Kevin Flashcards
(109 cards)
1
Q
Separating elements
A
- 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
2
Q
How did we get a solar system?
A
• 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
3
Q
Beta Pictoris
A
Star is 20-25 mya – very young – has a dust cloud – is consistent with galaxy creation hypothesis
4
Q
Solar System Formation:
A
- 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
5
Q
Chondritic meteorites:
A
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
6
Q
‘Chondritic’ Earth
A
- 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
7
Q
Differentiation of the Earth:
A
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)
8
Q
How do we know theres a core:
A
- 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)
9
Q
Siderophile elements:
A
- 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.
10
Q
Nickel - Cobalt experimental data:
A
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
11
Q
Deep magma ocean model of core formation
A
- 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
12
Q
Continuous core segregation during accretion
A
- 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
13
Q
Moderately volatile elements – Ga & Mn
A
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
14
Q
Core formation:
A
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).
15
Q
Accretion and planet growth:
A
- 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.
16
Q
When did the core form?
A
- 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).
17
Q
U-Pb isotopes
A
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)
18
Q
Pb in oceanic basalts
A
- 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
19
Q
Hf-W isotopes
A
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
20
Q
Peculiarities of the Earth – Moon system
A
• 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
21
Q
Lunar Geology
A
- 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)
22
Q
Lunar magma ocean:
A
- The Moon was molten after formation
- As the molten rock cooled, it crystallized
- Some crystals sank (pyroxene and olivine) others floated (plagioclase)
23
Q
Lunar Mare - impact origin:
A
- 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
24
Q
The Moon and the Apollo Missions
A
- 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
25
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
26
Current paradigm of moon formation
Impact origin
27
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
28
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
29
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
30
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
31
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.
32
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.
33
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.
34
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
35
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
36
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
37
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
38
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
39
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
40
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
41
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
42
Planetary scale differentiation
Metallic Core & early atmosphere appear to have formed at the same time, 30 Myr after the start of the solar system.
43
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
44
Planetary migration: the “Nice” model:
* Closer to the start of the solar system giant planets were much closer to the Sun surrounded by a disc of planetary material ‘cloud’ with a circular orbit
* Material from cloud smacked into larger planets like Jupiter – changed their orbit to more elliptical and pushed material into planetary zone
* This displaced material came in and caused the late heavy bombardment
* Newer idea is that these planets left, knocked into the asteroid cloud and came back but further from the sun – still brought material with it
45
Isua, whole-rock ages:
Long-lived chronometer:
147Sm --> 143Nd (t1/2 = 106 Ga)
147Sm abundance has decreased by only 3% in 4.56 Ga
Short-lived chronometer:
146Sm --> 142Nd
(t1/2= 103 Ma)
146Sm exists only in the first 500 Myr of solar system history
46
Zircons: both old and crustal
Know there must have been old, felsic rocks
- Melting of crustal material
- Already making granites at 4.3 bn years
- Low temps – had to be melting crustal rocks with water - water
47
Komatiites
* Komatiites are ultramafic lavas that were common on the early Earth but rare today. They have a high MgO and FeO content and olivines showing a spinifex texture (elongated, skeletal, branching crystals), indicative of the rapid crystallisation.
* Initial melting experiments in the laboratory showed they must have melted at very high temperatures.
* The progressive decline of
* komatiites was then used as
* evidence for progressive
* Cooling of the Earth’s mantle.
* Really high mg content – can only make a melt with that much mg if temp is really high
* Spinifex texture – rapid crystallisation – not due to cooling but undercooling of reaction – get to surface so fast they crystalize at temps far beyond their crystallisation temp
48
Decompression melting:
• Earth was hotter so melting is deeper because it hits the solidus sooner when rising – therefore more melt
49
Pt and Ru versus age of komatiites.
Maier et al. (2009)
• Older komatiites had lower platinum content
• Evolution of highly siderophile elements
• Probably addition of platinum from late veneer that took a while to be ‘mixed in’ to the mantle
50
m182W vs. mass fraction of late veneer
Moon has not received ‘Late Veneer’
| Isua data supports that ‘Late Veneer’ not fully mixed in during the past
51
Controls on MORB chemistry:
1. Degree of melting, reflecting variations in mantle temperature.
2. Fractional crystallisation, controlled by the phases that crystallise, depends on pressure and magma composition.
3. Variations in mantle composition, ‘fertile’ rock types melt first, these can then dominate magma chemistry.
Assimilation of seawater – sometimes effects basalts being produced
52
Fractional crystallisation
The common crystallization sequence is: olivine, olivine + plagioclase, olivine + plagioclase + clinopyroxene
MORB effects by fractional crystallisation controlled completely by which phases they form and in what order – based on composition, pressure, water content etc
Crystallise a phase – remove the composition of that phase – fractional crystalisattion
53
La/Sm in MORBs:
Different MORB types
E-MORBs La/Sm > 1.8; N-MORBs La/Sm < 0.7
Problem: Many MORB have incompatible element abundances that are too high for simple fractional crystallisation – magma chamber replenishment
Take 2 REE – La and Sm – incompatible – should both be effected the same by melting – any differences must be due to differences in mantle source
Difference seen – difference in incompatible/depletion of incompatible elements in source
Incompatible elements are good at showing differences in sources but not perfect – incompatible elements are replenished during waves of magma movement during fractional crystallisation – changes the composition constantly – not a closed system
54
Radiogenic isotopes:
Radiogenic isotopes are rarely used in isolation but have distinct advantages over conventional element concentration data
1. Radiogenic isotope ratios are NOT fractionated during partial melting.
2. Radiogenic isotope ratios are NOT fractionated during fractional crystallisation, but
3. Parent/daughter elemental ratios ARE fractionated during melting and Fractional crystallisation.
Only time (or mixing) can change Radiogenic isotopes
55
Sr-Nd isotopes in MORB:
* 87Sr/86Sr unradiogenic
* 143Nd/144Nd radiogenic relative to “chondritic” bulk Earth.
* This reflects the isotope composition of the mantle source of MORB
* Dark dots are MORBs
* Melting takes out neodymium preferably than Sr – due to compatible – so depleted in Nd melt rock compared to Sr
* Mantle has experienced long term depletion in incompatible elements
56
MORB Nd and Sr:
• Possesses unradiogenic 87Sr/86Sr and radiogenic 143Nd/144Nd relative to “chondritic” bulk Earth.
• Points to a long-term depletion of Rb>Sr and Nd>Sm in the mantle source.
• But quite variable compositions ….
Mantle Heterogeneity
57
Mantle heterogeneity:
Could be due to:
- Prior depletion producing harzburgite and dunite (depleted residue)
- Or enrichment producing clinopyroxene-rich rock types (melt that does not get all the way to the surface)
- Recycling, through subduction of oceanic (mafic) or continental (felsic) crust.
Prior depletion from prior melting as you have one piece that's already been depleted and one that’s not – especially if that melt does not lead to the melt being taking out of the mantle – leaves an enriched and depleted sections within mantle
58
The Pb paradox
1. Core formation – Pb siderophile, U lithophile - but difficult to estimate BSE, different age to Hf-W, also volatile
2. Volatile loss – Pb volatile, U lithophile, needs early Pb loss, and late Pb addition, but significant late addition (>2% Earth’s mass)
3. Hidden in the Mantle – U incompatibility > Pb, would leave a mantle with a low U/Pb that evolves to a relatively unradiogenic Pb isotope composition, but little evidence for unradiogenic Pb in oceanic basalts
59
Oceanic crust – isotope heterogeneity
* Systematic isotope variations of MORB, reflect long-term depletion/enrichment of the mantle source.
* Implication that many MORBs are sourced by the preferential melting of enriched rock types.
* Mantle pyroxenite melts before peridotite, may account for the bias to enriched compositions in MORB
* Observations suggest that mantle melts mix and homogenise through their ascent in the crust
* But, peridotite melts crystallise before pyroxenite melts, so might expect to see stratification in MORB, also accounting for the bias to enriched compositions in MORB
60
Ocean island basalts: Evolution in the Series:
Tholeiitic, alkaline, and highly alkaline
- Two main types of basalt:
- Tholeiitic basalt forms at low and high pressure and with higher melt fractions
- Alkali basalt forms only at high pressure and with small melt fractions
61
Effect of Pressure, Water, and CO2:
Increased pressure moves the
ternary eutectic (first melt) from
silica-saturated to highly undersaturated
alkaline basalts
Water moves the (2 GPa) eutectic
toward higher silica, while CO2
moves it to more alkaline types
Lower melt fractions – alkaline
Higher melt fractions – tholeitte
As you move from lower to higher you change and will get both in final poduct
62
Melt degree:
Problem: OIB Deeper melting, so higher temperature, and would normally expect more melting than MORB, but actually less.
- Limited upwelling (oceanic crust limits upwelling).
- Limited material to melt
Higher temp + pressure are small melt fractions even though you expect a longer melt column – it is deeper but has a smaller melt
Two potential reasons:
- Under crust so limited amount of upwelling
- Not much material to melt
63
Trace element variations in OIB
Increased conc of incompatible elements – shows lower degree of melting
MORB and OIB appear to have distinctive sources
64
MORB and OIB appear to have distinctive sources
- If you look at elements with similar compatibility their ratios should not be effected by degrees of melting, but they do differ between OIB and MORB
- Shows not just degree of melting is different but there are different source rocks that are being melting
- Generally enriched in incompatible elements (LILE, HFSE and LREE elements) relative to MORB, consistent with smaller degrees of melting.
- Variations in incompatible element ratios, that are not fractionated by melting or fractional crystallisation, suggests a distinct source from that of MORB.
65
Isotopes and OIB
- Isotopes do not fractionate during partial melting of fractional melting processes, so will reflect the characteristics of the source
- OIBs, which sample great expanses of the mantle, often in places where crustal contamination is minimal, provide the best evidence of the source of material in the deep mantle
66
Mantle reservoirs: Nd and Sr
- Earth started off in the cross-hair of bulk earth – taken from chondrite
- All differences are due to differentiation of silicate elements since formation
- Mantle in top left has experienced long term depletion in Nd – this is where OIBs sit - experienced long-time depletion – where have we put these elements from the manle? – in the EM sections in the crust
- Upper mantle is largely depleted and basalts that sample that mantle are in top right
1. BSE (Bulk Silicate Earth) or the Chondritic Uniform Reservoir (CHUR) with ‘chondritic’ composition
2. DM (Depleted Mantle) = N- MORB source
a. Often referred to as DMM the Depleted Mantle MORB source
3. 3. EMI = enriched mantle type I has lower 87Sr/86Sr (near chondritic)
4. 4. EMII = enriched mantle type II has higher 87Sr/86Sr (> 0.720, well above any reasonable mantle sources.
a. Sampling enriched basalts
- Radiogenic 87Sr/86Sr requires high Rb/Sr & long time to decay 87Sr
- Unradiogenic 143Nd/144Nd due to low Sm/Nd
- EMI (slightly enriched) thought to correspond with lower continental crust
- EMII is more enriched, especially in radiogenic Sr (indicating enriched in Rb parent) corresponds with the upper continental crust
- Continental crust made by differentiation of mantle – it is also differentiated itself – continental collision etc causes melting
- Lower rust is relatively depleted in incompatible elements – upper is vice versa
- Uranium,thorium etc enriched at top
- If some OIBs source enriched material, found deep in the mantle then it shows the crust has been recycled within the mantle – proves subduction – it also shows it retains its integrity as it stays enriched – OIBs can source both depleted material and enriched material in a single eruption
- Subduction can also occur by delamination – Yao Ling theory
5. PREMA (PREvalent Mantle)
a. FOZO (FOcal ZOne): A “convergence” reservoir toward which many trends approach, maybe a common mixing end-member
67
Pb isotopes - enriched sources:
- U and Th are concentrated in both oceanic and continental crust (U & Th highly incompatible)
- Oceanic crust has elevated U and Th content (compared to the mantle), over time all isotopes more radiogenic
- Continental crust is generally older than oceanic, so higher 207Pb/204Pb from the decay of 235U
- so 206Pb/204Pb and 207Pb/204Pb will both increase due to the presence of recycled oceanic crust, and 207Pb/204Pb will be relatively higher when recycling continental crust
- Recycled crust would have more 207/206 – would sit on a 2Byr geochron
68
Pb Isotopes - HIMU & EM sources:
HIMU reservoir:
- Very high 206Pb/204Pb ratio
- Source with high U, (too high for any mantle process). Old enough (> 1 Ga) to generate observed isotopic ratios
- Old recycled oceanic crust
EM reservoirs:
- high 207Pb/204Pb ratio relative to a given 206Pb/204Pb. Source with higher 235U, must be old relative to DM and HIMU
- Old recycled continental crust
69
Crustal recycling into the mantle:
EMI, EMII, and HIMU: too enriched for any known mantle process... must correspond to crustal rocks and/or sediments
- If the EM and HIMU = continental and oceanic crust, can only get to deep mantle by subduction and recycling
- To remain isotopically distinct: could not have rehomogenized or re-equilibrated with rest of the mantle
70
Primordial Helium in Earth’s mantle
Helium in the Earth’s mantle: inert and volatile
- Two isotopes: 3He (lower abundance) and 4He (greater abundance).
- U and Th decay to Pb via alpha decay to produce 4He nuclei (alpha particles)
- Little 3He produced in the Earth (mostly primordial).
- Therefore, 3He/4He in the Earth decreases with time.
- Absolute 3He/4He ratios in the solar system are small (10-3 to 10-8), so we normalize to 3He/4He ratio in atmosphere (Ra, 1.38x10-6).
The sun (solar wind) and the atmosphere of Jupiter have high 3He/4He (200-400), so high 3He/4He is thought to be primordial.
- 3Helium hardly changed since Earth formation
- Over age of Earth we have gone from a ratio similar to sun/gas giant – has gone down as we have produced loads of helium 4
- For every helium 3 isotope there are 1000000 helium 4 isotopes
- These ratios are normaliised to R/Ra
- In early Earth 3/4 was very high
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Primitive solar neon:
- Volatile-rich reservoir with primitive 3He/4He and solar-like Ne.
- If reservoir truly primitive what composition would you predict for Nd and Sr isotopes?
- If it is primitive and has not been affected by differentiation it should expect to be like BSE
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Mantle source of primitive high 3He/4He lavas
Two potential solutions:
Ancient depleted mantle from which lithophile elements have been lost (including Th & U).
- Depletion needs to be old >3 Ga
- Hard to maintain a He rich reservoir
“Primordial” material in the deep mantle or the core
- He (and Ne) have to be decoupled from lithophile elements (Nd-Sr-Pb).
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Ancient depleted mantle:
- Relationship between 3He/4He ratio and incompatible element enrichment, led to the hypothesis that high 3He/4He reflects ancient mantle depletion.
- Older the depletion the less growth of 4He from Th and U, signal then locked in due to loss of Th and U
- Highest ¾ ratios was from most depleted
- Makes sense as oldest reservoirs are most depleted
Possible link between ancient mantle melting event, and epsiodes of generation of continental crust
Spikes where you get crustal formation – so when the mantle was depleted you were making crust - no age correlation – why not really believed
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Deep Earth reservoirs for He:
(i) Deep layer in magma ocean – where He is separated from Th-U, without degassing
(ii) (ii) Earth’s metallic core – fractionation of He from Th–U, with little degassing
- First is very speculative – helium preserved but uranium and thorium lost
- 2. Helium is in core- becomes volatile rich reservoir
- Also – make core from many cores from planetesimals cores – this controls how much helium in core – then helium becomes siderophile
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Mantle source of primitive high 3He/4He lavas
- Ancient depleted mantle has to be old (>3Gyr) hard to deplete incompatible elements and not He
- “Primordial” material in the deep mantle or the core
o Either deep layer in the mantle, base of magma ocean
o Or, the core, but this requires high He concentrations in the the Early Earth.
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Solar-chondritic composition:
Most primitive meteorite types showing the least differentiation
Chondroles all held together in a fine-grained matrix
Compared to sun very similar
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Earth’s composition:
Earth was too close to sun when formed – so some elements could not condense – low abundance of these in earth now
Siderophile elements also depleted
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Chondritic Earth?
* Depletion in volatile & moderately volatile elements due to incomplete condensation
* Following accretion, a deep terrestrial magma ocean …
* Siderophile elements (Fe-Ni) to the core, leaving behind the early (primitive) silicate mantle/ BSE (bulk silicate Earth).
* From the primitive silicate Earth, the crust (continental and oceanic) was extracted from the early primitive mantle.
* But what is the bulk composition of the Earth, can this be matched to the composition of chondrites?
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Water:
Earths water content between that of enstatite chondrites and ordinary chondrites
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Chondritic Earth in terms of element comparison?
* Water content consistent with enstatite-ordinary chondrites
* Major elements match with carbonaceous chondrites
* Oxygen isotopes consistent with enstatite chondrites
* D/H and C closely matches carbonaceous chondrites.
* Osmium isotopes match with ordinary chondrites.
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Difficult to ascribe Earth to a particular class of chondrite, could be due to processes on Earth (e.g. core formation) or modification of meteorite compositions, or that there was some mix of different chondrites added to Earth.
But generally thought that refractory lithophile element abundances of the silicate Earth are chondritic.
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Chondritic 142Nd measurements
* Discovery: Boyet and Carlson (2005) found that 142Nd/144Nd ratios in accessible modern terrestrial lavas are 18±5 ppm higher than chondritic meteorites.
* 142Nd variation due to 146Sm decay. All modern accessible terrestrial samples evolved from a mantle reservoir with a Sm/Nd ratio ~6% higher than chondrites.
* Earth has a mantle composition with a sm/nd that is 6% higher than chondrites – small, does this matter?
* Difference was 20 ppm to generate difference in the first 500myr
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147Sm-143Nd evolution in the mantle with a 6% higher Sm/Nd ratio
eNd = ~ +7 (143Nd/144Nd = 0.5130), which is 630 ppm higher than chondritic 143Nd/144Nd at the present day
* Bottom right was mantle earths first composition – has fractionated left – done this because continents have been removed
* CHUR is the path the earth would take with a chondrite composition
* SCHEM is path if the earth had a 6% higher composition – implication that sm/nd is not fractionated in mantle as it is so close to the other rocks – changes whole thinking of mantle evolution
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Building a new Bulk Silicate Earth (BSE) composition
* The new compositional model relies on the difference in 142Nd/144Nd between Earth and chondrites.
* Fundamental assumption of model: Sm/Nd of Earth is ~6% higher than chondrites.
* Non-chondritic BSE (Bulk Silicate Earth) has an isotopic composition like high 3He/4He lavas (FOZO/PREMA).
* Problem solved?
* How do you do that? With an Earth 6% higher than chondrites
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The hidden reservoir:
Hidden enriched reservoir (enriched in incompatible elements – U, Th, K, LREE)
(i) Deep layer in magma ocean – rich in incompatible elements, low Sm/Nd
(ii) Early basaltic crust – rich in incompatible elements, low Sm/Nd, recycled to deep mantle
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Survival of a early enriched hidden reservoir:
Some potential problems with “hidden” reservoir hypothesis:
1. How would a “hidden” reservoir remain completely hidden at the bottom of the mantle during a giant impact event?
2. How to keep a hidden ENRICHED (U, Th, K) reservoir hidden?
Is there any evidence for “hidden” reservoir in ocean island volcanism at the present-day, either chemical or thermal.
Hard to hide a reservoir when earth has survived things like a moon collision – can be explained though
Bigger problem - how do you keep the reservoir hidden – no sign of it in isotopes or OIBs – need to keep it thermally hidden – enriched in heat generated elements (U, Th)
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Heat in Mantle plumes?
- >40% of Earth’s heat (47 TW) would have to be generated in this hidden layer, has to be transmitted through the overlying mantle in plumes.
- Plumes carry 7-14 TW of heat, of which 3-4 must come from the core to sustain the geodynamo.
- So if <14 TW in plumes not enough for a hidden layer
- Problem for geodynamo!
Dont see this massive heat source – no evidence for anything particularly hot in deep earth
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Problem for geodynamo!
- Only works if the heat-production at the CMB is fairly homogeneous
- If you had a deep mantle with this much heat it would mean the geodynamo would not work
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Hidden reservoir in mantle
* Maintains an overall chondritic composition for Earth
* No thermal or chemical evidence for an enriched reservoir at the present day.
* Difficult for the gedynamo to operate if there is an overlying layer in the mantle with high heat production
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Another Hidden reservoir - the core:
• Earth’s metallic core
| Requires susbstantial partitioning into metal, with Sm
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Sulphur in the core:
* Density versus pressure and bulk sound velocity for Fe92.5O2.2S5.3,
* Fe90O8S2, Fe90O0.5S9.5 and pure iron along the adiabatic geotherm, compared with the PREM model.
* Currently thought to be sulpfur with a 9.5% composition in core
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Sulfide in the core:
* Sulfide or sulfide-rich metal in the core may incorporate incompatible elements, including REE and U and Th.
* Solution to the 142Nd excess in the silicate Earth
* No problem with the overall heat-budget of the Earth
* Potentially provides a heat source for the geodynamo
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(2) Collisional erosion:
* Looks like a chondritic Earth but depleted of incompatible elements by melt extraction.
* These are the elements that would normally be enriched in the crust.
* Suggestion then that the non-chondritic Earth results from collisional erosion of Earths earliest crust.
Possibly that its been lost from earth
Early earth melted – made a crust that was rich in incompatible elements and low sm/nd– many impacts – oblated the earths surface – so this material is physically removed and ejected
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Thermal budgets of collisional model
* U, Th and K abundances in the Earth are 30-35% lower than the standard model.
* So, 30-35% less radiogenic heat than the standard model. Lower Urey ratio (radiogenic heat/total heat flux)
* The Earth produces ~47 TW.
* Chondritic model: ~20 TW radiogenic heat (Urey ratio 0.42)
* Collisional model: ~13 TW radiogenic heat (Urey ratio 0.3)
* The rest is primordial heat.
* But this gives extreme cooling rates for the Earth
A perceived other problem – if you lost 30-40% of heat producing heat would mean some extreme cooling -encore
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s-process
s-process, nucleus captures a neutron to form an isotope with one higher atomic mass. If the new isotope is stable, a series of increases in mass can occur, but if it is unstable, then beta decay will occur, producing an element of the next highest atomic number.
Happens in low mass stars in the latest stages of stellar evolution – when star begins to die
S-process
Produces 142 Nd
Prisodimum captures a neutron but this is not stable so 142 is produced
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Chondritic Earth argument overview
142Nd indicates that the Earth is distinct from most chondrites.
If due to differences in the Sm/Nd ratio and 146Sm decay. Then might be explained by:
(1) Hidden reservoir or
(2) Collisional erosion
If due to differences in s-process nucleosynthesis, then:
(3) Earth accreted from material that was slightly different from chondrites (more s-process 142Nd in the material
that made the Earth)
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Continental crust:
- Average continental crust has an andesitic composition, andesites dominantly produced during continental collision
- Continental crust has much more silica than ocean crust
- Continental arc lavas is very similar to continental crust
- Continental subduction zones are the preferred setting for the production of continental crust
- Continental crust enriched in incompatible elements
MORBs depleted in incompatibles
Pretty good match between subduction zones and continental crust
• Crust lies between oceanic sources and arcs
• Compression and crustal thickening – leads to melting and granite production that goes from lower crust to upper crust
• Upper and lower crust are no uniform – variation due to melting - In general upper crust is more silica rich
New crust made of 92% of arc magmas + 8% “OIB”
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Differentiation of continental crust:
Need 14% melting to produce the upper crust
- Melt model suggests that there is a crustal residue
- But not seen, so ether hidden or foundered
- Can you make continental upper crust from melting, differentiation and assimilation? No
- Foundered – lost it and sunk back into mantle – delaminated
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Continental crust formation:
* Major and trace element data suggest that most continental crust produced by arc volcanism with some produced by mantle plumes (making continental flood basalts).
* Need differentiation to produce andesitic magmas (water-rich), which dominantly occurs during continental arcs.
* Mass balance suggests that some part of the lower crust is either hidden or delaminated and lost back to the mantle
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Nd model ages: Clastic sediments
Clastic sediments provide an ‘average’ composition of the continents through time
* Problem: time of sedimentation sometimes difficult to date.
* Bigger problem: chemical bias towards younger rocks (a) fringing continents and (b) more susceptible to weathering
* Most are below the mantle curve – made from material that is made from material from recycled crust
* If you have enough measurements can make a histogram of fresh material to mantle material
* Records used to be taken from clastic rocks as they give a good average of present rocks through weathering but
* Physical bias – most continents are fringed by young material
* Younger material is also more easily weathered
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Nd model ages: Clastic sediments
* Continental growth curves for the Gondwana, from Nd isotope data for Australian shales (Allègre and Rousseau, 1984).
* Problem is that the growth curve is biased by preferential erosion of young crust in the sedimentary record
* Introduced a - Preferential erosion model, where the “erosion factor,” K, is defined by K = (y/[1 – y])/(x/[1 – x]).
* Erosion factor to account for preferential erosions
* Chemical bias makes a huge difference to ages and %s and it cant be very well calibrated – started to throw out clastic rocks
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Nd model ages - Igneous rocks:
Problem: many igneous rocks in crust formed by remelting of older rocks = so mixed model ages, with no geological significance.
102
Hf-Pb ages: Zircon
* In-situ measurement: Precise age from U-Pb chronology, crustal residence age from Hf isotopes.
* >80% zircons due to melting involving recycling of old crust
* Went from rocks to minerals
* Zircons – very resistant to erosion
* They also concentrate elements that tell us much – Uranium – also Hf –
* Done on insitu rocks using laser ablation
* Very precise concentrations
* Most zircons below line of the mantle so all crustal – made by remelting of crustal material – very few made fresh out of mantle
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Zircons age peaks and preservation:
* Volume of magmas depends on tectonic setting
* Preservation potential varies during supercontinent cycle
* Differences in preservation potential of crustal rocks may explain peaks in crustal ages previously attributed to enhanced crust generation
104
Mantle depletion ages:
* Osmium highly compatible during melting, so each mantle melting event preserved in sulfides and metal alloys in mantle rocks.
* Peaks in mantle melting match those in crustal zircons
* Preservation of events that made sulphides and alloys that match the ages of zircons in the crust
* Mantle preserving events that made continental crust
* Issue: idea concept focuses on plate tectonics and not melting of mantle
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Sulfides in diamonds:
* Os isotopes in sulfide inclusions in diamond, show that there was no eclogitic (high Re/Os) recycled material before 3 Gyr
* Marks the time of the onset of modern subduction?
* No ecologites/eclogitic diamonds before 3Gyr
* Ecologites signpost recycling of crustal material through subduction
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Onset of plate tectonics:
Prior to 3Gyr- no subduction – post there is and recycling of material
107
Composition of ancient crust:
- Rb/Sr ratio = proxy for crustal differentiation
- And silica content
- Cant just use silica content for silica content as its recycled
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Rb/Sr composition of juvenile crust:
* Nd concentration: when material came out of mantle
* Then figured out time of melting
* Simple two step model to determine if the rock was mafic or felsic
* What happened if it got remelted multiple times? Makes the calculation inaccurate – assumption
* Assumption = closed system
* Assumption = assuming Sr/nd are not affected by melting and use them for age
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Hot subduction - Slab (basalt) melting:
1. Dehydration D releases water in older arcs (lithosphere > 25 Ma) ……….. No slab melting!
2. 2. Slab melting M in arcs subducting young lithosphere.
Dehydration of chlorite or amphibole releases water above the wet solidus melts to produce andesites directly.
