The natural radioactive series which involve lead as a daughter element do offer a mechanism to test the assumptions. Common lead contains a mixture of four isotopes. Lead , which is not produced by radioactive decay provides a measure of what was "original" lead. It is observed that for most minerals, the proportions of the lead isotopes is very nearly constant, so the lead can be used to project the original quantities of lead and lead Lead is the final stable product of the Thorium series , so is not used in uranium-lead dating.
The two uranium-lead dates obtained from U and U have different half-lives, so if the date obtained from the two decays are in agreement, this adds confidence to the date. They are not always the same, so some uncertainties arise in these processes. There are powerful rationales for using lead isotopes as indicative of concentrations at the point when the lead-containing mineral was in the molten state.
Since the isotopes of lead are chemically identical, any processes that brought lead into the mineral would be completely indiscriminate about which isotope was brought in. The forming mineral will incorporate lead, lead and lead at the ratio at which they are found at that location at the time of formation. Any departure from the original relative concentrations of lead and lead relative to lead could then be attributed to radioactive decay.
This approach is generally considered to be the most precise for determining the age of the Earth.
Potassium-Argon dating has the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium. Since the argon will escape if the rock is melted, the dates obtained are to the last molten time for the rock.
The radioactive transition which produces the argon is electron capture. The rubidium-strontium pair is often used for dating and has a non-radiogenic isotope, strontium, which can be used as a check on original concentrations of the isotopes.mcash.flexi-parking.com/4453.php
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This process is often used along with potassium-argon dating on the same rocks. The ratios of rubidium and strontium to the strontium found in different parts of a rock sample can be plotted against each other in a graph called an isochron which should be a straight line. The slope of the line gives the measured age. The isotope 87 Rb decays into the ground state of 87 Sr with a half-life of 4.
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This is a rubidium-strontium isochron for a set of samples of a Precambrian granite body exposed near Sudbury, Ontario. The data is from T. To the intriguing question "How old is the Earth? While there are numerous natural processes that can serve as clocks, there are also many natural processes that can reset or scramble these time-dependent processes and introduce uncertainties.
To try to set a reasonable bound on the age, we could presume that the Earth formed at the same time as the rest of the solar system. If the small masses that become meteorites are part of that system, then a measurement of the solidification time of those meteorites gives an estimate of the age of the Earth.
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The following illustration points to a scenario for developing such an age estimate. Some of the progress in finding very old samples of rock on the Earth are summarized in the following comments.
But later in It is a compound of zirconium, silicon and oxygen which in its colorless form is used to make brilliant gems. Samples more than 3. Older ages in the neighborhood of 4. The graph below follows the treatment of Krane of Rb-Sr studies of meteorite samples from Wetherill in order to show the nature of the calculation of age from isochrons.
Considering the relative scale of nuclei and atoms , nuclei are so remote from the outer edge of the atoms that no environmental factors affect them. However, there are two obvious problems with radioactive dating for geological purposes: The relative amounts of strontium and are determined with great precision and the fact that the data fits a straight line is a strong argument that none of the constituents was lost from the mix during the aging process.
Similar results are also obtained from the study of spontaneous fission events from uranium and plutonium One of the standard references for modeling the age of the Earth is G.
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From such data, and from estimates of how long it would take to produce the quantities of various lead isotopes now found on the earth, geochronologists feel that the 4. The ages of rocks returned to Earth from the Apollo missions range from 3. Although the potassium-argon method has been used to date rocks on Earth for many decades, these types of measurements require sophisticated lab equipment that could not easily be transported and used on another planet.
Farley had the idea of performing the experiment on Mars using the SAM instrument. There, the sample was heated to temperatures high enough that the gasses within the rock were released and could be analyzed by an onboard mass spectrometer. Farley and his colleagues determined the age of the mudstone to be about 3. Indeed, prior to Curiosity's geochronology experiment, researchers using the "crater counting" method had estimated the age of Gale Crater and its surroundings to be between 3.
Crater counting relies on the simple fact that planetary surfaces are repeatedly bombarded with objects that scar their surface with impact craters; a surface with many impact craters is presumed to be older than one with fewer craters. Although this method is simple, it has large uncertainties.
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The researchers do, however, acknowledge that there is some uncertainty in their measurement. One reason is that mudstone is a sedimentary rock—formed in layers over a span of millions of years from material that eroded off of the crater walls—and thus the age of the sample drilled by Curiosity really represents the combined age of those bits and pieces. So while the mudstone indicates the existence of an ancient lake—and a habitable environment some time in the planet's distant past—neither crater counting nor potassium-argon dating can directly determine exactly when this was.
To provide an answer for how the geology of Yellowknife Bay has changed over time, Farley and his colleagues also designed an experiment using a method called surface exposure dating. Cosmic rays can only penetrate about two to three meters below the surface, so the abundance of cosmic-ray-debris isotopes in rock indicates how long that rock has been on the surface.
Using the SAM mass spectrometer to measure the abundance of three isotopes that result from cosmic-ray bombardment—helium-3, neon, and argon—Farley and his colleagues calculated that the mudstone at Yellowknife Bay has been exposed at the surface for about 80 million years. That is probably the most remarkable thing I've ever seen as a scientist, given the difficulty of the analyses," Farley says. This also helps researchers looking for evidence of past life on Mars.
Cosmic rays are known to degrade the organic molecules that may be telltale fossils of ancient life. However, because the rock at Yellowknife Bay has only been exposed to cosmic rays for 80 million years—a relatively small sliver of geologic time—"the potential for organic preservation at the site where we drilled is better than many people had guessed," Farley says. Furthermore, the "young" surface exposure offers insight into the erosion history of the site. The exposure of rock in Yellowknife Bay has been caused by wind erosion. Over time, as wind blows sand against the small cliffs, or scarps, that bound the Yellowknife outcrop, the scarps erode back, revealing new rock that previously was not exposed to cosmic rays.
At 80 million years ago, wind would have caused this scarp to migrate across the surface and the rock below the scarp would have gone from being buried—and safe from cosmic rays—to exposed," Farley explains. Geologists have developed a relatively well-understood model, called the scarp retreat model, to explain how this type of environment evolves.
Curiosity is now long gone from Yellowknife Bay, off to new drilling sites on the route to Mount Sharp where more dating can be done. In another paper in the same issue of Science Express , Grotzinger—who studies the history of Mars as a habitable environment—and colleagues examined the physical characteristics of the rock layers in and near Yellowknife Bay.