Time and Geology Quantitative Geologic Time Early Attempts Tried to place events in chronology of actual age James Ussher, Archbishop of Armagh and Primate of All Ireland 1650 The Annals of the World
-The Annals of the World Ussher calibrated the Julian calendar with the dates and events from the Old Testament. Determined Earth was created on Sunday, 23 October, 4004 BC
Time and Geology Quantitative Geologic Time Sir John Lightfoot 1642 Determined that the Earth was created at 9 am Geologic observations did not support this timeframe. Later disproved by the theory of evolution
Time and Geology Quantitative Geologic Time Other attempts used depositional rates of sediment If the depositional rate was known one could estimate the time required to deposit a given thickness of sediments The total geologic time elapsed could be estimated by dividing the annual sediment influx into the total thickness of sedimentary rock.
Time and Geology Quantitative Geologic Time Sir Edmund Halley 1715 Speculated that the ocean must be the same age as the Earth Ocean started out fresh Weathering of rocks on land produced salt If the amount of salt in the ocean was known it could be divided by the amount added each year John Joly tried this and came up with an age of ~90 my in 1899
Time and Geology Quantitative Geologic Time Lord Kelvin (William Thomson)1862 Thomson challenged the views on geologic and biological change of the early uniformitarians Argued Earth could not be that old Assuming that the Earth started out as a liquid at 7000°F and has been cooling since formation Using the cooling rate he determined the Earth was 24-40 my old
Time and Geology Quantitative Geologic Time Geologists felt the Earth had to be older and argued No PROOF! Discovery of radioactivity provided the answer. X-ray of Kelvin’s hand
Radioisotopic Methods for Dating Rocks A Quick Review The solid Earth is composed of rocks and minerals ROCKS - aggregates of minerals A MINERAL is a naturally occurring, inorganic solid, consisting of either a single element or compound, with a definite chemical composition (or varies within fixed limits), and a systematic internal arrangement of atoms.
MINERAL CHEMISTRY Radioisotopic Methods for Dating Rocks Minerals are composed of elements in specific proportions. Elements cannot be broken down into any smaller units and still be recognizable. Atoms of one or more elements combine in specific proportions to produce compounds. Specific proportions are expressed by the compound’s chemical formula. HALITE NaCl QUARTZ SiO2
Radioisotopic Methods for Dating Rocks MINERAL CHEMISTRY Atoms are extremely small. Atoms are composed of three particles: Protons and Neutrons in the nucleus of the atom. Electrons in energy levels around the nucleus. Protons have a positive charge. Electrons have a negative charge. Neutrons are neutral.
ATOMIC CHEMISTRY Radioisotopic Methods ATOMIC MASS of an element is the total number of protons and neutrons in the nucleus of the atom. ATOMIC NUMBER is the total number of protons in the nucleus of the atom. Always the same number of protons in every atom of the same element. Number of neutrons in the nucleus can vary. Variation in the number of neutrons in the same element produces ISOTOPES of that element.
ATOMIC CHEMISTRY Radioisotopic Methods for Dating Rocks Isotopes of Oxygen Always 8 protons in the nucleus. Oxygen has atomic number of 8 on the Periodic Table. 16O has 8 protons and 8 neutrons. 17O has 8 protons and 9 neutrons. 18O has 8 protons and 10 neutrons.
ATOMIC CHEMISTRY Radioisotopic Methods for Dating Rocks Some isotopes have very unstable nuclei and those break down spontaneously. This reaction is termed RADIOACTIVITY. Generally a large amount of heat is produced in the reaction.
Radioisotopic Methods for Dating Rocks Depends on the decay of radioactive isotopes. Isotopes are varieties of elements that differ by the number of neutrons in the nucleus. Radioactive isotopes have nuclei that spontaneously decay by emitting or capturing a variety of subatomic particles. The decaying isotope is known as the parent isotope. By decay, the parent isotope forms a daughter isotope.
Radioisotopic Methods for Dating Rocks Loss or gain of neutrons converts a parent isotope into a daughter isotope of the same element. Loss or gain of protons changes the parent isotope into a daughter isotope of a completely different element. Through this process, unstable radioactive isotopes decay to form stable, non-radioactive daughter isotopes.
Radioisotopic Methods for Dating Rocks ALPHA () DECAY Alpha () particles are composed of two protons and two neutrons (He nucleus) By expulsion of particles, the atomic mass decreases by 4 and the atomic number decreases by 2. Produces a daughter isotope that is a completely new element.
Radioisotopic Methods for Dating Rocks ALPHA () DECAY 238U92 decays by alpha ()decay to form 234Th90
Radioisotopic Methods for Dating Rocks BETA () DECAY Beta () particles are essentially electrons. These electrons are released from the nucleus of the parent isotope. Neutrons are composed of a proton and an electron. Neutron decays, releasing an electron, while at the same time produces a proton. Beta () decay increases the atomic number by 1. No change in the atomic mass.
Radioisotopic Methods for Dating Rocks BETA () DECAY 40K19 decays by beta() decay to form 40Ca20
Radioisotopic Methods for Dating Rocks ELECTRON OR BETA () CAPTURE Electron or Beta () capture involves capture of an electron from the surrounding orbiting cloud by the nucleus. These electrons join with a proton and form a neutron. Electron or Beta () capture decreases the atomic number by 1. No change in the atomic mass.
Radioisotopic Methods for Dating Rocks ELECTRON OR BETA () CAPTURE 40K19 decays by beta() capture to form 40Ar18
Radioisotopic Methods for Dating Rocks Radioactive isotopes are incorporated in minerals and rocks in a variety of ways. As minerals crystallize from magma, radioactive isotopes are included in mineral crystal structure. At the time of crystallization, only parent isotopes are included in the mineral. Radioactive parent isotopes then begin to decay producing daughter isotopes.
Radioisotopic Methods for Dating Rocks ISOTOPE DATING uses this process to measure the amount of time elapsed since the mineral’s formation. With time, the amount of parent isotope will decrease and the amount of daughter isotope will increase. The DECAY RATE is constant and acts like a “clock”. Decay rates are not affected by temperature, pressure, or chemical reaction with the parent isotope. By measuring the ratio of parent to daughter isotopes in the mineral and comparing it with the rate of radioactive decay, we can determine the numerical age of a rock.
Radioisotopic Methods for Dating Rocks The time it takes for HALF of the atoms of the parent isotope to decay into daughter isotopes is known as the isotope’s HALF-LIFE (t½).
1:1 parent to daughter 1:3 1:7
Radioisotopic Methods for Dating Rocks To calculate the numerical age of a rock, mineral, bone, etc., we determine the number of half-lives or fraction thereof and multiply the number of half-lives gone by by the known half-life (in years). Simply put: In a rock we find 23 atoms of 235U and 161 atoms of 207Pb Half-life (t½) is 713 million years. Age of the rock is 2.139 billion years.
Radioisotopic Methods for Dating Rocks FACTORS AFFECTING ISOTOPIC DATING Most useful in igneous rocks. As minerals crystallize, radioactive isotopes become incorporated in the minerals. No daughter isotopes at that time. Crystallization sets the isotopic “clock”. Doesn’t work in sedimentary rocks. How come?
Radioisotopic Methods for Dating Rocks FACTORS AFFECTING ISOTOPIC DATING Works best when a rock or mineral represents a “closed” system. Parent and daughter isotopes cannot move in or out of a mineral or rock. Igneous rocks best fit this criteria.