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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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