Isotope 5

Question 1

3 main types of short-lived isotopes

Answer 1

1. Those formed as intermediate daughter products of larger decay chains, e.g. 230Th in the 238U - 206Pb decay series
   • Dating and tracing physical processes such as melting
2. Recently-formed ‘cosmogenic’ isotopes
   • Formed at the surface, or in the atmosphere or in space, through spallation and neutron capture processes as a result of cosmic ray bombardment
   • Used to give surface exposure ages, sedimentation and sediment tracing in arcs
3. Extinct short-lived isotope systems
   • Can be used to investigate the early solar system (e.g. Hf-W)

Question 2

Secular equilibrium

Answer 2

Example
o Parent and decay of daughter 1 reach equilibrium after about 5 half-lives of daughter 1 – one decay of each at the same time – rate of decay is matched – will decay away at the same time
o Can be used to date between 0 and 5 half-lives of daughter 1

Question 3

Short-lived ‘cosmogenic’ radiogenic isotopes

Answer 3

• Generated by high energy cosmic ray bombardment
• NOT stable, decay fairly rapidly
• Exceptionally low abundance of such nuclides
• Requires specialised mass spectrometry
• Atomic decay counting required 1200 tonnes of ice, whereas only 10kg required for Accelerator mass spectrometry
• Ions travel at several % of speed of light, e.g. ~10,000,000 m/s (much faster than conventional mass spectrometry)
• Used to give exposure ages, and applied to sedimentation rates, glacial ice, hydrology and sediment tracing in island arcs

Question 4

Accelerator Mass Spectrometry

Answer 4

 Extremely large, but like all mass spectrometers AMS has 3 main components: Source, Mass analyser (x2) and Detectors.
 AMS also has the addition of a tandem accelerator – hence the name.
 Why use AMS?
 Almost complete elimination of molecular ions and isobars from the mass spectrometry.

Question 5

How does AMS work?

Answer 5

• The AMS source generates –ve ions, so, for instance, negatively charged 14C ions can be separated from the abundant 14N isobar, which doesn’t form negative ions in the AMS
• The accelerated -ve ions are converted to multiply-charged +ve ions by collisions with gas molecules, which are then accelerated. The ions can travel at several % of the speed of light.
Molecular ions do not exist as multiply charged +ve ions at high energies, so polyatomic interferences are completely eliminated.
• Result is AMS is able to measure single atoms in the presence of 1x1015 (a thousand million million) stable atoms.

Question 6

Radionuclide examples

Answer 6

Be10 - Exposure dating, ice cores, erosion rates

C14 - Organic matter

Al26 - Exposure dating of rocks

Cl36 - Exposure dating, tracing groundwater flow

Ca41 - Exposure dating of carbonate

I129 - Tracing groundwater

Question 7

14C

Answer 7

 Neutron bombardment:
10N + 147N = 146C + 11H
 14C decays to 14N (beta decay) with half-life of 5700 yrs
 Living tissue exchanges CO2 with atmosphere and is therefore in equilibrium with the atmosphere
 On death, exchange ceases and, over time, 14C decays to 14N
 If system remains ‘closed’ then age can be determined using law of radioactive decay:
A = Aoe-λt
(where A0 is initial activity of 14C, t is time and λ is the decay constant)

Applications:
 Dendrochronology
 Comparison with tree rings aids calibration of 14C dating
 Dating of organic material, e.g. charcoal, skin, peat
 But only up to 60,000 years old.

Question 8

10Be

Answer 8

 High cosmogenic production rate (0.02 atoms/cm2/sec) and relatively long half-life (1.36 Ma) decaying back to 10B by beta emission.
 Produced in the upper atmosphere by spallation reactions with N and O.
 Atmospheric residence time for 10Be ~1 year, removed by precipitation.
 Also produced in quartz by spallation reactions involving O and interactions with 28Si (10-100 atoms g-1 a-1). Used in conjunction with 26Al to determine exposure ages.
 After precipitation will dissolve into seawater and eventually form clay at the bottom of the ocean

Applications:
 Erosion studies
 Extent of accumulation in soils (and its erosion into rivers) indicates speed of erosion, and uplift.
 Tracing sediment subduction in arc magmas
 10Be adsorbed onto settling sediment, thus these seds have much higher 10Be than the mantle or mafic oceanic crust
 If subducted sediments contribute to arc magmatism, higher 10Be should be present in recent arc lavas
 1-2 x 106 atoms/g 10Be in MORB,
up to 24 x 106 atoms/g in arc lavas
 Comes into MORB by hydrothermal events
 High concentrations into arc magmas by subducting sediment into mantle and it returning to surface within 10 half lives
 Only hard proof of mantle recycling
 Can’t be used for tracing – OIB are nowhere near subduction zones – takes too long

Question 9

B/Be vs 10Be

Answer 9

 Combined boron and beryllium studies used to trace subduction fluid inputs in arc systems
 10Be enriched in sediments. B enriched, relative to Be in altered MORB
 Therefore:
 high 10Be/9Be ratios and low B/Be ratios indicate largely sediment input (fluid or melt)
 Low 10Be/9Be ratios and high B/Be ratios indicate fluid from the mafic part of the oceanic crust

Question 10

26Al

Answer 10

 Product of Ar spallation reactions. Very short half life of 717ka decaying to 26Mg by electron capture.
 Used to calculate terrestrial residence ages of meteorites:
 After parent body break-up, meteorites are bombarded
by intense cosmic rays
The meteorite becomes saturated in 26Al
 After falling to Earth, 26Al production almost ceases due to the protection of the atmosphere and 26Al decreases with time.
 Therefore, the measured 26Al activity in the sample can be used to calculate the time at which it arrived on Earth.
 Very low 26Al production in materials on Earth’s surface (0.0002 atoms/cm2/sec).
 26Al production by spallation reactions involving 28Si in quartz – production rate of about 10-100 atoms g-1 a-1 and as quartz is Al-poor the 26Al/27Al ratio changes sufficiently with time to be measured.
 Therefore, used 26Al can be used for exposure ages in quartz-rich materials (in conjunction with 10Be)

Question 11

36Cl

Answer 11

 Produced by spallation reactions with 36Ar in the atmosphere. Half-life is 301Ka.
 Also produced between 1952 and 1958 by marine nuclear bomb tests by neutron capture of 35Cl
 NOT removed from water (unlike Be).
 Therefore, good for tracing groundwater movement, which is no longer in contact with the surface
 Cosmogenic 36Cl used for ancient (ka age) systems
Anthropogenic 36Cl used for modern-day systems

Question 12

Extinct short-lived radioactive decay systems (cosmogenic)

Answer 12

• Formation in a stellar event ‘shortly’ before solar system formation or through spallation near early Sun
• Includes heavier isotopes than currently produced by cosmic bombardment
• Half-lives of hundreds of thousands to tens Ma
• Rapid decay leads to exhaustion of these isotopes ‘soon’ after solar system formation (unlike long-lived radiogenic isotopes)
• Potential for investigating early solar system processes including the age of the solar system and core formation in planetary bodies
• Combined with long-lived U-Th-Pb and Pb-Pb dating, short-lived isotope dating has achieved greater precision and resolution for the age of the earliest solar system materials
• 4568-4571 Ma (CA inclusions (CAIs) in meteorites)
• Earliest meteorites 5-10 Ma later