U-Th-Pb Decay Systems 9/9/10

U-Th-Pb Decay Systems 9/9/10 • Lecture outline: • U-Th-Pb systematics • 2) Concordant U-Pb dates • 3) Discordant U-Pb dates and open • system behavior • 4) Common Pb-Pb dating • 5) The Geochron Zircon Cross-section of a zircon grain from Antarctica, showing U-Th-Pb dates

Introduction to U, Th, and Pb Element Charge Radius (Å) U +4 (+6) oxic 1.05 Th +4 1.10 Pb +2 1.32 Th and U are highly incompatible and thus are concentrated in crustal materials and depleted in mantle Material U(ppm) Th Pb Chondrites 0.01 0.04 1.0 Troilite .009 <.01 5.9 Basalt 0.43 1.6 3.7 Galena trace trace HUGE Zircon HUGE HUGE trace Carbonates 1.9 1.2 5.6 Seawater (surface) 3 ppb 20 fg/g 2.7 pg/g Seawater (deep) 3 ppb 60 fg/g 5 pg/g Why are zircons and galenas the poster-children Of U-Th-Pb dating?

U-Th-Pb decay schemes -decays to 206Pb, half-life=4.47 Ga -decays to 207Pb, half-life=0.704 Ga 238U=99.27% 235U=0.72% 234U=0.0055% 232Th=100% 230Th=<0.0005% -decays to 208Pb, half-life=14.1 Ga We can simplify the multi-daughter decay chains to simple parent-daughter systems if and only if the system is in secular equilibrium (~5 half-lives of the longest-lived daughter): 238U --> 206Pb (234U = 248,000y) 235U--> 207Pb (228Ra = 5.75y) 232Th-->208Pb (231Pa = 32,500y)

U-Th-Pb decay chains

U-Th-Pb governing equations * Note that in all three decay schemes, 204Pb is used as a reference isotope After Smith and Farquhar (1989) You can measure a date with all three systems, and if those dates agree, then you have concordant dates. If x=(238U/1204Pb)m And y=(206Pb/204Pb)m We have y=b+mx Where intercept b=(206Pb/204Pb)i And slope m=(eλt-1) What processes can make U-Th-Pb dates Discordant?

If x=(206Pb/204Pb)m And y=(207Pb/204Pb)m We have y=mx+(y0+x0) Where (y0,x0)=primordial Pb isotopic composition And slope Wetherill’s concordia *cannot be solved algebraically, must use iterative solving or Tables (like 10.3 in book) The U-Pb concordia: line of concordant 206Pb/238U and 207Pb/235U ages Physical Interpretation: - at t=0 (crystallization), both ratios = 0 - system evolves along concordia, growing radiogenic Pb, as long as it remains a closed system - you can use either ratio to calculate age

A U-Pb discordia - open system behavior So what if you lose Pb during metamorphosis (very common – why?) For a zircon that formed at 4.0Ga experienced metamorphic event at 3.0Ga: Pb loss at 3Ga moves points from A to origin, For complete loss you move to origin, reset system. If the samples remain closed until present, they will follow a parallel concordia. discordia The discordia defined by altered samples will intersect The Concordia at the time of crystallization and the time of metamorphic event.

U-Pb discordia - open system behavior Tilton (1960) measured U-Pb isotopes in many minerals from Archaean shields across five continents yield discordant ages, with a discordia that implied a World-wide metamorphic event at 600Ma…. But there is no evidence for that…. What would be an alternative explanation?

U-Pb discordia - open system behavior Indeed, Pb loss does not often occur in a single metamorphic event - it can be quite complicated (i.e. multiple events or even continous Pb loss) The data fit a model of continuous diffusional Pb loss from the U-rich minerals. Why might that happen? What would a U-Pb concordia plot look like if you incorporated old (4Ga), partially reset zircons into a young (0.5Ga) melt? So what’s a geochronologist to do??

Common (whole-rock) Pb-Pb dating - for minerals with virtually no U or Th - single stage history: mantle contains mixture of radiogenic and common Pb, which is then “tapped” to form low-U galena Bulk Earth evolution: Where: T = age of Earth t = age of formation of low-U mineral C.Diablo = initial Pb isotope values at T Pb removed t years ago: or Can construct a similar equation for 235U-207Pb system… and divide the two equations….

Common (whole-rock) Pb-Pb dating To get the following equation: * In this equation, ‘t’ is time since galena crystallation, T is the age of the Earth co-genetic common lead samples will define an isochron So samples evolve along growth curve, which depends on: 1) The U/Pb ratio in the source (μ) 2) The time since Earth’s formation If x=(206Pb/204Pb)m And y=(207Pb/204Pb)m We have y=mx+(y0+x0) Where (y0,x0)=primordial Pb isotopic composition And slope

high µ low µ The Holmes-Houtermans Model: Common lead dating • “single stage” Pb assumes: • when Earth formed, U,Th, and P were evenly distributed; Pb isotopic ratios uniform • Earth solidified and small differences in U/Pb ratios develop • U/Pb ratio in a region thereafter changed only due to decay • sometime later, Pb isolated into low-U mineral, remained constant μ = 238U/204Pb Present-day μ = 6-14 high-μ materials will yield more radiogenic Pb isotope values during time T while low- μ yield lower radiogenic values over time T

The Geochron: a special Pb-Pb isochron In a stroke of genius, Patterson tested the following two assumptions by Pb-Pb dating meteorites and terrestrial sediments 1) Meteorites and Earth formed at the same time 2) Meteorite Pb ratios are representative of Bulk Earth initial ratios Fe-S meteorite stony meteorites terrestrial sediment Isotope ratios of Canyon Diablo Meteorite: 206Pb/204Pb 9.3066 207Pb/204Pb 10.293 208Pb/204Pb 29.475

Common (whole-rock) Pb-Pb dating **Remember that this model only applies to single stage leads (that’s one special lead!) What geological circumstances would favor single-stage evolution? Or where might you find these special leads? What if you encounter a set of samples that indicate “future ages?” with a single-stage Pb model? (see below plot) Implies 2-stage evolution: Bulk Earth was differentiated into high-μ and low-μ reservoirs a long time ago (episodic) or continually differentiated Very radiogenic Pb’s are due to increasing μ partway through source evolution. “future” ages?

U-Th-Pb Decay Systems 9/9/10