F13 quiz 1 condensed notes

Instead isotopic analysis would obtain an average age

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Unformatted text preview: y. In fact, the oceans are in a steady state, in which salt addition is roughly balanced by removal. Radioactivity, discovered in the 1890’s, gives us our most reliable means for measuring ages. Certain isotopes are unstable, and ultimately transform (“decay”) in various ways to become different isotopes. Isotopes are defined by their number of protons (which define which element they are) and neutrons; for example, the element lead (Pb) has 82 neutrons, but may have various numbers of neutrons. If a lead atom has 124 neutrons, it is referred to as 206Pb (as 82+124 = 206); 207Pb has 125 neutrons, and so on. A radioactive isotope is called a parent, and when it decays it becomes a daughter. The decay transformation by expulsion of particles from within the atom nucleus, as well as by transformation of the particles themselves. Radioactivity is an energy ­releasing phenomenon. Decay inevitably results in a change in the number of protons in the nucleus, thus changing the atom from one element to another. Decay is a statistical process; we can only characterize atoms’ behavior as a group. All atoms of a given radioactive isotope will decay if given sufficient time, but it is impossible to know when any single atom will decay. Each isotope decays at a specific overall rate, and this rate is not influenced by any external conditions (i.e. temperature, pressure, chemical reactions) that prevail on or inside the Earth, making it an ideal clock. The rate of decay for an isotope can be usefully summarized by its half life: the amount of time it takes for half of a group of radioactive parent isotopes to decay to daughter atoms. After one half ­life, by definition one ­half of the parent atoms will have decayed; after two half ­ lives, 1/2 x 1/2 = 1/4 of the parent remains; after three half ­lives, 1/2 x 1/2 x 1/2 = 1/8 of the parent remains, etc. During all of this, the stable daughter isotope accumulates (decay does not annihilate an atom, but simply changes its identity from parent to daughter). For example, seeing that 1/8 of the parent remains after three half ­lives have gone by, the other 7/8 of the original parent will have been transformed into daughter isotope. After three half ­lives, the daughter/parent ratio (d/p) = 7/8 ÷ 1/8 = 7. To be useful for determining ages of geologic events, a parent isotope must have a very long half ­life because the events themselves are so ancient. These long ­lived isotopes were incorporated into the earth at the time of its origin, and a portion of them persists u...
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This document was uploaded on 03/03/2014 for the course GEO 303K at University of Texas.

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