons generated by cosmic rays with nitrogen in the upper atmosphere and thus remains at a near-constant level on Earth.
The carbon ends up as a trace component in atmospheric carbon dioxide CO2. A carbon-based life form acquires carbon during its lifetime. Plants acquire it through photosynthesisand animals acquire it from consumption of plants and other animals. When an organism dies, it ceases to take in new carbon, and the existing isotope decays with a characteristic half-life years.
The proportion of carbon left when the remains of the organism are examined provides an indication of the time elapsed since its death. This makes carbon an ideal dating method to date the age of bones or the remains of an organism. The carbon dating limit lies around 58, to 62, years. However, local eruptions of volcanoes or other events that give off large amounts of carbon dioxide can reduce local concentrations of carbon and give inaccurate dates.
The releases of carbon dioxide into the biosphere as a consequence of industrialization have also depressed the proportion of carbon by a few percent; conversely, the amount of carbon was increased by above-ground nuclear bomb tests that were conducted into the early s. Also, an increase in the solar wind or the Earth's magnetic field above the current value would depress the amount of carbon created in the atmosphere. Fission track dating method[ edit ] Main article: This involves inspection of a polished slice of a material to determine the density of "track" markings left in it by the spontaneous fission of uranium impurities.
The uranium content of the sample has to be known, but that can be determined by placing a plastic film over the polished slice of the material, and bombarding it with slow neutrons. This causes induced fission of U, as opposed to the spontaneous fission of U.
The fission tracks produced by this process are recorded in the plastic film. The uranium content of the material can then be calculated from the number of tracks and the neutron flux.
- K-Ar dating calculation
This scheme has application over a wide range of geologic dates. For dates up to a few million years micastektites glass fragments from volcanic eruptionsand meteorites are best used.
Older materials can be dated using zirconapatitetitaniteepidote and garnet which have a variable amount of uranium content. The technique has potential applications for detailing the thermal history of a deposit.
The residence time of 36Cl in the atmosphere is about 1 week. Thus, as an event marker of s water in soil and ground water, 36Cl is also useful for dating waters less than 50 years before the present. Luminescence dating methods[ edit ] Main article: Luminescence dating Luminescence dating methods are not radiometric dating methods in that they do not rely on abundances of isotopes to calculate age.Potassium-argon (K-Ar) dating - Cosmology & Astronomy - Khan Academy
Instead, they are a consequence of background radiation on certain minerals. Over time, ionizing radiation is absorbed by mineral grains in sediments and archaeological materials such as quartz and potassium feldspar. The radiation causes charge to remain within the grains in structurally unstable "electron traps". Anything to the negative power is just its multiplicative inverse. So this is just the natural log of 2.
Radiometric dating - Wikipedia
So negative natural log of 1 half is just the natural log of 2 over here. So we were able to figure out our k. It's essentially the natural log of 2 over the half-life of the substance.
So we could actually generalize this if we were talking about some other radioactive substance. And now let's think about a situation-- now that we've figured out a k-- let's think about a situation where we find in some sample-- so let's say the potassium that we find is 1 milligram.
I'm just going to make up these numbers. And usually, these aren't measured directly, and you really care about the relative amounts.
But let's say you were able to figure out the potassium is 1 milligram. And let's say that the argon-- actually, I'm going to say the potassium found, and let's say the argon found-- let's say it is 0.
So how can we use this information-- in what we just figured out here, which is derived from the half-life-- to figure out how old this sample right over here?
How do we figure out how old this sample is right over there?
Well, what we need to figure out-- we know that n, the amount we were left with, is this thing right over here.
So we know that we're left with 1 milligram. And that's going to be equal to some initial amount-- when we use both of this information to figure that initial amount out-- times e to the negative kt. And we know what k is. And we'll figure it out later. So k is this thing right over here. So we need to figure out what our initial amount is.
We know what k is, and then we can solve for t. How old is this sample? We saw that in the last video. So if you want to think about the total number of potassiums that have decayed since this was kind of stuck in the lava. And we learned that anything that was there before, any argon that was there before would have been able to get out of the liquid lava before it froze or before it hardened.
So maybe I could say k initial-- the potassium initial-- is going to be equal to the amount of potassium 40 we have today-- 1 milligram-- plus the amount of potassium we needed to get this amount of argon We have this amount of argon 0. The rest of it turned into calcium And this isn't the exact number, but it'll get the general idea.
And so our initial-- which is really this thing right over here. I could call this N0. This is going to be equal to-- and I won't do any of the math-- so we have 1 milligram we have left is equal to 1 milligram-- which is what we found-- plus 0.
And then, all of that times e to the negative kt. And what you see here is, when we want to solve for t-- assuming we know k, and we do know k now-- that really, the absolute amount doesn't matter. What actually matters is the ratio. Because if we're solving for t, you want to divide both sides of this equation by this quantity right over here.
So you get this side-- the left-hand side-- divide both sides. Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control.
For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain. The results from a laser probe can be plotted in several graphical ways, including a map of a grain showing lateral argon distribution. Total fusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms.
For the J to be determined, a standard of known age must be irradiated with the samples of unknown age. Traditionally, this primary standard has been a hornblende from the McClure Mountains, Colorado a.
Some of these include other isotopic dating techniques e. This uncertainty results from 1 the branched decay scheme of 40K and 2 the long half-life of 40K 1. J Factor Because the J value is extrapolated from a standard to an unknown, the accuracy and precision on that J value is critical.
J value uncertainty can be minimized by constraining the geometry of the standard relative to the unknown, both vertically and horizontally. The NMGRL does this by irradiating samples in machined aluminum disks where standards and unknowns alternate every other position. J error can also be reduced by analyzing more flux monitor aliquots per standard location. This is caused by the net loss of 39ArK from the sample by recoil the kinetic energy imparted on a 39ArK atom by the emission of a proton during the n,p reaction.
Recoil is likely in every potassium-bearing sample, but only becomes a significant problem with very fine grained minerals e. For multi-phase samples such as basaltic wholerocks, 39ArK redistribution may be more of a problem than net 39ArK loss. In this case, 39Ar may recoil out of a low-temperature, high-potassium mineral e.