More about isotope geology
Radioactivity and isotopes
Most elements consist of several different isotopes, i.e. varieties of the same element with the same number of protons but different numbers of neutrons in the atomic nucleus, and thus different atomic masses.Example:
Carbon 12: 6 protons and 6 neutrons = mass 12Carbon 13: 6 protons and 7 neutrons = mass 13Carbon 14: 6 protons and 8 neutrons = mass 14Some isotopes are unstable, and disintegrate through radioactive decay in one or several steps to a stable daughter isotope of another element. This process occurs with a fixed rate, expressed as the half life of the isotope, the time it takes for half the number of atoms initially present to decay.
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Examples:
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Carbon 14
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Nitrogen 14
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5570 years
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Rubidium 87
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Strontium 87
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48.8 billion years
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Samarium 147
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Neodymium 143
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106 billion years
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Uranium 235
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Lead 207
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0.704 billion years
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Uranium 238
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Lead 206
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4.47 billion years
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These radioactive decays form geological clocks. By measuring the parent-to-daughter isotope ratio in a mineral or a rock, using a mass spectrometer, its age can be calculated. Such measurements are made at the Laboratory for Isotope Geology of the Swedish Museum of Natural History.Geological applications
The ability to quantify time is very important for geologists trying to determine the chronology and rate of various processes. By measuring the proportions between parent and daughter isotopes in a mineral or rock, we can calculate its age. The Laboratory for Isotope Geology is the only laboratory in Sweden making such analyses, using the radioactive decay systems U-Pb, Pb-Pb, Rb-Sr, and Sm-Nd. Such isotope data is also used to study the origin and evolution of rocks and ores. One special application is the use of Pb isotopes to study the origin of ores and ore-forming processes, of importance for ore prospecting. Isotope data is also used to study modern geochemical processes, e.g. in sea water and sediments.The main emphasis in the research carried out at the Laboratory for Isotope Geology has been on the geological evolution of the Fennoscandian Shield, an area encompassing Sweden, Norway and Finland as well as adjacent parts of Russia, where most of the bedrock formed between 3 and 1 billion years ago. Another field of interest has been the Scandinavian Caledonides, which are c. 400 million years old. In later years, our interests have widened to include the Arctic and North Atlantic regions, the concealed Precambrian bedrock of the Baltic States and Russia, as well as research in the Arabian Shield and elsewhere.
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Zircon (ZrSiO4) is a mineral often used for U-Pb-dating. To the left are some typical zircon crystals, c. 0.1 mm long, the top row being photo-graphed in a scanning electron microscope, the bottom row in glycerine in transmitted light in a standard light microscope.
Photo: Alexander Larionov.
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A new research program, using long-lived radioisotopes to study modern geochemical processes has been initiated. The overall objective is understanding of element transport from the continents to the ocean. Radioisotopes of elements such as Sr, Nd and U-Th have been used in a series of successful investigations regarding geochemical processes of transport and mixing in estuarine systems. This work has also demonstrated the importance of particles and colloids for the transport of elements such as U, Th and other trace elements through the hydrological cycle. Understanding of natural elemental cycles is important when addressing environmental problems.The laboratory has three mass spectrometers, two solid source thermal ionization mass spectrometers (Finnigan MAT 261 and Thermo Triton) for the radiogenic isotope systems U-Pb, Pb-Pb, Rb-Sr and Sm-Nd, and a Micromass IsoProbe multicollector ICP-MS (Inductively Coupled Plasma Mass Spectrometer) which can do isotopic analysis of many elements, including Pb, Sr Nd, Fe and Si in solution. In addition, the Nordic geological ion microprobe laboratory Nordsim is located here. This is a joint Nordic resource for geoscientific research based on a Cameca 1270 ion microprobe. This instrument allows direct in situ spot analysis of isotopes and elements on a micrometer scale.
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