Radiometric Dating Relative dating techniques are based on ______principles can be used to differentiate the relative age rock units and landforms. Relative dating techniques by themselves cannot be used to assign independent ______ages. Numerical or Radiometric dating allows geologist to assign actual ages to a deposit or landform. Many of the methods are based on the ratio of radioactive ______and their respective decay products measured in a material to be dated. In many cases relative dating techniques are used in tandem with numerical dating techniques to constrain the age of a landform or deposit. This is called “______.”
Unit A (225 m.y. old) Unit B (how old is it?) Unit C (275 m.y. old) Bracketing Certain layers in rock, particularly volcanic deposits like ash or lava flows, are more useful for radiometric dating techniques than others. For example, a volcanic ash layer: • Represent a very ______Dating of period of time in geologic volcanic ash history • Might be ______in an area • Contains radioactive elements that are ______for radiometric dating. Isotopes Fig. 3–3 • Isotopes = 2 atoms of the same element (same atomic #) that have different numbers of neutrons, and therefore different ______.
• Most elements have one isotope that is prevalent.
• Many elements have isotopes that are radioactive.
• All isotopes with the same atomic mass (number of Examples: ______) are the same H, D, T (see fig 3-3) element Carbon 12 & Carbon 14 (12C & 14C) • The number given before or after the element symbol is the U-235 & U-238 atomic ______Stable vs. Radioactive isotopes
• The atomic ______(number of protons) determines what element it is (i.e. Carbon will always have 6 protons)
• Different elements may have the same mass, depending on the number of ______. Radioactivity Radioactive decay refers to the process in which an atomic nucleus of an unstable atom loses energy by emitting ______particles.
The radioactive parent atom is converted to ______atom which can be stable, or radioactive (subject to further decay). Radioactive decay occurs via three decay processes: Alpha Decay, Beta Emission, Electron Capture. For the radioactive decay process to be useful for finding the age of a rock or fossil, the daughter isotope must only be able to be created through the ______of the parent isotope.
Otherwise, there would be no way to tell how much daughter was produced from the original radioactive parent isotope. Age Equation D = Do + N(eλt-1) D= number of daughter atoms N = number of existing parent atoms
Do = number of initial daughter atoms e = exponential function λ = decay constant t = time
• Radioactive isotopes will decay spontaneously to form daughter atoms. Radioactive decay is ______. Different radioactive elements will have different decay rates, but the decay curve will be the same (purple curve). • As radioactive parent atoms decay, daughter atoms are correspondingly created (blue curve). If you can measure the ______of parent to daughter atoms in a isotope system, you can solve for the age (T). T 1/λ l [(D D )/N + 1] Radioactive Decay Processes
Simple Decay: represents a decay process where a radioactive isotope will decay to a ______radiogenic daughter atom. For example, radiocarbon (14C) will always decay to nitrogen (14N).
Branched Decay: represents a decay process where the radioactive isotope can decay to ______radiogenic daughter atom. For example 40K can decay to either 40Ca (88.2% of the time) or 40Ar (11.8% of the time). Radioactive Decay Unstable isotopes undergo radioactive decay to form other isotopes, & in the process energy is emitted in three forms: alpha particle: __ protons + __ neutrons (He nucleus) beta particle: fast–moving, ______gamma rays: high–energy electromagnetic ______Alpha decay ______the atomic number and mass of the element, since a 2 P and 2 N are emitted.
Beta electron decay (ß-) does not ______the mass number. The atomic number ______because the electron is produced by the decay of a neutron which results in a ______. Half-Life: The amount of time that it takes half of a radioactive sample to decay to something else. Determining amount of radioactive isotope remaining 1 half-life ½ (1/21) 2 half- lives ¼ (1/22) 1 3 3 half-lives /8 (1/2 ) 1 4 4 half-lives /16 (1/2 )
t Formula: N = No (0.5) t= number of half-lives
N = amount left No = original amount http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html Radioactive Decay Processes A ______involves the radioactive decay of intermediate radioactive daughter atoms that eventually decay to stable daughter such as the decay of 238U to 206Pb.
T1/2 of U-238 = 4.5 billion years T1/2 of U-235 = 703.8 million years The decay constant and half-life (The half life is the time it takes one half of the existing parent atoms to decay to daughter atoms) of a radionuclide are related mathematically: T1/2 = 0.693/λ λ = decay constant Generally, the greater the half-life of a radioactive isotope pair the greater the ______for dating purposes, but the less useful it is for more ______events. Potassium-Argon dating can be used to date potassium-rich basalt flows that exceed ______years in age. 40K will also decay to stable 40Ca by beta decay (but that ratio is not used for dating purposes- Why?) K-Ar Dating
40Ca is the stable and common form of Calcium. It would be impossible to determine how much 40Ca was produced from the decay of 40K. K-Ar dating was utilized to assign chronology of early hominid finds in northern Ethiopia. Paleomagnetic data can be used to fill in gaps within the chronology. ______is used to date the age of the basaltic ocean floor. Although the K content of the ocean basalts is relatively low, there is enough potassium to yield meaningful ages. In the 1950’s and 1960’s “magnetic stripes” were discovered by geophysicists along the ocean floor. These stripes represent fluctuating polarity changes of the Earth’s magnetic field preserved in the basaltic ocean crust. The reversals have been dated using K-Ar dating, producing a paleomagnetic time scale. K-Ar dating has been very important in assigning ages to the timing of polarity changes and establishing the paleomagnetic time scale. The paleomagnetic time scale is subdivided into “______” which represent intervals of time defined by a given magnetic polarity. The steep front of this Hawaiian basalt flow represents an ice-contact front. The basalt flow abutted against glacial ice and its K-Ar age provides constraints on the timing of glacial advance on the Big Island of Hawaii. Up-thrown Block
The range front fault (trace shown by dotted line) exposed along the eastern Sierra Nevada cross-cuts a basaltic cinder cone that was dated using K-Ar dating. How could this age be used to constrain the uplift rate along this fault? ______Radiocarbon Dating • Radiocarbon is first produced in the atmosphere by collisions of ______with nitrogen atoms (Nitrogen has 7 protons and 7 neutrons in its nucleus). • The neutron will knock out a ______from the nitrogen atom’s nucleus. How many protons will now be present in the atom’s nucleus? • ______• How many neutrons? ______• The carbon atom is now radioactive 14C (radiocarbon) with a half-life of 5730 years. It will decay back to 14N via Beta decay. • Organisms keep a constant level of 14C while they are alive, but once they die, the level of 14C drops as it decays to 14N. Radiocarbon Dating Radiocarbon Decay Activity 14C decays to 14N as it emits a beta (β) particle (______). “Modern” carbon (1850 A.D.) has a decay activity of ~15 dpm/gm. The decay activity (Beta emission rate) will decrease by 50% every half-life (5,730 years). Radiocarbon ages can be determined for organic matter by directly counting β-emissions.
What will the approximate decay activity be for a sample that is 6000 years dpm = disintegrations/minute old? _____ dpm/gm
12,000 years old? ______dpm/gm
After about ______years, there is not enough radicarbon left to have enough activity to measure (see earlier chart). Organic samples are prepared for radiocarbon dating by combusting (paper or
wood) or dissolving (calcite in shells) the organics and producing CO2 gas (stored in glass containers). The decay activity in the CO2 gas will be directly related to the ______of the organic material. Improvement in accelerator mass spectrometry (AMS) have allowed geochronologists to directly measure the 14C to stable carbon(12C and 13C) ratio. The ratio of 14C to 12C or 13C will be reduced by ______after each half-life. AMS 14C dating requires a much smaller organic sample (milligrams versus grams) than the beta-counting method. Most 14C analyses today are measured using AMS. Why do you think we do not measure the ratio of parent (14C) to daughter (14N) ratio to determine a 14C age? Radiocarbon dating is useful for assigning ages to sediment that may incorporate organics during erosion and deposition, such as this log present in glacial till. How is the glacial till age related to the age of the log incorporated into the till? ______Radiocarbon dating is useful for assigning ages to young (<50,000 years old) volcanic ash layers, such as the Mazama (Crater Lake) tephra shown above. Why can’t we simply date the 6800 year old Mazama ash using K-Ar dating? ______