Isotope 3

Question 1

5 General Methods for dating samples:

Answer 1

□ Dating using the isochron equation for isotopic systems with low parent/daughter ratios (Rb-Sr and Sm-Nd)
□ Dating using isotopic systems with extremely high parent/daughter ratios (eg. zircon U-Pb)
□ Dating using daughter-daughter isotope variations (206Pb-207Pb dating of the age of the Earth)
□ Daughter only dating using seawater compositions through time as an indirect dating tool.
□ Introduce concept of model ages for melt extraction from chondritic and depleted mantle sources

Question 2

Requirements and assumptions for successful isochron dating

Answer 2

Requirements
1. A suite of rocks or minerals are required – they must have sufficient variation in P/D ratio, preferably some with some mineral phases or whole-rocks having P/D ~0
2. Decay constant or T1/2 needs to be appropriate for expected age
3. The elements must be present in sufficient abundance to allow precise and accurate isotope measurements
Assumptions
1. Decay constant is known and has remained constant (see Week 7)
2. The whole-rock or mineral suite had the same D0 at T=0 – They are derived from an isotopically homogenous source
3. The whole-rock or mineral suite is cogenetic and crystallised over a geologically short time and so have nearly the same age
4. The isotopic system has remained closed since T=0 with no addition or loss of either the parent or daughter isotopes

Question 3

What is a radiometric date dating?

Answer 3

• Date derived is the time at which the temperature of the system drops below the Closure Temperature (CT) for a particular element and mineral
• CT is the temperature at which a system has cooled so that there is no longer any significant diffusion of the parent daughter isotopes out of the system and into the external environment. Different minerals have different CTs and for a given mineral, the CT will be different for different isotopes.
• Igneous rocks: only dating crystallization if cooling below CT is very rapid relative to decay half-life for isotope system

Question 4

Which isotope system to use for dating a particular rock or formation?

Answer 4

• Geochronologists have a variety of decay systems to work with. Each is in some ways unique. Success in dating (rocks) depends on choosing the right tool. Factors to consider:
• Dating is only useful between 6-10 half lives
• Half-life: shorter half-live better for younger samples
• Chemical behavior of parent and daughter: specifically
 Abundance
 Fractionation of parent and daughter in different minerals (Kds)
 Mobility of parent and daughter
 ‘Closure Temperature’ of system: at what temperature is the clock reset?
 Sometimes it’s luck

Question 5

What happens when igneous rock is thermally metamorphosed

Answer 5

• Mineral phases will begin to isotopically re-equilibirate
• Will pivot around rock which is the same once re-equilibirated (WR-1 in this case)
• If the rock cools quickly then the clock will start ticking again

Question 6

Dating samples with very high Parent/Daughter ratios:

U/Pb dating of zircon

Answer 6

• Two radioactive U isotopes: 238U and 235U.
• Decay at very different rates to 206Pb and 207Pb, respectively
  • This dual radioactive clock is a major advantage as it allows post-crystallisation disturbance to be identified.
  • The pair of equations above can be used to define the locus of points for which 238U-206Pb and 235U-207Pb ages concur, the locus being called the Concordia

Question 7

Oldest rocks on Earth:

Answer 7

Why is not the gneiss in Canada?
Gneiss is metamorphic so the age dated is the metamorphism – protolith predates this
Jack Hills Conglomerate (3Ga but Zircons are contained in a conglomerate which are 4.4Ga)
• Also, interesting – suggests that 4.4 billion years ago – there were oceans – there must have been continents, so this was weathered and put in a conglomerate
• Oldest samples on Earth

Question 8

Age of the Earth: Daughter-Daughter dating using Pb-Pb

Answer 8

• In a stroke of genius, Clair Patterson tested the following two assumptions by Pb-Pb dating meteorites and terrestrial sediments
o Meteorites and Earth formed at the same time and that Earth formed from the same basic material as meteorites.
o Meteorite Pb-Pb ratio evolution is representative of the evolution of Bulk Earth.
o Mineral Troilite (FeS) in meteorites contains lots of Pb and no U so Pb likely to represent the primordial Pb isotope composition and the starting point for solar system Pb isotope evolution.

Question 9

Dating samples with extremely low P/D ratios:

Answer 9

Biogenic marine carbonates & seawater 87Sr/86Sr evolution