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26091 results in Geochemistry and environmental chemistry

Page 73 of 1305

  • 4 - Isotopics

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 33-39

  • 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.

  • 6 - Mass Spectrometry and Isotope Geochemistry

  • Kunchithapadam Gopalan
  • Book:
    Principles of Radiometric Dating
    Published online:
    21 October 2017
    Print publication:
    01 January 2017, pp 80-95

  • 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:

  • 1. An ion source for production, acceleration, and collimation of ions of different isotopes of an element(s);

  • 2. A mass analyzer for separation of ions according to their mass/charge (m/e) ratio in space or time; and

  • 3. A detector for the collection and measurement of mass-separated ions, sequentially or simultaneously.

    • PRINCIPLES OF MASS SPECTROMETRY

    Figure 6.1 is the schematic of a very old design of a mass spectrometer (Dempster, 1918) showing all the three basic components. Consider a beam of two singly charged isotopic ions ‘a’ and ‘b’ of mass ma and mb (ma + δm), respectively, produced with negligible kinetic energy in front of a stack of metal plates with narrow rectangular slits, known as the ion source. The beam is accelerated by falling through a potential of V volts distributed between the plates of the ion source, and collimated to emerge as a narrow rectangular beam from the final slit of width, ws, called the Source Slit (SS).

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

  • Part of
  • CHENGSHENG JIN, CHAO LI, THOMAS J. ALGEO, MENG CHENG, LIDAN LEI, ZIHU ZHANG, WEI SHI
  • Journal:
    Geological Magazine / Volume 154 / Issue 6 / November 2017
    Published online by Cambridge University Press:
    29 December 2016, pp. 1360-1370

  • 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.

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

  • ROMESH N. PALAMAKUMBURA, ALASTAIR H. F. ROBERTSON
  • Journal:
    Geological Magazine / Volume 155 / Issue 5 / July 2018
    Published online by Cambridge University Press:
    21 December 2016, pp. 997-1022
    • Article
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  • 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.

  • Preface

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp xv-xvii

  • 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).

  • 3 - How Are Minerals Identified?

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 37-60

  • 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.

  • 9 - How Do Igneous Rocks Form?

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 241-278

  • 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.

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

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 385-414

  • 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.

  • 11 - Sedimentary Rock-forming Minerals and Materials

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 333-352

  • Summary

    This chapter presents the systematic descriptions of uniquely sedimentary rock-forming minerals and materials. These are mainly the newly formed minerals that result from interactions of preexisting minerals with the atmosphere or precipitation from the oceans. Detrital minerals such as quartz, feldspar, and the micas, which were discussed in Chapter 7, are the major constituents of sedimentary rocks such as sandstones, arkoses, and graywackes. By studying hand specimens of these minerals in the laboratory while referring to the appropriate specimen descriptions in this chapter, you will further develop your mineral identification skills.

    In chapters 7, 9, and 10, we address many aspects of minerals and rocks of igneous origin. They are the result of crystallization from a liquid magma over a wide range of high temperatures (~1400 to 600°C) in an environment in which oxygen gas (O2) is nonexistent and in which H2O is generally a minor constituent. This chapter deals with minerals that (1) are newly formed as a result of chemical reactions (under atmospheric conditions) that slowly destroy earlier minerals (e.g., feldspars) and recombine various ions in solution in water to form clay minerals, and oxides or hydroxides, and (2) those that are chemical precipitates in depositional basins such as carbonates, evaporite minerals, and Precambrian iron-formations. However, we must also mention those minerals that have survived the physical and chemical weathering processes while exposed to the atmosphere; these are known as detrital minerals.

    We systematically treat 13 sedimentary rock-forming minerals. These include ice, the solid form of H2O; a hydroxide; a clay mineral (representative of the large clay mineral group); two polymorphs of CaCO3; four more carbonates; two halides; and two sulfates. Clearly, this group of common sedimentary minerals is totally different from the igneous rock-forming minerals discussed in Chapter 7.

    We close with discussions of three sedimentary materials: chert and agate, phosphorite, and soil.

  • 12 - Formation, Transport, and Lithification of Sediment

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 353-384

  • Summary

    Sedimentary rocks form through the compaction and cementing together of loose sediment, which can be derived from the weathering of preexisting rocks, the hard parts of organisms, or chemical precipitates. Most sediment is formed from weathered rock, and this detrital material must be transported by water, wind, or ice to depositional sites before it can begin the process of turning into solid rock. Grain size is an important property of sediment because it determines how easily sediment can be moved. During transport, sediment is sorted by grain size. In rivers, which are the main agent of sediment transport, the coarser fraction travels on the bottom, whereas the finer fraction travels in suspension, which leads to separate deposits of coarser sand and gravel and finer mud, respectively. Most detrital sediment is transported to the sea, but some accumulates in basins on continents. Along shorelines, wave action further abrades and sorts sediment. In warm climates, coastal sediments may consist almost entirely of the hard parts of calcareous organisms. Sediment eventually accumulates in basins, most of which have a specific plate tectonic setting, such as forearc basins at convergent plate boundaries, subsiding passive continental margins, and rift and pull-apart basins on continents. Sediments accumulating in these basins begin to compact under their own weight as pore fluids are expelled. These fluids are commonly saturated in the minerals with which they are in contact, and as the fluids rise, they precipitate minerals that cement the detrital grains together to form a rock. In this chapter, we discuss the formation, transport, deposition, and eventual solidification of sediment to form sedimentary rock.

    Importance of Sediments in Understanding the History of the Earth

    Sedimentary rocks, which are the topic of Chapter 13, are formed by the accumulation and burial of sediment in depositional basins. Most sediment is formed from the weathering of preexisting rocks as a result of physical, chemical, and biological processes. Some sediment is formed directly by organisms, and some may be a direct chemical precipitate. Sedimentary rocks have a diverse compositional range, not only because of differences in the type and source of the sediment but also because of processes associated with the transport and burial of sediment. Before we discuss sedimentary rocks, we must, therefore, understand how sediment is formed, transported, deposited, and turned into rock (Fig. 12.1).

  • 15 - Metamorphic Rocks

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 439-476

  • Summary

    Metamorphism includes all changes that affect rocks as a result of changes in pressure, temperature, or composition of fluids in the environment. These changes occur in sedimentary, igneous, and even former metamorphic rocks. The Earth's tectonic plates are constantly on the move, and consequently most rocks experience change at some time in their history. Metamorphic rocks are abundant, constituting approximately 60% of the continental crust. Even igneous rocks of the ocean floor are metamorphosed by circulating ocean water that cools them near midocean ridges. Metamorphic rocks preserve an important record of past conditions in the lithosphere, so it is important that we learn how to read that record. In doing this, we address the following important questions: Why do metamorphic rocks change when they find themselves in a new environment? Do these changes go to completion; that is, do they reach equilibrium under the new conditions? Do minerals in the rock tell us what those conditions were? And what are the chances that new minerals formed at depth in the Earth survive the trip to the surface during exhumation? We will learn that metamorphic rocks closely approach thermodynamic equilibrium and, as a consequence, contain only a small number of minerals. Mineral assemblages can be used to determine pressures, temperatures, and fluid compositions at the time of peak metamorphism. These assemblages are normally preserved during exhumation. Textures of metamorphic rocks provide important information about stresses in the crust during metamorphism.

    Metamorphism is the sum of all changes that take place in a rock when it experiences changes in temperature, pressure (both lithostatic and directed), or composition of fluids in the environment. The important word in this definition is change. The changes may be physical, chemical, or isotopic, or any combination of these. The original rock, known as the protolith, can be igneous, sedimentary, or a previous metamorphic rock. Most metamorphic reactions are very slow, so time is important to determine how complete a change may be. Some rocks are more reactive than others, and higher temperatures and the presence of fluids also speed up reactions. Changes that take place while rocks are heating are referred to as prograde and those occurring during cooling are referred to as retrograde.

  • 8 - The Direction and Rate of Natural Processes: An Introduction to Thermodynamics and Kinetics

  • Cornelis Klein, University of New Mexico, Anthony Philpotts, University of Connecticut
  • Book:
    Earth Materials
    Published online:
    25 September 2020
    Print publication:
    15 December 2016, pp 197-240

  • Summary

    Have you ever wondered why change occurs and why some processes are rapid whereas others are slow? Why does a match burn when struck, or why does a tombstone weather? Both processes involve reaction with oxygen in the air, but the match burns rapidly, whereas the tombstone weathers slowly. Why should the match or the tombstone react with oxygen, and why do the processes have such different rates? In this chapter we examine the fundamental principles governing all change. Thermodynamics is the subject that provides the answers to the questions about the direction of natural processes, whereas kinetics deals with the rates of processes. Thermodynamics is the study of energy and its transformations. It allows us to determine the direction of natural process and the equilibrium states toward which they strive. The rates of natural processes vary considerably, from the rapid explosion of volcanic gas to the slow motion of tectonic plates. Despite this great variation, these processes obey similar laws. We will also introduce some of the fundamental thermodynamic principles that allow us to determine what to expect if equilibrium were achieved. We then outline some of the important factors that control the rate at which equilibrium is approached. Finally, we look at one of the most remarkable rate processes, that of radioactive decay, and how it is used to determine absolute ages.

    The elements that constitute the Earth can combine in a large number of ways to form many different minerals, which, in turn, can combine in different ways to form many different rocks. In addition, one set of elements might form one mineral under one set of conditions and another mineral under another set; e.g., graphite and diamond are both forms of carbon (polymorphs, see Sec. 5.8), but graphite is the stable form at low pressure and diamond the stable form at high pressure. Or a group of minerals that form a rock under one set of conditions might change into a different group of minerals, or even melt, under another set of conditions (Chapter 15). And under still other conditions these minerals may weather and be transported and deposited as sediment (Sec. 11.17).

Page 73 of 1305