CEE 1030 1st Edition Lecture 8 Two ways to date geologic events Relative dating determine whether a rock is older or younger than surrounding rocks field relationships fossils Absolut dating determine actual number of years since rock was formed radioactive decay radiometric dating Structure of an atom nucleus contains most of the mass of the atom has protons positive charge has neutrons no charge neutral electrons with negative charge cloud around nucleus Atoms and Isotopes atomic number number of protons all atoms of a particular elements have the same atomic number mass number number of protons and neutrons atoms of a particular element can have different numbers of neutrons isotopes Radioactive decay radioactivity spontaneous changes decay in the structure of certain unstable atomic nuclei parent an unstable radioactive isotope daughter product s the isotopes resulting from decay of a parent isotope usually of a different element unstable element will decay into a stable element can change elements in order to be stable Three main types alpha decay beta decay electron capture alpha decay loss of 2n 2p atomic mass top number on left of element goes down by four atomic number goes down two beta decay n p electron atomic number goes up by one atomic mass stays the same electron capture p electron n opposite of beta atomic number goes down by one How is radioactive decay useful for dating the rate at which radioactive decay occurs for a particular unstable isotope is constant if we know the rate of decay and the amount of parent and daughter isotopes present in a sample we can calculate an age half life of a radioactive isotope is the time taken for half of the atoms initially present Requirements for isotope dating decay rates constant and precisely known no evidence or theoretical basis to expect decay rates to vary under the physical conditions within the Earth or over time initial concentration of daughter known best to choose minerals that do not include daughter element during initial formation can also use isochron techniques separate and analyse several different minerals from rock sample closed system parent and daughter elements must not diffuse leak in or out of the mineral diffusion stops below a certain temperature 150o 800o C which varies for different elements and different minerals chemical weathering of sample a problem minerals in slowly cooled metamorphic rocks can give different ages by different techniques radiometric dating is a complex procedure that requires precise measurements careful and detailed sample preparation sophisticated and expensive equipment Carbon 14 or radiocarbon dating 14C half life is 5730 years does not tell us anything about the age of the earth only for very young samples 40 000 years is being formed actively in the atmosphere atoms moving at the speed of light by high energy cosmic rats incorporated into all living organisms can date wood charcoal bone shells useful tool for archeologists and geologists who study very recent earth history Agreement between systems two best dating systems 40Ar 39Ar and U Pb done on the same samples excellent agreement Uranium Lead Dating Method probably best method used 238U 206Pb 4470 million years 235U 207Pb 704 million years minerals with high U and low no Pb Earth s oldest rocks Arcasta Gneisses Yellowknife area of the NW territories near Great Slave Lake in Canada 4 03 Billion years old Absolute ages for the geological time scale obtaining absolute ages for boundaries between units on the geological time scale is not simple time scale is based mainly on fossils found in sedimentary rocks absolute ages are best determined on igneous and some metamorphic rocks Sedimentary rocks not easy detrital mineral grains will give age of original source rock cement minerals give younger ages than sediment deposition age date indirectly from volcanic igneous rocks interbedded with sedimentary rock units pyroclastic ashes lava flows Volcanic Ash Layers blanket a wide geographical area distinctive time marker horizon often contain minerals that can be dated by radiometric techniques