geochemie i - 2 igneous processes

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

� �

� �

�+

� '

� #� )

�� #

� $� 2

� 3� #

�%� +

!" 2

�#�� 5�(

� #� '

�� 2

�$

� 0

� #�

�2

�&

� /� '

� %�5

�'� 3

� .�.

� 0�+

� *� &

�2� 4

� 5�&

� 5"/

)�&

5�)

�$ 7

� #� '

�/

�/

�-

�$

� 0� 2

3� '

�$� '

� +� 0

� .! $

� 2

�

4

� 5

��

-�+

��

��

�

� -

� �

� �

� �

� �

��

��

��

��

� �

��

� �

��

�����

��

��

� ���� ��� �

��

��

��

� �����

� ���

� ����

���

� ���

��

� �� ���

��� ��

��

��

��

��

� ���

��

��

� ���

� �

� �

� �� �

� �

�

�

��

����

����

� �� �

�

��

� ���

�� �

%� *�#��1� 5 .� .� ,� (� 3� .� &�0

� 6� �� �

%�2

� '

� '

2

� 2

'

� /

� #��

� �

�

��

4

6

8

10

12

14

2

16

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).