GEOL 562 Notes: U-Series and Th-series nuclides. Guide Questions: Reading: White, Lecture 10

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1 GEOL 562 Notes: U-Series and Th-series nuclides Reading: White, Lecture 10 Motivation: Up to now we have dealt with long half-life nuclides (all left over from the birth of the solar system). What are our options if we want time information about short timescale processes? ll short half-life nuclides present in the pre-solar nebula are gone. We then have only: 1) Short half-life nuclides in the U-series and Th-series decay chains 2) Short half-life nuclides produced by cosmic rays Guide Questions: Why do U, 235 U, and 232 Th decay to produce chains of decay products instead of just decaying in one step to daughter nuclides? What is the range of half-lives of the nuclides in these chains? What is the rough upper limit of ages that might be determined using U,Th-series nuclides? Why do we use activities, or activity ratios of these nuclides instead of just talking about concentrations, or isotope ratios? If a system remains closed and is allowed to sit undisturbed for millions of years, how do the activities of the nuclides in the chain relate to each other? If a system remains closed and is allowed to sit undisturbed for millions of years, how do the concentrations of the nuclides in the chain relate to each other? If a crystal or other system is created and the nuclides are initially NOT in secular equilibrium, how do the activities of the nuclides evolve over time? How does a parent/daughter activity ratio evolve over time? How much do the activities of U, 235 U, and 232 Th change during the time spans measureable by U, Th-series methods? How well does U remain dissolved in water with oxygen present? How well does Th remain dissolved in water? Is the U/Th ratio of a recently formed coral large or small? How does this affect age dating of corals? In a very rough way, how does the 230 Th/ U ratio evolve with time in a coral? What is the final 230 Th/ U activity ratio? In a more detailed sense, how does the 230 Th/ U activity ratio evolve with time in a coral that starts out with 234 U and U in secular equilibrium? In a more detailed sense, how does the 230 Th/ U activity ratio evolve with time in a coral that starts out with ( 234 U/ U) = 1.15 (parentheses indicate activity ratio here)? How does ( 234 U/ U) evolve with time in a coral that starts out with ( 234 U/ U) = 1.15? How is ( 231 Pa/ 235 U) age dating similar to, and different from, 230 Th/ U age dating? Why do we often observe unsupported 230 Th adsorbed onto sediments? How can this 230 Th be used to get ages of sediments or sedimentation rates? What two methods are used to measure U,Th-series nuclides? What are the special features needed for measurements by mass spectrometry? Why do 210 Pb excesses often occur in sediments? How can this be used to get sediment ages? Over what range of age, roughly? How can buildup of alpha particles in U-rich, Th-rich minerals be used to determine when the rocks were exhumed from deep in the earth s crust?

2 Introduction to the U and Th decays series Diagram of the U decay series and Th decay series nuclides: Chart of the nuclides - Visit and zoom in to the portion of chart for Hg to Pu. - There are no stable nuclei with mass greater than U, 235 U, and 232 Th are unstable, BUT they have long half-lives and thus these nuclides are left over from the birth of our solar system - When a U nucleus decays, it emits an alpha particle to become 232 Th, but this daughter nuclide is quite unstable, and beta-decays with a half-life of 24 days to 234 Pa, but THIS daughter has an even shorter half-life (odd Z, odd N), and so on... This chain of unstable nuclei continues until we reach 206 Pb. See figure 10.1 in White s notes U and 232 Th have similar decay chains. Interestingly, the three decay chains are completely independent; they do not share any isotopes Of the dozens of U + Th series nuclides, We use a few that have useful half-lives for our work: Nuclide Half-life Decay Chain Other info 234 U 250 ka U First major nuclide in chain 230 Th 75 ka U Daughter of 234 U 231 Pa 32 ka 235 U First major nuclide in chain 226 Ra ka U Daughter of 230 Th 210 Pb 22 y U One of the last nuclides in the U series. Concepts necessary to interpret U-series isotope data: 1) Decay chain Decay of U is the only source of 234 U, Decay of 234 U is the only source of 230 Th, Decay of 230 Th is the only source of 226 Ra, and so on... - The amount of each nuclide formed per unit time depends on the amount of parent nuclide and the parent nuclide s decay constant. - The amount of nuclide lost per unit time depends on the nuclide s decay constant The Chain of Buckets nalogy: One analogy that help one to understand this is the linked series of buckets model: Here are the rules Rate of flow through the drain pipe in each bucket depends on: 1) The amount of water in the bucket (= number of atoms) 2) The size of the pipe (= decay rate)

3 Series of Buckets analogy Buckets with small drain pipes will tend to accumulate more water Buckets with very large drain pipes will accumulate almost no water. In fact, it s almost like having no bucket at all. Starting with empty buckets, it takes a while for the series of buckets to fill up, but eventually, the buckets end up filled to levels that remain steady and that depend on the drain pipe sizes. There s a faucet that puts out a constant flow of water to supply the bucket chain U-series nuclides Nuclides with slower decay tend to build up to greater concentration Nuclides with very fast decay hardly build up at all. In fact, we can ignore their presence in many cases. Starting with only U on a system, the concentrations of the U-series nuclides build up and eventually reach levels that remain steady and depend on the decay constants. U, 235 U, and 232 Th are long-lived nuclides which are present in relatively large concentrations and which provide effectively constant inputs to the decay chains. 2) We use ctivity, or ctivity Ratios, for U-series and Th-series analysis. They are more convenient than concentrations, or isotope ratios. ctivity = ln ctivity is the number of decays per unit time (in the early days, measurements were made by counting alpha decays per unit time, so this was natural unit to use) ctivity also = number of atoms passed down to the next step of the chain per unit time ctivity is also the amount lost per unit time ctivity of the parent is the amount gained per unit time. Thus, if we have hypothetical chain of only 2 nuclides: dn 2 = l 1 N 1 - l 2 N 2, Solve this for N 2 N 2 = Or.. in the case where there s no N 2 at the start: N 2 = See White s Lecture 10 notes for more detail. l 1 N 0 1 e - l 1 t - e - l 2 t l 2 - l 1 ( ) + N 2 0 e -l 2 t l 1 N 0 l 2 - l 1 e - l 1 t - e - l 2 ( t ) 1 3) Isotopic equilibrium = secular equilibrium Imagine what happens to a particular nuclide s concentration as time moves forward: If the concentration starts out high, there won t be enough input from the parent to support that concentration, and it will decrease. If it starts out low, the loss rate (decay rate) will be low, and the input from the parent will increase the concentration until the loss rate eventually matches the loss rate. Eventually the loss due to decay and the gain from the parent will be equal and the system will settle into a steady state- We call this secular equilibrium (secular means temporal, or related to time, here). This is in contrast to, say, a chemical equilibrium between two chemical phases.

4 VERY IMPORTNT: Mathematically, the secular equilibrium can be described by: dn - 2 = 0 (i.e., no change as time passes) Th U ˆ =1-0 = dn 2 = l 1 N 1 - l 2 N 2 l 1 N 1 = l 2 N 2 1 = 2 =1 e.g., - Very important: t secular equilibrium, the activities of nuclides are equal. - The activity in excess of the equilibrium amount is called unsupported or an excess of a nuclide - The concentration that would be maintained at equilibrium is called the supported concentration. So how can we use this to get age dates? 1) If a crystal that starts out with disequilibrium in the decay chain, the chain will evolve toward equilibrium in a predictable manner. 2) We have mathematical equations to describe this evolution. 3) The same closed system requirements apply as with other geochronology method. 4) s with the geochronology methods we discussed earlier, a major challenge is knowing the initial conditions. What should we measure? 1) We could just measure concentrations of individual nuclides, but ratios are usually better behaved because they normalize out some of the variables. 2) Daughter over parent ratio is useful; increases with time. Example: 230 Th/ 234 U 3) Over the time spans measured with these nuclides (< 1 million years), U, 235 U, and 232 Th do not decay significantly. These nuclides gives constant inputs to the decay chains. They also can serve as nearly stable isotopes to which other, more variable nuclides can be compared. So we often measure ratios with U, 235 U, and 232 Th in the denominator. Examples: 234 U/ U, 230 Th/ U, 226 Ra/ U, 231 Pa/ 235 U Example: Finding ages of corals (important in paleoclimate studies). Important feature of carbonate minerals: 1. U is taken in readily as carbonates form (in O 2 -bearing waters U is present as soluble U(VI); UO 2+ = Uranyl ion) 2. Th is present as Th 4+ in all waters, is highly insoluble, and therefore has very small concentrations in water. It also does not fit into carbonate mineral structures well. So there s very little Th in most carbonate minerals like those made by corals. So... Consider an idealized case where inherited Th is zero: Th/ U starts at zero Th/ U increases with time as it is produced by the U decay chain 3. Eventually 230 Th/ U reaches a maximum value ( 230 Th/ U) = 1 (parentheses mean activity ratio here)

5 1) Example one: If 234 U is in secular equilibrium with U in the water, then... l 234 N 234 = l N dn 230 = l 234 N l 230 N 230 = l N - l 230 N 230 d 230 = l 230 ( ), but is constant... d 230 ˆ = l ˆ R Thˆ = 1 - e - l 230t U ( ) simple exponential approach toward equilibrium NOTE: I have included the subscript so you know these are activity ratios; in the U- series literature, parentheses alone, without the subscript, mean activity ratios. 2) Example two: Most waters deviate a bit from 234 U/ U equilibrium, and we can use the approach toward 234 U/ U equilibrium as a dating tool. 234 U ˆ =1.15 in the ocean; (groundwater has a wide range of values from 0.6 to 5.0) U Digression: What causes this? 1) Recoil from ejection of an alpha particle can send a 234 U directly into the water from a crystal at the moment of alpha ejection 2) 234 U tends to reside in defects in crystals (created by recoil)- these are dissolved preferentially during weathering We can use this to get age dates: The initial excess 234 U decays away: 234 U ˆ =1 + Uˆ ˆ R - 1 e - l U 234 t 1 U symptotic approach toward 1, starting at the initial value. Problems with this approach in reality: 1. Molluscs and forams tend to inherit U after burial. (Corals are O.K.) 2. Need to estimate initial ratio. No problem in the oceans, but in fresh water systems, initial ratio can vary greatly. 3. Not very precise for younger ages ( 234 U half life is long). 3) Example 3: Finding 230 Th/ U ages of corals that inherited U with 1) Similar to example 1 above 230 Th/ U approaches 1 asymptotically 2) BUT... the initial excess of 234 U will give us extra 230 Th above this 234 U U ˆ =1.15

6 230 Thˆ = 1 - e - l l ( ) È U 230t Uˆ l l Í Î U e- l 234 t - e - l 230 ( t ) This equation has two parts: 1) first term is buildup to equilib. taking into account only the amount supported by U 2) second term is 230 Th resulting from decay of excess 234 U- Very similar to the equation above for the second nuclide in the hypothetical two-nuclide decay chain The above form is good if you know the initial 234 U/ U activity ratio. Usually we would prefer to have a form with today s 234 U/ U activity ratio. We get this by substituting for the initial 234 U/ U activity ratio and combining terms Thˆ = 1 - e - l l È ( ) U 230t Uˆ l l Í Î U 1- ( e- l l 234 )t ( ) We can also derive an equation for 230 Th/ 234 U by dividing both sides by 234/: È 230 Thˆ 230t Í ( ) 234 U = 1-e-l l 234 U ˆ Í 1 Í 1- l l U ˆ 1- e - ( l 230 -l 234 )t ( ) Í U Í U Î This the most commonly used form. Practical considerations: Small amounts of inherited Th may be present- need to correct for this Small amounts of detritus- bearing Th- may be found in the carbonate -especially soil carbonates- need clean carbonate, or need to correct for dirt 4)_Example 4: Pa age dating of corals. Just like U-Th dating: Pa is insoluble like Th 231 Paˆ = ( 1- e - l ) 231t 235 U Shorter half-life- more sensitive, but shorter time range (<200 ka) You can compare U-Pa and U-Th ages to check consistency. 5) Example 5: Excess 230 Th; ges of deep ocean clastic sediments - Th is rare in carbonates because it leaves seawater and sticks to surfaces. - t the same time, U is soluble and does not stick as much to surfaces. - There should be an excess of 230 Th relative to parent 234 U in sediments, provided they are not carbonate-rich. - nother way of saying this is, There is unsupported 230 Th in the sediments. This excess decays over a time scale of about t 1/2 = 75 ka We can measure 230 Th/ 232 Th of Th recovered by gently leaching sediments. 230 Th / 232 Th is greatest at the top of a sediment core, and decreases with depth.

7 230 Th/ 232 Th depth This plot can then be translated roughly into an age vs. depth relationship. Why do we plot 230 Th/ 232 Th? This may seem odd; 230 Th is NOT in the 232 Th decay chain. However, different sediment layers may have different surface area per gram sediment, and thus would yield different amounts of adsorbed Th. Normalizing to 232 Th helps remove this problem, but then we must assume that initial 230 Th/ 232 Th (as opposed to 230 Th concentration) is constant. t some depth, the curve levels off at the value supported by U decay. See Figure 10.2 in White (taken from Faure, 1986). NOTE: White says the y axis is ( 230 Th/ 232 Th), but it is actually log( 230 Th/ 232 Th). - Excess 231 Pa, from the 235 U decay series, can be used in the same way. - Excess 228 Th, from the 232 Th decay series, can be used in the same way. 228 Th has a half-life of 1.9 years, so this is useful for rapidly accumulating sediments, not deep-sea sediments. Digression: Measurement of U-series and Th-series isotopes. 1) The OLD method is alpha decay counting. The number of alpha decays per unit time is measured. However, in many cases, the rates of decay are small and the samples must be counted for perhaps weeks to get good data. 2) For the nuclides with longer half-lives, we now use mass spectrometry, in which we count the number of atoms present for each nuclide, instead of the decays. This can be more precise. There are some challenges, however: a) The mass spectrometer must be capable of measuring small amounts. Specialized ion counting systems are used to detect small amounts, and the noise level in the instruments must be very low. b) The mass spectrometer must be capable of measuring a rare isotope well when the amount of another isotope is much larger. Example: 230 Th/ 232 Th might be < 10 5, and the much, much larger 232 Th ion beam has some stray ions that can enter the 230 Th detector. This noise in the 230 Th signal is minimized using special filters that increase the so-called abundance sensitivity reject the lower-energy stray ions.

8 Sediment ages from 210 Pb measurements Half-life = 22.3 years Thus, not useful for slow deposition in deep ocean basins Useful for coastal, lacustrine, and estuarine sediment dating Parent for 210 Pb is (effectively) 222 Rn, half-life = 4 days Thus, 222 Rn in atmosphere produces 210 Pb in atmosphere 210 Pb in atmosphere settles out. Causes a 210 Pb excess in surficial sediments - This unsupported 210 Pb is greatest at the surface - t depth, the unsupported 210 Pb is gone (decay) - mount of decay relative to the initial amount of unsupported gives age ( 210 Pb ) u =( 210 Pb ) 0 u x e (-lt) Practical procedure: Collect sediment Sample Digest sediment to release Pb Purify Pb Measure 210 Pb by counting (ctually we usually count a daughter or granddaughter isotope) To get ( 210 Pb ) 0 u :ssume current value at top of sediment gives the starting value for the deeper seds you are analyzing. To get ( 210 Pb ) that is supported by ( U ): ssume deep, old sediments give the supported value. U,Th-He thermochronology - 4 He (alpha particles) builds up in minerals over time due to U,Th decay - But, only if minerals are below He closure temps. He closure temps are lower than r closure temps because He is smaller and thus more mobile. - So, we can measure amount of time that has elapsed since a mineral grain cooled below closure temperature - pplication: Uplift rates. o Closure T for apatite = 70 C, depth of about 2km (depends on gradient) o Titanite = 200 C = depth of about 6 km o Thus, you can tell how long ago a rock was buried to these two depths - See this review article: (U-Th)/He Dating: Techniques, Calibrations, and pplications, Kenneth. Farley, Reviews in Mineralogy and Geochemistry, Volume 47 (

GEOL 562 Notes: U-Series and Th-series nuclides. Guide Questions: Reading: White, Lecture 10 Faure and Mensing Ch. 20

GEOL 562 Notes: U-Series and Th-series nuclides Reading: White, Lecture 0 Faure and Mensing Ch. 20 Motivation: Up to now we have dealt with long half-life nuclides (all left over from the birth of the