Geology 323 Final-University of Calgary

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Geochemistry (GLGY 323-UCAL) Final Exam

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Credits: Based on the excellent class notes provided by, Dr. Rajeev Nair during Winter 2012.
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Concepts and Additional Questions for Winter 2012 Final

-What is secular equilibrium?
-Which step defines the rate of uranium decay once it has reached secular equilibrium?
-How isotopic fractionation is differ from elemental and mineral fractionation?
-What are the three stages of nucleosythesis?
-What are isobars, isotopes and isotones?
-How is p-process differ from the r & s processes occurs in supernovae or explosive nucleosynthesis?
-What are the distinctions between a meteorite and a comets?
-What are the four basic ways to ID a meteorites?(Ni con, Chondrules, CAI-Refractory incl, Matrix)
-What is Oddo-Harkins Rule(effect) and how is it demonstrated on REE abundance patterns?
-What is the difference between an undifferentiated and a differentiated meteorite?
-What are LILEs, HFSEs and how REEs fit it there?
-Why are HFSEs in MORB and subduction zone basalt are depleted while LILEs enriched in MORB?
-What is closure temperature and what conditions must be met in order to use radioactive isotopic dating?
-What is solidus and liquidus indicate during; a) Melting of rocks and b) crystallization of magma?
-What are/is source process/es and what are/is differentiation process/es?
-Why Zircon is commonly used for U-Pb dating?
-How carbon dating is used and once the “clock” of 14C going to 12C starts in organisms?
-While 12C going and 13C are both stable isotopes of carbon, why 12C is the most abundant(98.89%) form?

Did you miss anything? (key concepts)

-Know how do do stability diagrams, Eh-pH diagrams, Activity-Activity and Harker diagrams.
-Eh of natural systems (+ = e move out of SHE and natural env is oxidizing)
-Be able to describe all stages of nucleosynthesis in detail.
-Big Bang Nucleosynthesis occurred ~14 billion years ago and formation of first atomic nuclei after 3 min.
-Know how to calculate delta for stable isotopes.
-Know the types of mass dependent isotopic fractionations.
-Know the differences between OIB, MORB, subduction zone basalt compositions and how the were formed.
-Be able to describe identifiable parts of Chondrites, how they form and what they composed of..
-Know that Fe is the most stable atom because it has the highest nuclear binding energy (NBE).
-Any element up to Fe undergo fusion to form new elements while elements after Fe undergo fission.
-Large atoms often undergo fission not fusion.
-Know H, He was the first two elements to form with some minor amounts of Li and Be from BBN.
-Alpha decay, beta decay, positron decay, electron capture, nuclear fission and gamma ray emission.
-Redox reactions and Eh calculations.
-Describe the Tipple-Alpha process and why we need it to explain some nucleons.
-Franhuafer lines and how it helps geochemists.
-Difference between classes and groups in classification of meteorites.
-Fractionation process of mineral and isotopes.
-Be able to describe the 3 types of magma; primary, primitive(parental) and evolved.
-Know what is the liquid line of decent on element-element diagrams.
-Know that most REEs have a charge of 3+ except for Ce4+ and Eu2+.
-Difference between a radiogenic isotope, a radioactive isotope and a stable isotope.
-Know how to calculate the relative mass difference between isotopes.
-Tc and Pm anomalies…
-Know the Carbon-14 dating techniques, calibrations and limitations.
-Be able to interpret and give a graphic and dirty description of “spider diagrams”
-Know Pb-U concordia diagram (concordia curve/discordia line/the alpha & beta intersections)
-Know how to use decay equations.
-Know the concepts related to isotope fractionation in the hydrosphere
-All the types of standards used in stable isotope geochemistry
-Equilibrium and kinetic isotopic fractionation.
-Methods of radiometric dating and the manipulation of formulae.
-Isotopic fractionation due to progressive rain and isotopic variations in meteoric water.
-Ice core and carbonate sediments data analysis.
-Know the standards used in stable isotope geochemistry. (more info-click here)
-Be familiar with all the formulae.
-Know how to use every concept we learned in labs, specially for the last 5 labs
Spend more time on Lecture 17 to 34(last lecture)

Midterm I Exam Concepts

Basic Chemistry

There are three conventions on how the electrons are arranged around an atom; Aufbau, Pauli and Hunds.

1. The Aufbau principle: electrons placed in orbitals with the lowest energey first and additional electrons are placed in orbitals that minimize the energy of the atom.

2. The Pauli Exclusion principle: no two electrons in an atom can have the same value for all the four quantum numbers; n, l, ml, ms.

3. Hund’s rule of maximum multiplicity: orbitals are singly occupied first before they are paired (filled) with electrons of opposite spins.

  • Nuclear stability is determined by the nucleons(proton/neutron) number ratio in the nucleus of the atom and the geochemical stability is determined by the electronic configuration of the atom.
  • Mass of the nucleus is always less than the sum of the individual mass of protons and neutrons because some of the theoretical mass is converted in to nuclear binding energy during the formation of the atom.
  • Physical and chemical characteristic of compounds/minerals depend on the type of bonds, ionic or covalent and the elements involved.
  • Electron affinitydef increases from left to right along a period on the periodic table of elements. Since addition of an electron is generally exothermic, higher electron affinity (large negative number) indicate an electron acceptor.

    An Earth Scientist’s Periodic Table of the Elements and Their Ions: web | table

    Identification of Elements and Minerals

    This is an overlap between geochemistry and mineralogy. However, in geochemistry the analytical quantitative methods are used to identify elements and minerals while mostly qualitative analysis is used in mineralogical assessments. These analytical methods includes:

    1. X-ray Fluorescence (XRF): web
    -destructive bulk analysis
    -detection is at ppm level (can detect major, minor and trace elements)

    2. Instrumental Neutron Activation Analysis (INAA): web
    -destructive bulk analysis
    -detection is at ppm level (can detect major, minor and trace elements)
    -can detect large number of elements

    3. Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES): web
    -destructive bulk analysis
    -not as good in precision as XRF, INNA and ICP-AES

    4. Atomic Absorption Spectrophotometry (AAS): web

    5. Electron Microprobe Analysis (EPMA): web
    -non-destructive in-situ analysis
    -detects elements from B to U
    -cannot detect light elements and bad for trace/minor element detection

    6. Mass Spectrometry: web

    Please use Wikipedia for more information on each of these instruments and methods. I think it is redundant to post information that is already available.

    Chemical Composition of Earth Layers

    Earth started with homogeneous chemical composition and later chemically differentiated into distinct layers; crust, mental and core. The core formed first by segregating mostly iron from silicates causing the resulting composition mostly made of silicon. This silica rich composition known as Bulk Silicate Earth(BSE). The BSE also known as primitive mantle. Because the mental was molten the heavy mental such as iron separate and sink to the core of the Earth hence contributing more iron to the core formation. The BSE further differentiated in chemical composition forming the crust and the mantel.

    The composition can be analyzed using weight percentages(Wt%) of each chemical compound. Geochemist observed that silicon rich SiO2 is the most common form of compound found in the Earth’s crust while mantle is about 45.1% SiO2 with 38.1% MgO. These data is complied based on many laboratory studies on petrology. The following table list the composition in Wt%:

    Wt% SiO2 TiO2 Al2O3 FeO1 MnO MgO CaO Na2O K2O P2O5 CO2 H2O
    Upper Crust 50.5 1.56 15.3 10.5 < 1 7.47 11.5 2.62 0.13 0.13 ? ?
    Lower Crust 60.6 0.7 15.9 6.7 0.10 4.7 6.4 3.10 1.80 0.10 ? ?
    Mantle 45.1 3.3 8.0 38.1 3.1 ? ?
    Primarily Fe-Ni Alloy with some light elements2 such as Si, S, O, C, K.

    1. Fe form in two different ways in nature; Fe2+ and Fe3+ and depend on which Fe, the element ratio between F:O will charge.

    2. this account for the disparity in the measured density of the core from the seismic data composed from a pure Fe-Ni alloy.

    During oceanic crust formation, the melting process undergo fractionation of chemicals. Continental crust is more felsic(silica rich) than oceanic crust but felsic magma is produced by melting of pre-existing basaltic rocks. Oceanic crust is formed as a result of decompression melting of silica rich mantle at spreading mid-ocean ridges producing basaltic rocks (peridotites).

    Distribution of Elements in Rocks and Minerals

    Victor Moritz Goldschmidt (1888-1947) pioneered in classification of elements in Earth by their partitioning behaviour.

    Distribution or partitioning of elements between two different phases can be calculated using distribution coefficient (aka partitioning coefficient) which is defined as for element i, the coefficient:

    Kdi = concentration of i A/concentration of i B, where A and B are two phases and Kd can be determined by experimental values or theoretical calculations. When partition coefficient is between two solid phases of minerals it doesn’t matter which mineral is on the denominator and which one is on the numerator.

    If the partitioning occurred between a melt and a solid, then the solid always has to be on the numerator. During melting of solid rocks, the incompatible elements preferentially moves into the melt. In solid mineral-melt situation, the following rules are used by geochemists to determine the condition of the partition:

  • If the Kdi > 1 then,
    -element preferentially partition into the mineral/solid
    -compatible elements
  • If the Kdi < 1 then,
    -element preferentially partition into the melt
    -incompatible elements
  • If the Kdi = 1 which is extremely rare, then,
    -no preference
  • Note: There are several factors that effect the Kd value. These are temperature, pressure and composition of minerals/composition of magma.

    Bulk partition coefficient determines the distribution of elements between a melt and its minerals in multiple phases. It is defined as for element i, the coefficient:

    Di = WAKdiA + WBKdiB or Di = WjKdiJ, where Wj is the weight fraction and KdiJ is the partition coefficient of element i in mineral j. The following rules are used by geochemists to determine the condition of the partition:

  • D > 1 = compatible (solid favored; preferentially petition into the mineral)
  • D < 1 = incompatible (melt favored; preferentially petition into the melt)
  • Goldschmidt’s classification(ref) of elements based on their partitioning behavior has four phases; gas phase, silicate phase, sulfide phase and metallic phase.

    Phase Discription
    Gas elements preferentially partition into a fluid phase (found in atmosphere, called atmospheric elements)
    Silicate elements preferentially partition into silicate phase (lithophile elements)
    these elements also preferentially binds with oxygen
    Sulfide elements preferentially partition into sulfide (chalcophile elements)
    Metallic elements preferentially partition into metallic (siderophile elements)

    Rare Earth Elements (REE)

    Rare Earth Elements are seventeen natural chemical elements found on Earth. The name “rare earth elements” were coined by geologists because often ore deposits containing these elements are not found in high concentrations to be mined. (with exception of promethium, all other elements are abundant across the world, but not found in deposits of high concentrations). It is misleading because are actually not rare at all. However in nature, they are not found in high concentrations.

    These elements have specific atomic number and appears on the periodic table of elements. Some textbooks may refer this group as rare earth metals, rare earth oxides, etc. Most REE are in the Lanthanide(ref) group.

    A list of REE:

    21 Sc Scandium
    39 Y Yttrium*
    57 La Lanthanum
    58 Ce Cerium
    59 Pr Praseodymium
    60 Nd Neodymium
    61 Pm Promethium
    62 Sm Samarium
    63 Eu Europium
    64 Gd Gadolinium
    65 Tb Terbium
    66 Dy Dysprosium
    67 Ho Holmium
    68 Er Erbium
    69 Tm Thulium
    70 Yb Ytterbium
    71 Lu Lutetium

    The element Yittrium(39) is not an REE, but usually grouped with REEs because of similar geochemical properties and behavior as of other elements in this group. Exception to Eu and Ca which occur in 2+/3+ and 4+ states respectively, REEs are commonly occurred in 3+ valance state.

    Atomic radii decreases when moving from atomic number 57 down to 71. This increases the number of crystallographic sites which these elements can occupy. Even though REEs are incompatible elements, within the REE group of elements the relative compatibility increase with reduction in atomic radii.

    REE are further divided into three sections based on the atomic weight; Light REE(LREE), Middle REE(MREE) and Heavy REE(HREE). The HREEs are more compatible (relatively larger D than other sections of REE.

    Isotopic Geochemistry

    Table of Standards used in stable isotopic geochemistry
    [table id=1 /]

    Standing on the shoulders of giants