Lecture 1 - Application Of Elemental And Radiogenic Isotop Geochemistry To Igneous Petrogenesis - Formation Of Elements And Trace Element Distribution In Rocks Flashcards
Atomic number
Z
Proton number
Always the same
Number on bottom
Mass number
A Top number Sum of protons and neutrons Z +N = A Can change when an isotope = different number of neutrons
Origin of the elements 1
Big Bang Nucleosynthesis
H, He, Li fusion
Z =< 3
Big Bang nucleosynthesis
<1 second: No Hadrons, only quarks
> 1 second: T=100bn K, quarks combine to form protons and neutrons (proton neutron ration 6:1)
3-20 mins: T=10bn K, proton neutron fusion forms light nuclei (H,He,Li)
+20mins: T too low for fusion, H/He/Li proportions for the universe are set
~100,000 years: T < 5000 K, neutral atoms of H and He formed, no more nucleosynthesis until first stars formed at 200Ma
Origin of the elements 2
Stellar nucleosynthesis
Tc detected in 1952 in red stars first real evidence (has no stable isotopes) (formed by S process)
Fusion: C - Fe
Z = 6-26
S-process neutron capture
Z= 26-83
Stellar nucleosynthesis
-fusion
Stars formed by gravitational collapse of molecular clouds
Stellar interior temperatures are high enough for fusion of H into He. 3 processes:
- deuterium fusion (lower temp than PC and easier than PC, v sensitive to temp, allows stars to grow, doesn’t make a lot of solar wind so matter acretes a lot of mass, starts with 1 proton and 1 neutron, ends with a He4 and 2protons which go back to the cycle) about 4 seconds
- proton chain fusion (protons coming together to form deuterium nucleus, starts off with 2 protons, v long process) ~ a billion years (then leads to DF)
- CNO Cycle (carbon, nitrogen, oxygen) (catalytic process, CNO increase speed of fusion of protons to make He4, (CNO aren’t actually used), why high mass stars fuse so quickly
Once all the Hydrogen has fused to produce He the core will contract gravitationally
Temperature increase so He can fuse, H fusion continues in a shell surrounding the core
Stars < 8 solar masses fusion stops at He, enters a red giant phase where S-process can occur in 2nd generation stars (needs element Fe)
Stellar nucleosynthesis
- S-process neutron capture (slow process)
2nd generation stars
Or
Stars >8 solar masses
Zig zag lines that go up to the right with the occasional step to the left (B decay)
Stages of burning, each stage the rate of process increases and time decreases:
- Carbon burning 6000 years
- Neon burning 1 year
- Oxygen burning 6m
- silicon burning 1d: ends with production of 56Ni which decays rapidly to 56 Fe
Fe cannot fuse, as it is endothermic, so it’s the last part: core collapse a type 2 supernova
origin of the elements 3
nucleosynthesis supernovae and kilonova
R-process Neutron capture
everything heavier than Fe (Z= 26- 92)
Nucleosynthesis Supernovae and Kilonova
R-process (rapid)
Heavy element synthesis
Once all material in star converted to Fe it can no longer produce energy so star collapses
Heat released from gravitational collapse causing a massive explosion/supernova
explosion yields a large flux of neutrons
origin of the elements 3
Spallation
Li, Be & B
Spallation
cosmic ray exposure causing ejection of matter from nuclei
spallation of O, N, C causes lithium, beryllium and Boron production
mostly occurred between Big Bang and the start of our solar system, is a minor process today
Major elements
> 1.0 wt %
control important properties such as phase relationships, melting temp, densities and viscosities
critical for determining when/whether a magma forms or whether a magma will ascend
Minor elements
> 0.1 - 1.0 wt %
commonly accessory minerals in rocks (e.g. Zircon)
abundances too low to affect phase equilibria significantly
Trace Elements
<0.1 wt %
too low in concentration to form their own mineral phases
substitute for minor/major elements in common minerals
generally expressed in mass of given mass
TEs show different preferences for different minerals and either solid or liquid phase of a system - very useful
TE in rocks/magmas don’t affect chemical or physical properties of the system as a whole
TE present at such low concentrations they behave passively and don’t influence geochemical processes
TE don’t control appearance/disappearance of major mineral phases
behavior controlled by element-mineral reactions (not element-element)
different TE have different chemical properties so behave differently during different geological processes
chondrite element averages
chondrite element averages removed from element concentrations in rocks to create a smooth comparable graph
chemical fractionation
the uneven distribution of an ion of any trace element between two competing phases
can be quantified and sued to trace earth and environmental processes in both qualitative and quantitative manner
the distribution/partition coefficient
a relative measure of the way an element distributes itself between 2 different phases
can be solid-solid, solid-liquid, liquid-liquid
depends on the ionic potential (=charge/radius) (‘field strength’)
general partitioning rules (Goldschmidts rules for igneous phases):
- ions will substitute readily for each other in a mineral lattice if….
- ions will substitute readily for each other in a mineral lattice if….
- of two ions with similar charge and radius to occupy a lattice site..
- the ion with the most similar…
- their ionic radii differ by <15% (size)
- have the same charge (or +/- 1 unit charge difference) - rarely with larger difference (Charge)
- the one with higher ionic potential is favored as it will make stronger bonds
- electronegativity to the major element being replaced will be favored because it destabilizes the crystal lattice least
because ions with similar charge and ionic radius can substitute in a crystal lattice …
a huge range of mineral compositions based on a single lattice can be produced
henrys law
at low concentrations (therefore TEs) the behavior of TE doesn’t depend on its own concentration.
The partitioning behavior of TE between two phases is defined by the concentration of the TEs in the two coexisting phases
most commonly done for coexisting crystal and melt phases
Kd>1
compatible (prefers solid)
Kd<1
Incompatible (prefers liquid)
Kdi = (partition coefficient)
Ci(mineral)/Ci(liquid)
Ci is the conc of element i
elements with a higher partition coefficient =
more compatible elements…TE whose charge and radius most closely match that of the major elements
REE distribution have…
…a systematic size variation (smooth pattern)
most REE have 3+ ions (except Fu3+ and Eu2+ in magma)
different bulk distribution coefficients are due to…
… different mineral proportions within rocks
MRFE
mantle rock forming elements, compatible
LILE
Large ion lithophile elements, fluid mobile
HFSE
High field straight elements, fluid immobile
OIB
smaller % melt than MORB
TE in crystallisation
Indicator of crystallizing phase
often trace elements strongly partitioned into a single mineral
Origin of the elements summary
Nucleosynthesis in big bang (H, He)
Fusion in stars (C-Fe)
Neutron capture in stars/supernovae/kilonae (>Fe)
spallation (Li, B, Be)
