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Modul B.Geo. 109 Vorlesung :
For skripts to part 1 see :
http://134.76.75.184
1. „Hot“ Geochemistry: igneous processes and planetary differentiation 2. Low-temperature geochemistry: Surface processes, soils and weathering
3. Organic geochemistry : The geochemistry of life and its signals in rocks
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70605040
Cox et al. (1979)
SiO2
phonolites
trachytes
benmoreites
mugearites
hawaiites
basalts andesites
dacites
rhyolites
Nomenclature based on major elements
4
6
8
10
12
14
2
16
70605040
SiO2
basalts
The behaviour of trace elements in partial melting and weathering
2.0
1.5
1.0
0.5
01 2 3 4 5 6
Wertigkeit
Na
Li
Sr
Ca
Mn Fe
Co Ni
Mg
La
EuLu
NdY
Th
U
Zr, Hf
Nb Mo
ScCuVCrGa
Al
Ti
Ge
SiBSe
CsRbTl
K Sc
LILEmobil in Wasser
HFSEimmobil in Wasser
2.0
1.5
1.0
0.5
01 2 3 4 5 6
Charge
Na
Li
Sr
Ca
Mn Fe
Co Ni
Mg
La
EuLu
NdY
ThU
Zr, Hf
Nb Mo
ScCuVCrGaAl
Ti
Ge
SiBSe
CsRbTl
K Ba
LILEmobile in water
HFSEimmobile in water
Mg
Si Ti
AlFe
Sm
YbTmTbGdLu
Na
SrLa
UTh
Ba
Nd
Fe
Mn
Sc
Ca
Ce
Ca
0,001
0,01
0,1
1,0
10
K
1,01A
0,4 0,6 0,8 10 12 14 16
Ionic Radius, A
0,79 A
( Henderson 1984 )
In
Partition coefficients are controlled by the size and charge of the atoms and the available sites in the crystal lattice. Partition coefficients are determined (1) empirically by analyzing minerals in matrix in natural rocks, (2) by experimental crystal-melt equilibrium and (3) by calculations based on the crystal-lattice thermodynamic model of Blundy and Wood (1994, 2003). See GERM - Website for reference. Blundy, J., Wood, B. 1994. Nature, 372, 452-454. Blundy, J., Wood, B. 2003. Earth Planet Sci. Lett. Frontiers, 210, 383-397.
1.3
1.2
1.1
1.0
0.9
57 60 65 70
The elementpromethium(Pm, Z=60) has nostable isotope
Europium has twostable valence states:II and III, Eu2+ isKnown to occur inigneous melts underlow fO2 conditions, and
substitutesfor Ca2+ inplagioclase.
Cerium is the only REEto form a IV valence state(in oxidizing sedimentaryenvironments)
La Ce3+
PrNd
SmEu3+ Gd
Tb DyHoEr Tm
YbLu
Y3+
´LANTHANIDECONTRACTION´
Ionic radii fromWhittaker andMuntus
AtomicnumberZ
Eu2+
Ce4+
Gill (1996), p.231
400
100
50
10
1
0
NdCe SmEuGd DyErYb
Clinopyroxene
Zircon
Garnet
Apatite
Hornblende
Hypersthene
Biotite
Oivine
Partition coefficients for typical minerals in igneous petrogenesis
0,1 0,1 NdCe Sm Eu GdDyErYb
Anorthoclase
Plagioclase
K-Feldspar0,05
0,01
1
4
00,1
0,005
0.5 1.00
BATCH PARTIAL M ELTING
F
D = 10D = 4D = 2
FRACTIONAL M ELTING
Cl/C
0= F(D-1 )
0.50
F
0.01
100
0.01
0.1
1.0
D = 10
D = 4
D = 2
D = 1
D = 0.75
D = 0.25
D = 0
1
Cl/C0 = 1/[D(1-F)+F]
0.1
10
D = 1
D = 0.75
D = 0
1
10
100
D = 0.25
Incompatible elements fractionate only during low degrees of partial melting (unless accessory minerals are involved). Incompatible elemt concentration decreases with increasing degree of partial melting.
Cl/Co=1/D*(1-F)(1/D-1) Cl/Co=1/(F+D-F*D)
Batch partial melting Fractional crystallization Cl/Co=1/(F+D-F*D) Cl/Co=F(D-1)
00,10,20,30,40,50,60,70,80,91
F
0
2
4
6
8
10
12
14
16
18
20
Batch partial melting
Incompatible elements fractionate only during low degrees of partial melting (unless accessory minerals are involved). Incompatible elemt concentration decreases with increasing degree of partial melting.
Incompatible elements fractionate only during low degrees of partial melting (unless accessory minerals are involved). Incompatible elemt concentration decreases with increasing degree of partial melting.
Incompatible elements fractionate only during low degrees of partial melting (unless accessory minerals are involved). Incompatible elemt concentration decreases with increasing degree of partial melting.
0 20 40 60 80 100
50%
0%20% 10%
30%40%
PARTI AL
M ELTING
FRACT IONAL
CRYSTALLI ZATI ON
0
20
40
60
80
100
R (D = 4) ppm
15%
30%
40%
10%
20%
Partial melting and crystallization are not reversible processes for trace elements ! The algorithms are different to describe these processes and the melt/crystal mass rations are different.
Cl/Co=F(D-1)
Fractional Crystallisation:
Batch Partial Melting: Cl/Co=1/(F+D*F-D)
Batch
10
100
La Ce Nd SmEuGd Dy Er YbLu
Primi ti ve mantl e source
10%
5%
2%
1%
0.1%
Roll inson (1993)
Chondr i te-normal ized REE patterns calcu lated for
moda l batch melting of a primitive
mantle sou rce wi th ca. 2.15 condri tic concentra-tion of REE and with the mineralogy -
oli vine 55%, orthopyroxene 25%, cl in opyroxen11% and gar net 9%.
Incompatible elements fractionate during low degrees of partial melting. Thus, incompatible trace element ratios change in magma series from the same source, but only at small degrees of melting. At large degrees of melting (>5 to 10%) trace element rations will not change (much) during melting and can be used to distinguish different magma sources.
90%
70%
50%
30%
Komati i te-336
Chondri te-normalized REE pattern s cal culated for ol ivin e fr action ati on
fr om a komatiite melt (komatiite-336) at 30%, 50%, 70% and 90%
fr action al crystall ization .Rol l i nson (1993)
10
100
La Ce Nd SmEuGd Dy Er YbLu
Incompatible elements do not fractionate during crystallization (unless accessory minerals are involved) Thus, incompatible trace element ratios do not change in magma series and can be used to distinguish magma suites from different degrees of melting or different sources.
Komatiite =ultra-mafic magma formed by high degrees (>30%) of partial mantle melting that was possible only during Archean and Proterozoic times
Ta (ppm)
0 1 2 3 4 5 6 7 8 9 10
10
8
6
4
2
0
Inkompatible Elemente
Titanit-Fraktionierung
Incompatible elements
Incompatible elements do not fractionate during crystallization (unless accessory minerals are involved) Thus, incompatible trace element ratios do not change in magma series and can be used to distinguish magma suites from different degrees of melting or different sources.
La/Sm
La (ppm)
0 5 10
2.0
0
1.0
Island
Reykjanes Rücken
fraktionierte Kristallisation
“Chemical Geodynamics”
The relation between crust and mantle geochemical reservoirs is mostly based on our knowledge on the geochemical composition of
continental crust and mantle-derived basalts
... and thus
starts with a review of Basalt Classification and Petrogenesis and geochemical discrimination diagrams
Basalt Classification
• Basalt is the most common volcanic rock at the Earth’s surface.
• How do you safely recognize a basalt ? • How do you distinguish between a primitive and a primary
basalt ? • How do you distinguish basalt types (tholeiites, alkali
basalts, arc basalts, MORB, OIB, IAT CAB ? • How about exotic mantle-derived magmas : kimberlite,
boninite, komatiite,carbonatite, and other extreme compositions
In late 1960’s it was recognized that basalts formed in several plate tectonic environments.
Basalts and Plate Tectonics
Alk MgO
FeO*
Calc-Alkaline
Tholeiitic
Myoshira suggests simple plots of certain major element oxides could provide a better basis for discrimination.
AFM diagram could provide a sensitive means of differentiating island arc (calc-alkaline and MORB (tholeiitic) basalts.
Alk = Na2O + K2O
MgO = MgO
FeO* = FeO + Fe2O3 + MnO
This works well for Japan, but has shortcomings:
• does not discriminate OIBs at all
• does not allow for the effect of continental crustal contamination
Myoshira – AFM Diagram
• Advantages: – Much more sensitive as concentrations vary over
orders of magnitude – A lot more of them and some are compatible with
basalts and some incompatible • Disadvantages
– Require a much more sophisticated analytical apparatus
First trace element basalt classifications are simple
and utilize few elements
Minor and Trace Elements as Petrogenetic Indicators
MnO*10 P2O5*10
TiO2
CAB
IAT
MORB
OIT
OIA
Mullen diagram plots the minor elements TiO2, MnO and P2O5.
Discriminates a variety of plate tectonic environments and basalt types:
• OIT = ocean island tholeiite
• MORB = mid-ocean ridge basalt
• OIA = ocean island alkaline
• IAT = island arc tholeiite
• CAB = continental arc basalt
Mullen Diagram
Zr Y*3
Ti/100
C
D A
B
Island- arc A,B
Ocean-floor B
Calc-alkali B,C
Within-plate DD
One of many Pearce diagrams. This one uses Ti, Zr and Y. Does a good job of discriminating basalts. Ocean floor = MORB; calc alkali = OIB
Pearce Diagram
Use and abuse of Ternary Discrimination diagrams
The basic problem with such diagrams is that the basalts often don’t plot in fields that are consistent with geologic evidence (i.e. a basalt that can be directly tied to a MORB origin ends up plotting as a CAB). Too many geologists have obtained geochemical data from geochemist, plotted these in ternary cassification diagramms ...., and failed badly with their interpretation ! GeoComics....
• Solution 1) a better understanding of what tace element patterns really mean (alteration, melting, fractional crystallization, source differences). • Solution 2) use more and more sensitive elements, construct “spider diagrams” (better “trace element patterns”) • Don’t consider recent papers with ternary discrimination diagrams, it may too simplistic for its geochemistry !
Along came the “Spider” ...
• Spider diagrams have become the norm. • They display a spectrum of trace and minor
elements. • Elements are selected to cover a range
from incompatible to compatible. • They can be compared easily against any
standard.
Ocean island basalt plotted on a mid-ocean ridge basalt (MORB) normalized spider diagram of the type used by Pearce (1983). Data from Sun and McDonough (1989).
Pearce-Spider Diagram : separate LIL and HFS elements, order in increasing incompatibility towards the center of the diagram
Spider diagram for oceanic basalts
increasing incompatibility Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989).
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