Origin and Evolution of Meteorites and the Terrestrial Planets (L19-24) Flashcards
(113 cards)
1
Q
Outline the formation of solar systems
A
An event triggers gravitational collapse of a cloud of dust and gas (nebula)
Nebula collapses to form a spinning disk (conserving angular momentum)
Collapse releases GPE, centre heats up
Central hot portion forms a star
Outer, cooler particles repeatedly collide, accretion to planet-sized bodies
2
Q
Outline the formation of a star
A
Results from gravitational collapse of a giant molecular cloud
As T and P increase, fragments condense into a rotating sphere of superhot gas (protostar)
If large enough, core T rises to fuse hydrogen
Nuclear fusion -> hydrostatic equilibrium
3
Q
Outline the star evolution of H fusion
A
Continuous fusion of H into He causes a build up of He
Accumulation of denser He causes gravitational self-compression
Core exhausts supply of H, contracts until hot enough for He fusion
He fusion initiation depends on star’s mass
4
Q
What are the conditions in a star for C burning?
A
> 4x solar mass + used up lighter elements in their core
High T following collapse once He levels decrease
5
Q
What are the principle reactions of C burning?
A
12C + 12C -> 20Ne + 4He
12C + 12C -> 23Na + 1H
12C + 12C -> 24Mg + γ
6
Q
What happens in a star once C density drops below C burning levels?
A
Core cools and contracts
Contraction heats the core to Ne and then O ignition T’s -> formation of discrete element burning shells
7
Q
What were the products of primordial nucleosynthesis during the Big Bang?
A
Light nuclei: 1H, 2H, 3H, 4He
8
Q
How are elements up to 56Fe produced?
A
Stellar nucleosynthesis - nuclear fusion in stars
9
Q
How are elements heavier than 56Fe produced?
A
Neutron addition reactions in stars
10
Q
Define:
Isotope
Isobar
Isotone
A
Isotope: same number of protons, different number of neutrons
Isobar: same number of total protons and neutrons
Isotone: different number of protons, same number of neutrons
11
Q
What are the conditions for heavy element synthesis?
A
In large stars with high abundance of heavy nuclei
Requires high neutron flux (supernovae)
Isotopes with high N for given Z
12
Q
S process:
Conditions?
Source of neutrons?
A
Slow neutron flux, AGB stars: burnt-out, supported by a He burning shell
22Ne + 4He -> 25Mg + n
13C + 4He -> 16O + n
13
Q
R process:
Conditions?
Products?
A
Neutron capture»_space; average beta-decay half life
Intense neutron flux, produced after core-collapse supernovae
Produces heavy isotopes = isotopes with high ratios of neutrons to protons
14
Q
Which observations can help estimate the composition of the solar system?
A
EM spectrum of solar radiation
Direct sample of solar wind
Meteorite sample
15
Q
Outline the size and timescale of solar system formation
A
Condensation phase Initial coagulation, planetesimal formation: ~10^5 years ~10km Orderly growth: ~10^6 years, Moon size Runaway growth: ~10^7 years, Mars size Late-stage collisions: ~10^7-8 years
16
Q
What is the “Giant Impact”?
A
An event where a Mars-sized impactor planet hit the Earth, creating a debris disk that formed the Moon.
17
Q
What are the phases that condense from a nebula dependent on?
A
Composition of the nebula
T
Oxidation state
18
Q
What is compositional zonation in a solar nebula related to?
A
Condensation temperature
19
Q
What is the “snow line” in a nebula?
A
~180K, water ice condenses
Ice is ~10 times more abundant by mass than rock in the solar nebula
Icy exoplanets formed beyond the snow line
20
Q
How do gas and ice giants differ in compositional structure?
A
Gas giant: core (rock, ice), metallic hydrogen, molecular hydrogen
Ice giant: core (rock, ice), mantle (water, ammonia, methane ices), “crust” (hydrogen, helium, methane gas)
21
Q
What is the order of condensation in elements?
A
1st: Platinum group = Os, Ir, Ru that condense as metals
2nd: oxides and silicates of Ca, Al and Ti
3rd: metallic Fe-Ni, olivines and pyroxenes
4th: S, which reacts with Fe to form sulfides
5th: Fe reacts with O to form magnetite
22
Q
What is the Goldschmidt classification of elements?
A
Terms to describe element volatility and the degree with which they concentrate into planetary mantles vs metal cores
23
Q
Define Goldschmidt classification terms: Siderophile Lithophile Chalcophile Hydrophile Atmophile
A
Siderophile = "iron-loving" = partitions into Fe-Ni metal Lithophile = "rock-loving" = partitions into silicates Chalcophile = "sulfur-loving" = sulfides Hydrophile = water and ices Atmophile = gases
24
Q
What can isotope systems that involve parent and daughter isotopes of
elements with different properties be used for?
A
Dating planetary processes
25
Define volatility of an element
T at which 50% has condensed from gas to solid for a gas of nebular composition
26
How can the solar photosphere be used for nebular composition?
Assumed to show bulk solar composition
Obtain compositions through spectroscopy
Only way to see volatile elements H,C,N,O that are depleted in meteorites
27
```
Define meteorite terms:
Chondrites
Achondrites
Irons
Stony-irons
```
```
Chondrites = primitive, metal-silicate segregation has not yet taken place
Achondrites = differentiated, silicate and derived by melting of planetesimal silicate mantles
Irons = relict cores of planetesimal bodies
Stony-irons = relict core-mantle boundaries
```
28
What are the two major groupings of meteorites?
Undifferentiated (chondrites) VS differentiated (achondrites, iron, stony-iron)
29
Chondrites:
3 groups?
Contain what?
Carbonaceous, enstatite and ordinary
| Contain chondrules, CAIs = Calcium-Aluminium-Inclusions, and are enriched in volatile elements
30
What are chondrules?
| How did they form?
Spherical bodies, glass and quenched crystals, mostly olivine
Once-molten droplets formed in brief high T events in the nebula
31
What are CAIs enriched in?
How do they relate to nebular condensation?
What is special about them?
Refractory elements
First ~5% of material to condense from nebular gas at high T
Oldest objects in the solar system
32
What is the structure of chondrites?
An assemblage of chondrules, CAIs and matrix
33
How are meteorites classed?
By thermal metamorphism:
Grade 3 = least modified
Grades 1 and 2 = aqueous, low T alteration
Grades 4-6 = increasing thermal metamorphism and equilibration
34
What might a higher metamorphic grade of a meteorite mean?
Formed deeper in the parent body
35
What are the two types of stony-iron meteorites and what is the difference in structure and formation?
Pallasites: Fe-Ni metal with nodules of olivine, probably formed at the interface between molten metal and molten silicate melts
Mesosiderites: brecciated pyroxene and plag with Fe-Ni metal, may have formed by two differentiated asteroids colliding
36
Iron meteorites:
What are they?
Formation?
How are they classified?
Remnants of the metal cores of disrupted asteroids
From liquid cores that were fragmented then reformed during impacts
Classified by composition (siderophile and chalcophile elements)
37
What is special about CI chondrites?
Composition matches solar photosphere
Have no chondrules or metal
Only fine-grained matrix and silicates
38
What is thought about the composition of carbonaceous chondrites?
Most complete composition that may have formed the planets
39
Regarding the Earth, how does volatility relate to abundance?
Increasing volatility = decrease in abundance
40
Why are isotopes in chondrites often studied?
To find isotopic anomalies that can relate to nucleosynthesis
Presence of enrichments in neutron-rich isotopes suggests material derived from giant stars or supernovae
41
Achondrites:
What are they?
Most common group?
Igneous rocks formed by crystallisation of melts on asteroidal parent bodies
HED (Howardites, Eucrites, Diogenites) meteorites from Vesta
42
What kind of material are the lunar samples?
Mainly highland material (feldspar-rich lunar anorthosites), mare basalts, rare "fire fountaining" glasses, soils from impacts
43
What are the two different terrains on the Moon?
Highlands: mountainous, scarred by craters, highly feldspathic rocks
Lowlands: ~3km lower, smooth surfaces = basins flooded by younger basic lava flows
44
Half life:
Definition?
Time taken for 'complete decay'?
Time taken for the activity of an amount of a radioactive substance to decay to half the initial value
5 half lives
45
How are isotope systems chosen when dating a process of interest?
```
Half life (5x = extinct)
Parent-daughter element fractionation
```
46
What equation relates the number of daughter atoms to number of parent atoms?
D = D_0 + N(e^λt -1)
47
Define incompatible
Partitions into melts preferentially to crystal structures
48
When performing radioactive dating, what can be measured more precisely than concentration, and how is this utilised?
More precise to measure isotope ratios
| Normalise concentrations to a stable reference isotope
49
Why is 26Al a useful radionuclide for dating?
Short half-life and abundance:
Can date the earliest objects in the solar system
Abundant enough to provide a significant source of heat
50
What does the presence of former 26Al in the solar system mean for its formation?
26Al is produced in red-giant stars
| So presence means red-giant star material contributed to the nebula
51
Why is the Cr-Mn decay system useful?
Dating planetary volatile depletion
52
Assuming time 0 in the solar system was when CAI's formed, what happened in the first 5 Ma after?
Chondrule formation after ~2 Ma
| Parent bodies of achondrites, formed, melted and differentiated within 5 Ma
53
How is time zero in the solar system calculated?
| What age is used for time zero?
Using isotope ratios of Pb isotopes
| ~4.55 Ga
54
How are isotopes measured?
| Why are isotope ratios measured?
Mass spectrometry: separating charged ions by mass and charge
Corrects for changes in signal intensity
55
What are the three stages of terrestrial planet formation?
| How can stages (2) and (3) be dated?
(1) dust condenses out of the hot nebular disk
(2) dust aggregates to form chondrites, planetesimals and protoplanets
(3) protoplanets grow into planets
Isotope systems that involve lithophile vs siderophile parent and daughter elements
56
Angrites:
What are they composed of?
Which isotope system is relevant and why?
When did they form?
Plag, Ca-Ti pyroxene, olivine, oxidised but depleted in volatiles
182Hf-182W, help to date planetesimal formation
Between CAIs and chondrite formation
57
What is shown from using 182Hf-182W dating on Mars?
It reached half its current size in <2 Myr after CAIs formed
| Stunted growth must be from lack of accretionary material
58
Which model helps explain why Mars is small?
Grand Tack: involves the early inward-then-outward migration of Jupiter and Saturn causing the planetesimal disk to be truncated at ~1 AU
59
What is the inner structure of the Moon?
Solid inner core 240km, liquid outer core 90km, partial melt 150km, mantle 1200km, crust 50km
60
What data is available about the Moon's core?
Limited data from moonquakes -> tiny core and unclear if molten
Weak magnetic field
61
Which theory is used to explain the Earth-Moon system?
```
Earth close to final size
Mars-sized impactor
Moon predominantly derived from impactor
Both bodies already differentiated
Both bodies form at ~1 AU
```
62
How does differentiation come about in terrestrial planets?
Accreting mass becomes big enough to melt
Iron metal melt sinks to centre, cools and solidifies into a core
Impacts add and remove material
Differentiation into distinct reservoirs
63
Why does differentiation occur?
Minimises potential energy moving denser material closer to the centre
64
What are the conditions for meteorite core formation?
| Are they applicable to planetary interiors?
Low P
| Probably not due to PT dependence of element partitioning
65
How can the conditions of planetary core formation be reconstructed from mantle composition?
Calibrating the partitioning behaviour of different elements between metal and silicate under different conditions
66
What is a distribution coefficient?
A relative measure of the way an element distributes itself between 2 different phase e.g. D_metal /D_silicate
67
What is important about the D values of Ni and Co?
Overlap > 25 GPa
High-P metal-silicate equilibration during core formation
The range matches the Ni/Co ratio of chondrites
68
How can deep-metal silicate equilibrium above the core-mantle boundary be explained?
Metal segregation at the base of a deep magma ocean
| Chondritic materal may still be added to the Earth and mix with the magma ocean
69
Outline the model of single-stage core formation
Want to match Earth/CI trace elements with experimental partition coefficients at a fixed set of conditions
Can't match it all = conditions change over time; planet is increasing in size so P, T increase
70
Outline the model of continuous core formation and accretion
Earth accretes initially from strongly-reduced and volatile-free material
Then accretes from more oxidised volatile-bearing material
If true, core volatiles should low
Volatiles must have been added late
71
Light elements in Earth's core:
Potential light elements?
How do they partition?
What are the constraints?
Si, O, S, C, P and H
Preference for liquid outer core
Constraints from densities and sound velocities for different alloys
72
When is oxygen not a volatile element?
| Why?
In a nebula condensation species
| Forms metal oxides and bonds with Si
73
Can Si isotopes be used as evidence for Si in the core?
Stable isotopes partition according to bonding environment
Si will be in a different environment in metal alloy compared to silicate melt
If Si present in the core, then the mantle has a distinct Si isotope signature
74
Why does equilibrium partitioning occur?
Bonds between different isotopes in similar environments have different energy due to quantum effects
Heavy isotope forms a lower energy bond; less vibration = stronger bond
Heavy isotope preferential to strongest bond compounds
Strong bond compounds typically lower coordination number and more oxidised
75
What is the Si isotope evidence for Si in the core?
Experiment: isotopically light is concentrated in metal relative to silicate
Heavy values of mantle rocks relative to chondrites means light values in the core
76
How were volatiles and metals brought to Earth?
By meteorite impact called "late veneer"
77
What is Earth's HSE problem?
Excess highly siderophile problem
HSE concentrations in Earth's mantle are higher than expected (should be in core)
HSE signature looks like chondrites, not mantle residue from core formation
78
When was the late veneer added to the Earth?
How is this known?
What was it made of?
After Moon-forming giant impact
Impact likely mixed 182W/184W isotopes, but Earth's ratio lower than Moon's
Chondrites
79
Which melting events do igneous rocks give information about?
Formation of oceanic and continental crust
Continental rifting
Mantle plumes
Subduction
80
What are the peridotitic lithologies found in the mantle?
Dunite
Lherzolite
Harzburgite
81
How are lherzolites and harzburgites distinguished?
Lherzolite: fertile "mantle" rocks
Harzburgite: Ol-opx rocks, residues of partial melting
82
How are mantle rocks and chondrites used to estimate about the Earth?
They constrain major lithophile elements
Si condenses at lower T than Al or Mg
Chondrites: Al/Si proportional to Mg/Si
Peridotites: Al/Si decreases as Mg/Si increases as melt is extracted
Intersection of lines = approximation of bulk silicate Earth
83
What do mantle xenoliths usually contain?
Olivine
Pyroxene
Garnet
High-P phases like diamond
84
Knowing the rough bulk composition of the BSE, what is a 'reasonable' assumption to make about the Earth's mantle?
What is the problem?
Most of Earth's mantle is composed of Mg-Si phases like olivine
Mantle xenoliths contain other phases and studies show pure olivine can't account for composition range of modern oceanic basalts
85
What conditions is an multi anvil apparatus used to simulate?
T up to 2500K
P up to 25GPa
Spinel-bridgmanite transition zone, upper mantle base
86
What conditions are diamond anvil cell experiments used to simulate?
T up to 6000K
| P up to 250 GPa (inner core P)
87
What is the result of phase transitions at higher P?
More closely packed, denser polymorphs = increased seismic velocity
88
When does melting take place w.r.t. solidus?
| What does the solidus position depend on?
When the mantle adiabat cross the solidus
| Mineralogy and volatile content
89
What is mantle potential T?
T of the mantle adiabat extrapolated to the surface
90
What can be used to understand melting processes and reconstruct the composition of mantle and crustal source regions?
Trace and major elements
91
Define:
Trace element
Major element
Trace: low concentrations, follow Henry's Law
Major: high concentrations, form essential structural components of minerals, influence phase relationships
92
Define compatible and incompatible
| w.r.t. variation diagrams
Compatible: -ve correlation with SiO2
Incompatible: +ve correlation with SiO2
93
How does partition coefficient relate to compatibility?
D > 1 = compatible in solid
| D < 1 = incompatible in solid
94
How can partition coefficients be determined?
Analyses of coexisting phases in natural rocks
Analyses of phases in experimental systems
Crystal chemistry and an ionic model
95
How can element partitioning be predicted?
Assuming solid phases are simple ionic lattice structures
1) Can swap ions if radii differs by <15%
2) 2 ions, different radii, same valence, smaller ion preferentially in solid over melt
3) 2 ions, similar radii, different valence, ion maintaining charge in crystal structure is preferred
96
What is a simplified view of crystal-chemical controls on element partitioning?
Atoms connected by springs, minimise strain energy
Too large element = compression = strain
Too small element = extension = strain
97
What does experimental data show about element partitioning?
Larger cations have lower partition coefficients
Optimal size for partitioning exists
Cation charge is important
98
```
Large Ion Lithophile Elements:
Which elements?
Compatibility?
Why?
Exceptions?
```
K, Rb, Sr, Cs, Ba
Incompatible
Large size
Mica/amphibole
99
```
High Field Strength Elements:
Which elements?
Compatibility?
Why?
Exceptions?
```
Ti, Zr, Nb, Hf, Ta
Incompatible
Small radii but high charge
Zircon, rutile, titanite
100
Platinum Group Elements:
Which elements?
Goldschmidt classification?
Seen where?
Re, Os, Ir, Pt
Highly siderophile and/or chalcophile
Concentrated in core, also in sulphide and other ores
101
Rare Earth Elements:
Which elements?
Compatibility?
Substitution?
Lanthanides
Incompatible
If smaller radii, substitute for Al3+ in garnet
102
What is the point of trace element modelling?
Reconstruct compositions of primary melts = mantle sources = mantle heterogeneity on Earth, crust formation and Earth origins
Apply to achondrites = planetary processes
Constrain magmatic differentiation processes = magma chamber behaviour + eruption release and impact
Formation of granites and ore deposits
103
What happens to compatibility during fractional melting?
| How does it compare to batch melting?
Progressive melts become depleted in incompatible elements
Solid depleted in incompatible elements faster than for batch melting
Accumulated fractional melts are similar to batch melts
104
W.r.t. REE and compatibility, what happens as ionic radius decreases?
Partition more easily into lattice structures
| More compatible
105
Why are REE useful when studying compatibility?
Coherent behaviour
Garnet: very depleted REE
Plag - Eu anomaly
106
What are the concentrations of REE in crustal rocks and mid ocean ridge basalt (MORB)?
How can this be justified?
Crustal rocks: enriched in light REE, crust formed from melts of primitive mantle
MORB: light REE depletion, matches LREE enrichment in crust - depleted mantle
107
What is the cause of Eu anomalies?
When plag is a fractionating phenocryst
| Source may have had prior melt extraction
108
Why is it confusing that lunar basalts have a negative Eu anomaly?
What does this mean?
Primitive melts without plag phenocrysts and are too MgO-rich to have fractionated much plag
Widespread extraction of plag from mantle source = anorthosite crust formation = magma ocean
109
What are the patterns of REE in MORB melt inclusions?
| What is the application?
Smooth patterns, *melts* are depleted in LREE
| Large range of depletions in melt inclusions -> fractional melting
110
What are the patterns of ocean island basalt (OIB) w.r.t. REE?
Application?
Strong enrichment in incompatible elements
| Steep slope of MREE-HREE = garnet in source
111
What are OIB associated with?
Mantle plume tracks
112
What are island-arc basalts (IAB) depleted in?
Where is this also seen?
What does it relate to?
HFSE particularly Nb, Ta and Ti
Seen in continental crust
Relates to (a) residual rutile/titanite in subducting slab, (b) fluids enriching the source of the lavas, as HFSE not mobile in fluids
113
What are island-arc basalts enriched in?
Fluid-mobile elements
| Very incompatible elements enriched in sediment melts but not mobile in fluids
