Start studying Lesson 3 Absolute Dating. Learn vocabulary, terms, and more with flashcards, games, and other study tools. scientist can find out how old the rock is depending on the radioactive isotopes half life what are index fossils and how are they used to determine the absolute age of sedimentary rock. One of the most widely used and well-known absolute dating techniques is carbon (or radiocarbon) dating, which is used to date organic remains. This is a radiometric technique since it is based on radioactive decay. Cosmic radiation entering the earth’s atmosphere produces carbon, and plants take in carbon as they fix carbon dioxide.
Radioactive decay[ edit ] Example of a radioactive decay chain from lead Pb to lead Pb. The final decay product, lead Pbis stable and can no longer undergo spontaneous radioactive decay. All ordinary matter is made up iwotopes combinations of chemical elementsaee with its own atomic numberindicating the number of protons in the atomic nucleus. Additionally, elements may exist aboriginal cultural communication rules dating different isotopeswith each isotope of an element differing in the number of neutrons in the nucleus. A particular isotope of a particular element is called a nuclide. Some nuclides are inherently unstable. That is, at some point in time, an atom of such a nuclide will undergo radioactive decay and spontaneously transform into a different nuclide.
In theory, the age of any of these minerals can be determined by: It illustrates how the amount of a radioactive parent isotope decreases with time. This amount is a percentage of the original parent amount. Time is expressed in half-lives. Experiment by dragging on the graph. Note that this half-life can be obtained from the graph at the point where the decay and growth curves cross. Determine the half-lives for the other three isotopes and enter your estimate into the text fields below each graph.
Note the differences in scale between the various graphs Re-setting the Clock - Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusion, setting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature. This field is known as thermochronology or thermochronometry. Radiocarbon Dating The radiocarbon dating method was developed in the 's by Willard F. Libby and a team of scientists at the University of Chicago. It subsequently evolved into the most powerful method of dating late Pleistocene and Holocene artifacts and geologic events up to about 50, years in age.
The radiocarbon method is applied in many different scientific fields, including archeology, geology, oceanography, hydrology, atmospheric science, and paleoclimatology. The Rb-Sr method. Rubidium occurs in nature as two isotopes: Rb decays with a half-life of Which minerals and rocks can be dated with the Rb-Sr method? The minerals must contain Rb, which is a rather rare element. Examples include the mica family biotite and muscovite and the feldspar family plagioclase and orthoclase.
Select a fresh, unweathered rock sample. Sample Selection A geologist collects a fresh, unweathered hand sample for age dating. Fresh is the key word here, and means that the chemistry of the sample has NOT been changed since the sample formed. Weathering alters the chemistry of rocks including their isotopic compositions. Therefore, a highly weathered rock may yield unreliable age information. Crush the rock and separate the Rb-bearing minerals.
Getting a Rock Sample Ready for the Mass Spectrometer For reliable age determination, careful sample preparation is an important and often tedious process. The rock is mechanically crushed into small fragments. Fragments of the Rb-bearing minerals are then separated from the whole rock using a variety of methods, such as a magnetic separator. These materials are then used to prepare a "whole-rock" sample and several "mineral separate" samples.
The whole rock sample will yield the weighted average isotopic composition of all the minerals in the rock. Each mineral separate will yield the composition of that particular mineral. Other Steps There are other steps that must be carried out to prepare a sample for analysis by a mass spectrometer, such as converting the sample to a solution by dissolving the mineral separates in selected acids, using techniques of column chemistry to increase the concentration of the small amounts of Rb and Sr in the solution and then precipitating the concentrated solution as a "salt" compound.
It's this compound of Rb-Sr salts that can be attached to a special filament and placed into the mass spectrometer for analysis. Analyze the isotopic compositions of the whole rock and mineral separates on a mass spectrometer.
A Mass Spectrometer is used to Measure Isotopic Ratios The gas source mass spectrometer includes three fundamental parts, 1 a "source" of positively charged ions or molecular ions, 2 a magnetic analyzer, and 3 ion collectors. Once formed, the ions are accelerated and focused by charged plates into a beam that enters a flight tube. Multiple ion detectors are arranged to collect the ion beams of interest.
These collectors measure each beam as a current that can be amplified and determined with high precision. A Mass Spectrometer is a very powerful and sophisticated instrument. Many types exist. Below is a simplified diagram of the electro-mechanical mass spectrometer system and a picture of a modern instrument. Without getting too much into the quantum physics of it, the reason decay happens is because this process causes an atom to lose energy, which tends to be a more stable state.
There are a few different modes of radioactivity, but for our purposes we'll focus on two: In alpha decay, an atom loses two neutrons and two protons an alpha particle from the nucleus, reducing its atomic number by two and its weight by four. In beta decay, a neutron changes into a proton, releasing an electron a beta particle in the process, increasing its atomic by one but not changing its weight. Radiometric dating is the method of using this radioactive process to date things from the past.
Take carbon dating. Carbon occurs in three different isotopes on the Earth. Most of it is carbon Carbon and carbon are stable, meaning they don't radioactively decay. Carbon is radioactive, and decays, via beta decay, into nitrogen It's half-life is around 5, years.
This means that in 5, years about half of the carbon in any sample will have decayed. So the amount of carbon is exponentially decreasing over time. Animals and plants incorporate carbon which they get from their food or from carbon dioxide into their bodies as part of the various organic molecules that make up their cells and other structures. They incorporate all the isotopes in the same ratio as the environment around them.
This ratio is well known both for the present, and for the past. Once an organism dies it stops taking in new carbon. The carbon that is part of their bodies starts to radioactively decay, bringing the ratio of carbon to carbon down.