LECTURE 05 minerals in geology

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Copyright ©The McGraw-Hill Companies, Inc Permission required for reproduction or display Evolution of the Earth Seventh Edition Prothero • Dott Chapter NUMERICAL DATING OF THE EARTH • Rocks contain radioactive minerals which are constantly disintegrating at a steady rate • Under certain circumstances, these atomic “clocks” can be red to give a “time” • The meaning of the “time” depends on what has happened to the rock since the “clock” was set Fig 5.1 Establishing absolute geologic age sandstone shale e k i d Example of crosscutting relationships that establish relative ages: an igneous dike cuts through red shales and is truncated by overlying sandstone A radiometric date on the dike will give a minimum age for the shale and a maximum age for the sandstone Note the combination of “Geologic” age and absolute age techniques Radioactive elements • Not all elements are radioactive Those that are and are the most useful for geologic dating are: • U-238 • K-40 • C-14 Half-life = 4.5 By Half-life = 1.25 By Half-life = 5.73 years • Also, Sm-147, Rb 87, Th-232, U-235 U-238 DECAY • Often elements decay according to a complex decay scheme in which a host of intermediate products, many themselves radioactive, are produced • U-238 is such and element, and given its importance to geologic dating, it is worthwhile to examine it decay scheme • Keep in mind that u-238 has a half-life approximately equal to the age of the earth, 4.5 By Fig 5.3 Half-life for decay from U-238 all the way to Pb-206 is 4.5 b.y (billion years) U-238 Decay Series Decay rates for intermediate daughter products range from [...]... alive When they die, the ingestion stops, and the radioactive C-14 clock begins to count down Fig 5.9 Fission tracks in an apatite crystal They are produced when an atom of U-238 disintegrates emitting an alpha particle, a Helium nucleus (He-4) This massive atomic particle causes massive structural damage in the crystal that can be revealed by etching The number of tracks in a given area is proportional... the age of the mineral (Why not just use the U238 to Pb-206 method directly in such cases?) Fig 5.10 Metamorphic redistribution of daughter isotopes 1 Mineral crystallizes 1000 mya (1 billion yrs ago) 2 After 500 my (million yrs) some parent isotopes have decayed 3 480 mya (million yrs ago) metamorphic event redistributes daughter atoms out of crystal into adjacent rock 4 Dating of the mineral would now... dating The ratios of 3 radiogenic lead isotopes to non-radiogenic lead-204 all change but at different rates These ratios can also be used to date a rock or mineral Fig 5.8 Constant generation of C-14 in the upper atmosphere by cosmic particle bombardment of N (nitrogen) Nitrogen (N-15) emits a proton and becomes C-14 This is radioactive with a halflife of about 5,730 years Plants and animals ingest... metamorphic event 5 But a whole rock age would provide the original age of the rock/mineral (1000 mya) Fig 5.11 Illustration of how radiometric dating can establish a geologic time scale Fossils establish that the granite is Silurian (a) A date for the granite establishes that the Silurian is about 425 my old (b) The date for the lave flow in the Old Red sandstone establishes that part of the Devonian
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