Kevin Flashcards
Separating elements
- Partial Melting
- Siderophile – Likes to bond to iron – affinity
- Density and gravity
- Melting temperature and condensation temperature
- Volatility
- Compatibility (during melting) – compatible or incompatible
- Redox state
- Speciation
How did we get a solar system?
• Began as a cloud of dust and gas that was the size of a galaxy– held apart by hydrostatic forces – collapsed due to supernova (thought due to elemental composition) – due to conservation of energy a spinning disc was created – heat accumulates at centre = sun – dust and gas orbits this – starts to accrete and formed early planets that form their own gravity and pull in material
Beta Pictoris
Star is 20-25 mya – very young – has a dust cloud – is consistent with galaxy creation hypothesis
Solar System Formation:
- Some event (e.g. supernova) triggers gravitational collapse of a cloud (nebula) of dust and gas
- As the nebula collapses, it forms a spinning disk (due to conservation of angular momentum)
- The collapse releases gravitational energy, which heats the centre –forms a star
- The outer, cooler particles suffer repeated collisions, building planet-sized bodies from dust grains (accretion)
- These arguments suggest that the planets and the Sun should all have (more or less) the same composition
Planets closer to the sun are relatively depleted in volatile elements – unlike planets further away from the sun – Nebulla hypothesis does not hold up for volatile content due to condensing temps – Besides these volatile elements all planets have the same elements
Chondritic meteorites:
Stony meteorites containing chondrules – small spherical particles that were once molten. They consist of:
• High T refractory components (chondrules and calcium-aluminium inclusions (CAIs)) Aggregates of metal, sulphides, oxides and silicates
• Fine grained matrix of minerals
CAIs are the first minerals to condense in the solar system. Chondrules formed by transitory heating of nebular dust
CAIs earliest solids in solar system
‘Chondritic’ Earth
- Long held assumption that:
- Composition of Earth = composition of chondritic meteorites (primitive solar system material) = composition of the Sun
- Except for the depletion of volatile elements in the Earth and chondrites
Differentiation of the Earth:
No shear waves in outer core = liquid
Differentiation: Earths metallic core
• 32.3 % mass of the Earth
• Outer core liquid, inner core solid
• Consists of Fe + Ni and about 10% of some light element (O, S and/or Si)
How do we know theres a core:
- Seismic
- Conservation of mass and momentum calculation
- Magnetic field
- Density/moment of inertia – torque needed to spin
- Mineral physics
- (indirect) Compositional models – bulk earth = silicate earth + metal earth (core)
Siderophile elements:
- Highly siderophile elements are present at a much higher abundance than might be expected from low pressure experimental data
- Moderately siderophile elements are strongly dependent upon the P-T of metal silicate equilibration. For example, Ni and Co partition much too strongly into the core at low pressures and appropriate oxygen fugacity (metal/silicate parition coefficient DNi~500, DCo~100, both need D~5) to explain their observed mantle abundances.
Nickel - Cobalt experimental data:
Metal/silicate partition coefficients (D) as a function of pressure for nickel (Ni) and cobalt (Co).
Mantle has a Ni/Co ratio ≈ 1.1, indicating metal-silicate equilibration at about 28 GPa
- Metal/silicate partition coefficients (D) as a function of pressure for nickel (Ni) and cobalt (Co).
- Data suggest metal-silicate equilibration between 30 to 50 GPa, corresponding to depths of 800 to 1300 km
Deep magma ocean model of core formation
- Accreting planetesimals break-up.
- Droplets of liquid Fe falling through liquid silicate should stabilise with diameters of about 1 cm and should fall at 0.5 cm/s (Rubie et. al. 2003).
- They will continuously requilibrate with the silicate until they reach a depth at which they can form a thick layer.
- Heat from large amounts of radioactive material, accretion and early core formation (gravitational kinetic energy) caused a lava ocean
Continuous core segregation during accretion
- Changing pressure of core formation during accretion plus the possible effect of changing the composition of accreted material
- Earth took a long time to accrete and grow – up to 100mya
Moderately volatile elements – Ga & Mn
Ga/Mn ratio ≈ 1.0, but at low pressures Ga is highly siderophile and Mn is lithophile, so Ga and Mn must have been added to the Earth late during accretion
Core formation:
Overall, experimental data suggest that accretion was heterogeneous, and Earth began as a small body formed from highly reduced material, depleted in volatile elements, and became more oxidised as it increased in size, and relatively rich in volatiles consistent with:
- Planetary dynamic modeling suggesting that significant amounts of volatile rich material originating at >2.5 AU was accreted to the Earth during the later stages of growth (O’Brien et al. 2006).
Accretion and planet growth:
- Gravitational accretion; Earth grows by attracting smaller planetisimals
- Planetary accretion simulation from Raymond et al. (2006), using 1054 initial planetesimals from 1 to 10 km radius. Earth like planets are formed.
When did the core form?
- For any radioactive decay system to be of use in dating a process, that process must fractionate the parent element from the daughter element. So, to investigate the timing of core formation, radioactive systems are needed in which one of either the parent or the daughter elements is siderophile and the other is lithophile.
- The two decay schemes with elements of contrasting properties are hafnium– tungsten (Hf–W) and uranium–lead (U–Pb).
U-Pb isotopes
Siderophile and lithophile elements fractionated by core formation.
Siderophile Pb incorporated into core.
Lithophile U retained in mantle.
238U an 235U, which decay to 206Pb and 207Pb respectively (235U t½ = 704 Ma; 238U t½ = 4.55 Ga)
Pb in oceanic basalts
- the “Pb paradox”
- Oceanic basalts lie to the right of the geochron
- Means there is some lead on the other side of the geochron that is missing – possible its in the core
Pb in Earths metallic core
Pb siderophile, U lithophile
Pb age of the core:
- Core formation – Pb siderophile, U lithophile - but difficult to estimate BSE, also volatile
- Volatile loss - early Pb loss, late Pb addition
- needs significant addition (>2% Earth’s mass)
- Hidden in the Mantle – U incompatibility > Pb
- No unradiogenic Pb in oceanic basalts
Hf-W isotopes
Metal-loving and silicate-loving elements fractionated by this process.
Siderophile W incorporated into core.
Lithophile Hf retained in mantle.
182Hf decayed to 182W during first ~60 million years (half life ~ 9 Myr) (We measure 182W/184W – sometimes normalised to the chondritic value, e182W)
Tungtsen (W) isotope evolution of chondrites:
Both Hf and W are refractory and should therefore occur in chondritic relative proportions in bulk planets
Assume that any bulk planet started with the W isotope composition of chondrites
It is defined by:
(1) The initial e182W of the solar system
can be determined from Ca-Al-rich inclusions (CAIs)
(2) The present-day 182W/184W of chondrites
can be directly measured on chondritic meteorites
Peculiarities of the Earth – Moon system
• The Moons orbit does not coincide with the Earth’s equatorial plane
• It has a large size compared to the Earth
• Earth-Moon orbital system strongly coupled (high-angular momentum)
- Stabilises Earth’s rotational axis (get seasons)
- Tidal drag slows down the Earth’s rotation (days are getting longer, 60 seconds every 4 Ma)
- Moon moving away from Earth (3.7 cm year)
• 500 myrs ago a day was 22 hours – in theory and some evidence
• Drag and movement is slowing down
• Relationship with Earth is unusual
• Rocks on Moon are older than any on Earth
• - Rocks on Earth have had 3 billion years of plate tectonics so if they were ever present they have been reworked – therefore moon rocks have secrets about early solar system
Lunar Geology
- Basic observation
- 2 rock types – lighter and darker – lighter is older
- Light is anorthosite – very rich in plagioclase – flotation cumulates – low density – Moon once had a molten surface
- Of course, history is more complex
- Lunar highland rocks can be divided into those that are slightly iron rich (formed soon after Moon formed – oldest is 4.5 billion years) and those that are magnesium rich and range in age from 4.5 to 4.3 – indicates different magma sources
- Cooled by about 4.3 – KREEP rocks – consistent in mineralogy but scattered across highlands (potassium, rare earth elements, phosphorous)
Lunar magma ocean:
- The Moon was molten after formation
- As the molten rock cooled, it crystallized
- Some crystals sank (pyroxene and olivine) others floated (plagioclase)
Lunar Mare - impact origin:
- Impacts of asteroids form huge basins.
- Shock waves create fractures in the rock beneath the basin
- Upwelling caused partial melting
- Magma rose along the fractures, filling the basin
The Moon and the Apollo Missions
- Oldest Moon rocks > 4.4 Billion years
- Depleted in volatile elements (H2O, Rb, K, Na)
- Enriched in high-temperature refractory elements (Ti, Ca, Mg, Al, Si)
- Lower density than Earth
- Same oxygen isotope composition to the Earth
- Very small core – 10% mass – earth is 30%
- Same oxygen isotope composition to the Earth – curious as planets have different
Origin of the Moon (previous ideas)
• Co-accretion: Earth and Moon accreted side by side. Does not explain the differences in density or the difference in the depletion of volatile elements, or angular momentum.
• Capture: originally Moon in heliocentric orbit captured when it came close to Earth. Difficult to do without Moon colliding with Earth, difficult to explain the oxygen isotope composition.
• Fission: Moon split off as a blob during the rapid rotation of a molten Earth. Explains why the Earth and Moon have identical oxygen isotope compositions, and why the Moon has a lower density. But the Moon could not split from Earth without large input of energy.
o Fission: proposed by son of Charles Darwin – thought what formed the Pacific
Current paradigm of moon formation
Impact origin
Giant impact – predictions:
• Collisions in early solar system are likely
• Explains the depletion in volatile elements and Fe
• Explains the angular momentum of the Earth-Moon system (spin + orbital distance)
• Problems:
o Only one Moon
o Isotope composition similar to Earth
- Most models predict two moons – there is a paper that suggests they collided and the other was destroyed
- Isotope composition should be similar to Thea (impactor) not the Earth
Core formation - Hf-W age:
- Early Hf-W data suggested that the Moon was formed between 30 to 50 Myr after the start of the solar system
- But 182W also produced by cosmic ray induced neutron capture of 181Ta
- Earth has Magento sphere – protects us from cosmic rays – Moon doesn’t have this – if a cosmic ray hits 181Ta it produces 182W
- W isotope data for metal from the same samples
- Found that same samples with differing Hf/W (parent/daughter ratios) had identical 182W, also identical to the Earth
- Only possible if they were formed after about 60 Myr (i.e. when 182Hf was extinct)
Moon has a e182W value that is 27±ppm higher than Earth. Also has a higher Hf/W ratio (~30-50) than Earth (~26), so either;
(1) Small difference because Moon formed while 182Hf was still live, so formed within 60 Myr of the start of the solar system, or
(2) Different amount of late accretion (“late veneer”) to the Earth and Moon
– said it adhered to the moon forming late
Core formation - Hf-W age:
Has a higher Hf/W ratio (30-50) than earth
- Suggests that Moon formed while 182Hf was still live, so formed wihin 60 Myr of the start of the solar system
Young age for lunar anorthosites!
- New data suggest that the old ages for the old anorthosites are incorrect. New age is 4,360±3 Myr – 200 Myr after the start of the solar system.
- Either the Moon is young, or the magma ocean was sustained for >100 Myr, or the magma ocean was formed very late, or the Sm-Nd ages have been reset during cooling.
When dating a rock only the rock and not a planet
Some minerals survive though – unlike rocks
Zircon ages - U-Pb and Lu-Hf: 4.51 billion years old – mineral ages – 60myr after start of solar system
Age of the Moon:
- If e182W difference is due to difference in Hf/W ratio, then the Moon must have been formed before 182Hf became extinct, between 40 to 60 Myr after solar system formation (or due to the late veneer).
- Zircon Lu-Hf ages of 4.51 Gyr, suggest the Moon was formed no more than 60±0.1 Myr after solar system formation.
Refractory element isotope composition:
- Oxygen isotopic composition of Moon identical to Earth, inconsistent with numerical models estimating that more than 40% of the Moon-forming disk material was derived from Theia.
- But is the same for more refractory elements such as W, Ti, Cr and Si.
Moon’s isotope composition:
Silicate Earth and Moon possess identical oxygen, silicon, neodymium, and titanium isotope composition, requires either;
(1) Impacting planet with an orbit similar to Earth ….
(2) Post-impact mixing between vaporized parts of Earth and Theia, before the Moon forms …. Difficult to do
(3) More recent modelling (Canup, 2012) shows that if the impactor is larger – comparable in size to the Earth – then the collision is symmetrical, and can produce a debris disk with the same composition as Earth’s mantle.
Volatile element isotope composition
- Moon is depleted in volatile elements, and volatile loss will fractionate the stable isotopes of volatile elements.
- Volatile loss would lead to preferential loss of the light isotope of any volatile element isotope system, leaving the Moon enriched in heavy isotopes.
- Lunar rock – heavy isotope composition – lost their light composition rocks
Volatile isotope behaviour:
a) Canonical model, disk forms from the impactor
b) Equilibration of a silicate atmosphere.
c) High-energy model, mantle, atmosphere and disk form a well-mixed disk
Roche limit – if you had a moon within this zone it would be broken up by tidal forces
Could lose volatiles through impacts or volcanism
a) Giant-impact – loss of volatile elements and their light isotopes
Alternatively:
b) Volcanism – loss of volatile elements (for example from a magma ocean)
c) Impacts – after formation of the Moon
Water in the solar system:
Dteurium is the heavy isotope of hydrogen – not very abundant – its ratio differs in diff minerals so can be used to fingerprint
- Water didn’t come from comets
- Very similar to chondrites – came from chondritic material
- Think Earth accreted from this chondritic material, so this makes sense
Mass of Earth: 61024 kg; Mass of oceans: 1.41021 kg Ordinary chondrites - 0.1 wt% H2O: 61024 kg (0.001) = 61021 kg = 4 oceans
Carbonaceous chondrites - 15 to 20 wt.% H2O:
61024 kg (0.15) = 91023 kg
= 600 oceans!
So, only need a few carbonaceous-type planetesimals to get Earth’s water
Possible we got it through accretion throughout or just at the end of the main accretion event
Another possibility is that it came from accretion during the late veneer
Accretion of volatiles:
The planet formed at 1 AU in this particular simulation is extremely water-rich: oceans would be 10’s of kilometers deep!
Principle works but the results don’t work for Earth
Water in the late veneer?
Carbonaceous chondrites – 15 wt.% H2O:
= 61024 kg 0.005 (late veneer) x 0.15
= 4.5 x 1021 kg water added in the late veneer
= 4.5 x 1021 / 1.4 x 1021
= 3.2 Oceans
So, late addition of carbonaceous chondrite material can supply all of Earth’s water
Don’t know when the water arrived
Likely later because it was too hot and dry in very early earth but this is speculative
Earth’s Very Early Atmosphere
- Earth’s very early atmosphere was probably composed of hydrogen and helium (the most abundant gases in the universe).
- If present, it would have quickly been lost into space because Earth’s gravity is insufficient to retain these and because Earth had no magnetosphere until its core formed.
- Without a magnetic field, the solar wind would have swept away any atmospheric gases
The atmosphere post hydrogen and helium would have come from the degassing of the accreted material
• The ability of a molecule to be retained is strongly dependent on the temp of that molecule
• If the temp exceeds one-sixth of the escape velocity it will be lost from the atmosphere
Nitrogen and the inert gases
Nitrogen is the most abundant gas in the atmosphere (78.7%), Argon (Ar) 3rd most abundant is one of the inert gases (Ne, Kr and Xe in trace amounts), all relatively unreactive.
Inert gases can tell us much on the timing of degassing of the Earth.
Decay of 129I 129Xe (t1/2 = 15.7 million years)
Iodine is lithophile, xenon is volatile (atmophile)
So, if outgassing occurred within the first 80 Myr of Earths accretion then the atmosphere would contain 129Xe that differs from the mantle.
- I129 decays to Xe 129
- Iodine decays and Xe is outgassed – Potassium stays in the mantle
- Can date when there was a lot of potassium in the mantle and a lot of Xe in the atmosphere to get an age of atmopshere
I-Xe age of the atmosphere
- 129Xe/130Xe evolution suggests very early formation of Earths atmosphere, about 30 Myr after the start of the solar system.
- Overlaps with the age of the core formation
- Formation of earths core and atmosphere are probably due to the same process
Planetary scale differentiation
Metallic Core & early atmosphere appear to have formed at the same time, 30 Myr after the start of the solar system.
The Earth’s earliest crust:
- The oldest rocks on Earth are about 3.85 billion years old
- It has been suggested that the absence of any older rocks is due to the late heavy bombardment, seen in crater ages on the Moon
- Coincidence between oldest rocks and the cratering event on the moon
- If the moon experienced this then the Earth would have – it would have destroyed rocks at the surface
- These rocks recycled by plate tectonics
