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LU1 INTRODUCTION TO GEOCHEMISTRY

◊INTRODUCTION

“The primary purpose of geochemistry is on the one hand todetermine the composition of the earth and its parts and on theother to discover the laws which control the distribution of individual elements” 

 –  V. M. Goldschmidt, 1933.

Geochemistry deals with the

• Abundance and distribution of the elements and their isotopes.

• Distribution between core / mantle / crust / hydrosphere / atmosphere

• Geochemistry is the study of the sources and fates of chemical species

in natural environments and their evolution through Earth history.

• Geochemistry is a set of tools for helping to understand the Earth; thesetools are based upon chemical, instead of the physical observations.

• Viewing natural phenomena and natural materials at the chemicallevel gives us insights into the underlying processes that can often not beascertained from other forms of observation.

• Understanding something about the chemistry of matter and theparameters that affect it help us to explain how a natural environmentfunctions.

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The field of geochemistry involves study of the

• C hemical  composition of the Earth and other planets,

• Chemical processes and reactions that govern the composition of rocks and soils, and

• Cycles of matter and energy that transports the Earth's chemicalcomponents in time and space.

The most important fields of geochemistry are:

1. Determination of the relative and absolute concentrations of theelements and their isotopes in the earth and on earth´s surface.

2. Examination of the distribution and movements of elements in

different parts of the earth (crust, mantle, hydrosphere etc .) and inminerals with the goal to determine the underlying legalities of 

distribution and movement.

3. Analysis of the distribution of  elements and their isotopes in thecosmos (cosmochemistry).

4. A study of the role of processes and compounds that are derived fromliving or once-living organisms (organic geochemistry)

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◊HISTORY OF GEOCHEMISTRY 

• The term “geochemistry” was first used by the Swiss chemist Schönbein

in 1838.

• You might guess, merely from the etymology of the word, that the field of geochemistry is somehow a marriage of the fields of  geology and

chemistry . That would be a good guess.

• Geochemistry really couldn’t develop as a science until both chemistryand geology had developed into mature sciences.

• But just how are chemistry and geology combined within geochemistry;what is the relationship between them?Perhaps the best explanation would be to state that in geochemistry, we

use the tools of chemistry to solve geological problems; that is, we usechemistry to understand the Earth and how it works.

 • The Earth is part of a family of heavenly bodies, our Solar System, that

formed simultaneously and are closely related. Hence, the realm of geochemistry extends beyond the Earth to encompass the Solar System.

 • Geochemistry is a very broad topic. So broad in fact that no one can

really master it all; geochemists invariably specialize in one or a fewaspects, such as atmospheric chemistry, geochemical thermodynamics,isotope geochemistry, marine chemistry, trace element geochemistry etc.

• Much of the early development of geochemistry occurred in the thenUSSR as an outgrowth of geochemical prospecting (AlexanderFersman and V.I. Vernadsky).

• Geochemists then adopted theoretical and experimental approaches (the latter best exemplified by Norman L. Bowen of the GeophysicalLaboratory of the Carnegie Institution of Washington, the Father of Experimental Petrology).

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Geochemistry has both pure and applied components.

Pure Geochemistry: concerned with “How and Why did the Earth andSolar System reach their current chemical state?” Current research topicsinclude:• Chemical cycles:

Fluxes of elements between earth’s reservoirs, especially between crustand mantle, and of greenhouse gases between atmosphere andlithosphere

• Paleoclimates:Application of stable isotope thermometry

• Astrobiology:Despite the name, the search for life on other planets involvesgeochemistry in a very significant way.

Applied Geochemistry: benefits humanity in many ways, including:• Geochemical prospecting: search for natural resources (fossil fuels, ore

deposits)• Environmental geochemistry: behavior of anthropogenic contaminants in

the environment (air and water pollution)

Note:

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◊ELEMENTS

• Generally, an element is a basic part that is the foundation of something.• For a long time, elements (classical element ) were believed (by the

pythagoreans and alchemists for example) to be the building blocks of allmatter in the universe.

• In chemistry, an element (chemical element ) is a substances that cannot be separated into simpler substances and that singly or in combination constitute all matter.

The elements never created or destroyed. The Earth can be envisioned as arecycling machine that redistributes these elements.

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Classification of the Elements

1. The Periodic Table.This is the most universal classification.Elemental Grouping: These elements may be bracketed together intoseveral chemical grouping. This grouping also serves as a useful guide tothe geochemical behavior of the elements

Widely Used Terms for Element GroupingsA. Alkali Metals,

B. Alkaline Earths,C. Transition Metals/Heavy Metals,

D. PGE's, (Platinum Group Elements) E. Halogens,

F. Noble (Inert) Gases,G. REE's (Rare Earth Elements),

H. Actinides,

I. Metalloids,J. Precious Metals

 1H

 F2

He

A3Li

B4

Be 

C5B

C6C

C7N

C8O

E9F

10Ne

11Na

12Mg

(Transition Metals/Heavy Metals in Bold) 13Al

14Si

I15P

16S

17Cl

18Ar

19K

20Ca

C21Sc

C22T

C23V

C24Cr

C25Mn

C26Fe

C27Co

C28Ni

C29Cu

C30Zn

31Ga

32Ge

I33As

I34Se

35Br

36Kr

37Rb

38Sr

39Y

40Zr

41Nb

42Mo

 D44Ru

D45Rh

D46Pd

J47Ag

48Cd

49In

50Sn

I51Sb

I52Te

53I

54Xe

55Cs

56Ba

57La

72Hf 

73Ta

74W

75Re

D

76Os

D

77Ir

D

78Pt

J

79Au

80Hg

81Tl

82Pb

83Bi

84Po

85At

86Rn

87Fr

88Ra

89Ac

 

GREE

58Ce

59Pr

60Nd

  62Sm

53Eu

64Gd

65Tb

66Dy

67Ho

68Er

69Tm

70Yb

71Lu

 

H

Actinides

90

Th 

92

U 

However, for some specific studies, we have other classifications to helpus understand how the elements behave.

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Important Ocean Elements

Important Atmospheric Elements

Important Biological Elements

Important Lithospheric Elements

2. Geochemical Classification

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(based on which part of Earth the element tends to reside in)Goldschmidt (1922), the father of geochemistry, suggested aclassification based on where elements were usually found. Onlyqualitative, but this classification works because of the similarities inelectronic configuration within the groups:

Lithophiles - found in rocksFound as silicate minerals(Form ionic bonds with oxygen,Have valance electrons that are outside shell of 8 electrons)

Siderophiles - found in metalsFound as native elements(Valence electrons are complete s subshell, not available forcombination)

Chalcophiles – found in sulfide depositsFound as sulfides(Form covalent bonds with sulfur,Valence electrons are outside a shell of 18 electrons)

Atmophiles – found in the atmosphereFound as gases (inert gases, halogens, C, N, O)These are shown as below.

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The Geochemical Classification of the Elements

 ALithophile

BLarge IonLithophil 

e

C HFI 

DChalcophile

 E 

Siderophil e

F  Atmophile

F1H

 F2

He

3Li

4Be

 C5B

E6C

F7N

A8O

A9F

10Ne

A11

Na

A12

Mg

 A13

Al

A14

Si

E15

P

D16

S

17Cl

18Ar

B19K

A20Ca

A21Sc

A22T

A23V

A24Cr

A25Mn

E26Fe

E27Co

E28Ni

D29Cu

D30Zn

D31Ga

E32Ge

D33As

D34Se

35Br

36Kr

B

37Rb

B

38Sr

B

39Y

C

40Zr

C

41Nb

42Mo

 

E

44Ru

E

45Rh

E

46Pd

D

47Ag

D

48Cd

D

49In

E

50Sn

D

51Sb

D

52Te

53I

54Xe

B55Cs

B56Ba

B57La

C72Hf 

C73Ta

C74W

E75Re

E76Os

E77Ir

E78Pt

E79Au

D80Hg

D81Tl

D82Pb

D83Bi

D84Po

85At

86Rn

B87Fr

B88Ra

B89Ac

 

REEB58Ce

B59Pr

B60Nd

 B

62Sm

B53Eu

B64Gd

B65Tb

B66Dy

B67Ho

B68Er

B69Tm

B70Yb

B71Lu

 

Actinides

C

90Th

 

C

92U

 

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Geochemical Affinity

• In the classification scheme of Goldschmidt, elements are divided according to

how they partition between coexisting silicate liquid, sulfide l iquid, metallic

liquid, and gas phase…defined by examining ore smelting slags and meteorites

Silicate Liquid

Sulfide Liquid

Metallic Liquid

Gas Phase

Siderophile

Chalcophile

Lithophile

Atmophile H, He, N, Noble gases

Alkalis, Alkaline Earths,

Halogens, B, O, Al, Si, Sc, Ti,

V, Cr, Mn, Y, Zr, Nb,

Lanthanides, Hf, Ta, Th, U

Cu, Zn, Ga, Ag, Cd, In, Hg,

Tl, As, S, Sb, Se, Pb, Bi, Te

Fe, Co, Ni, Ru, Rh, Pd, Os, Ir,

Pt, Mo, Re, Au, C, P, Ge, Sn

• To first order, the distribution of elements between core and mantle resembles

equilibrium partitioning between metal liquid and silicates…conf irmed by iron

and achondrite meteorites (but at high P, no separate sulfide phase)

• Melting a chondrite gives 3 immiscible liquids plus vapor:

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◊SOLAR SYSTEM COMPOSITE ABUNDANCE CURVE

Main features:

1. H and He are the most abundant elements.2. Li, Be, and B have very low abundance because of low stability3. Very high abundance of Fe 4. Abundance does not change much for heavier nuclides (Z>40) 5. Zig-zag pattern:

In terms of elements, elements with even Z are more abundant thanthose with odd Z.

6. The general decline in abundance as Z increases is interrupted by a sizablepeak around Z=26, comprising elements in the neighbourhood of iron.

Relative Abundances in the Solar System and Universe

o abundances of the first 50 elements decrease exponentially with

atomic number

o abundances of the heavier elements independent of atomic number

o H and He are the most abundant elements.In atomic terms, He has one-tenth of the abundance of H and togetherthey comprise 98% of the solar system.

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o progressing to higher atomic numbers leads to an overall decrease

in abundance, making the heaviest nuclei among the least abundanto note anomalously low abundance of Li, Be, B as compared to other light

elementso note anomalously high abundance of Fe

o

zig-zag pattern:Elements having even atomic numbers are on average about ten timesmore abundant than elements with similar but odd atomic

numbers.

References:

Brownlow, A.H., 1979, Geochemistry, Prentice Hall.Krauskopf, K.B. & Bird, D.K., 1995, Introduction to Geochemistry, WCB

McGraw HillMason, B. and Moore, C.B., 1989, Prinsip-Prinsip Geokimia, DBP.

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