Absolute Dating. What is an isotope?

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1 Absolute Dating any method of measuring the age of an event or object in years radiometric dating (which uses the concept of radioactive decay) is the most common method of absolute dating used to determine the age of rocks and fossils uses the concept of isotopes uses the concept of half-life What is an isotope? Most elements have the same number of protons and neutrons An isotope is an element that has the same number of protons as other atoms of the same element do but have a different number of neutrons Some isotopes are stable meaning that they do not change over time Other isotopes are unstable o Scientists call unstable isotopes radioactive o These unstable isotopes tend to break down into stable isotopes over time

2 How are isotopes used to calculate the age of rocks? The process of the break down of radioactive isotopes into stable isotopes is called radioactive decay Unstable isotopes of one element break down or decay to form a stable isotope of the same element or a different element Examples: o Carbon-14 decays to form Carbon-12 o Uranium-238 decays to form Lead-206 The unstable isotope is referred to as the parent material The stable material produced by the decay of parent material is called the daughter isotope Radioactive decay occurs at a steady rate Scientists use the relative amounts of stable (daughter) and unstable (parent) isotopes in an object to determine an object s age A old object will have more daughter isotopes compared to the amount of parents isotopes

3 What is a half-life and how is it used to determine the age of an object? A half-life is the time needed for half of a sample of a radioactive substance to undergo radioactive decay In other words, a half-life is the time needed for half of the parent material to become daughter material The rate of radioactive decay is predictable The rate of radioactive decay depends on what parent material is decaying The rate of decay is different for different types of parent material Radiometric Dating = the method of determining the age of an object based on the ratio of parent material to daughter material Radioactive parent isotopes and their stable daughter products Radioactive Parent Stable Daughter Potassium 40 Argon 40 Rubidium 87 Strontium 87 Thorium 232 Lead 208 Uranium 235 Lead 207 Uranium 238 Lead 206 Carbon 14 Nitrogen 14

4 Each radioactive isotope has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product. Half Lives for Radioactive Elements Radioactive Parent Stable Daughter Half life Potassium 40 Argon billion yrs Rubidium 87 Strontium billion yrs Thorium 232 Lead billion years Uranium 235 Lead million years Uranium 238 Lead billion years Carbon 14 Nitrogen years

5 Radioactive decay occurs at a constant rate. The rate of decay is proportional to the number of parent atoms present.

6 The proportion of parent to daughter tells us the number of halflives, which we can use to find the age in years. For example, if there are equal amounts of parent and daughter, then one half-life has passed. If there is three times as much daughter as parent, then two halflives have passed. Example: A rock sample contains an isotope that has a half-life of 100,000 years You analyze the sample and find that it has equal amounts of the radioactive parent material and the stable daughter material This means that half of the radioactive isotopes have decayed so the rock sample must be 100,000 years old Knowing that it will take another 100,000 years for half of the remaining parent material to decay You analyze a different sample and find that ¾ of the material is made of daughter material and only ¼ is made of parent material How old is this sample? o 100,000 years + 100,000 years = 200,000 years

7 Potassium Argon Dating Method Method for determining the age of igneous rocks based on the amount of argon-40 in the rock Radioactive potassium-40 decays to argon-40 with a half-life of about 1.3 billion years this method useful for dating rocks that are billions of years old Used to date rocks older than 100,000 years Uranium Lead Dating Method Uranium-238 is a radioactive isotope that decays in a series of steps to lead-206 Half-life of Uranium-238 is 4.5 billion years Method of dating very old rocks by means of the amount of common lead they contain Used to date rocks more than 10 million years old By this method, the age of the Earth has been estimated to be approximately 4.6 billion years The oldest rock sample on record is a metamorphic gneiss from northern Canada, which is dated at 3.9 billion years old

8 Rubidium Strontium Dating Method Method of estimating the age of rocks, minerals, and meteorites Uses measurements of the amount of the stable isotope strontium-87 formed by the decay of the unstable isotope rubidium-87 that was present in the rock at the time of its formation This method is applicable to very old rocks because the transformation from rubidium to strontium is extremely slow The half-life, or time required for half the initial quantity of rubidium-87 to disappear, is approximately 49 billion years This method is used to date rocks older than 10 million years

9 Carbon 14 Dating Method Method of determining the age of once-living material Developed in 1947 Depends on the decay of the radioactive isotope carbon-14 (radiocarbon) to nitrogen-14 (stable carbon) Cosmic rays from the sun strike Nitrogen 14 atoms in the atmosphere and cause them to turn into radioactive Carbon 14, which combines with oxygen to form radioactive carbon dioxide All living plants and animals continually take in carbon - Some of this carbon is radioactive carbon-14, which slowly decays to the stable isotope nitrogen-14 o green plants absorb it in the form of carbon dioxide from the atmosphere. Plants use this carbon dioxide to form sugar through photosynthesis. o it is passed to animals through the food chain All living things contain a constant ratio of Carbon 14 to Carbon 12. (1 in a trillion) When an organism dies it stops taking in carbon, so the amount of carbon-14 in its tissues steadily decreases Because carbon-14 decays at a constant rate, the time since an organism died can be estimated by measuring the amount of radiocarbon in its remains. The half-life of carbon-14 is only 5,730 years This method is a useful technique for dating fossils and archaeological specimens from 500 to 50,000 years old It is widely used by geologists, anthropologists, and archaeologists

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11 Dating Method Used to date objects... Potassium-Argon Uranium-Lead Rubidium-Strontium Carbon-14 older than 100,000 years older than 10 million years older than 10 million years less than 50,000 years What absolute dating method would you use to date the rocks at the bottom of the Grand Canyon? What absolute dating method would you use to date the rocks within which dinosaur fossils are found? What absolute dating method would you use to date ancient Native American items found at an archeological dig? Is radiometric dating used to determine the age of sedimentary rocks? Radioactive isotopes don't tell much about the age of sedimentary rocks (or fossils). The radioactive minerals in sedimentary rocks are derived from the weathering of igneous rocks. If the sedimentary rock were dated, the age date would be the time of cooling of the magma that formed the igneous rock. The date would not tell anything about when the sedimentary rock formed.

12 Summary During radioactive decay, an unstable isotope decays at a constant rate and becomes a stable isotope of the same or a different element The unstable isotope is called the parent material The stable isotope is called the daughter material Half-life is the time it takes for half of the parent material to decay to form daughter material Radiometric dating, based on the ratio of parent to daughter material, used to determine the absolute age of a sample Depending upon the perceived age of a sample, scientists use different methods of radiometric dating, including carbon-14 dating The Geologic Column was produced using both relative and absolute dating methods