Granite age dating

At the contact, this limestone is converted to marble with high-temperature metamorphic minerals, but remnants of the stromatolites can still be found Richard Squires, oral communication, Thus, it is very clear from the above examples that some granite masses are the same age as or even younger than the "Noachian Flood deposits. Absolute ages of granite bodies, rather than relative ages, can be obtained by using various radioactive isotopes; i. For example, trace amounts of uranium and lead are dissolved in the granite melts. Uranium and lead ions have entirely different chemical characteristics, and they normally crystallize in entirely different minerals.

Because the uranium ion is about the same size as the zirconium ion, uranium will substitute for zirconium and crystallize in zircon, but the lead ion goes elsewhere, commonly in potassium feldspar, as the granite magma crystallizes. But the isotope of uranium U is radioactive and eventually decays to form lead Pb. When the granite first crystallizes and the radioactive uranium enters the zircon crystal devoid of Pb , the clock is set and "ticking," and the uranium is constantly breaking down, eventually to produce new lead Pb atoms trapped in the zircon crystals.

Because this U-Pb decay-scheme is a constant, the ratio of uranium to lead in zircon populations in granite can be used to determine the age of a granite. World-wide the absolute ages of various granite bodies are consistent with the relative ages described above. Granites in the bottom of the Grand Canyon give Precambrian ages of 1.

The Narragansett Pier granite that contains million-year-old Pennsylvanian flora fossils Brown et al. And granites in the Sierra Nevada give Jurassic and Cretaceous ages of 66 to million-years-old that are younger than the rocks about million years-old containing upper Triassic ammonites, which these granites intrude. Occasionally, some granites give apparently anomalous isotopic "ages," including even some of which indicate an age greater than the 4.

This fact is commonly harped on by creationists who are critical of isotopic age-dating methods. But in these places logical explanations suggest reasons why the dates are unusual. Close examination generally shows that, where unusual age "dates" are obtained from granite samples, other processes have affected the granite to cause the anomalous dates. For example, the granite may have been deformed and fractured so that fluids have entered and altered the isotopic ratios. Where granites have been dated by the Rb-Sr age-dating method, anomalous measurements are not unusual because of the susceptibility of rubidium and strontium to be added or subtracted by introduced fluids moving through fractures and deformed crystals in the granite Collins, ; Hunt et al.

The K-Ar age-dating method can also give values that differ from U-Pb age measurements because heat generated from the intrusion of another nearby igneous mass has allowed some of the argon gas to leak. In each of these places, the unusual or unexpected age dates is not the failure of the dating method, but an indication that other events have occurred in the geologic history of these rocks. Geologists realize that apparently inconsistent "dates" can occur and seek to find out why they occur, knowing that the isotopic age-dating technique, itself, is not at fault.

For example, the following analogy can be used. Water-proof wrist watches that can be worn by scuba divers generally keep good time, but occasionally these watches fail and give faulty time.

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When that happens, an examination of the watch shows that it has been damaged so that a crack in the holding case has occurred, and water has leaked into the clock mechanism. The faulty time is not because the watch is improperly designed but because water has corroded the gears in the clock. On that basis, a person does not throw out all clocks or watches or cease to buy them.

ovahiryripen.tk #18 - Absolute radiometric age dating of rocks and geologic materials

Likewise, when isotopic age-dating of granites or other igneous rocks produces unexpected or illogical age dates, one does not throw out the whole system of isotopic age-dating. In some disturbed and deformed rocks, the "clock timing mechanism" has been "upset" by "corrosion" or some other factor, and the faulty date is a clue to the geologist to look for the cause. The primary reason for accepting the isotopic age-dating methods is because, in many places, world-wide, where several different kinds of isotopic age-dating methods have been applied to the same rock, all age determinations were found to be the same.

This equality of measured dates gives confidence that the isotopic age-dating methods are valid scientific procedures. The vastly different half-lives of the radioactive isostopes in each age-dating method and the completely different chemical characteristics of the isotopes make the coincidence of producing the same age dates not a pure-chance situation. The age dates must be controlled by physical laws that are very dependable. Heat capacity of granite. The intruded rocks have to be there first before the granite can cut through them.

In some places granite masses of one type cut across other granite bodies, which also shows that some granites are younger than others. The fact that granites also have several possible different origins, as described earlier, also implies different ages of granite. For example, if some granites are derived by melting of sediments, erosion of a continental land mass must occur first to produce the sediments.

Then, the sediments must be deeply buried, and a strong heat source must be found before the granite can be formed from them. At any rate, it is clear that all granites are not formed necessarily at the same time as in Day Three of the Genesis Week. Precambrian granite bodies in the bottom of the Grand Canyon in Colorado have an erosion surface on which the horizontal, Paleozoic, fossil-bearing sediments are deposited, with the Cambrian Tapeats sandstone at the bottom and the Permian Kaibab limestone at the top.

The eroded surface indicates that these granites are older than these sediments, the so-called "Noachian Flood deposits. The same kinds of metamorphic contact-relationships are found in the granites that intrude fossil-bearing sediments in Maine, Connecticut, and Rhode Island Harrison et al. The Narragansett Pier granite in Rhode Island surrounds inclusions of Pennsylvanian metamorphosed sediments containing flora fossils, Annularia stellata Brown et al. The flora fossils are now totally carbonized as graphite, indicating the high temperature of the granite body that metamorphosed the sedimentary inclusions.

The fact that the granite contains inclusions of these fossil-bearing sediments makes the granite younger than these supposed "Flood" sediments. The Sierra Nevada granite intrusions in California also have intruded and metamorphosed supposed "Flood sediments" in roof pendants containing Ordovician graptolite fossils Frazier et al.

In other places, the Sierran granites have intruded and metamorphosed "Flood sediments" containing Triassic ammonites coiled cephalopods Smith, A granite in the Mojave desert in California near Cadiz intrudes Cambrian limestone containing stromatolite fossils. At the contact, this limestone is converted to marble with high-temperature metamorphic minerals, but remnants of the stromatolites can still be found Richard Squires, oral communication, Thus, it is very clear from the above examples that some granite masses are the same age as or even younger than the "Noachian Flood deposits.

Absolute ages of granite bodies, rather than relative ages, can be obtained by using various radioactive isotopes; i. For example, trace amounts of uranium and lead are dissolved in the granite melts. Uranium and lead ions have entirely different chemical characteristics, and they normally crystallize in entirely different minerals.

Because the uranium ion is about the same size as the zirconium ion, uranium will substitute for zirconium and crystallize in zircon, but the lead ion goes elsewhere, commonly in potassium feldspar, as the granite magma crystallizes. But the isotope of uranium U is radioactive and eventually decays to form lead Pb.

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When the granite first crystallizes and the radioactive uranium enters the zircon crystal devoid of Pb , the clock is set and "ticking," and the uranium is constantly breaking down, eventually to produce new lead Pb atoms trapped in the zircon crystals. Because this U-Pb decay-scheme is a constant, the ratio of uranium to lead in zircon populations in granite can be used to determine the age of a granite. World-wide the absolute ages of various granite bodies are consistent with the relative ages described above.

Granites in the bottom of the Grand Canyon give Precambrian ages of 1. The Narragansett Pier granite that contains million-year-old Pennsylvanian flora fossils Brown et al. And granites in the Sierra Nevada give Jurassic and Cretaceous ages of 66 to million-years-old that are younger than the rocks about million years-old containing upper Triassic ammonites, which these granites intrude.

Potassium-argon dating of the cape granite and a granitized xenolith at sea point.

Occasionally, some granites give apparently anomalous isotopic "ages," including even some of which indicate an age greater than the 4. This fact is commonly harped on by creationists who are critical of isotopic age-dating methods. But in these places logical explanations suggest reasons why the dates are unusual.

Close examination generally shows that, where unusual age "dates" are obtained from granite samples, other processes have affected the granite to cause the anomalous dates. For example, the granite may have been deformed and fractured so that fluids have entered and altered the isotopic ratios. Where granites have been dated by the Rb-Sr age-dating method, anomalous measurements are not unusual because of the susceptibility of rubidium and strontium to be added or subtracted by introduced fluids moving through fractures and deformed crystals in the granite Collins, ; Hunt et al. Is it the single group's results, or is it the line based on the class average?

U is found in most igneous rocks.

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Unless the rock is heated to a very high temperature, both the U and its daughter Pb remain in the rock. A geologist can compare the proportion of U atoms to Pb produced from it and determine the age of the rock. The next part of this exercise shows how this is done. Each team is given a piece of paper marked TIME, on which is written either 2, 4, 6, 8, or 10 minutes. The team should place each marked piece so that "U" is showing. This represents Uranium, which emits a series of particles from the nucleus as it decays to Lead Pb- When each team is ready with the pieces all showing "U", a timed two-minute interval should start.

During that time each team turns over half of the U pieces so that they now show Pb This represents one "half-life" of U, which is the time for half the nuclei to change from the parent U to the daughter Pb A new two-minute interval begins. Continue through a total of 4 to 5 timed intervals.

That is, each team should stop according to their TIME paper at the end of the first timed interval 2 minutes , or at the end of the second timed interval 4 minutes , and so on. After all the timed intervals have occurred, teams should exchange places with one another as instructed by the teacher. The task now for each team is to determine how many timed intervals that is, how many half-lives the set of pieces they are looking at has experienced. The half life of U is million years. Both the team that turned over a set of pieces and the second team that examined the set should determine how many million years are represented by the proportion of U and Pb present, compare notes, and haggle about any differences that they got.

Right, each team must determine the number of millions of years represented by the set that they themselves turned over, PLUS the number of millions of years represented by the set that another team turned over. Pb atoms in the pegmatite is 1: Using the same reasoning about proportions as in Part 2b above, students can determine how old the pegmatite and the granite are. They should write the ages of the pegmatite and granite beside the names of the rocks in the list below the block diagram Figure 1.

This makes the curve more useful, because it is easier to plot it more accurately. That is especially helpful for ratios of parent isotope to daughter isotope that represent less than one half life. For the block diagram Figure 1 , if a geochemical laboratory determines that the volcanic ash that is in the siltstone has a ratio of U If the ratio in the basalt is 7: Students should write the age of the volcanic ash beside the shale, siltstone and basalt on the list below the block diagram.

Why can't you say exactly what the age of the rock is? Why can you be more precise about the age of this rock than you could about the ages of the rock that has the trilobites and the rock that contains acritarchs and bacteria?


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Based on cross-cutting relationships, it was established that the pegmatite is younger than the slate and that the slate is younger than the granite.