2a.
Geochemistry of the geosphere
Geochemistry on the Earth’s surface
for analytical geochemists
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Task
• Draw a picture of planet Earth with its internal
structure
• Think about how we can know what the
interior of the earth looks like…
Topics
• Structure and composition of the solid part of
the Earth (= geosphere)
– Crust
– Mantle
– Core
• The origin and development of the planet
Earth
• Magmatic processes
Warning : the term geosphere is sometimes used as a general designation for all
earth's spheres (hydrosphere, lithosphere, atmosphere...)
GEOCHEMICAL INTRODUCTION
Geochemical nature of the elements
• Most elements show affinity to one group:
1. Lithophilic elements – form the silicate phase
2. Siderophiles – form the metallic phase
3. Chalcophilic – form the sulphide phase
4. Atmophilic – gaseous elements, volatile
• Some elements have a mixed nature (e.g. O,
Fe)
• This division describes the distribution of
elements in the Solar System and on Earth
Adapted from Gill (2015
Volatile and refractory elements
Volatile elements
• Low boiling point
• H, He, N2
• Cd, Pb, S
• Most alkali metals
Refractory elements
• Solid even at high
temperatures
• Platinum metals
• Ca, Al, Ti – form very
refractory oxides and
silicates
Two extremes, the remaining elements form a
gradual transition
Převzato z McSween & Huss (2010)
STRUCTURE OF THE EARTH
Structure of the Earth
Rust Country adapted from the original : By Kelvinsong - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=23966175
+ Biosphere
Sources of knowledge of the Earth's
interior
• Abrupt breaks in seismic wave velocity
• Differentiation of meteorite parent bodies
– captured in a varied spectrum of possible
compositions
• Mineralogical and petrological properties of
rock samples raised to the surface by
magmatic processes
– Reconstruction of pressure-temperature (P-T)
conditions
Seismic waves
• Pressure (P) and shear (S)
waves
• The influence of density
and compactness of the
environment (or elasticity)
on the speed of wave
propagation
• Earth's density in the
range of 2.8 to 12 g/cm3
– Effect of compositional
change and
recrystallization on more
compact minerals with
depth
Public domain (USGS)
Seismic waves
Public domain (USGS)
Brews ohare – Üleslaadija mom töö , CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9536500
Discontinuities
1. Base of the crust (Mohorovičić disc., Moho)
– 40 km below the continents
– 5-7 km under the oceans
2. The base of the lithosphere
– 80 km under the oceans
– A gradual transition
– The lower edge of solid lithospheric plates
– Below it is the asthenosphere
3. Transition of olivine structure to spinel
structure (410 km)
4. Base of the upper mantle (660 km)
– Transition of all minerals to perovskite and
additional magnesio-wüstite (Fe-Mg oxide)
5. D'' (mantle/core interface, 2900 km)
6. Inner/Outer Core Interface (5150 km)
– Fe and Ni in metallic form
– The movement of the outer fluid core creates
the Earth's magnetic field Adapted from Misra (201
Crust
• Only 0.7% of Earth's mass
• Dominantly from silicates
• 99% made up of 8 elements (O, Si, Al, Fe, Ca,
Mg, Na, K)
A. Oceanic crust
B. Continental crust
Plate tectonics
The processes of plate tectonics change the chemical composition of individual
parts of the Earth.
Adapted from USGS image (Public Domain )
Oceanic crust
• 70% of the surface
• Average thickness 7 km (least on mid-ocean
ridges ~4 km, most on oceanic volcanic
plateaus over 10 km)
• Young <200 Ma
• Relatively homogeneous
• Higher density – basalts (enriched in Ca, Mg
and Fe)
Continental crust
• Topographically higher, thicker – average of 40 km (20-
80 km)
• Older – oldest parts > 3.8 Ga
• Varied lithology – averageable to "andesitic
composition"
• Enrichment by Si, Na, K, volatile substances and
radioactive elements
• Enriched with incompatible elements
– cont. crust contains approx. 50% of the original contents of
the primary mantle
• The existence of a developed continental crust is a
unique feature of Earth compared to other rocky
planets
Public domain (USGS)
Mantle
• Represents 83% of the volume and 68.3% of
the mass of the Earth
• Compared to the crust, it is enriched in Mg,
contains less Fe and Si, and is depleted in
volatile and radioactive elements
• On the basis of seismics divided into
1. Upper mantle
2. Transition zone
3. Lower mantle
Upper coat
• Begins at the Moho – gradual transition rather
than a sharp boundary
– Uncertainty in the position of the crust/mantle divide
• Includes the lower part of the lithosphere
– The solid outer layer of the Earth
– Brittle deformations (rigid material)
• Beneath the lithosphere lies the asthenosphere
– Elastic deformation (elastic material)
– Low wave speed zone – possible presence of melt
– Zone of convective movements
Mantle composition
• Estimates based on
– P-wave speed,
– composition of ophiolite complexes
and xenoliths from basalts and
kimberlites and
– conditions of formation of basaltic
magma
• We assume a composition of
ultramafic rocks – peridotite (ol.+px.)
and eclogite (gr.+px.)
• Different composition under oceanic
and continental crust
• Peridotite under Moho depleted of
incompatible elements during magma
formation
• Primitive peridotite called pyrolite
(pyroxene-olivine) Adapted from Misra (201
Mantle composition
• Overall chemical composition
very similar across the mantle
• Mineralogical composition is a
function of depth (i.e. pressure)
• In the transition zone, olivine
changes to a spinel structure
(ringwoodite )
• From approx. 300 km px. forms
solid solution with gr.
• In the lower mantle, a mineral
mixture of Mg-perovskite
(MgSiO3), Ca-perovskite (CaSiO3)
and magnesiowüstite (MgFeO)
Adapted from Misra (2012
Core
• It makes up 16% of the volume and 31% of the mass of the Earth
• Earth's average density is 5.515 g/cm3 – the highest of any planet, with
the crust and mantle being less dense on average
– The heavy core assumption
• A lower content of siderophilic elements in the mantle and crust than
in chondrites
– We assume they are in the core
• An alloy of Fe, Ni (4-5%) and Co (0.2%), smaller amounts of S, C, O
and Si.
– Other estimates indicate higher contents of O (5%) and S (2.7%),
possibly also Si (up to 7.3%)
• Due to the low viscosity and impermeability to S waves, we assume
that the outer core is molten
• The inner core is solid and the crystals have a remarkably uniform
orientation
• The source of Earth's magnetic field
AdaptedfromAlbarède(2003)
The overall composition of the Earth
• The ratios of elements and isotopes in Earth roughly correspond to
chondritic meteorites
• The average composition of Earth will be similar to the average
composition of chondrites
• Conversion to individual parts of the Earth based on density and
elastic properties – different approaches, e.g.:
– The core corresponds to the diameter of FeNi in chondrites
– The silicate part of the Earth (bulk silicate earth = BSE) representing
mantle and crust corresponds to silicates in chondrites
– Problem: which meteorite should be used as default? How should the
average be weighted?
– Other studies demonstrate that the composition of contemporary
meteorites does not correspond to the Earth and the material that
dominantly formed it.
• Therefore, estimating the composition of the Earth from meteorites
is problematic without knowledge of a meteorite that clearly
represents the original material
The overall composition of the Earth
• The second approach is to combine the average
composition of differentiated parts of the Earth
– We know the atmosphere, the hydrosphere and the
crust
– The composition of the mantle and core depends on
the used model
• The data presented correspond to a combinationbased
procedure
1. BSE estimation based on the composition of basalts,
ophiolites, komatiites and xenoliths, including the
effects of core formation, etc.
2. Volatility trend (the process of condensation of
elements from the nebular disk)
Adapted from Misra (201
Adapted from Misra (201
90% of the Earth is
made up of only 4
elements:
Fe, O, Si, Mg
Adapted from Misra (201
90% of the Earth is
made up of only 4
elements:
Fe, O, Si, Mg
3 other elements
make up more
than 1% of Earth
Ni, Ca, Al
Adapted from Misra (201
90% of the Earth is
made up of only 4
elements:
Fe, O, Si, Mg
3 other elements
make up more
than 1% of Earth
Ni, Ca, Al
Another 10
elements make up
0.1-1% of Earth
Na, P, S, Cl, K, Ti, V,
Cr, Mn, Co
Adapted from Misra (201
90% of the Earth is
made up of only 4
elements:
Fe , O, Si, Mg
3 other elements
make up more
than 1% of Earth
Ni, Ca, Al
Another 10
elements make up
0.1-1% of Earth
Na, P, S, Cl, K, Ti, V,
Cr, Mn, Co
99.9% of the Earth
is made up of 17
elements
RelativeabundanceofelementsintheEarth'suppercrust.Notshown:NobleGases,Tc(43),Pm(61),andallelementsafterBi(83),exceptforTh(90)&U(92).
ByGordonB.Haxel,SaraBoore,andSusanMayfieldfromUSGS;vectorizedbyUser:michbich-http://pubs.usgs.gov/fs/2002/fs087-02/,PublicDomain,
https://commons.wikimedia.org/w/index.php?curid=11215468
Composition of the Earth
Crust
cont.
Crust
ocean
Mant
le
Core
Meteorite
s
SiO2 69 48 43 – 33,32
Al2O3 14 15 – – 2.41
Fe2O3
+ FeO
4 11 12 90 35,47
CaO 4 11 3 – 2.3
MgO 4 9 37 – 23.5
NiO – – – 8 1.9
Other 5 6 5 2 1.1
In total 100 100 100 100 100
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
0
0
0
0,01
0,1
1
10
100
1000
10000
1E+05
1E+06
1E+07
1E+08
1E+09
1E+10
1E+11
0 5 10 15 20 25 30
Atomové číslo
atmů/106
atomůSi
Chondrit Země Kůra Vesmír
Composition of the Earth - comparison
Earth Crust Crust Crust
Element Mass % Mass % Atomic % Volume %
O 30 46 61 90.95
Si 15 28 20 0.84
Al 1.1 8 6.2 0.46
Fe 35 6 1.9 0.5
Mg 13 4 1.4 0.38
Ca 1.1 2.4 1.9 1.44
K – 2.3 1.8 1.19
Na – 2.1 2.5 1.08
Other – < 1 – –
S 1.9 – – –
Ni 2.4 – –
In total 99.5 98.8 96.8 96.84
Composition of the
Earth's crust
0
5
10
15
20
25
30
35
40
45
bazalty…
granity…
ruly
břidlice
mramory
jíly
pískovce
vápence
hornina
obsah(objemová%)
0
5
10
15
20
25
30
35
40
45
50
O
Si
Al
Fe
Ca
Mg
Na
K
prvek
obsah(hmotnostní%)
0
5
10
15
20
25
30
35
40
45
plagioklasy
křemen
živce
pyroxeny
slídy
amfiboly
olivín
jílovém.
magnetit
kalcit
dolomit
minerál
obsah(objemová%)
GENESIS AND DIFFERENTIATION OF
THE GEOSPHERE
The formation of the Earth
Planetary
accretion
• The protoplanetary disk
cools
• Condensation
• Composition of forming
bodies determined by
the distance from the
Sun
– Refractory substances (rock
planets)
– Volatiles (gas and ice giants)
• The main phase of
planet formation took
place during the first 30
Ma
The formation of the Earth
• Formed shortly after formation of the Solar
System
– Comparison of W isotopes in the mantle and meteorites
ByPublicDomainPictures-
http://pixabay.com/en/mercury-venus-earth-
mars-jupiter-163610/,CC0,
https://commons.wikimedia.org/w/index.php
?curid=37984303
The formation of the moon
• Giant impact theory
– Earth's edge-on collision
with a Mars-sized body
(Theia)
– Chondritic composition
– The Earth has about 90%
of its final mass before the
collision
– Both bodies differentiated
into a metal core and a
silicate mantle
• Age based on isotopes
4.48 ± 0.02 Ga
Public domain (NASA)
The formation of the moon
• The Moon is formed from the mantle material of an impactor ejected into space around the Earth
• Formed in 10-100 years
• A metal-depleted moon
– 8-12% Fe compared to 31% on Earth
– Theias metal core incorporated into Earth (10% mass)
Geochemistry of the Moon
• The isotopic composition of oxygen
corresponds to that of the Earth
– Originating at the same distance from the Sun?
– Blending on impact?
• The energy released by the collision would
melt the released material
– The moon is formed as a molten body – a
magmatic ocean
– The origin of a magmatic ocean on Earth too?
Accretion models
• Models of the formation and division of the
Earth into mantle and core
• Two basic views
1. Homogeneous
2. Heterogeneous
• Both models assume formation from meteoric
material from the asteroid belt
• The difference lies in the mechanism of
division into silicates and metals
Homogeneous accretion
• Condensation of material from the
protoplanetary disc was completed
before accretion began
• The composition of the resulting
material is a function of distance from
the Sun – the further away, the more
volatile the material, the excess gas
carried away
• The earth was then created by
accretion as a cold, homogeneous and
undifferentiated mixture of silicates
and metals
• Subsequent melting (meteor
bombardment, gravitational
compression, radioactive decay)
• The molten material has split into the
core and the mantle – the heavier
material sinks to the bottom, the
lighter remains on top
• The model explains the density distribution
well
• Problems
• It is unlikely that a planet the size of
Earth would attract condensates
corresponding to only one
temperature
• Venus, Earth and Mars shouldn‘t have
atmosphere – no volatiles would be
present at the appropriate
condensation temperature
Adapted from Misra (2012
Heterogeneous accretion
• The gradual growth of the planet
by simultaneous accretion and
condensation of material as the
original nebula cools
• The result is a stratified planet:
– The most refractory material is
in the middle (Fe+Ni) because it
condensed first
– The less refractory silicates
follow
– Finally, the atmosphere (and
hydrosphere) from volatile
phases
• The model does a good job of
explaining the expanding material
source zone as the planet grows
• A widely accepted theory, but
problems arise
• The contents of siderophilic
elements in the mantle do not
correspond to expectations
• The zonality of a planet cannot
be estimated from
condensation temperatures
Adapted from Misra (2012
Magmatic ocean
• We assume that the Earth
was melted on a large
scale at least once
• Melting homogenized the
composition created by
heterogeneous accretion
– most features were not
preserved
• Drops of molten metal,
sink to the bottom where
they form larger
formations and descend
through the solid mantle
into the core
AdaptedfromMisra(2012)
Primitive cloak
• The composition
corresponds to BSE
Earth at the beginning of the Archean
• Thick basaltic crust
• Ocean
• Small extent of
land (domestic
granitoids)
• Atmosphere with
CO2
• Sun 25%
brightness
• 1 day = 10 hours
BySimoneMarchi(SwRI),SSERVI,NASA
Formation and development of the crust
• From a comparison with other planets, we
assume a basaltic primordial crust similar to the
current oceanic crust
• No leftovers available
• Probably formed soon after accretion
• Warmer upper mantle – faster cycle of formation
and destruction
– The question of plate tectonics is not clarified
– The oldest preserved oceanic crust ca. 1 Ga
– Xenoliths indicator of some form of rock circulation
Early continental crust
• Oldest continental crust
from partial melting of
primitive mantle
• The oldest preserved crust
3.8-4.0 Ga
• The oldest minerals – 4.4 Ga
old zircons , probably from
granitoid rocks
– The O isotopes within indicate
the existence of oceans
ByJohnValley,UniversityOfWisconsin-Madison
Zircon from the Jack Hills region of
Western Australia - the oldest
known mineral on Earth.
Bark composition and growth
The composition of the oceanic and
continental crust relative to the
composition of the primitive mantle shows
a surprisingly complementary evolution,
where the continental crust is significantly
enriched with incompatible elements.
Schematic model of continental crust
growth and development on a global scale.
Adapted from Misra (2012
Adapted from Misra (2012)
FORMATION AND EVOLUTION OF
MAGMA
And a little about volcanism
Formation of magma
• The basic environment of magma formation from the point of view of plate
tectonics
1. Constructive board edges
2. Destructive board edges
3. Oceanic intraplate environment
4. Continental intraplate environment
Adapted from Misra (2012)
Causes of melting
• The mantle is made up of solid rock – only approx. 1%
in the asthenosphere is melt
• The main source of melt is decompression melting and
the drop in temperature of the solidus (melting point
of the rock) due to the presence of volatile substances
• Beneath oceanic rifts, melt is produced by a drop in
pressure on upwelling peridotite
– Similarly, intraplate magmatism
• In subduction zones, the descending plate is heated,
water is released from hydrated minerals and partial
melting is initiated by the resulting fluids
Hot spots
Partial melting
• Formation of basaltic magma by
melting of the upper mantle
(olivine composition)
• Olivine is heated and reaches the
temperature on the solidus curve
(c1)
• A melt with a composition of m1 is
formed; with a further increase in
temperature, olivine continues to
melt and amount of molten
forsterite increases – the melt
moves to m2 , the remaining
crystals have a composition of c2
• The complete transition to the
melt occurs at a temperature over
1800 °C (m3)
• The gap between the onset of
melting and complete melting is
crucial for magma genesis
• Simple melting occurs only for end
members that do not behave like
solid solutions
Adapted from Gill (201
Partial melting
• The result is a magma
(m2) poorer in Mg
than the source rock
(c1) and a refractory
residue richer in Mg
(c2)
• What does the
composition of the
resulting phases
depend on?
Adapted from Gill (201
Partial melting
• Mixtures of minerals melt earlier than
individual minerals
• All minerals from the beginning of melting
contribute to the composition of the resulting
melt (magma)
• Minerals don't melt one by one!
Decompression melting
• Mid-ocean ridges
• The material in the solid mantle
performs convective movement
under the influence of temperature
and the related density difference
• Movement to the surface will cause
a drop in pressure on the rock and a
transition across the solidus curve
(X–Y line)
• There is no need to supply heat for
melting – on the contrary, the
emerging material is cooled by the
expansion work performed
• Intraplate volcanism
• Combined effect of warmer material
transport and decompression
AdaptedfromGill(2015)
Primary magma
• Melting of the mantle produces basaltic
primary magma
• Processes of differentiation and mixing give
rise to other advanced types of magma
• Properties determined by origin, intensity and
mechanism of partial melting
– Dependent on tectonic position
• The variety of igneous rocks is related to the
origin and differentiation of the magma
Trace elements in magma . processes
• An important source of
information on
additional processes
• E.g. rare earth elements
• Fractionation into
melt/crystals
• Equilibrium
fractionation
• Rayleigh fractionation -50
-45
-40
-35
-30
-25
-20
-15
-10
-5
0
00,20,40,60,81
f (zbývající pára)
δ(‰)
α = 1,01
Rayleighova frakcionace
rovnovážná kondenzace
Lava
• Rock melt on the Earth's surface
• Forms parts of volcanoes, lava
flows, etc.
• The way lava flows and thus the
geometry of lava formations
depends on viscosity
Kīlauea (Hawaii)
Arenal Volcano (Costa Rica)
ByMatthew.landryandEnglishWikipedia,CCBY-SA2.5,
https://commons.wikimedia.org/w/index.php?curid=4398466
Melt viscosity
• It increases with the degree of polymerization
of silicates
– Acidic melts (rhyolites) – more viscous
– Basic melts (basalts) – less viscous
• Shorter polymers "flow better"
• Viscosity is drastically reduced by the
presence of water
H 2 O + O 2– = 2 OH –
Volatile components
• The role of other volatile
components is also crucial
for viscosity
• H2O, CO2, H2, HCl, N2, HF, F2,
Cl2, SO2, H2S, CO, O2, NH3, S2,
He, Ar
• Critical point –> supercritical
fluid
– At depths below 1 km, the
difference between the liquid
and gaseous state disappears
– water: 21.8 MPa, 371 °C
– CO2: 7.3 MPa, 31 °C
• Fluid pressure
ByMatthieumarechal,CCBY-SA3.0,https://commons.wikimedia.org/w/index.php?curid=4623701
Solubility of volatile components in
silicate melts
• Melt with dissolved volatiles = melt + volatiles
V t+f < V t + V f
• A large amount of volatile substances are
stable in the melt only at high pressures
Separation of volatile substances from
the melt
• Pressure drops with upward
movement.
• The stability of dissolved volatile
components decreases.
• Unsaturated magma ->
saturated magma ->
supersaturated magma.
• The excess is separated into a
fluid phase – exsolution, boil
• Crystallization of anhydrous
minerals under constant p –>
fluid supersaturation &
retrograde, secondary boiling; it
can also occur when the
temperature drops!
Eruption
• The leakage of
volatile substances
will also cause an
increase in viscosity
– Polymerization
– Presence of bubbles
• Highly viscose
magma leads to
explosive eruptions
Plinian eruption
Stromboli
Pahoehoe(Hawaii)
Summary
• The Earth's interior is made up of concentric shells differing
in density and composition: core (liquid outer and solid
inner), mantle and crust (continental and oceanic)
• Differentiation into primitive shell (BSE – bulk silicate Earth)
and metallic core is best explained by magmatic processes
under high PT conditions in a magmatic ocean
• The oldest oceanic crust is 200 Ma old, the primary crust
was basaltic and was recycled by the same processes that
affect oceanic crust today
• The oldest continental crust is 4.4 Ga old and was formed
by partial melting of the primitive mantle, growing
episodically by gradual accumulation
• Primary magma is formed in the upper mantle by partial
melting of mantle rocks, especially by decompression
melting
Tento učební materiál vznikl v rámci projektu Rozvoj doktorského studia
chemie
č. CZ.02.2.69/0.0/0.0/16_018/0002593
Resources
• Images without a specified source are public domain,
with a free license or copyright or used with the
permission of doc. Zeman.
• Albarède, F. (2003). Geochemistry: an introduction.
Cambridge: Cambridge University Press. ISBN 0-521-
89148-5.
• Gill, R. (2015). Chemical Fundamentals of Geology and
Environmental Geoscience . 3rd Edition . John Wiley
and Sons . 288 p. ISBN: 978-0-470-65665-5
• Misra, K. (2012). Introduction to geochemistry:
principles and applications. Wiley-Blackwell. 438 p.
ISBN 978-1-4443-5095-1.