2NUMERICAL DATING OF THE EARTH Rocks contain radioactive minerals which are constantly disintegrating at a steady rateUnder 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
3sandstone shale dike Establishing absolute geologic age. Fig. 5.1Establishing absolute geologic age.sandstoneExample of cross-cutting 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.shaledike
4Radioactive elementsNot all elements are radioactive. Those that are and are the most useful for geologic dating are:U-238 Half-life = 4.5 ByK-40 Half-life = 1.25 ByC-14 Half-life = 5.73 yearsAlso, Sm-147, Rb 87, Th-232, U-235
5U-238 DECAYOften 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.
6Fig. 5.3Half-life for decay from U-238 all the way to Pb-206 is 4.5 b.y. (billion years).U-238 Decay SeriesDecay rates for intermediate daughter products range from <1 sec (polonium) to 1,622 years (radium 226).
7Fig. 5.4Schematic diagram showing decay of radioactive parent isotope (e.g. U-238) to a daughter (e.g. Pb-206). The original isotope was sealed in a mineral grain at time of crystallization. Note changing ratio of parent/daughter after 2 half-lives. Note that to get an estimate of the geologicc age, you need the ratio of the parent isotope to the daughter isotope, e.g. two measurements.
8Fig. 5.5Simple arithmetic plot of a universal isotopic decay curve. After 1 half-life 50% of parent isotope remains; after 2 half-lives, 25% remains.What happens if the vertical axis is changed from linear to logarithmic?
9BLOCKING TEMPERATURES The “Blocking Temperature” is an important concept; it refers to processes that result in a “resetting” of the atomic clocks in a rock.Essentially, it is possible to heat igneous and metamorphic rocks to high enough temperatures that they no longer behave as “closed systems”. That is some of the daughter products can “leak” out of the primary mineral, giving an erroneous parent/daughter ratio and hence a wrong age. (Age for what? How could the age be interpreted in a rock in which the blocking temperature has been reached?)
10Blocking temperatures for some common minerals and decay series. Fig. 5.6Blocking temperatures for some common minerals and decay series.The blocking temperature is the temperature above which a mineral or rock no longer behaves as a closed system and the parent/daughter ratios may be altered from that due to pure radioactive disintegration.This can result in resetting the isotopic clock and/or give what are called discordant dates.These types of problems have given opponents of the radiometric dating of the Earth ammunition to attack the 4.5 By age geologists cite.
11These ratios can also be used to date a rock or mineral. Fig. 5.7Use of daughter lead isotopes for 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.
12Fig. 5.8Constant 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 half-life of about 5,730 years.Plants and animals ingest this radioactive C-14 while they are alive. When they die, the ingestion stops, and the radioactive C-14 clock begins to count down.
13Fission tracks in an apatite crystal. Fig. 5.9Fission 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 to the age of the mineral.(Why not just use the U-238 to Pb-206 method directly in such cases?)
14Metamorphic redistribution of daughter isotopes. Fig. 5.10Metamorphic redistribution of daughter isotopes.Mineral crystallizes 1000 mya (1 billion yrs ago)After 500 my (million yrs) some parent isotopes have decayed.480 mya (million yrs ago) metamorphic event redistributes daughter atoms out of crystal into adjacent rockDating of the mineral would now yield the age of the metamorphic eventBut a whole rock age would provide the original age of the rock/mineral (1000 mya).
15Thus the Silurian must be younger than 425 My and older than 370 My. Fig. 5.11Illustration 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 is about 370 my old.Thus the Silurian must be younger than 425 My and older than 370 My.