F13 quiz 1 condensed notes

Life that is easily detected in the rocks due to the

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Unformatted text preview: ntil today. Useful parent ­daughter pairs include 40K ­40Ar, with a half ­life of 1.3 billion years, and 87Rb ­87Sr, with a half ­life of 49 billion years. The “gold standard” for isotope dating is the uranium ­lead system. There are two principal uranium isotopes that decay into two distinct lead isotopes (238U ­206Pb and 235U ­207Pb), so a crystal that incorporates a mix of these two parent isotopes contains two independent isotopic clocks that are ticking at two different rates. If both clocks agree (i.e. parent ­daughter ratios indicate the same age), we can be very confident that the clocks have not been disturbed, and that the age is correct. Certain minerals are very useful as “boxes” for radioactive isotopes that we can use as clocks. Zircon, for example, is captures uranium into its mineral structure during igneous crystallization, but but not lead, which is uranium’s ultimate daughter after many intermediate decays. A measurement of the amount of uranium and lead in a zircon, allows us to figure out how long ago that zircon formed. LECTURE 8: EARLY EARTH HISTORY The meaning of the isotopic age of an igneous rock is connected with cooling and crystallization from a magma, which is the geologic event by which igneous rock originated. For example, consider a deep source being melted to produce basaltic magma. 40K (potassium) is present in the source, decaying to 40Ar (argon), but during eruption at the earth’s surface, the lava loses the Ar (an inert gas) to the atmosphere. Upon cooling, crystals of feldspar in basalt begin to retain argon even though it is a gas: the K ­Ar clock has been re ­set. In a cooling body of granite deep underground, the rock could be fully solid for millions of years before it has cooled to the point that Ar would be retained in crystals. For both an extrusive igneous rock (such as basalt) or intrusive rock (such as granite) the K ­Ar age refers to the time of cooling, not crystallization. The age of a sedimentary rock is understood to be the length of time since it was deposited. In most cases, we cannot use isotopic age methods to date clastic deposition. Instead, isotopic analysis would obtain an average age of the source rock being eroded to make the sediment. Again, take the K ­Ar clock as an example. Transporting a grain of K (potassium) feldspar down a stream would not cause the accumulated daughter 40Ar to be separated from the mineral; the isotopic clock would not be re ­set. Ages of deposition of clastic sediments and of their containe...
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