Isotope 1 Flashcards
Solar system formation processes
Big Bang fusion
Spallation
Stellar fusion
Supernovae - neutron capture
Solar System Concentrations – ‘Oddo-Harkins Sawtooth effect’
• Concentrations relative to silica
• Uneven proton elements are less stable and therefore less abundant
• Th and U from neutron stars
Big Bang nucleosynthesis
o Prior to 1 second no Hadrons (Protons or Neutrons) only quarks
o After the first second (T = 100 billion K) quarks can combine to form protons, neutrons…P/N ratio 6:1
o ~3-20mins (T = 10 billion K) = fusion of protons and neutrons forms light nuclei of H, He and Li (Z ≤ 3)
o Produced atoms of the lightest elements (H and He, after about 100,000 years)
o After 20mins T dropped to point were fusion could no longer take place. H/He/Li proportions for universe set.
o ~100,000 years (T< 5000K). Neutral atoms of H and He formed… No more nucleosynthesis until first stars @ 200Ma
o Neutral atoms coalesce
o Point big bang stopped – stars formed – no other elements
Stellar nucleosynthesis General
o In stars Tc should not exist – found in 1952 so therefore neutron capture and fusion must exist
o Fusion (C to Fe; Z = 6-26)
Done in massive stars (>8 sun masses) under great P and T
o s-process neutron capture (Z = 26-82) (82 = lead)
o In the stellar interior temperatures and pressures are high enough to allow fusion and conversion of H to He.
Nucleosynthesis in supernovae and kilonova.
o r-process Neutron capture
o (everything heavier than Fe; Z = 26-92)
o After all material in star’s core is converted to Fe, the star can no longer produce energy
o The star collapses; heat released from gravitation collapse causes a massive explosion - a supernova.
o The explosion yields a large flux of neutrons
o Heavy elements (Z > 26) synthesised by rapid neutron capture.
o Gravitational waves = merge of two neutron stars
Formation of gold and platinum
Spallation
o (Li, Be & B)
• Causes their anomalously low abundaces compared to other light elements
o Ejection of matter from nuclei due to cosmic ray exposure
o Lithium, beryllium and boron produced by spallation of O, N, C
o Mainly occurred between the Big Bang (13.7 Ga) and the start of our solar system (4.56 Ga)
o But also continues to be a minor process today
Element classification:
- Major elements > 1.0 wt. % (weight percent)
- Minor elements > 0.1 – 1.0 wt. %
- Trace elements < 0.1 wt. %
Major elements: control important properties such as phase relationships, melting temperature, densities, viscosities (melt).
These properties are critical in determining, for example, when/whether magma forms or whether magmas will ascend!
Minor elements: commonly form accessory minerals in a rock, such as, apatite (P), zircon (Zr) etc., but their abundances are too low to affect phase equilibria in any significance.
Trace elements: so low in concentration that they do not form their own mineral phases, but substitute for major (or minor) elements in common minerals
Trace element expression
• Generally expressed in terms of mass in a given mass of sample:
o e.g. µg/g = parts per million (ppm),
o ng/g = parts per billion (ppb)
o pg/g = parts per trillion (ppt).
o µg/g = µg g-1 = ppm
• TE in rocks/magmas do not affect the chemical or physical properties of the system as a whole.
• TE are present at such low concentrations that they behave passively and do not influences geochemical processes.
• TE do not control the appearance or disappearance of major mineral phases.
• Behaviour is controlled by element-mineral reactions, not element-element reactions.
• Different TE have different chemical properties (charge, size etc) so behave in different ways during different geological processes.
• Relationship compared to condrites
o Condrites = bulk earth – Earth before chemical reactions
The distribution coefficient
The distribution coefficient …or partition coefficient
• A relative measure of the way an element distributes itself between 2 different phases
• Can be solid-liquid, solid-solid or even liquid-liquid
• Depends on:
o Charge
o Size
o Concept of ionic potential (“field strength”)
o Ionic potential = Charge/radius
General Partitioning rules:
“Goldschmidt’s Rules”
These outline the conditions for trace element partitioning between igneous phases.
Ions will substitute readily for each other in a mineral lattice if…
1. Size: Their ionic radii differ by <15%.
2. Charge: They have the same charge or ±1 unit of charge
difference (substitution with greater charge differences may occur but to a significantly lesser degree).
Of two ions with similar charge and radius to occupy a lattice site…
3.The ion with the higher ionic potential (z/r) is favoured because it will make stronger bonds.
A fourth rule was added more recently by Ringwood:
4. The ion with the most similar electronegativity to that of the major element being replaced will be favoured because it destabilizes the crystal lattice the least.
- Ions with similar charge and ionic radius can substitute in crystal lattice
- Provides huge range of mineral compositions based on single lattices
At low concentrations, relevant for trace elements, the behavior of a trace element does not depend on its own concentration (Henry’s Law).
The partitioning behavior of a trace element between two phases can be simply defined as the ratio of the concentration of the trace element in the two coexisting phases. Most commonly we do this for coexisting crystal and melt phases.
Partition coefficient
Ci Mineral/ Ci Liquid = kd
If Kd is >1 element i prefers to be in the mineral (solid) phase.
The Element i is said to be COMPATIBLE.
If Kd is <1 element i prefers to be in the melt (liquid) phase.
The Element i is said to be INCOMPATIBLE.
Stellar nucleosynthesis processes
o 3 main processes:
Deuterium Fusion: Occurs at lower T than PC fusion and allows stars to grow.
• (4 secs) – easiest
• Uses larger element
• Without would not have large stars
• Solar winds don’t blow matter away so it can accrete
Proton chain: Dominant in stars with ≤1M☉
• Forms deuterium fusion but stars off with two protons
• Very long process – 1bln years
CNO cycle: Catalytic process dominant in stars with >1.3M☉
• Increase the speed of fusion by protons
o Once all the H has fused to produce He the core will contract gravitationally, T will increase so He can fuse. H fusion will continue in a shell surrounding the core.
o For <8M☉ stars fusion stops with He. Star enters red giant phase when s-process neutron capture can take place in 2nd generation stars to form elements heavier than Z=38. Star eventually cools to form White Dwarf.
o For >8M☉ stars fusion of elements heavier that He can take place…
o Carbon-burning (6000yrs)
o Neon-burning (1yr)
o Oxygen-burning (6m)
o Silicon burning (~1d)
o Si burning ends with production of 56Ni which decays very rapidly to 56Fe.
o Fe cannot fuse as this process is endothermic not exothermic.
o The star has run out of fuel and core collapse takes just a matter of seconds – Type II Supernova
o S-process – requires iron (seed element) – slow processes – slow neutron capture
o Iron is the end of stella fusion
