Engineering Geology - Geochemistry

7/25/2019 Engineering Geology - Geochemistry

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Republic of the Philippines

Technological University of the Philippines

Km. 14 East Service Road, Western Bicutan, Taguig City 1630

A TECHNICAL REPORT ENTITLED, ENGINEERING GEOLOGY: GEOCHEMISTRY

UNDERTAKEN AT TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES TAGUIG

CITY OF TAGUIG, MANILA, PHILIPPINES

A Report Submitted to

ENGR. ADOLPHCOLLIE Z. TANTE

Civil Engineering Department

COLLEGE OF ENGINEERING

TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES

Taguig Campus

City of Taguig, Philippines

In Partial Fulfillment of the Requirements for the Subject

ENGINEERING GEOLOGY

of the Course

BACHELOR OF SCIENCE IN CIVIL ENGINEERING

Agui la, Mary Joy S.

Carabbacan, Aaron Paul I.

Durante, Ajielyn C.

Hisona, Mariel T.

2015


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GEOCHEMISTRY | ENGINEERING GEOLOGY | J uly 8, 2015

GEOCHEMISTRYOF THE C ONSTANT-C HANGING EARTH


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TABLE OF CONTENTS

Table of Contents ................................................................................................ i

Foreword ............................................................................................................ iv

Introduction to Geochemistry ........................................................................... 1Definitions and Func tions ............................................................................................................... 1

Fields & Disciplines ........................................................................................................................... 1

Review of Fundamental Chemistry ................................................................... 3

The Physical Properties of Elements ............................................................................................. 3

The Periodic Table of Elements ..................................................................................................... 3

The Chemical Properties of Elements .......................................................................................... 5

States of Matter and Atomic Environments of Elements .......................................................... 6

Geochemical Classifications ......................................................................................................... 6

Decay Processes .............................................................................................................................. 7

Mass Conservation & Elemental Fractionation .......................................................................... 7

Review of Fundamental Geoscience ............................................................... 7

Earths Layers and Components .................................................................................................. 7

Earths Atmosphere Layers and Components .......................................................................... 9

Earths Geography and Wind Resources .................................................................................. 10

Geologic Time Scale ..................................................................................................................... 11

The Earth in the Solar System ....................................................................................................... 12

Geochronology and Radiogenic Tracers ...................................................... 14

Dating of Radioactive Nuclides ................................................................................................. 15

System With High Parent/Daughter Ratios ............................................................................... 15

The Isochron Method .................................................................................................................... 16

Radiogenic Trac ers ........................................................................................................................ 16

Helium Isotopes ............................................................................................................................ 16

Element Transport ............................................................................................ 16Advection ....................................................................................................................................... 17

Diffusion ........................................................................................................................................... 17

Chromatography ........................................................................................................................... 17

Adsorption ...................................................................................................................................... 18

Geochemical Systems ..................................................................................... 18

Single-Reservoir Dynamics ........................................................................................................... 18

Interaction of Multiple Reservoirs and Geoc hemical Cycles .............................................. 18

Mixing and Stirring .......................................................................................................................... 18

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The Chemistry of Natural Waters ..................................................................... 18

Basic Concepts .............................................................................................................................. 19

Dominance Diagrams ................................................................................................................... 19

Speciation in Solutions .................................................................................................................. 19

Water-Solid Reactions ................................................................................................................... 19

Electrolyte Chemistry ..................................................................................................................... 19

Biological Activity ........................................................................................................................... 19

The Carbonate System ................................................................................................................. 19

Prec ipitation, Rivers, Weathering, & Erosion ............................................................................. 19

Elements of Marine C hemistry ..................................................................................................... 20

Biogeochemistry............................................................................................... 20

The Geologica l Rec ord ................................................................................................................ 20Some Specifics of Biological Ac tivity ......................................................................................... 21

The Chemistry of Life ..................................................................................................................... 21

Biominerals ...................................................................................................................................... 21

Biological Controls on the Ocean Atmosphere System ...................................................... 21

Diagenetic Transformation of Organic Mineral ....................................................................... 21

Biomakers ........................................................................................................................................ 21

Metals in Organic Matter ............................................................................................................. 22

Environments ..................................................................................................... 22

Phanerozoic Climates .................................................................................................................. 22

The Rise of Atmospheric Oxygen ................................................................................................ 22

The Geochemical Environment of the Origin of Life ............................................................. 22

Mineral Reactions ............................................................................................. 22

Early Diagenesis ............................................................................................................................. 22

Hydrothermal Reactions ............................................................................................................... 22

Metamorphism ............................................................................................................................... 22Water/Rock Ratios ......................................................................................................................... 22

The Solid Earth ................................................................................................... 24

The Geochemical Variability of Magmas ................................................................................ 24

Magmatism of the Different Tectonic Sites ............................................................................... 25

Mantle Convection ....................................................................................................................... 25

The Growth of Continental Crust ................................................................................................ 25

The Element Barn .............................................................................................. 26

Silicon ............................................................................................................................................... 26

Aluminum ......................................................................................................................................... 26

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Potassium ......................................................................................................................................... 26

Sodium ............................................................................................................................................. 27

Magnesium ..................................................................................................................................... 27

Calcium ........................................................................................................................................... 27

Iron .................................................................................................................................................... 28

Sulfur ................................................................................................................................................. 28

Phosphorus ...................................................................................................................................... 29

Carbon .............................................................................................................................................. 29

Bibliography ...................................................................................................... 30

Geochemistry in Civil Engineering .................................................................. A

Construction Materials Properties ................................................................................................ A

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FORWORD

Modern geochemistryis a discipline that pervades nearly all of Earth science,from measuring geological time through tracing the origin of magmas to unravelling

the composition and evolution of continents, oceans and the mantle, all the way tothe understanding of environmental changes. It is a comparatively young disciplinethat was initiated largely by Goldschmidt in the 1930s, but its modern developmentand phenomenal growth started only in the 1950s.

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GEOC HEMISTRY: INTRODUCTION

1. Definitions and Functions of Geochemistry

Victor M. Goldschmidt(Found er & Fa ther of Mo d ern Ge oc hem ist ry )defined the study ofgeochemistry as: the la ws gove rn ing the d ist rib u t ion of the c hem ic a l e lem ents a nd the i risoto p es through out the e a rth.

The term Geochemis t ry was first used by the Swiss chemistChristian FriedrichSchnbeinin1838.

The major task of geochemistry is to investigate the composition of the Earth as a whole and ofits various components and to uncover the laws that control the distribution of the variouselements.

To solve these problems, the geochemist needs a comprehensive collec tion of analytical dataof terrestrial material, i.e. rocks, waters and atmosphere.

Furthermore, he uses ana lyses of meteorites, astrophysica l data about the composition of othercosmic bodies and geophysical data about the nature of the Earths inside. Much useful

information also came from the synthesis of minerals in the lab and from the observation of theirmode of formation and stability conditions.

2. Fields & Disciplines of Geochemistry

Trac e Elem en tGeochemistry For igneous and metamorphic systems (and sedimentaryrocks for that matter), an operational definition might be as follows: trace elements are thoseelements that are not stoichiometric constituents of phases in the system of interest

IsotopeGeochemistryis an aspect of geologybased upon study of the natural variationsin the relative abundances of isotopesof various elements. Variations in isotopic abundance aremeasured by isotope ratio mass spectrometry, and c an reveal information about the ages andorigins of rock, air or water bodies, or processes of mixing between them.

Petrochemistryis a branch of chemistrythat studies the transformation of crudeoil(petroleum) and natural gasinto useful products or raw materials. These petrochemica ls havebecome an essential part of the chemical industry today.

SoilGeochemist ry is the study of the chemicalcharacteristics of soil. Soil geochemistry is

affected by mineralcomposition, organic matterand environmentalfactors.

SedimentGeochemist ry The composition and physical properties of sedimentary rocksare to a large extent controlled by chemical processes during weathering, transport and alsoduring burial (diagenesis). We cannot avoid studying chemica l processes if we want tounderstand the physical properties of sedimentary rocks. Sediment transport and distribution ofsedimentary fac ies is strongly influenced by the sediment composition such as the content ofsand/clay ratio and the c lay mineralogy. The primary composition is the starting point for thediagenetic processes during burial.

MarineGeochemist ry studies the role of various elements in watersheds,including copper, sulfur, mercury, and how elemental fluxes are exchanged throughatmospheric-terrestrial-aquatic interactions.

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Atmospheric Geochemist ry is a branch of atmospheric sciencein which the chemistryofthe Earth's atmosphereand that of other planets is studied. It is a multidisciplinary field ofresearch and draws on environmental chemistry, physics, meteorology, computermodeling, oceanography, geologyand volcanologyand other disciplines. Research isincreasingly connected with other areas of study such asclimatology.

PlanetaryGeochemist ry focuses on processes, which occurred before, during and afterthe formation of the Earth. It includes the Earth's earliest evolutional stages, such as the formationof the core, the creation of the Moon and the development of the first continents.

Cosmochemistry the analysis of the distribution of elements and their isotopes in thecosmos(universe).

Geochem i c a l Thermodynamics the study of the interrelationof heatand workwith chemical reactionsor with physical changes of statewithin the confinesof the laws of thermodynamics. Geochemical thermodynamics involves not only laboratory

measurements of various thermodynamic properties, but also the application of mathematicalmethods to the study of geochemical questions and the spontane i tyof processes.

Geochem i c a l Kinetics the study and discussion of geochemica l reactions with respec t toreaction rates, effec t of various variables, re-arrangement of atoms, formation of intermediatesetc.

Aquatic Chemistry studies the role of various elements in watersheds,including copper, sulfur, mercury, and how elemental fluxes are exchanged throughatmospheric-terrestrial-aquatic interactions.

Inorganic Geochemist ry the study of the synthesis and behavior of inorganic andorganometallic compounds. This field covers allchemical compoundsexcept the myriadorganic compounds(carbon based compounds, usually containing C -H bonds), which are thesubjects of organic geochemistry. The distinction between the two disciplines is far from absolute,and there is much overlap, most importantly in the sub-discipline of organometallic chemistry. Ithas applications in every aspec t of the chemica l industryincluding c atalysis, materials science,pigments, surfac tants, coatings, medicine, fuel, and agriculture.

OrganicGeochemist ry involves the study of the role of processes and compounds that

are derived from living or once-living organisms.

Biogeochemist ry is the field of study focusing on the effec t of life on the chemistry of theearth.

Environmental Geochemistry includes applications to environmental, hydrological andmineral exploration studies.

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3. Review of Fundamental Chemistry

3.1 The Properties of Elements The 92 naturally occurring chemical elements(90, in fact, because promethium and

technetium are no longer found in their natural state on Earth) are composed of anucleus of subatomic nucleons orbited by negatively charged electrons.

Nucleonsare positively charged protons and neutral neutrons. As an atom containsequal numbers of protons and electrons with equal but opposite charges, it carries nonet electrical charge. The mass of a proton is 1836 times that of an elec tron.

The chemical properties of elements are largely, although not entirely, determined bythe way their outermost shells of electrons interact with other elements. Ionsareformed when atoms capture surplus elec trons to give negatively charged anions orwhen they shed elec trons to give positively charged cations. An atom may formseveral types of ions. Iron, for example, forms both ferric (Fe3+) ions and ferrous (Fe2+)ions, while it also oc curs in the Fe0elemental form.

3.2 The Periodic Table of Elements The atomic numberof an element is equal to the number of its protons. The atoms

mass number is equal to the number of particles making up its nucleus. The Avogadro

numberNis the number of atoms contained in 12 g of the carbon-12 isotope. Theatomic mass of an isotope is the weight of a numberN of atoms of that isotope. The atoms of an element can differ in mass from each other because they have

differing numbers of neutrons. These are isotopes. Those with more neutrons will weighmore and be more massive. The a tomic mass (often referred to as atomic weight) ofan element is calculated by adding together the number of protons and the numberof neutrons.Examples of Stable Isotopes: H-1, H-2 (D), H-3 (T) (or 1H, 2H, 3H), C -12, C -13, C -14 (o r12C , 13C , 14C ), O-16, O -18.

Examples of Radiogenic Isotopes:Fe -54, Fe -56, U-235, U-23 The First Ionization Potentialis the energy required to remove the least tightly bound

electron from the atom.

Valenceis the number of electrons given up or accepted. Transition metals oftenhave more than one valence. Example: Fe(II) and Fe(III)

Electron Affinityis a measure of the desire or ability of an atom to gain electrons. It isan energy concept. It is the amount of energy released when an electron as addedto an atom.

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The concept of Electronegativityrefers to the ability of a bonded atom to pullelec trons towards itself. It is defined as the relative ability of an atom in a molecule toattract electrons towards itself. As atoms bond, electrons are shared or transferred.

The atom with the higher electronegativity will dominate the electrons. Cations have smaller radii than anions.Ionic radiusdecreases with increasing charge.

Ionic radius is important for geochemical reactions such as substitution in crystallattices, solubility, and diffusion rates.

Chemical Bondings: Ionic Bond is the total transfer of electrons from one atom toanother. Covalent Bond is the outer electrons of the bound atoms are in hybrid orbitsthat encompass both atoms. Due to different electronegativity, covalent bonds areoften polar --> dipole interac tions (Van der Waa ls interactions).Metallic Bond is thevalence electrons are not associated with any single atom, but are mobile (electronsea). This bond type is less important in geochemistry than the other bonds.

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Chemical Properties of the Elements1. Hydrogen:Hydrogen is unique as it is the simplest possible atom c onsisting of justone proton and one electron.

2. Alkali Metals:These are very reactive metals that do not occur freely in nature.These metals have only one elec tron in their outer shell, therefore they are readyto lose that one electron in ionic bonding with other elements. The a lkali metalsare softer than most other metals. Cesium and francium are the most reactiveelements in this group.

3. Alkaline Earth Metals:The alkaline earth elements are metallic. All alkaline earthelements have an oxidation number of +2, making them very reactive. Bec auseof their reactivity, the alkaline metals are not found free in nature.

4. Transition Metals:The transition elements are both ductile and malleable, andconduct electricity and heat. The interesting thing about transition metals is thattheir valence elec trons, or the electrons they use to combine with other elements,are present in more than one shell. This is the reason why they often exhibit severalcommon oxidation states.

Com pa rison o f som e a tom ic a nd

re sp ec t ive ionic rad ii ( in na nom et ers)

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5. Other Metals:The 7 elements classified as other metals, unlike the transitionelements, do not exhibit variable oxidation states, and their valence electrons areonly present in their outer shell. All of these elements are solid. They have oxidationnumbers of +3, +4, -4, and -3.

6. Metalloids:Metalloids are the elements found along the stair-step line thatdistinguishes metals from non-metals. This line is drawn from between Boron andAluminum to the border between Polonium and Astatine. Metalloids haveproperties of both metals and non-metals. Some of the metalloids, such as siliconand germanium, are semi-conductors.

7. Non-Metals:Non-metals are not able to conduct electricity or heat very well. Asopposed to metals, non-metallic elements are very brittle. The non-metals exist intwo of the three states of matter at room temperature: gases (such as oxygen)and solids (such as carbon). They have oxidation numbers of +4, -4, -3, and -2.

8. Rare Earth Metals:The thirty rare earth elements are c omposed of the lanthanideand actinide series. They are transition metals. One element of the lanthanideseries and most of the elements in the actinide series are called trans-uranic, andare synthetic or man-made.

9. Halogens:The term halogen means salt-former and compounds containinghalogens are called salts. All halogens have 7 electrons in their outer shell,giving them an oxidation number of -1. The halogens are non-metallic and exist,at room temperature, in all three states of matter.

10. Noble Gases:All noble gases have the maximum number of elec trons possible intheir outer shell (2 for Helium, 8 for all others), making them stable and preventingthem from forming compounds readily.

3.3 The States of Matter and the Atomic Environments of Elements The many crystallized silicate and alumino-silicate structures are classified according

to the pattern formed by their tetrahedra. The most important ones in geology are:1. Isolated-tetrahedra silicates: the most common minerals in this family are the

various sorts of olivine, such as forsterite Mg2SiO4, and of garnet, such as pyropeMg3Al2(SiO4)3.

2. Single-chain silicates: these are the pyroxenes, which fall into two groups with twodifferent crystallographic systems; orthopyroxenes, such as enstatite Mg2Si2O6, andclinopyroxenes, such as diopside CaMgSi2O6.

3. Double-chain silicates: amphiboles, such as tremolite Ca2Mg5Si8O22(OH)2orhornblende Ca2Mg4Al2Si7O22(OH)2. The formation of these hydroxylated mineralsrequires some degree of water pressure.

4. Sheet silicates: micas and clay minerals usually containing aluminum, potassium,and smaller ions such as Fe2+and Mg2+. A distinction is drawn between di-octahedral micas like muscovite (common white mica) K2Al6Si6O20(OH)4and tri-octahedral micas like biotite (ordinary black mica) K2Mg6Al2Si6O20(OH)4, thedifference being the proportions of 2+ and 3+ cations and therefore siteoc cupancy. This family is extremely diverse.

5. Framework silicates: these silicates are interconnected at each of their apexes.This family includes quartz SiO2and the feldspars, the most important of which arealbite NaAlSi3O8, anorthite CaAl2Si2O8, and the various potassium feldspars whoseformula is KAlSi3O8.

3.4 Geochemical Classifications The most widespread c lassification is probably that of Victor Goldschmidt.

Goldschmidts scheme rests on the observation by Berzelius, a Swedish eighteenth-century chemist, that some elements tend to form oxides or carbonates whereasothers form sulfides.

The lithophileelements (Na, K, Si, Al, Ti, Mg, C a) generally concentrate in the rock-forming minerals of the crust and mantle.

The siderophileelements (Fe, C o, Ni, Pt, Re, Os) have an affinity for iron and therefore

concentrate in the Earths core The chalcophileelements (Cu, Ag, Zn, Pb, S) readily form sulfides. The atmophileelements (O, N, H, and the inert gases) concentrate in the

atmosphere.

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3.5 Decay Processes The (alpha) process, which is the emission of a helium nucleus (two protons and two

neutrons), is common at high mass.(147Sm143Nd + .)

The (beta minus) processinvolves the emission of an electron by the parentnucleus. In a vac uum, a free neutron does not survive more than 15 minutes before itturns into a proton and an elec tron.

(87

Rb87

Sr +

+) Capture of an elec tron of the K shell is a less frequent process, and was identified byvon Weizscker during his investigation of excess argon-40 in the Earths atmosphere.For example, 40K + e40Ar. Electron capture affects the nuclide in much the sameway as positron emission does.

Spontaneous fission of some heavy atoms like uranium-238 or plutonium-244 is a ratherrare and very slow process; it forms the basis of the fission-track dating method.

3.6 Law of Conservation of Mass According to the Law of Conservation of Mass, M a ss is nei ther c rea ted nor de stroye d

in a ny tra nsform a t ion o f ma tter . According to the Law of Definite Compositions, A pure co m po und is a lwa ys

c om po sed o f the sam e e lem en ts c om b ined in a de f in it e p rop o rt ion b y we igh t . According to the Law of Multiple Proportions, When to e leme n ts c om b ine to fo rm

m ore tha n one c om po und , the d i ff e ren t we igh ts o f one tha t c om b ine wi th a f ixed

we ights of one that c om b ine w ith a f ixed we ight o f the othe r are in the ra t io o f sm al l

w ho le num b ers.

4. Review of Fundamental Geoscience4.1 Earths Layer and Components

It is impossible to have a completely clear picture of Earths internalstructure. However, study of the transformations of the

planets surface and analysis of other planets inthe Solar System have supplied muchinformation about the interior of Earth.Our planet has a total mass ofabout 6 trillion tons and isformed of three concentriclayersfrom densest tolightest, core, mantle,and c rust. Each hasan individualchemicalcomposition and

specificphysicalproperties.Earthscrust,

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composed of oceanic c rust and continental crust, represents barely 3%of the planets volume.

Schematic Section of the Earth1 Continental Crust A Mohorovii discontinuity2 Oceanic Crust B Gutenberg discontinuity3 Upper Mantle C Lehmann-Bullen discontinuity4 Lower Mantle

5 Outer Core6 Inner Core

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4.2 Earths Atmosphere and Components Troposphere: Thetroposphereis the lowest layer of the atmosphere that

lies next to the Earths surface. Most of the air that makes up theatmosphere is found in the troposphere. As you move up into thetroposphere the temperature dec reases. At the top of this layer the a irtemperature is about - 60C (-76 F).

Stratosphere:The stratospherebegins at the top of the troposphere (14

km/ 9 miles) and extends to about 50 kilometers (31 miles) above thesurface of the Earth. As you move up into thestratosphere the a ir temperature actuallyincreases. This oc curs because of ozone.Ozoneis a gas that absorbs solar radiationand releases it as heat. The stratospheric or"good" ozone protec ts life on Earth from thesun's harmful ultraviolet (UV) rays. The ozonelayer/ ozonosphere is located at the bottomof the stratosphere.

Mesosphere:The mesospherebegins at about50 kilometers (31 miles) above the Earths

surface and extends to 80 kilometers (50miles). As you move up into themesosphere, the airtemperature decreases. Temperaturesat the top of this layer can drop to -90C (-130F ). Interestingly, this layer also protects theEarth. Meteoroids entering Earths atmosphereusually burn up in the mesosphere.

Thermosphere:The thermosphereis theoutermost layer of the Earths atmosphere. Itbegins at 80 km (50 miles) above the Earth andextends outward into space. Sometimes thethermosphere and exosphere are listed asseparate layers. The higher you movein this layer, the higher thetemperature. Temperatures inthe thermosphere can reach1,800C (3,300F)! The beautifulcolors of the aurora borea lis ornorthern lights occur in thisatmospheric layer. This is alsowhere the space shuttle orbits the Earth!

The ionosphere is found at the bottom ofthe thermosphere (close to the mesosphere).

The exosphere is located at the top of thethermosphere.

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4.3 Earths Geography and Wind Resources

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4.4 The Geologic Time Scale

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4.5 The Earth in the Solar System Spectroscopic measurements of elementsfrom the distant stars are

strongly biased towardsonly those elements inexcited states at or nearthe stellar surface. Thoseelements principally inthe interior do not

contribute to surfaceradiation in the sameproportions as actuallyexist in a star. Thesituation is better for theSun. When elementdistributions are statedasCosm ic Abunda nces,they actually are roughestimates made fromSolar Ab unda nc es .

From the figure, we see

four patterns:1. An overwhelming

abundance of lightelements

2. A strong preferencefor even-numbered elements

3. A peak in abundanceat iron, followed by a steadydecrease.

4. Elements 3-5, Lithium,Beryllium and Boron, are very low in abundance.

These patterns have to do with nucleosynthesis (element formation) in the

stars. If the Sun and Solar System formed from the same material, we would

expect the raw material of the planets to match the composition of theSun, minus those elements that would remain as gases. We find such acomposition in a c lass of meteorites called chondrites, which are thoughtto be the most primitive remaining solar system material. Chondrites areconsidered the raw material of the inner Solar System and probablyreflect the bulk composition of the Earth.

Differentiation has created a light crust depleted in iron and enriched inoxygen, silicon, aluminum, calcium, potassium, and sodium.

The abundance of elements in the Earth's crust is much different from theabundance of elements that are to be found on the other planets andour Sun. The c ontinental crust of the Earth also differs radically from theoverall composition of the Earth.

Our Earth as a whole and its crust, in particular, have extraordinaryconcentrations of elements, all associated with silicate minerals likeolivine, pyroxene, amphibole, plagioclase, the micas, and quartz.Although there are a vast number of silicate minerals, most silicateminerals are made from just eight elements.

The two most common elements in the Earth's crust, oxygen and silicon,combine to form the "backbone" of the silicate minerals, along with,

occasionally, aluminum and iron. These four elements alone account forabout 87% of the Earth's crust. This silica te or alumina-silicate "backbone"carries excess negative charge, however. Positive charge in the form of

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In orthosilicates such as olivine, thetetrahedra are separate and eachoxygen is also bound to other metalions that oc cupy interstitial sitesbetween the tetrahedra.

In pyroxenes, the tetrahedral eachshare two oxygen and are bound

together into chains. Metal ions arelocated between the chains.

In sheet silicates, such as talc, mica,and clays, the tetrahedra eachshare 3 oxygens and are boundtogether into sheets.

cations has to be brought in to balance this negative charge. The fourmost important elements that fit in themineralogical structures of the silicates arecalcium, sodium, potassium andmagnesium. Taken all together, constitutingnearly 99% of crustal elements, leaves littleroom for all of the other elements.

As a consequence, all other elements areeither nearly absent from the Earth's crust orare found primarily in non-silicate rocks.

The silica tetrahedron consists ofa central silicon atom bound to 4oxygens.

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From Mohs Scale of Mineral Hardness:

5. Geochronology and Radiogenic Tracers

We have seen that radioactivity is not dependent on the chemical bonding of atoms, or ontemperature, or on pressure. Radioactivity can be described as an event whose probability ofoccurrence per unit time is invariant. The probability that aradioactive nuclidewill decay per unit oftime is denoted. This probability, better known as the decay constant, is specific to the radioactive

nuclide under consideration. Radioactive decay, like incoming calls at a telephone exchange, is aprime example of a Poisson process, in which the number of events is proportional to the time overwhich the observation is made. In the absence of any other loss or gain, the proportion of parentatoms (or radioactive nuclides) disappearing per unit of time t is constant:

dP / P dt = For a number of parent atomsP =P0at time t = 0, this equation integrates as:

P =P(t) =P0et

Note that physical time elapsing in the real world is normally given in seconds (s), which is nota very helpful unit in the Earth sciences, while geological ages, through which we go back throughtime, are noted inanni (a), from the Latin annus. Derived units kyand ka(thousand years), MyandMa (million years), Gyand Ga (billion years)apply to physical time and time interval (or age),

respectively.

SCALE MINERAL CHEMICAL FORMULA

1 Talc Mg3Si4O10(OH)2

2 Gypsum CaSO4 2H20

3 Calcite CaCO3

4 Fluorite CaF3

5 Apatite Ca5(PO4)3(OH-,Cl-,F-)6 Feldspar/Orthoclase KAlSi3O8

7 Quartz SiO2

8 Topaz Al2SiO4(OH-,F-)2

9 Corundum Al2O3

10 Diamond C

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5.1 Dating by Radioactive Nuclides Carbon-14:This method of dating, which is certainly the most familiar to the general

public, is not the oldest historica lly. However, it has revolutionized archeology andearned its inventor, Libby (see Arnold and Libby, 1949), the Nobel Prize for Chemistryin 1960. The Earth is subjec ted to bombardment from high-energy galactic cosmicrays, mostly protons and particles, which react with the Earths atmosphere. Theinteraction of these particles with nitrogen and oxygen produces sec ondary particles,mostly neutrons. In spite of a limited lifetime, neutrons do not have to overcome the

Coulomb barrier of the nucleus and react more easily than charged particles of thesame energy, such as protons. An important source of 14C is the reaction betweenneutrons and nitrogen, which produces radioac tive carbon-14 and a proton:

14N + n 14C + p

Beryllium-10:The radioactive nuclide 10Be is one of the pieces produced byspa llation, the breaking of atmospheric 14N and 16O nuclei by cosmic rays. Metallic Beis rapidly oxidized to BeO, scavenged by atmospheric particles, and finallyincorporated into soil and sediments by rain water and run off. The 10Be decayconstant is 4.62 107y1and it is customary to normalize its abundance to that of thestable isotope 9Be. Beryllium is an element similar to aluminum and is found in clay

and soils.Beryllium-10dating is muchused inoceanographyfor measuringsedimentationrates ormanganese-nodule growthrates.The thorium-

230 excessmethod:This is asomewhatdifferent case, asthis nuclide, witha radioactivehalf-life of 75 000years, is notcreated byradiation but bythe decay of aparent nuclidewhose half-life islong enough for

its rate of production to be considered constant on time scales of less than one

million years. It is one of the examples of c locks based on a chain of radioactivedecay in which nuclides decay from one to another by or radioactiveprocesses.

5.2 Systems With High Parent/Daughter Ratios The Potassium-Argon Method:This is the workhorse of geochronology. The method

relies on the potassium-40 nucleus capturing a K-shell electron so that 40K + e 40Ar.The radioactive constant or probability of decay per unit time for this process is=5.81 1011y1. Potassium-40 also decays by an ordinary process into 40Ca (dualdecay), for which the radioactive constant is= 4.96 1010y1. The proportion of 40Katoms taking the 40Ar pathway is equal to the relative probability/(+) or 10.5%.Using N for the number of nuclides, equation then becomes:

N40Ar(t) =N40Ar(0) + ( / +)N40K(t) [e(+)t 1]

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Dating zircons by the uraniumlead method:This method is the Ferrari ofgeoc hronology for long geological time sca les; it is prec ise and resistant againstdisturbances occurring after closure of the system, but difficult to implement. Inrecent years, its development has considerably benefited from the improvedcleanliness of chemical extractions and in situ methods of analysis (secondary ionmass spectrometry and laser-ablation inductively coupled plasma massspectrometry, ICP-MS). The advantage of the method lies in the radioactive (238U and235U) and radiogenic nuclides (206Pb and 207Pb) being isotopes of the same elements:uranium for one and lead for the other.

5.3 The Isochron Method When minerals and rocks form, they already contain some of the radiogenic isotopes

used for dating. The daughter nuclide may be present in large, yet unknown,concentrations. The isochron methodwas devised to provide an age, even when theamount of radiogenic isotope initially present in the system is not negligible withrespect to that produced by radioactive decay after its formation. The keyassumption is that the initial isotopic composition of the element to which theradiogenic nuclide belongs is unknown, but constant, in all the samples analyzed.Isotopic homogenization is assumed to be complete at t = 0, which may be the case

where minerals crystallize from a magma or from seawater within a time interval thatcan be considered as very short compared to the age of the rocks.

5.4 Radiogenic tracers The property that phase separation, such as melting and crystallization, fractionates

parent/daughter ratios has received enormous attention and created the fertileconcept of radiogenic tracers. We have previously discussed the point that phasechange does not fractionate the normalized isotopic ratios themselves, a commonsource of confusion. For this reason, this method was commonly used for dating allsorts of sedimentary and magmatic rocks until superseded by zircon geochronology.It will be seen that this was the first method ever to yield the age of the Solar Systemand it is still widely used to date meteorites and planetary samples.

5.5 Helium Isotopes Radiogenic helium 4He is produced by the decay of the two natural isotopes of

uranium (238U and 235U) and of the only long-lived isotope of thorium (232Th).We haveseen earlier that these decay proc esses are in fact the start of a chain of events. Forexample, in order to pass from 238U to its distant descendant 206Pb, the initial nuclidemust lose (238206)/ 4 = 8 particles, which, by capture of the rock matrix elec tronstorn off during particle expulsion, become that number of 4He atoms. The only stablereference isotope is3He. The isotope geoc hemistry of helium is an incomparable toolfor investigating the outgassing of the mantle and a lso to model water circulation inaquifers. Note that, by force of habit, many geoc hemists continue to use the inverse3He/4He ratios standardized to the value for atmospheric helium (1.4 106) instead of

the 4He/3He ratios.

6. Element Transport

The theory of element transport is a way of representing the spatial changes in geochemicalproperties in various contexts, such as movement in the ocean or mantle, the migration of geologica lfluids or magmatic liquids within a rock matrix, or the attainment of chemica l and isotopic equilibriumamong minerals within the same roc k, etc.

The essential concepts forming the core of this theory are those of conservation, flux, sources,and sinks. A conservative property is additive and can only be altered by addition or subtraction at thesystem boundaries or by the presence of sources and sinks. Massor number of moles are conservative

properties; concentration is not: if a mole of salt is added to a solution already containing one mole, theresulting solution will contain two moles, regardless of how the salt is added. In contrast, two solutions ofone mole per liter combine to form two liters of a solution a t one mole per liter. A fluxis a quantity ofsomething (mass, moles, energy, etc.) crossing a unit surface per unit time. The most familiar of these

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fluxes is volume flow, which is quite simply the velocity v(in cubic meters per square meter per sec ond).Mass fluxis the masscontent of volume flow,i.e.v, where is thedensity of the medium.

The flux of an element i isvC i , where C i is its localconcentration in kilograms(or moles) per kilogram. Ifthe flux of a compound oran element changessuddenly at one point,then a source or sink ofthis compound or elementis present: generally, achemical reaction orradioactive proc ess isresponsible for this.

6.1Advection

Advectionis bulktransport, which is easierto understand in onedimension. Let us considera medium of densitymoving at speed v. If, at apoint x and over an areaA , we consider a slice of

matter of thickness l, the balance of variation of mass of i per unit time in this slice isequal to the difference between the incoming and outgoing fluxes:

A l [dC i (x)/dt] =AvC i (x) AvC i (x + l)

6.2 Diffusion Diffusive transferis transfer over short distances caused by the thermal agitation ofatoms or the turbulence of the medium.

6.3 Chromatography In analytical chemistry, chromatography is a technique of element separation that

involves the percolation of a liquid (the eluent or mobile phase) through a porousmatrix (the stationary phase composed, for example, of ion-exchange resin) and

exchange elements with this matrix. The variable affinity of the stationary phase forthe elements lets each of them percolate through the matrix with a different veloc ity,which eventually ensures their separation. In rocks, similar processes take place whengeological fluids move through the pores of a sediment during diagenesis or through

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rock layers during metamorphism: because of the large volumes of fluid involved andbecause of the broad range of reactivity from one element to another, considerablechemical separation in the fluid and strong mineralogical and geochemicalmodifications of the rock matrix are commonplace.

6.4 Adsorption Capture of atoms, ions, and molecules by surfac es is known asadsorption.This

process is critica l to the understanding of geochemical cyclesbecause it accounts

for why the bulk of many elements carried by rivers do not contribute to the inorganicsalts dissolved in seawater as much as expected: upon mixing of freshwater andseawater in estuaries, iron hydroxides prec ipitate and entrain, adsorbed on theirsurface, most of the transition elements, the rare-earth elements, and high-field-strength elements dissolved in the runoff. This is also a process at work in carbonatedsprings: decompression removes CO2and increases the pH; iron hydroxidesprec ipitate and clean up the spring water from many of its constituents. Likewise,the scavenging of shallow waters by wind-blown particles carried from the desertsand by organic material depletes the surface ocean in the same elements.

7. Geochemical Systems

This looks at the changes that over time affect the geochemical properties of a system or a setof systems, such as the mantle, the c rust, or the oc ean, when subjected to disturbances whethercaused naturally or by human activity. The essential concepts utilized residence time and forcing aretaken from chemical engineering. Viewing system Earth as a chemical factory composed of reactors,valves, sources, and sinks, has proved to be a simple and robust model. The theory goes by variousnames, with the box model probably the most widely used.

7.1 Single-Reservoir Dynamics Let us begin by considering a lake containing a mass of waterM that we will take to

be constant. A river flows through the lake with a rate of flow Q, which we will expressin kilograms per year;Q is therefore the same upstream and downstream. We areinterested in the balance of a chemical species in the lake. A chemical elementi

introduced upstream with a concentration Ci in is either lost through the lake outlet orentrained into sediments. The sedimentation rate P is also expressed in kilograms peryear. The lake itself is considered homogeneous, being well mixed by turbulent flowand by convection.

8.The Chemistry of Natural Waters

The external aspects of geochemical cycles, the phenomena that occur at relatively lowtemperatures (typically from 0 to +30C) in the ocean, the atmosphere, and in rivers, are largelygoverned by chemica l equilibria in solution or at the watermineral interface. The c ycles themselvesimply transfer controlled primarily by waterrock interaction (erosion, sedimentation, hydrothermal

reactions) and by biological activity. A central role is played by the carbonate system.

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8.1 Basic Concepts Acidityis the concentration [H+] (mol kg1) of protons in a solution. The exact form, H+

or H3O+, in which these protons occur is of little significance. A scale of acidity isdefined by the potential pH of the protons in the solution, such that pH =log [H+]. At25C, pure water has a pH of 7. A lower pH indicates an acidic solution and a higherpH a basic solution.

Ion behavioris dictated by the dissoc iation of ac ids and bases. In an acidbasereaction, the acid is the proton donor and the base is the acceptor. A strong acid

such as HCl or a strong base such as NaOH become completely dissoc iated toproduce C land Na+ions, which behave essentially like inert species and are ofrelevance only in terms of charge. Weaker acids become partly dissociated byreleasing one, two, or possibly more protons.

Ion complexationis the assoc iation of ions carrying charges of opposite sign. In thenocean, for instance, the copper ion Cu2+may be surrounded by different anions OH,Cl, HCO3, which form different species of copper, but also by humic acids from thesoil. In this sense, H2CO3and HCO3may be viewed as carbonate complexes of theproton. C omplexation a lso obeys the mass action law with it successive constants.

Redox reactionsrelate to electron exchange: a reductant gives up electrons to anoxidant. Oxidationof Fe2+into Fe3+is a common result of elec tron acceptance byoxygen atoms:

Fe3++ e Fe2+ Atmospheric gasesare scarcely soluble in water, except for CO2. The solubility of

gases is described by Henrys law, which establishes a simple proportionality betweenthe partial pressure Piof the gas i above the solution and its concentration in thesolution.

The solubility of solidsprecipitating from solutions is expressed using anothercoeffic ient of the mass action law, the product of solubilityKs.

The condition of electrical neutrality: this condition is normally written by calculatingthe charge balance of fully dissoc iated speciesA lk, which is known as alkalinity (notto be confused withbasicity, whichcharacterizes a solutionwith pH > 7)

All the thermodynamicconstantsdepend ontemperature and, to alesser extent, onpressure. Thoseanalogous to anequilibrium coefficientmay be expressed by asimilar law to ElementalFractionation.

8.2

Water Solid Reactions Waterhas the effec t onthe minerals of igneousor metamorphic roc ks ofputting the more solublecations into solution andproducing residual clay minerals. A notable exception is quartz. A first type ofreaction c ontrolling interaction between the lithosphere and hydrosphere essentiallyinvolves reactions of proton and cation exchange such as:

2NaAlSi3O8+ 2H++ H2O Al2Si2O5(OH)4+ 4SiO2+ 2Na+(albite) (solution) (kaolinite) (silica) (solution)

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8.3 Precipitation, Rivers, Weathering, and Erosion The hydrological cycleis well understood. Evaporation caused by intense solar

heating of the oceans between the tropics produces water vapor. When the hot,moist air rises, the water vapor condenses into fine droplets forming clouds, but onlyfalls to the ground as rain when the droplets coalesce to a ttain sufficient size. Themasses of hot air migrate toward the poles and as they cool precipitationprogressively drains the moisture from the atmosphere. This can be seen from themeasurement of isotopic concentrations of hydrogen and oxygen in precipitation,

i.e. rain (meteoric) water and snow.

8.4 Elements of Marine Chemistry Rivers carry their dissolved load to the ocean. It is prac tical to group all of this added

mineralization under the heading of erosion-related alkalinity flux. Many elements donot pass the filter of estuaries; as discussed above, at this point, fresh water and salt

water mix, causing asubstantial dec rease inthe dielectric constantof the water. Massiveprecipitation ofcolloids takes place,

with the adsorption ofmany elements suchas the transitionelements, rare-earths,etc . This phenomenonis amplified by therichness of estuaries inorganic matter. Theocean surface ischaracterized byabundant life,sustained throughoutthe depth of waterwhere light canpenetrate (known asthe photic zone, some50 m deep) by primaryphotosyntheticproduction by algae.

This primaryproductivity allows other planktonic forms or larger organisms to develop by grazingand predation. As indicated above, organic matter concentrates light isotopes,particularly 12C relative to 13C and 14N relative to 15N, and the intensity of thisproductivity can be measured with the increased 13C and 15N of surface watercompared with deep water.

9. Biogeochemistry

9.1 The Geological Record Oxidized rocks, limestones, cherts, and phosphates contain the biological materials

with the most spectacular contribution to the geological record. Modern limestonesare largely formed by the accumulation of carbonate tests of foraminifera andunicellular algae such as coccolithophores. Diatom frustules contribute massiveamounts of silica to sediments at the bottom of the Southern oceans. The giganticphosphorites of Africa represent fossil hard parts (teeth and bones) or theirremobilization by diagenetic fluids: they are mined to produce fertilizers for

agriculture. On the sea floor, these three types of rocks are often assoc iated witheach other in areas rich in nutrients, continental platforms, wind-driven upwellings ofdeep seawater such as next to the coasts of Morocco and Peru and the olderseawater from the Southern oceans.

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Reduced carbon is another important remnant of biological ac tivity. Different typesof organic material are found in rocks:1. Humic sub sta nc es are common components of soils and waters. The precursors of

these very complex compounds are diverse organic compounds resistant tobiodegradation such as lignin and tannins.

2. Kerogen consists of a variety of polymers and mac romolecules formed duringdiagenesis by microbial degradation and condensation of humic substances,which make the products progressively more insoluble in diagenetic fluids.Different types of kerogens are distinguished, which may eventually lead to theformation of liquid hydrocarbons and coal. Bitumen is the fraction of carboncompounds that can be extracted from a rock by liquid solvents.

3. Liquid hyd roc a rb ons (petroleum) evolve by loss of water, carbon dioxide, andmethane from some types of kerogen upon heating at temperatures of 60150 C(the oil window). The nature of the end-product depends on the rate oftransformation.

4. Ga seo us hydroc a rb ons such as methane evolve from kerogen and coal attemperatures of 150230 C.

5. Coa l is the residual carbon-rich material left behind after hydrocarbons have leftthe rock. The most abundant humic c oals derive from vascular plants, whereassapropelic coal forms from fine-grained organic particles.

9.2 Some Specifics of Biological Activity Some additional defining effects of biogeochemistry may be summarized as follows:

1. The most abundant elements in the biomass are reduced carbon, hydrogen,nitrogen, and phosphate. Fossil biomolecules are of particular economicimportance since, as every living c reature, humans like to turn reduced carboninto energy.

2. Biological ac tivity affects sediment composition through the deposition ofbiominerals (carbonate, phosphate) or by interfering with diagenesis.

3. Over the Earths history, biological ac tivity has modified the composition of theatmosphere (notably by adding oxygen and removing carbon dioxide) and ofthe ocean. It is still today an essential parameter in c limatic regulation.

4. Biological activity maintains chemical gradients such as those of nutrients in thewater column.

5. Some of the originalbiomolecules, mostlyinsoluble lipids, are leftuntouched bydiagenesis. They aretrue geochemical fossilsand as such are calledbiomarkers.

9.3 The Chemistry of Life Carbohydrates (sugars)

derive their name fromtheir generic formulaCn(H2O)n. The commonand simple sugar glucoseC6H12O6 is amonosaccharide whosering structural form leads topolysaccharides viacondensation reactions.

The commonest of theselatter is cellulose, aubiquitous component of

cell walls, notably in plantsbut also in chitin used byarthropods and insec ts as

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shell material. Carbohydrates are the primary reserve of energy and are used toproduce adenosine triphosphate (ATP).

Amino acids are organic (carboxylic) acids in which one carbon is attached to anamino NH2 group. They contain most of the nitrogen present in organic matter.Polymerization of amino acids via their NH2group produces proteins. Enzymes areproteins with a catalytic function, i.e. molecules which reversibly provide the littleincrement of energy that helps a particular reaction go over the activation energybarrier.

Lipids include the fats and the glycerides, or esters of glycerol esters with one or morefatty ac ids. When PO34 groups attach themselves to one chain or more of carboxylicacids, they are known as phosphoglycerides. Tetracyclic compounds made of fourpyrrole units (a five-membered ring with a nitrogen atom opposing two doublecarbon bonds) are biologically important as they form pigments known asporphyrines: chlorophylls (Mg) and hemoglobin (Fe) are closely related compounds.

Terpenes are a broad class of lipids based on the isoprene chain (C5H8), which hasone unsaturated bond a t each end. Many form important compounds with multiplecyclic bonds (sterols, hopanes).

Aromatic alcohols (phenols) condense to form lignin, the second most abundantcomponent in plants.

Nucleotides contain nitrogen-bearing compounds, a pentose sugar (ribose ordeoxyribose), and a phosphate. Nucleic acids are polymers of nucleotides.

9.4 Metals in Organic MatterMetallic deposits of probable biological origin are known, such as the Precambrian

banded iron formations (BIF). It is suspected that this is not an isolated case and theemerging field of the stable isotope geochemistry of metals may soon be able to comeup with biomarkers of its own. Among the trace metals that contribute to differentfunctions in the c ell, some may contribute to the sedimentary record. For example:

1. Iron enters multiple molecules such as porphyrin, a ring of nitrogen-bearingchelates. Porphyrin groups are present for electron transfer in cytochrome and fordioxygen transport in hemoglobin. Proteins containing both S and N (ferredoxin)perform more specialized functions such as photophosphorylation duringphotosynthesis.

2. Because of its property of super-acidity, Zn is one of the most prevalent traceelements of life and has been found in over 600 proteins, notably assoc iated withthe immune system. It is found in carbonic anhydrase, which dramatica llyincreases the rate of conversion of CO2 into HCO3 .

3. Copper is present in cytochrome oxidase for elec tron transfer in the energy cycleand replaces iron in hemocyanin for dioxygen transport in some species.

4. Magnesium concentration in the cytoplasm is in the percent range. It is the corecation of chlorophylls.

5. Potassium is used to maintain gradients and controls transfers across the cellmembranes (biological pumps).

10.Mineral Reactions

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Take a look at the chemical and mineralogical changes accompanying the formation ofsedimentary andmetamorphic rocks.Marine mud isconsolidated intosediment throughearly diageneticreactions. Thesesediments and theother rocks that formthe bedrock may beaffected bypercolating hotwater, mineralized tovarying degrees, andacids. This ishydrothermalmetamorphism. Whenrock is dragged deepdown by subductionand thereby heated

and dehydrated, it istransformed,producing a greatvariety of differentmineral assemblages.

This process is termedmetamorphism.Some of thesethermal processesconcern thetransformation oforganic matter, whose ultimate products are the fossil fuels such as natural gas, petroleum, and coa l.

The principal geoc hemical issues raised by these processes are to identify the nature of the rock beforeits transformations, the physical conditions (temperature and pressure) of the transformations, and thenature and intensity of exchanges between the transformed rocks and the interstitial solutions.

Transformations are often controlled by water pressure and temperature. Let us take theexample of the important reaction whereby muscovite (white mica) disappears from gneiss and schist,and which characterizes the entry of metamorphic rocks into granulite facies:

KAl3Si3O10(OH)2+ SiO2 KAlSi3O8+ Al2SiO5+H2O(muscovite) (quartz) (K-feldspar) (sillimanite)

The behavior of oxygen is particularly important. The redox reaction between two types of

common oxides in igneous and metamorphic roc ks:6Fe2O3 4Fe3O4+O2

(hematite) (magnetite)

Relationships like this can be used either for estimating temperature if the redox state of thesystem is known, or vice versa for measuring oxygen pressure if temperature is known. The fugacity ofoxygen in many natural rocks is distributed around the famous QFM (quartzfayalitemagnetite) buffer:

3Fe2SiO4+O2 3SiO2+ 2Fe3O4

(fayalite) (quartz) (magnetite)

Balancing hydrothermal reactions requires a combination of cation-exchange reactionsbetween the rock and the solution and the principle of electrical neutrality. First we need to draw up an

inventory of proton/cation exchange reactions such as (7.22), which can be simply re-written:

2NaAlSi3O8+ 2H++H2OAl2Si2O5(OH)4+ 4SiO2+ 2Na+(albite) (solution) (kaolinite) (silica) (solution)

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Hydrothermal reactions at the mid-oc ean ridges play an important role in the magnesium cycleand in controlling the a lkalinity of the ocean. It has been observed that water from the black smokers ofthe oc ean ridges is particularly acidic, i.e. its pH is much lower (typica lly 3) than that of deep oc eanwater, and that it is totally devoid of magnesium. This can be explained by reactions of seawater withcommon basalt minerals:

Mg2SiO4+ Si(OH)4+H2O+ Mg2+ Mg3Si2O5(OH)4+ 2H+(olivine) (solution) (solution) (serpentine) (solution)

11.The Solid Earth

11.1 The Geochemical Variability of Magmas Magma, the common term for a molten rock, may contain crystals in suspension,

usually called phenocrysts.Molten magma reaching the surface is known as lava. Ifthe magma crystallizes completely, most commonly as an effec t of slow cooling, therock produced is said to be intrusive or plutonic. If cooling is too fast for crystallizationto be completed, for example in a submarine eruption, the liquid is quenched to aglass. After cooling, it forms an effusive or volcanic rock.

The two most abundant types of magmatic rocks are basalt in ocean areas andgranite in continental areas. A basalt reaches the surface at a temperature of 1150

1250 C, and a granitic liquid (rhyolite) at about 1000 C gives the characteristicmajor element compositions of magmatic rocks in their standard form, i.e. by oxideweight. Basalt is rich in iron, magnesium, and calcium, whereas granite is rich in silicaand alkaline elements. We will now look at the two mechanisms responsible for thevariability of magmatic rocks, melting of the mantle and c rust, and magmaticdifferentiation.

The factors of chemica l variability of the primary melts extracted from the mantleand the crust are multiple and not always independent.1. The na ture o f the source roc k. Partial melting of the peridotitic upper mantle,

composed mainly of olivine, pyroxene, and aluminous minerals (feldspar, spinel,and garnet) yields basalts. Melting of the continental crust, formed by wetsediments accumulated by erosion, and by their metamorphic equivalents (schistand gneiss) produces granitic liquids.

2. Pressure. The pressure P at any depth of melting z is the weight of the overlyingrocks; P and z are related through the hydrostatic condition dP =gdz, where isthe rock density. Different planets have different gravities and therefore differentpressuredepth relationships! At shallow depths, melting of the mantle producestholeiitic basalts, i.e. basalts that are somewhat richer in Si and poorer in Mg thanother types of basalts. At greater depths the trend is reversed and magma isricher in Mg and Fe and poorer in Si; these are alkali basalts.

3. Tem p erature. For a system of a given composition at a given pressure,temperature isdirec tly related to the d eg ree o f mel t ing: the major elements that

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enter the melt most readily (Si, Al, Ca , K, Na, Ti) and incompatible trace elements(Rb, Zr, Ba, rare-earths, Th, U, etc.) are relatively abundant in the first melts. Asmelting proceeds, the liquid becomes richer in refractory elements (Mg, C r), freshmelt dilutes incompatible elements, and basalt gives way to picrite. Melttemperature and composition are not independent. Close to the minimumeutec tic temperature, the melt fraction is small and the liquid composition isbuffered by the residual minerals.

4. Wate r and c a rbon d iox ide c on ten t o f the sourc e. The effect of water is similar tothat of elements with low melting points (e.g. alkali elements). Water may formhydrous minerals such as mica and amphibole in the upper mantle and a numberof unnamed minerals (alphabet phases known by letters) at depth, but it is alsodissolved in the lattice of the so-called nominally anhydrous minerals such asolivine and pyroxene and greatly reduces their viscosity. Water is also very solublein melts because it breaks the SiOSi double bond into two SiOH single bonds.

The presence of such impurities lowers the melting point of the rocks.5. Fertility. This is the abundance of minerals with a low melting temperature, and it

affec ts the capacity of the rock to produce magma (melt productivity). At agiven temperature, a mantle rock rich in pyroxene and aluminous minerals willproduce more liquid than a peridotite with abundant refractory olivine.

11.2 Magmatism of the Different Tec tonic Sites The temperature at which rock melts increases with pressure and decreases sharply in

the presence of water. The low water content of the mantle (about 500 ppm) limits itseffect on upper-mantle melting. Mechanica l effec ts of trace amounts of water,however, stand out: dry olivine is stiffer than wet olivine and melting leaves behind afairly rigid residue. The production of lava is assoc iated with dec ompression of themantle in plumes and beneath oceanic ridges but also with dehydration of wetmaterial (weathered sediments and basalts) sinking in subduction zones.

These three sites produce basalts, i.e. lavas rich in Mg, Ca , and Fe and poor in Si.Other types of magma are assoc iated with continental collision zones: these are felsicmagmas rich in Si and poor in Mg, whose characteristic product is common granite.Continental margins are often assoc iated with mixed magmatism, which goes by thename of orogenic magmatism.

Examples are the deadly eruptions of Mount Pele, Mount Pinatubo, or El Chichon. Acommon type of lava erupted by these volcanoes is andesite.

12.The ElementBarn

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12.1 Silicon Silicon is, after oxygen and iron, the third most abundant element in the Earth.

Although Si concentration in the core may reach several percent, this ubiquitouselement is essentially lithophile and refractory. It is the most abundant cation amongthe silicates that constitute the mantle at shallow depth (olivine, pyroxene,plagioclase, garnet), at intermediate depth (ringwoodite, majorite), and in the deepmantle (perovskite). It also forms the major igneous minerals in the crust of both mafic

igneous roc ks (olivine, pyroxene, amphibole, plagioc lase) and felsic igneous roc ks(quartz, feldspars). Silicon is a major constituent of clastic sediments (quartz and clayminerals) and makes up a substantial fraction of metamorphic minerals.

Most common form: Si4+ Ionic radius: 0.26 Stable isotopes: 28 (92.23%), 29 (4.67%), 30 (3.10%) Atomic weight: 28.086 Condensation temperature: 1311K Dissoc iation of silicic acid in water:

H2SiO3HSiO3+ H+(log K = 9.86) No significant complex in waters Reactions limiting solubility in water:

SiO2+ H2O

H2SiO3 (log K = 4.0) (quartz)SiO2 + H2O H2SiO3 (log K = 2.7) (amorphous silica) Residence time in seawater: 20 000 years

12.2 Aluminum Aluminum is the sixth most abundant element in the Earth. It is a highly refractory

lithophile element. The radioactive isotope 26Al quickly decayed into 26Mg in the firstmillions of years of the Solar Systems evolution. It provided substantial heating to theearly planetary bodies, and the isotopic composition of Mg is one of the most widelyused extinct radioactivity chronometers. It is unlikely that Al enters in large proportionsin the core, but it is a major constituent of many major minerals at any depth in themantle and in the crust. In the mantle, it enters plagioc lase up to pressures of about 1

GPa, spinel to 2 GPa, and garnet beyond. At these high pressures, Al also entersclinopyroxene in large proportions: garnet and c linopyroxene dissolve into each otherto form majorite, an essential mineral phase of the mantle above the 660 kmdiscontinuity. At higher pressure, Al is hosted in a perovskite structure, but its prec isebehavior is still largely unknown. The major mineral that hosts Al in igneous rocks isfeldspar: only plagioclase occurs in basalts, while plagioclase and alkali feldspar mayoc cur together in felsic rocks. Biotite mica may occur in both types of roc ks butnormally accounts for only a small part of the Al inventory. In sedimentary rocks, Al ishosted in clay minerals such as kaolinite and illite and, occasionally, in detritalfeldspars. In metamorphic gneisses and schists, Al largely resides in feldspars andmicas.

Most common form: Al3+

Ionic radius: 0.39 (tetrahedral) and 0.54 (oc tahedral) Stable isotope: 27 (100%) Atomic weight: 26.982 Condensation temperature: 1650K Complexes in water: hydroxides Reactions limiting solubility in water:

Al2Si2O5(OH)4+ 7/2 H2O H4SiO4+ Al(OH)4+ H+(log K = 19.5) (kaolinite) Residence time in seawater: 620 years

12.3 Potassium Potassium is an alkali element, i.e. both volatile and lithophile. Its concentration in the

Earth is therefore poorly constrained. As for other volatile elements, K is depleted in

the Earth with respect to carbonaceous chondrites and enriched with respect toMars and the Moon. The dual dec ay of the radioactive isotope 40K into either 40Ca byemission or 40Ar by elec tron c apture makes this element, after U and Th, one of the

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three significant sources of heat in the Earth and accounts for about 20% of theradioactive heat production.

Most common form: K+ Ionic radius: 1.51 (octahedral) and 1.64 (dodecahedral) Stable isotopes: 39 (93.26%), 40 (0.011%), 41 (6.73%) Atomic weight: 39.098 Long-lived isotope: 40 (T1/2 = 1.25 109a) Condensation temperature: 1000K No significant complex in waters Residence time in seawater: 12 106years

12.4 Sodium Sodium is a volatile and lithophile alkaline element. As with K, the Na terrestrial

abundance is therefore not well known. Breaking down the inventory of Na amongmantle minerals is very difficult. Most major mineral phases do not accept thiselement but, under upper mantle conditions, Na solubility in c linopyroxene increasessubstantially with pressure. In the crust, Na is essentially hosted in the albitecomponent of plagioc lase feldspar. In contrast with potassium, there is no major Na-rich c lay mineral of major geological importance and most sedimentary Na resides indetrital feldspar. Evaporitic rock salt NaCl represents another significant surfacerepository of sodium.

Most common form: Na+ Ionic radius: 1.02 (octahedral) Stable isotope: 23 (100%) Atomic weight: 22.990 Condensation temperature: 970 K No significant complex in waters Residence time in seawater: 83 106years

12.5 Magnesium Magnesium is a refractory and lithophile alkaline-earth element. It does not enter

core composition in substantial concentration. After oxygen, magnesium is the

sec ond most abundant element in the mantle where it is hosted in most majorminerals (olivine, pyroxenes, garnet, spinel, ringwoodite, etc.). Its concentration in theaverage c rust is relatively low: as a consequence, amphiboles, micas, Mg-rich c lays(smectite), and carbonates (dolomite) normally remain minor mineral phases.Magnesium-rich calcite is a major form of carbonate precipitated by marineorganisms. After Na, Mg is the second most abundant cation of seawater in which it ispartly complexed by carbonate and sulfate ions.

Most common form: Mg2+ Ionic radius: 0.72 (octahedral) Stable isotopes: 24 (78.99%), 25 (10.00%), 26 (11.01%) Atomic weight: 24.305 Condensation temperature: 1340 K

Complexes in water: hydroxides, carbonates, sulfates Reactions limiting solubility in water:CaMg(CO3)2Ca2++ Mg2++ 2CO23 (log K = 1.70) (dolomite)

Residence time in seawater: 13 106years

12.6 Calcium Calcium is a refractory and lithophile alkaline-earth element. The abundance of the

isotope 40 produced by radioactive decay of 40K is oc casionally used as achronometer. The relative abundances of non-radiogenic isotopes have also beenused as a tracer of certain biological processes. J ust like Mg, Ca does not enter corecomposition in substantial quantities. In the mantle, C a is stored in clinopyroxene andin its high-pressure equivalent C a-perovskite. In igneous rocks, as in the c rust in

general, calcic plagioclase (anorthite) and amphibole are major hosts for calcium.There is no major Ca-rich clay mineral. The major low-temperature Ca-rich phase iscalcium carbonate in its two forms of calcite and aragonite. Ca lcium phosphates area ubiquitous form of Ca storage in igneous (fluorapatite) and sedimentary rocks

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(carbonate-apatite). Apatite is the essential ingredient of vertebrate hard parts(bones and teeth). Calcium sulfates (gypsum and anhydrite) are an essentialcomponent of evaporitic sequences.

Most common form: Ca2+ Ionic radius: 1.00 (octahedral) Stable isotopes: 40 (96.94%), 42 (0.65%), 43 (0.14%), 44 (2.09%), 46 (0.004%), 48 (0.19%) Atomic weight: 40.078 Condensation temperature: 1518 K Complexes in water: hydroxides, carbonates, sulfates Reactions limiting solubility in water:

CaCO3Ca2++ CO23 (log K = 8.22) (aragonite)CaCO3Ca 2++ CO23 (log K = 8.22) (calcite)

CaSO4.2H2O Ca2++ SO24+ 2H2O (log K = 4.62) (gypsum) Residence time in seawater: 1.1 106years

12.7 Iron Iron is the most abundant element in the Earth. It is refractory and, by definition,

siderophile. It is the most abundant element of the core, both in the solid inner coreand the liquid outer core in which convec tion generates the terrestrial magnetic field.In contrast with the core where Fe oc curs in its metallic and most reduced form Fe0,iron in the mantleis essentially in its Fe2+form. Ferrous iron (Fe2+) substitutes for Mg2+inmost silicatemineral phases. Iron is, after Si and Mg, the third most abundant cation inthe mantle. In the upper mantle, ferrous iron is found in olivine, pyroxene, garnet, andamphibole. In the deep mantle, it enters with Mg into the perovskite structure ofringwoodite and a lso into the oxide structure of magnesio-wstite (Fe, Mg) O. Inigneous roc ks, as in the crust in general, it is hosted in amphibole and biotite and also,together with Fe3+, Al3+, Cr3+, and Ti4+, in oxide minerals (magnetite, ilmenite). Ferriciron easily substitutes into the tetrahedral site of alkali feldspars, which is why so manygranites turn reddish upon incipient weathering. When exposed to the atmosphere orseawater at low temperature, Fe is normally oxidized to Fe3+. It is found in differentforms of iron hydroxide (such as goethite hematite, and limonite) that dominate soils,sediments, as well as ferromanganese nodules and enc rustations from the deep sea .Iron-rich clay minerals and carbonates are uncommon. Organic compounds containimportant Fe-rich proteins that have different functions, notably oxygen transport inthe cell (porphyrins). Iron concentration in seawater is very low, again bec ause of thevery low solubility of hydroxides.

Most common forms: Fe0, Fe2+, and Fe3+ Ionic radius: 0.61 for Fe2+and 0.55 for Fe3+(octahedral) Stable isotopes: 54 (5.90%), 56 (91.72%), 57 (2.10%), 58 (0.28%) Atomic weight: 55.847 Condensation temperature: 1336 K Complexes in water: hydroxides, chlorides Reactions limiting solubility in water:

Fe(OH)3Fe(OH)+2+ OH(log K = 16.5)Fe(OH)3+ H2O Fe(OH)4+ OH(log K = 4.4)

Residence time in seawater: 55 years

12.8 Sulfur Sulfur is strongly chalcophile and volatile. It has been repeatedly suggested that very

large quantities of this element are dissolved in the core and c ontribute to therelatively low seismic velocities of the core with respect to those of pure iron. Sulfurdoes not readil dissolve in silicates. Terrestrial sulfur is therefore stored in sulfides. Athigh temperatures, solid solutions of Ni and Fe dominate (monosulfide solid solution,MSS). At ambient temperature, sulfur enters a variety of sulfides. The major repositoryof sulfur in sediments is sulfides, notably pyrite. Because sulfates, the oxidized form ofsulfur, are relatively soluble, these minerals play a minor role in the making ofcontinental crust, with the exception of gypsum and anhydrite in evaporites, and

barite in hydrothermal veins. In seawater, river, and rain water, in which substantialamounts of dissolved oxygen are present, the stable form of sulfur is the oxidized form,sulfate SO24 , which is the third most abundant ion of seawater.

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Most common forms: S0, S2, and SO24 Ionic radius: 0.31 (tetrahedral), 0.29 (oc tahedral), and 1.84 (S2) Stable isotopes: 32 (95.02%), 33 (0.75%), 34 (4.21%), 36 (0.02%) Atomic weight: 32.07 Condensation temperature: 648K Dissociation of H2S in water:

H2SSHS+ H+(log K = 7.02)HSS2+ H+(log K = 13.9)

Reactions limiting solubility in water:CaSO4.2H2O Ca 2++ SO24+ 2H2O (log K = 4.62) (gypsum)

FeSFe2++ S2(log K = 18.1) (pyrrhotite) Residence time in seawater: not known but oceans are well mixed for sulfate

12.9 Phosphorus Phosphorus is a lithophile and moderately siderophile element. Substantial amounts of

this element are probably dissolved in the liquid core. It is almost exclusively hostedincalcium phosphate (apatite) Apatite may be of igneous origin. Although apatite iscertainly present in the upper mantle, P repository in the deep mantle is not wellunderstood. Biogenic (fish teeth and bones) and diagenetic apatites are the essentialrepositories of sedimentary phosphorus. They occasionally form huge deposits, as inWest Africa, are actively mined to provide agricultural fertilizer. Some of thesedeposits, found in particular in the Late Precambrian of China, are chemicalprecipitates and seem to be assoc iated with episodes of global glaciation. In low-temperature waters, phosphates form numerous complexes and, as indicated by thedissociation reactions above, speciation is pH-dependent. Phosphorus concentrationin seawater and river water is limited by the very low solubility of apatite. Phosphateradicals often attach themselves to the surfac e of iron oxyhydroxide colloids whenthey precipitate in estuaries.

Most common form: PO34 Ionic radius: 0.17 (tetrahedral) Stable isotope: 31 (100%) Atomic weight: 30.974 Condensation temperature: 1230K Dissociation of H3PO4 in water:

H3PO4H2PO4+ H+(log K = 2.15)H2PO4HPO24+ H+(log K = 7.20)HPO24PO34+ H+(log K = 12.35)

Reaction limiting solubility in water:Ca5(PO4)3OH5Ca2++ 3PO34+ OH(log K = 55.6) (hydroxylapatite)

Residence time in seawater: 70 000 years

12.10 Carbon Carbon is not an extremely abundant component of the Earth. It is both siderophile

(in its reduced form) and atmophile (in its oxidized form). Substantial amounts of thiselement may be dissolved in the core. In the mantle, carbon occurs as graphite and,at depth in excess of about 120 km, as diamond when the conditions are reducing. Inoxidizing conditions, carbon occurs as carbon dioxide, which, at depths of about 70100 km, reacts with mantle silicates to form carbonates, e.g:

Mg2SiO4+CO2MgCO3 + MgSiO3(olivine) (magnesite) (pyroxene)

Carbon dioxide solubility in magmas rapidly changes with pressure and thereforedepth.

Carbon dioxide outgassing from mantle-derived magmas starts at a depth ofapproximately 60 km and quickly strips the magma of many volatile species, such asrare gases, well before eruption. Carbon dioxide is found as fluid inclusions in olivinephenocrysts (the prime target for He isotope measurements) and makes up a veryimportant component of the gas phases in mid-oc ean ridge basalts and ocean

island basalts.

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BIBLIOGRAPHY

Albarde, Francis (2009), Geo c hem ist ry : An Int rod uc t ion. 2ndEd. Cambridge:Cambridge University Press

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