U-Pb Concordia/Discordia Dating
Techniques
surface ocean
prod uction ocean biota
lan d bi ota
sol uble wxi ng
ocean decay
•Decay of 238U to 206Pb and 235U to 207Pb
mixin g
•System behavior
= exponential decay
decay terr prod
soi ls
sed imen ts
upl ift
dee p ocean
•If/then/else logical statements
ling
introduced upwel
to remove
Pb or U at
sin kin g
specifiedsedtime
imen tati on
•Start with simple 2 and 3 isotope decay
series to learn about secular equilibrium
weath ering
238U
decay to 206Pb
prod uction ocean biota
surface ocean
lan d bi ota
234U
234Pa
230Th
sol uble wxi ng
238U
ocean decay
234Th
226Ra
218Rn
218At
decay 210
terr prod
soi lsPo
214Po
210Bi
206Pb
sed imen ts
218
214Bi
210Pb
206Tl
222Rn
upl210
ift
Tl
206Hg
214Pb
mixin g
half-life
Uranium 238
4.468*109 years
Thorium 234
24.5 days
dee p ocean
Protactinium 234
Po
upwel ling
1.14 minutes
Uranium 234
2.33*105 years
Thorium 230
8.3*104 years
Radium 226
1590 years
Radon 222
3.825 days
Polonium 218
3.05 minutes
Lead 214
26.8 minutes
Bismuth 214
19.7 minutes
Polonium 214
1.5*10-4 seconds
Lead 210
22 years
Bismuth 210
5 days
Polonium 210
140 days
Lead 206
Stable
sin kin g
sed imen tati on
weath ering
Isotope
Basic model of U
decay
Lead_206(t) = Lead_206(t - dt) + (Decay) * dt
INIT Lead_206 = 0
INFLOWS:
Decay = Uranium_238__Decay_Constant*Uranium_238
Uranium_238(t) = Uranium_238(t - dt) + (- Decay) * dt
INIT Uranium_238 = 1*10^9
OUTFLOWS:
Decay = Uranium_238__Decay_Constant*Uranium_238
Uranium_238__Decay_Constant = LOGN(2)/Uranium_238_Half_Life
Uranium_238_Half_Life = 4468
dN
 N
dt
•Model incorporating both decay systems and
•Metamorphic loss or gain of isotopes (e.g. Lead_206_Outflow =
IF(TIME=1500)THEN(0.1*Lead_206/DT)ELSE(0)
U-Pb Concordia-Discordia Diagram
0.8
Age of rock = 3.5 billion
years
0.7
20%
0.6
206Pb/238U
10%
30%
0.5
0.4
0.3
Time of lead loss = 1.5 billion years, ratios give time = 2
billion years, which reflects the time elapsed since lead
loss
0.2
0.1
235U
half life shorter than 238U so 207Pb/235U ratio larger than 206Pb/238U
0
0
5
10
15
20
207Pb/235U
25
30
35