Geochemistry and environmental chemistry

Summary

Geology is the study of complex natural experiments conducted on a large scale in both time and space. The experiments are neither reversible nor repeatable. They cannot be directly observed; but they must be reconstructed historically.

S A Schumm

We view samples of the accessible universe as a library of experiments that have already been done (at different times and sites). Our art must be to select those materials which have a persistent memory and which together can bring testimony to the natural experiment of interest.

G J Wasserburg

INTRODUCTION

Simply stated, the present Earth is very different from the early Earth. The main objective of geology is to reconstruct the time sequence and nature of the key or major processes that shaped the Earth over 4.5 Gy. Schumm (1991) cautions that this task will be quite formidable by quoting a metaphorical statement of Pretorius in a different context, as follows:

It is the nature of the history of the earth that a geologist has available to him only partial information. Occasional lines from disconnected paragraphs in obscuranist chapters are what can be read. Violence in the handling of the book through time has caused many of these chapters to be ripped and reassembled out of context. That the gist of early chapters can be deciphered at all is a credit to the perseverance and imagination not always associated with other sciences. The geologist operates at all times in an environment characterized by a high degree of uncertainty and ornamented with end products which are the outcomes of the interactions of many complex variables. He sees only the end, and has to induce the processes and responses that filled the time since the beginning.

To extend this metaphor further, some of the missing pages of the book of genesis of the Earth may have to be found in the libraries of experiments carried out on other planets.

In this chapter we will consider how radiogenic isotope geochemistry has provided firm constraints on the timing of some major or large scale events in Earth's history.

Summary

It is as if Nature had taken a sample at each step of the planetary formation process, had kept it intact somewhere for 4.5 billion years, and had finally sent it to us from the sky so that we could study it in our laboratories

C J Allegre

INTRODUCTION

From the point of view of a present-day observer in the Solar System, the time scale of the universe can be split into two major subdivisions—presolar and solar—as shown schematically in Figure 8.1. The presolar period begins with the big bang at time to (as measured from the present) and ends with the isolation from the Interstellar Medium (ISM) at time tt of a small portion of the dust and gas in the ISM to form the Solar System. The immediately following solar era begins at time ti with the formation of the solar nebula as a separate entity, and ends at the present time. Within these two broad time zones, one can distinguish discrete events and protracted processes even if some of the processes are yet unfamiliar. For example, the presolar era includes the cosmological or primordial synthesis of hydrogen (H) and helium (He) immediately after the Big Bang, formation of galaxies, synthesis of heavier elements from H and He in the interior of stars, and its termination in the material destined to form the Solar System. The solar era comprises the collapse of the isolated gas cloud into a central massive body (the potential sun) and a thin disc around the central object, condensation of the material in the disc into chemical compounds at tc, aggregation and accretion of the chemical species into the planets and other objects of the solar system at ta, and subsequent internal evolution of individual planets to the present time.

As geologists recognized the immensity of geologic and, hence, the solar time from observations of large scale geologic structures and processes, astronomers and astrophysicists relied on observations of large scale structures in the universe, like stars, star clusters, and galaxies to infer an even longer presolar time scale. The best known of such indirect dating methods is the mutual recession of galaxies, as evidenced by their spectral red shift. The most precise result, so far, for the Big Bang, as based on the rate of expansion of the universe, is 13.4 ±1.6 Ga (Lineweaver, 1999).

Summary

The discovery of radioactivity provided an additional (and continuous) heat source for earth's interior and revolutionized studies of earth history by permitting quantitative dating.

C H Langmuir and W Broecker (2012)

INTRODUCTION

Certain nuclides with a specific combination of protons and neutrons may transform spontaneously or be transformed by external agents to another nuclide with a different combination of neutrons and protons. We will consider such transformations as relevant to radiometric dating in the present chapter.

SPONTANEOUS NUCLEAR TRANSFORMATIONS

Radioactivity

The spontaneous transformation or decay of a potentially unstable nuclide to a more stable nuclide is called radioactivity. The energy released by the decay is carried mainly by particles and radiation. The decaying nuclide and its product nuclide are customarily labelled parent (p) and daughter (d), respectively. If d is radioactive, it decays to another nuclide until a stable d is produced. The nuclear decay process obeys the following conservation laws of Physics: (1) Mass/energy, (2) Electric charge, (3) Linear momentum, (4) Angular momentum, and (5) Nucleon number (Kaplan, 1955; Beiser, 1973; Leighton, 1959).

It was noted in the first chapter that stable nuclides define a narrow band in a Z vs N plot (Figure 1.3) corresponding to the greatest stability of Z/N ratio as a function of N. Unstable nuclei that deviate from the path of stability eventually transform into stable nuclei by different decay modes and rates. A diagonal section across the path of the stability valley will contain isobaric nuclides with the most stable isobar in or close to the path. Neutron-rich isobars will fall below the path and proton-rich isobars above it. Unstable nuclides heavier than 209Bi along the path are also neutron-rich. Although nuclides decay in many modes, the modes most common and relevant to radiogenic isotope geochronometry are few (Dalrymple, 1991), and are shown in Figure 2.1.

Beta (β-) decay

Neutron-rich nuclei below the stability zone reach stability by converting one of their neutrons into a proton, an electron (β-), and an antineutrino (v), and then emitting the latter two. The electron created in the nucleus just before its emission is called a beta or b particle to distinguish it from an orbital or extra-nuclear electron.

Summary

The beginning of wisdom is to call things by their right names.

Confucius

INTRODUCTION

The basic measurement in isotope geochemistry is the quantitative variation of the relative and absolute numbers of isotopes of an element caused by natural physical and chemical processes and created artificially in the laboratory. This chapter deals with various topics related to this end.

ISOTOPIC ABUNDANCE

The fundamental unit of chemistry is the atom or molecule. It is, therefore, necessary to be able to measure and express the number of atoms or molecules in a chemical system (natural or artificial). However, the numbers of atoms, or atomic groups (molecules), even in a very small chemical system are far too numerous to be counted. For example, just one microgram of sodium chloride (NaCl) will contain ∼1016 sodium chloride molecules, each with one sodium and one chlorine atom. Chemists have, thus, developed a practical method to count such enormous numbers of chemical units by simply weighing them. This is based on the fundamental atomic theory that equal numbers of any atom will be contained in one gram atomic weight (atomic weight expressed in grams) of its element. This ‘equal number’, measured experimentally, is 6.023 × 1023, and is called the Avogadro's number after its discoverer.

The fundamental unit of isotope chemistry is the isotope or isotopic molecule. Numbers of isotopes of an element will also be far too numerous to count, like atoms in a chemical system. So, like the gram atomic weight in chemistry, it is more convenient to use, in isotope chemistry, the unit ‘mole’ (gram molecular weight) defined as the amount of material which contains as many particles as there are carbon (C) atoms in exactly 12 gram of pure 12C. This number is the Avogadro's number, 6.023 × 1023. This definition emphasizes that the mole refers to a fixed number of any type of identical items (e.g. mole of electrons, mole of molecules, or even a mole of men). The product of mole and Avogadro's number gives the number of a given isotope in a chemical system, and the product of mole and atomic weight of the isotope gives the mass of that isotope in atomic mass units in the system. The definition of a mole permits the calculation of the value of the unified atomic mass unit, u.

Summary

Much of the most promising modern Earth Science depends on the measurements and observations that can no longer be obtained with a hammer, Brunton compass, and hand lens alone. Although much that is important is still being done and will continue to be done in this way by first class minds, new expensive and frequently unavailable instrumentation is called for

Preston Cloud.

INTRODUCTION

Basic analytical measurements in isotope geochemistry are absolute and relative abundances of isotopes of an element available in a solid, liquid, or gaseous form. X-ray Fluorescence (XRF) Spectrophotometer, Atomic Absorption Spectrophotometer (AAS), Inductively Coupled Plasma Atomic Emission Spectrophotometer (ICP-AES), and Electron Probe Micro Analyzer (EPMA) are commonly used for elemental analysis (Potts, 1987). These instruments rely on the excitation of orbital electrons and detection of the resulting emission of radiation, or absorption of external radiation, characteristic of each element. Isotopes of an element have the same electronic structure (Chapter 1), and, hence, are not distinguished by the above instruments. Mass sensitive instruments, called mass spectrometers, have become the preferred instruments for isotope geochemistry (De Laeter, 1998). Many types of mass spectrometers have been developed over the last century, but most of them consist of three main components:

The early Cambrian Period was a key interval in Earth history with regard to changes in both ocean chemistry and animal evolution. Although increasing ocean ventilation has been widely assumed to have played a key role in the rapid appearance, diversification and spatial colonization of early animals, this relationship is in fact not firmly established. Here, we report a high-resolution Fe-C-S-Al-Ti geochemical study of the lower Cambrian Wangjiaping section from an outer-shelf setting of the Yangtze Sea of South China. Iron speciation data document a redox transition from dominantly euxinic to ferruginous conditions during Cambrian Age 3 (c. 521–514 Ma). Interpretation of coexisting pyrite sulphur isotope (δ34Spy) records from Wangjiaping reveals relatively high marine sulphate availability at Wangjiaping. Furthermore, Wangjiaping section shows lower δ34Spy (‒2.1±5.3‰) and lower TOC (2.4±1.1%) values but higher positive correlation (R2 = 0.66, p < 0.01) between TOC and Fepy/FeHR relative to deeper sections reported previously, suggesting that euxinia developed at Wangjiaping in response to increasing marine productivity and organic matter-sinking fluxes. Our reconstructed redox conditions and fossils at Wangjiaping in comparison with previously well-studied strata in the inner-shelf Xiaotan and Shatan sections suggest that planktonic and benthic planktonic trilobites with bioturbation appeared in the oxic water columns, whereas only planktonic trilobites without bioturbation occurred within the anoxic (even euxinic) water columns during Cambrian Age 3. This finding indicates that spatial heterogeneity of redox conditions in the shelves had an important effect on early animal distribution in the Yangtze Block.

The Mesaoria (Mesarya) Basin exemplifies multi-stage basin development within a regional setting of diachronous continental collision. The Plio-Pleistocene represented a period of major sediment accumulation between two topographic highs, the Kyrenia Range in the north and the Troodos Massif in the south. During Pliocene time, open-marine marls and chalks of the Nicosia (Lefkoşa) Formation accumulated in a shelf setting. The Early Pleistocene period was characterized by a relative fall in sea level and a change to shallower-water bioclastic deposition of the Athalassa (Gürpınar) Formation. The northern margin of the basin was approximately delineated by the E–W neotectonic Ovgos (Dar Dere) fault zone. A carbonate ramp system formed directly to the south of this structural feature. During Early Pleistocene time, the basin evolved from an open-marine shelf to semi-enclosed lagoons with deltaic deposits, and finally to a non-marine aeolian setting, flanked by the rising Kyrenia Range to the north. Synthesis of geological evidence from the Mesaoria (Mesarya) Basin as a whole, including outcrop and borehole evidence from the south, adjacent to the Troodos Massif, indicates that the Pliocene – Early Pleistocene represented a relatively quiescent period. This intervened between Late Miocene – earliest Pleistocene southward thrusting–folding of the Kyrenia Range and Pleistocene intense surface uplift of both the Kyrenia Range and the Troodos Massif. The basin development reflects flexurally controlled collapse during Late Miocene – earliest Pliocene time related to southward thrusting, followed by strike-slip during westward tectonic escape of Anatolia, and finally regional uplift controlled by under-thrusting of continental crust from the south, as collision progressed.

Summary

Over the past two decades, many curriculum changes have occurred in geology, Earth science, and environmental science programs in universities. Many of these have involved the compression of separate one-semester courses in mineralogy, optical mineralogy, and petrology into a single-semester offering that combines mineralogy and petrology, commonly called Earth Materials. Such a course is a challenge to the instructor (or a team of instructors) and the students. This is especially so when few, if any, textbooks for such a one-semester course have been available.

This text, Earth Materials, is an introduction to mineralogy and petrology in which both subjects are covered with a roughly even balance. To keep this textbook reasonably short and applicable to a one-semester course, we decided against providing a shallow survey of everything and instead concentrated on what we consider the most fundamental aspects of the various subjects.

In the writing of this text, we assumed that the students who enroll in an Earth materials course would have previously taken an introductory physical geology course, as well as a course in college-level chemistry.

Coverage

Basic aspects of mineralogy must precede the coverage of petrology. This sequence is obvious from the chapter headings. After a brief, general introduction in Chapter 1, minerals and rocks are broadly defined in Chapter 2. That is followed by three chapters that relate to various mineralogical aspects and concepts. Chapter 3 covers the identification techniques that students must become familiar with to recognize unknown minerals in the laboratory and in the field. It also includes discussion of two common instrumental techniques: X-ray powder diffraction and electron beam methods. Chapter 4 covers the most fundamental aspects of crystal chemistry, and Chapter 5 is a short introduction to basic aspects of crystallography. Chapter 6 covers optical mineralogy. This subject is included so that instructors who plan to introduce thin sections of rocks in their course can give their students quick access to the fundamentals of optical mineralogy and the optical properties of rock-forming minerals.

The sequencing of subsequent systematic mineralogy chapters is completely different from that most commonly used in mineralogy textbooks. In these chapters, minerals are discussed in groups based first on chemistry (native elements, oxides, silicates, and so on) and, subsequently, for the silicates, on structural features (layer, chain, and framework silicates, and so on).

Summary

The ability to recognize common minerals in hand specimens is basic to much that follows in this text. The tests that you will become familiar with are exactly the same as those used worldwide by field and research geologists, be it in the field, in the research laboratory, or in the home office. Your course instructor may discuss the various techniques for the unique identification of a mineral (or several minerals as in a rock) in lectures, but personal expertise can be gained best by hands-on work that you will likely do in the laboratory that accompanies your course. There you will have small test samples that you will be allowed to scratch (to test hardness), to hold in your hand (“to heft” the specimen so as to assess its average specific gravity), and to evaluate their reaction to dilute HCl, and so on. Even better is that you will probably also see much larger, better mineral hand specimens in which you can observe other properties such as crystal form, habit, cleavage, range of color, and state of aggregation. You will learn to combine these observations and develop the skills to identify unknown minerals correctly.

Once you know what mineral you are dealing with, or which several minerals, as most common rocks contain a mix of minerals, you will be well prepared for understanding how rocks are classified and the conditions under which different rock types (igneous, sedimentary, and metamorphic) are formed. For example, once you have identified all four major minerals in a specific, relatively coarse-grained, light-colored rock as (1) quartz, (2) two types of feldspar, and (3) a mica, you can conclude that you are dealing with a granite. Such knowledge of common rock-forming minerals is basic to much of what is presented in this text. The four later chapters that deal with the systematic mineralogy of igneous rocks (Chapter 7); sedimentary rocks (Chapter 11); metamorphic rocks (Chapter 14); and ore deposits, coarse-grained pegmatites, and quartz veins (Chapter 16) give detailed information on the diagnostic properties that allow us to identify minerals.

We introduce you to various observations and tests that can be made of minerals in hand specimens. In many instances a combination of the results of several of these leads to the identification of the mineral at hand.

Summary

Igneous rocks are the most abundant rocks in the Earth's crust, formed by the crystallization of melts that have risen from the planet's interior. The crust itself has formed over an extended period of time, in fact billions of years, by just such crystallization. The outer half of the planet is normally solid, so it is natural to wonder where magmas come from. Where does this melting take place, and why do normally solid rocks become molten? What composition do rocks have in the source region? Is melting partial or total? What magma compositions are formed when these rocks melt? What makes magma rise toward the surface, and how fast does it travel? As magma ascends and begins to cool, it becomes a mixture of liquid and crystals that can separate from one another and change the composition of the magma. Eventually, magma cools and solidifies to form igneous rock. The processes leading to the formation of this rock are of great importance because they have controlled the differentiation of our planet. The composition of the crust is very different from that of the planet as a whole, and life as we know it would be very different if these processes had not taken minor elements from the Earth's interior and concentrated them in the crust. In this chapter we look into each of these igneous rock-forming processes. We leave to Chapter 10 a discussion of the diversity and classification of igneous rocks.

Igneous rocks are those formed by the solidification of molten rock. This molten material, which we call magma, is formed at depth in the Earth and rises toward the surface, where it cools and solidifies, either beneath the surface, where it usually has time to crystallize, or on the surface as volcanic rocks, where cooling may be rapid enough to form glass. We classify igneous rocks on the basis of the minerals they contain, which reflect the composition of the magma. Magma compositions are determined in the source region by the type of rock that undergoes partial melting, but they can be modified during ascent and solidification, especially in large magma chambers, where solidification can take thousands of years. Throughout geologic time, the rise of magmas from the mantle has slowly generated the Earth's crust, whose composition is therefore determined by the composition of magmas.

Summary

In the previous chapter, we looked at how sediment is formed, transported, deposited, and lithified to form rock. In this chapter, we discuss the classification of sedimentary rocks, which fall into three main groups: (1) the siliciclastic sedimentary rocks, which include mudrocks (shale), sandstones, and conglomerates; (2) the biogenic sedimentary rocks, which include limestones, dolostones, cherts, and coals; and (3) the chemical sedimentary rocks, which include evaporites, phosphorites, and iron-formations. We learn how each type can be identified and examine the textures of the most important types in photomicrographs of petrographic thin sections. Correct identification of sedimentary rocks is the first step to working out the geologic history of an area, because these rocks preserve a record of the source of sediment, its mode of transport, and its site of deposition, which in many cases is determined by plate tectonic processes. We examine the geologic record preserved in some specific sedimentary rock sequences. Sedimentary rocks are the source of all fossil fuels, most iron ore, material used for fertilizers, much of the world's gold ore, and many other placer mineral deposits. Most of the world's construction material comes from sediments and sedimentary rocks in the form of building stones and raw material (lime and aggregate) for making cement. Porous sedimentary rocks form important aquifers both for agricultural use and for sustaining urban areas. In this chapter, we indicate with which sedimentary rocks each of these economically important resources is associated. We also discuss the plate tectonic setting in which each sedimentary rock is formed.

Sedimentary rocks are formed from the lithification of sediment (Fig. 12.1). They are classified into three main groups based on the type of sediment from which they are formed. Those formed from the detritus of weathered rocks are called siliciclastic because most of their minerals are silicates. Those formed from sediment derived from organisms are described as biogenic, and those formed by chemical precipitation are referred to as chemical sedimentary rocks.

Geochemistry and environmental chemistry

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26098 results in Geochemistry and environmental chemistry

11 - Chronology of Earth History

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Index

8 - Meteorites: Link between Cosmo- and Geochemistry

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Frontmatter

Acknowledgments

2 - Nuclear Transformations

Summary

5 - Radioactivity and Radiometric Dating

References

Dedication

4 - Isotopics

Summary

6 - Mass Spectrometry and Isotope Geochemistry

Summary

Evidence for marine redox control on spatial colonization of early animals during Cambrian Age 3 (c. 521–514 Ma) in South China

Pliocene–Pleistocene sedimentary–tectonic development of the Mesaoria (Mesarya) Basin in an incipient, diachronous collisional setting: facies evidence from the north of Cyprus

Preface

Summary

3 - How Are Minerals Identified?

Summary

Minerals and Varieties

Index

9 - How Do Igneous Rocks Form?

Summary

Common Units of Measure

13 - Sedimentary Rock Classification, Occurrence, and Plate Tectonic Significance

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Geochemistry and environmental chemistry

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