DATING – IMPACT GEOLOGY
by: Charles O’Dale
- IMPACT MELTS AND GLASSES,
- DATING METHODS,
- DATING GLOSSARY
1. CRATERS (example)
The Manson Meteorite Impact and the Pierre Shale
In the Cretaceous Period, a large meteorite struck the earth at a location near the present town of Manson, Iowa. The heat of the impact melted some of the feldspar crystals in the granitic rocks of the impact zone, thereby resetting their internal radiometric clocks. These melted crystals, and therefore the impact, have been dated by the 40Ar/39Ar method at 74.1 Ma (million years; Izett and others 1998), but that is not the whole story by a long shot. The impact also created shocked quartz crystals that were blasted into the air and subsequently fell to the west into the inland sea that occupied much of central North America at that time. Today this shocked quartz is found in South Dakota, Colorado, and Nebraska in a thin layer (the Crow Creek Member) within a thick rock formation known as the Pierre Shale. The Pierre Shale, which is divided into identifiable sedimentary beds called members, also contains abundant fossils of numerous species of ammonites, ancestors of the chambered nautilus. The fossils, when combined with geologic mapping, allow the various exposed sections of the Pierre Shale to be pieced together in their proper relative positions to form a complete composite section (Figure 1). The Pierre Shale also contains volcanic ash that was erupted from volcanoes and then fell into the sea, where it was preserved as thin beds. These ash beds, called bentonites, contain sanidine feldspar and biotite that has been dated using the 40Ar/39Ar technique.
The results of the Manson Impact/Pierre Shale dating study (Izett and others 1998) are shown in Figure 1. There are three important things to note about these results. First, each age is based on numerous measurements; laboratory errors, had there been any, would be readily apparent. Second, ages were measured on two very different minerals, sanidine and biotite, from several of the ash beds. The largest difference between these mineral pairs, in the ash from the Gregory Member, is less than 1%. Third, the radiometric ages agree, within analytical error, with the relative positions of the dated ash beds as determined by the geologic mapping and the fossil assemblages; that is, the ages get older from top to bottom as they should. Finally, the inferred age of the shocked quartz, as determined from the age of the melted feldspar in the Manson impact structure (74.1 ± 0.1 Ma), is in very good agreement with the ages of the ash beds above and below it.
Relative dating to determine the age of rocks and fossils: Geologists have established a set of principles that can be applied to sedimentary and volcanic rocks that are exposed at the Earth’s surface to determine the relative ages of geological events preserved in the rock record. For example, in the rocks exposed in the walls of the Grand Canyon there are many horizontal layers, which are called strata. The study of strata is called stratigraphy, and using a few basic principles, it is possible to work out the relative ages of rocks.
3. IMPACT MELTS AND GLASSES
Impact melts and glasses (or minerals that have recrystallized from the melt; e.g., Krogh et al., 1993; Izett et al., 1994) have another important use, as they often are the most suitable material for the dating of an impact structure. The methods most commonly used for dating of impact melt rocks or glasses include the K-Ar, 40Ar-39Ar, fission track, Rb-Sr, Sm-Nd, or U- Th-Pb isotope methods.
Is a common technique of radiometric dating and is applied to date certain events, such as crystallization, metamorphism, shock events, and differentiation of precursor melts, in the history of rocks. The initial amount of the daughter product can be determined using isochron dating.
RADIOMETRIC DATING Reports of the National Center for Science Education – Brent Dalrymple,
The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are listed below:
|Parent Isotope||Stable Daughter Product||Currently Accepted Half-Life Values|
|Uranium-238||Lead-206||4.5 billion years|
|Hafnium-182||Tungsten-182||9 Million years|
|Uranium-235||Lead-207||704 million years|
|Thorium-232||Lead-208||14.0 billion years|
|Rubidium-87||Strontium-87||48.8 billion years|
|Potassium-40||Argon-40||1.25 billion years|
|Samarium-147||Neodymium-143||106 billion years|
Potassium-40 decays slowly into argon-40, so that the more argon-40 present, the older the sample is. However, measuring the ratio of potassium-40 to argon-40 has the disadvantage of the potassium and argon needing to be measured separately. A more reliable variant of this method is to convert the potassium into argon-39. The rock sample is heated to release both the argon-39 and argon-40, so that the two isotopes can be measured at the same time. The amount of argon-39 that it is released indicates how much potassium-40 was originally in the rock.
|Name of Method||Age range of Application||Material Dated||Methodology|
|Radiocarbon||1 – 70,000 years||Organic material such as bones, wood, charcoal, shells||Radioactive decay of 14C in organic matter after removal from bioshpere|
|K-Ar dating||1,000 – billion of years||Potassium-bearing minerals and glasses||Radioactive decay of 40K in rocks and minerals|
|Uranium-Lead||10,000 – billion of years||Uranium-bearing minerals||Radioactive decay of uranium to lead via two separate decay chains|
|Uranium series||1,000 – 500,000 years||Uranium-bearing minerals, corals, shells, teeth, CaCO3||Radioactive decay of 234U to 230Th|
|Fission track||1,000 – billion of years||Uranium-bearing minerals and glasses||Measurement of damage tracks in glass and minerals from the radioactive decay of 238U|
|Luminescence (optically or thermally stimulated)||1,000 – 1,000,000 years||Quartz, feldspar, stone tools, pottery||Burial or heating age based on the accumulation of radiation-induced damage to electron sitting in mineral lattices|
|Cosmogenic Nuclides||1,000 – 5,000,000 years||Typically quartz or olivine from volcanic or sedimentary rocks||Radioactive decay of cosmic-ray generated nuclides in surficial environments|
|Magnetostratigraphy||20,000 – billion of years||Sedimentary and volcanic rocks||Measurement of ancient polarity of the earth’s magnetic field recorded in a stratigraphic succession|
|Tephrochronology||100 – billions of years||Volcanic ejecta||Uses chemistry and age of volcanic deposits to establish links between distant stratigraphic successions|
How do we know the age of the Earth?
In geology, zircon is used for radiometric dating of zircon-bearing rocks (using isotopes of U which is often present as an impurity element, as is Th, radiogenic Pb, Hf, Y, P, and others). Zircon contains the radioactive element uranium, which converts to the element lead at a specific rate over a long span of time, “the most reliable natural chronometer that we have when we want to look at the earliest part of Earth history.”
6. DATING GLOSSARY (from “The Nature Education“)
absolute dating: Determining the number of years that have elapsed since an event occurred or the specific time when that event occurred
atomic mass: The mass of an isotope of an electron, based on the number of protons and neutrons
atomic nucleus: The assemblage of protons and neutrons at the core of an atom, containing almost all of the mass of the atom and its positive charge
daughter isotope: The isotope that forms as a result of radioactive decay
electrons: Negatively charged subatomic particles with very little mass; found outside the atomic nucleus
electron spin resonance: Method of measuring the change in the magnetic field, or spin, of atoms; the change in the spin of atoms is caused by the movement and accumulation of electrons from their normal position to positions in imperfections on the crystal structure of a mineral as a result of radiation.
elements: Chemical substances that cannot be split into a simpler substances
fault: A fracture in a rock along which movement occurs
geomagnetic polarity time scale: A record of the multiple episodes of reversals of the Earth’s magnetic polarity that can be used to help determine the age of rocks
half-life: The amount of time it takes for half of the parent isotopes to radioactively decay to daughter isotopes
index fossil: A fossil that can be used to determine the age of the strata in which it is found and to help correlate between rock units
isotopes: Varieties of the same element that have the same number of protons, but different numbers of neutrons
magnetic field: A region where lines of force move electrically charged particles, such as around a magnet, through a wire conducting an electric current, or the magnetic lines of force surrounding the earth
magnetism: The force causing materials, particularly those made of iron and other certain metals, to attract or repel each other; a property of materials that responds to the presence of a magnetic field
normal polarity: Interval of time when the earth’s magnetic field is oriented so that the magnetic north pole is approximately in the same position as the geographic north pole
neutrons: A subatomic particle found in the atomic nucleus with a neutral charge and a mass approximately equal to a proton
optical stimulating luminescence: Dating method that uses light to measure the amount of radioactivity accumulated by crystals in sand grains or bones since the time they were buried
paleomagnetism: Remanent magnetization in ancient rocks that records the orientation of the earth’s magnetic field and can be used to determine the location of the magnetic poles and the latitude of the rocks at the time the rocks were formed
parent isotope: The atomic nucleus that undergoes radioactive decay
polarity (magnetic polarity): The direction of the earth’s magnetic field, which can be normal polarity or reversed polarity
potassium-argon (K-Ar) method: Radiometric dating technique that uses the decay of 39K and 40Ar in potassium-bearing minerals to determine the absolute age
principle of cross-cutting relationships: Any geologic feature that cross-cuts across strata must have formed after the rocks they cut through were deposited.
principle of faunal succession: Fossil species succeed each other in a definitive, recognizable order and once a species goes extinct, it disappears and cannot reappear in younger rocks.
principle of original horizontality: Layers of strata are deposited horizontally, or nearly horizontally, and parallel or nearly parallel to the earth’s surface.
principle of superposition: In an undeformed sequence, the oldest rocks are at the bottom and the youngest rocks are at the top.
protons: Positively charged subatomic particles found in the nucleus of an atom
radioactivity (radioactive): An unstable isotope spontaneously emits radiation from its atomic nucleus
radioactive decay: The process by which unstable isotopes transform to stable isotopes of the same or different elements by a change in the number of protons and neutrons in the atomic nucleus
radiocarbon dating: Radiometric dating technique that uses the decay of 14C in organic material, such as wood or bones, to determine the absolute age of the material
radiometric dating: Determination of the absolute age of rocks and minerals using certain radioactive isotopes
relative dating: Rocks and structures are placed into chronological order, establishing the age of one thing as older or younger than another
reversals (magnetic reversals): Changes in the earth’s magnetic field from normal polarity to reversed polarity or vice versa
reversed polarity: Interval of time when the earth’s magnetic field is oriented so that magnetic north pole is approximately in the same positions as the geographic south pole
strata (singular: stratum): Distinct layers of sediment that accumulated at the earth’s surface.
stratigraphy: The study of strata and their relationships
thermoluminescence: Dating method that uses heat to measure the amount of radioactivity accumulated by a rock or stone tool since it was last heated