Lecture 1 - Application Of Elemental And Radiogenic Isotop Geochemistry To Igneous Petrogenesis - Formation Of Elements And Trace Element Distribution In Rocks

Question 1

Atomic number

Answer 1

Z
Proton number
Always the same
Number on bottom

Question 2

Mass number

Answer 2

A
Top number 
Sum of protons and neutrons 
Z +N = A
Can change when an isotope = different number of neutrons

Question 3

Origin of the elements 1

Answer 3

Big Bang Nucleosynthesis
H, He, Li fusion
Z =< 3

Question 4

Big Bang nucleosynthesis

Answer 4

<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

Question 5

Origin of the elements 2

Answer 5

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

Question 6

Stellar nucleosynthesis

-fusion

Answer 6

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)

Question 7

Stellar nucleosynthesis

- S-process neutron capture (slow process)

Answer 7

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

Question 8

origin of the elements 3

Answer 8

nucleosynthesis supernovae and kilonova
R-process Neutron capture
everything heavier than Fe (Z= 26- 92)

Question 9

Nucleosynthesis Supernovae and Kilonova

R-process (rapid)

Answer 9

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

Question 10

origin of the elements 3

Answer 10

Spallation

Li, Be & B

Question 11

Spallation

Answer 11

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

Question 12

Major elements

Answer 12

> 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

Question 13

Minor elements

Answer 13

> 0.1 - 1.0 wt %

commonly accessory minerals in rocks (e.g. Zircon)

abundances too low to affect phase equilibria significantly

Question 14

Trace Elements

Answer 14

<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

Question 15

chondrite element averages

Answer 15

chondrite element averages removed from element concentrations in rocks to create a smooth comparable graph

Question 16

chemical fractionation

Answer 16

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

Question 17

the distribution/partition coefficient

Answer 17

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’)

Question 18

general partitioning rules (Goldschmidts rules for igneous phases):

1. ions will substitute readily for each other in a mineral lattice if….
2. ions will substitute readily for each other in a mineral lattice if….
3. of two ions with similar charge and radius to occupy a lattice site..
4. the ion with the most similar…

Answer 18

1. their ionic radii differ by <15% (size)
2. have the same charge (or +/- 1 unit charge difference) - rarely with larger difference (Charge)
3. the one with higher ionic potential is favored as it will make stronger bonds
4. electronegativity to the major element being replaced will be favored because it destabilizes the crystal lattice least

Question 19

because ions with similar charge and ionic radius can substitute in a crystal lattice …

Answer 19

a huge range of mineral compositions based on a single lattice can be produced

Question 20

henrys law

Answer 20

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

Question 21

Kd>1

Answer 21

compatible (prefers solid)

Question 22

Kd<1

Answer 22

Incompatible (prefers liquid)

Question 23

Kdi = (partition coefficient)

Answer 23

Ci(mineral)/Ci(liquid)

Ci is the conc of element i

Question 24

elements with a higher partition coefficient =

Answer 24

more compatible elements…TE whose charge and radius most closely match that of the major elements

Question 25

REE distribution have…

Answer 25

…a systematic size variation (smooth pattern)

most REE have 3+ ions (except Fu3+ and Eu2+ in magma)

Question 26

different bulk distribution coefficients are due to…

Answer 26

… different mineral proportions within rocks

Question 27

MRFE

Answer 27

mantle rock forming elements, compatible

Question 28

LILE

Answer 28

Large ion lithophile elements, fluid mobile

Question 29

HFSE

Answer 29

High field straight elements, fluid immobile

Question 30

OIB

Answer 30

smaller % melt than MORB

Question 31

TE in crystallisation

Answer 31

Indicator of crystallizing phase

often trace elements strongly partitioned into a single mineral

Question 32

Origin of the elements summary

Answer 32

Nucleosynthesis in big bang (H, He)
Fusion in stars (C-Fe)
Neutron capture in stars/supernovae/kilonae (>Fe)
spallation (Li, B, Be)