Module 5

Question 1

Describe the rate order of radioactive decay reactions.

Answer 1

Radioactive decay is a first-order (exponential) reaction

 Decay rate decreases as parent atoms (N) decreases

Question 2

Define parent and daughter isotopes, Ds, decay constant and half life.

Answer 2

lambda= decay constant (time–1)
-Same as the rate constant (k)

-probability of 1 atom decaying per amount of time

Half-Life (T1/2)
Time it takes for half of an initial number of radioactive atoms (N0) to decay to daughter atoms (D*)

N = Radiogenic isotopes
D= daughter product of radioactive isotopes
 Daughter can be RADIOACTIVE or STABLE
D0 = initial daughter
Ds = initial stable daughter for ratio

Question 3

 Describe the requirements for an accurate geochronology estimate.

Answer 3

Accurate decay constant
-> no large changes in T, P, etc., during or since formation)

-All samples must have been formed at the same time

-all samples must have the same initial daughter isotope composition D0/Ds

-closed system
->No disturbance to the isotope system since formation of the rock or mineral

-Good range in present-day concentrations (N and D) or isotope ratios (N/DS and D/DS)

Question 4

isochron
Rb-Sr or Re-Os systems.

Answer 4

-solve for t and D0 (daughters initial) simultaneously

Needs at least 2 samples from same rock/mineral (preferably more!

-radioactive atom N decays, daughter atom D is produced.
-increased daughter, and decreased parent

Steeper slope = older age

initial daughters not always consistent in each mineral
use ratio initial daughter to stable isotope ratio
->will be same at formation

Solve for t to get the age of the rock/mineral

Question 5

 Determine when using the Rb-Sr, Re-Os or U-Pb system would be appropriate to obtain an
absolute age of a geological material and explain why

Answer 5

Rb-Sr System
-Rb substitutes for K in feldspars, etc.
-49 billion year half life
-Manicouagan Impact Event

Re-Os System
- siderophilic and chalcophilic
– used to date sulfide minerals in black
shales and in ores
 Half-life 41.6 billion years
-Os tends to be high in ultramafic/mafic igneous rocks
-Molybdenite (MoS2):
-best sulfide mineral for Re-Os
-robust: survives high-grade
metamorphism and deformation

U-Pb System in Zircon (ZrSiO4)
-time of volcanism or magma crystallization
-precise
-date zircons as old as the Earth
 Zircons are very hard and durable, resist melting during metamorphism

how can pb be lost?
-radiation damage, alpha decay in zircon lattice
pb accumulation in zircons
->if crystal is heated up or interacts with groundwater
-high temperature metamorphism

concordia/discordia
->discordant line for lost pb

Pick isotopes relevant to your system (think, Goldschmidt!)

Organic tissue – 14C
Black shales – 187Re
-relevant to the age range of interest 14C maybe too short)