Download Differentiation 2: mantle, crust OUTLINE

11/9/16
Differentiation 2: mantle, crust
OUTLINE
Reading this week:
White Ch 12
Today
1.  Core last lecture, now the rest:
2.  Mantle, crust
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QoD
?
Light elements in the core
Contenders: O, S, Si, C, P, Mg and H.
Hotly debated, but many people like S, O:
FeS is miscible with Fe liquid at low and high temperatures
• S more depleted in silicate Earth than similar volatility elements
• Iron meteorites contain FeS (troilite)
FeO miscibility requires high pressures and temperatures
Together with S affects how much sidero/chalcophile elements
enter core: needed to explain mantle
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What the chalcophile elements say
Chalcophiles are depleted in the silicate Earth relative to
chondrites, but not as depleted as many of the siderophiles
are.
⇒ could argue against much S in the core (if there’s more S
loving elements in the mantle than expected, S probably same)
– ongoing problem
What the siderophile elements say
Siderophiles not as low in the mantle as expected from pure
metal-silicate equilibration.
•  5-350 times more enriched than expected
for complete silicate-Fe equilibrium
•  Volatile siderophiles even more enriched
than non-volatile ones.
⇒ 3 possible causes:
1)  incomplete equilibration
2)  an impure Fe phase
3)  addition of a volatile rich component after
core formation, aka “late veneer”
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When did differentiation happen?
•  About 4.5 billion years ago
•  After beginning of Earth’s accretion at
4.568 Ga
•  Before the formation of the Moon’s
oldest known rocks, 4.47 billion years
ago
⇒  ~100 Ma window
Formation of The Moon
Giant impact as last major
event, aka starting point:
• impactor’s core largely
transferred to Earth
• Moon accretes from debris in
orbit (85% impactor, 15%
Earth)
• High temperatures: evaporated the most volatile elements
• Lunar siderophile element depletion: it formed a core twice:
once prior to impact, once after impact
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Formation of our moon
Highland anorthosites (white),
explained by low density feldspar
floating to surface of magma ocean
=hot!
Crust formed by time of oldest
lunar rocks ~4.47 Ga.
Heavy impact bombardment
continued until ~3.9 Ga.
3.8-3.1 Ga: Basalts fill some of the
large craters (Mare)
=> Use this for Earth analog!
Earth’s Mantle
•  Lies between the crust and the core.
•  Depth range is 40 km to 2900 km.
•  The mantle consists of rocks of
intermediate density, mostly compounds
of O, Mg, Fe, Si
•  New continental crust may be produced
during partial melting of mantle material.
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Evidence for mantle composition:
•  Sampled by xenoliths, occasionally exposed by crustal
deformation (ophiolites)
–  Peridotite
–  Eclogite
! What is eclogite?
•  Seismic velocities match both rocks
•  Must melt to form basaltic magma
–  Peridotite melting – max 40%
–  Eclogite melting – nearly 100%
http://www.wild-rocks.com/images/Ophiolite.gif
Mantle compositional estimates
Models on the
right are still
reasonable
today:
• Pyrolite: a mix
of mantle
samples
• Anderson’s
model adds
eclogite, to
“undo” melt
depletion
Recycled crust
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Earth’s Crust
•  Lighter rocks floated to the surface of the magma ocean.
•  The crust is formed of light materials with low melting
temperature and is up to 40 km thick.
•  Generally compounds of Si, Al, Fe, Ca, Mg, Na, K, O
•  4.3-4.4 Ga zircons from western Australia have δ18O
isotopes characteristic of liquid water:
=> Earth cooled enough for solid crust + liquid water within
100 Ma after the Giant impact (Moon > 4.47Ga)
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Bimodal distribution
of topography
a hypsometric curve:
two modes (left) or
two plateaus (right) on
curve with little
transition
from: http://www.personal.umich.edu/~vdpluijm/gs205.html
continental crust: ~1km
oceanic crust: ~ -4km
Topography and isostasy
Crust is less dense than the mantle,
and basically “floats” on it.
Continental crust = numerous rock
types, but its mean density =2.7 g/
cm3. Continent = granodioriteandesite, not really granitic
Oceanic crust = largely basaltic, its
mean density = 2.8 g/cm3.
Isostasy = equal standing: column of mantle + crust = equal at a
reference depth; thick lower density continent “floats higher”
Low density relates to different chemical composition
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Continents are complex, oceanic crust systematic
•  Forms at mid-ocean
ridges, cools away from
ridge until ~180Ma
•  Made ~entirely of basalt expected from (partial)
melting of the mantle
•  All other solar system
“crusts” are basaltic
Hot spots are also largely basaltic (e.g. Hawaii).
Hotspot melting
probably deeper
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How to make continental crust
Mantle melting makes “basalt” (45-55% SiO2), so how to
make rocks with SiO2 > 60%
Continental crust age distribution
Low density continental crust does not subduct, it just folds.
⇒ Continents up to 4 Ga, only continental mass recycled is small
amounts of sediment on oceanic plates (small flux)
⇒ Land
keeps
being
added
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Where to add to a continent?
• At convergent plate margins (volcanic arcs) – water added to the
mantle from the subducted lithosphere causes melting - flux
melting - calc-alkaline basalt (so still not silicic)
Adding mass to a
continent
Step 1: accrete terranes to
the continental margin; i.e.
blocks of unrelated origin
got assembled together
Model would be initially to
have island arcs collide
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Make the granitoids
•  Within the continental arcs
•  Great example: coastal batholiths
•  What we think happens:
–  Existing low(er) SiO2 rocks get reheated by
repeated intrusion and remelt/mix (just the
low-temperature melting components)
Compositions by Goldschmidt’s classes
Split “primitive mantle” to crust, mantle; elements divided:
Lithophiles mostly in crust; ionic bonds; large ions. O, Mg, Fe, Si
in mantle too
Chalcophiles
split between
mantle, crust,
core; covalent
Siderophiles
mostly in the
core (metal)
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