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Geosphere and Geochemistry

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  • 8/8/2019 Geosphere and Geochemistry




    How did the elements on earth originate?

    These originated with the creation of the universe.

    Therefore, before considering the elements on Earth, weshould consider the origin of the universe and the solar


    Information on the composition of this nebula has come from

    meteorites and information on solar composition fromspectroscopy.


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    Beginning of the Universe(1) The Big Bang

    Formation of the Galaxies

    (2) Star Formation

    Formation of the Solar System:

    (3) Supernova explosion: Explosion of a massive star(4) Solar Nebula Condensation

    (5) Formation of the Sun and Planetary ringsFormation of the Earth


    Note: nebulous - cloudy, misty, or hazy


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    Beginning of the Universe:

    (1) The Big Bang

    The Universe is thought to have begun in a "Big Bang" approx.

    15 billion years ago.

    According to the Big Bang Theory the Universe expanded to itspresent enormous volume from an initial miniscule starting


    Formation of the Elements: H and He

    1. The cosmic explosion produced, among other particles,

    protons, neutrons and electronsthat rapidly becameorganized into the elements.

    2. Firstthesubatomic particles (quarks, electrons, etc.) wereable to form,

    when temperature dropped further quarks were able to

    organize intoprotons and neutrons,

    then these were able to formsimple atomic nucleiand

    finally neutral atoms ofhydrogen and helium could form

    when temperatures had dropped even lower.

    The formation of helium from hydrogen


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    Formation of the Galaxies:

    Regions began to emerge which had concentrations of the

    gases hydrogen and helium higher than elsewhere.

    These formed the basis for the development of what we nowrecognize as galaxies.

    A galaxy starts to form by the accumulation of hydrogen gas ina very large cloud, called a nebula.

    Galaxies formed (perhaps within the first million years of the

    history of the Universe) by the inward gravitation collapse of

    the early matter of the Universe, thus enhancing the difference

    between regions where there was matter and regions wherethere was none.

    (2) Star Formation

    The in homogeneities within galaxies, coupled with further

    gravitational collapse, are the basis for star formation.

    As matter aggregates in a growing nebula the internal gravitydraws in more gases.

    Eventually the nebula develops localized 'clumps' of gas whichcontinue to grow into even denser gaseous bodies stars.


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    Formation of the Elements: Remaining Elements of the Periodic


    The remaining elements of the periodic table wereproduced viasuccessive nuclear fusionin stars (up to Fe),

    and neutron capture (larger than Fe).

    Early in star development hydrogen is utilized tomanufacture the element helium.

    The nuclei of the primitive light elements combined to formheavier elements.

    The fusion reactions proceeded in stages producing

    successively heavier elements.

    As the hydrogen in the star is used up, the star contracts and

    its temperature rises so that nuclear reactions can take placewhich permit the synthesis of the elements carbon, nitrogen

    and oxygen, from helium.

    When the helium is almost completely consumed the carbon

    and oxygen can be transformed into elements with masses upto that ofsilicon.

    Increasing nuclear reactions, at higher temperatures lead to

    the formation of elements with masses up to that ofiron (Fe).

    Beyond this point no heavier elements can be formed by the

    process of nuclear fusion because the temperatures requiredare higher than those found in stars.


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    Nucleosynthesis (Stellar Nucleosynthesis)

    This is the process through which elements were manufactured

    inside the stars.

    This is basically the fusion of small atomic nuclei to make

    bigger ones.

    The very high temperatures caused by gravitational collapse

    ignited the fires of thermonuclear fusion in which the nuclei ofthe primitive light elements combined to form heavier


    Elements up to Fe formed by fusion

    In the largest of these stars, the fusion reactions proceeded in

    stages producing successively heavier elements.

    Nuclear burning is under way at all interior layers of such stars.

    The Figure below is a cutaway drawing of such a multiple

    burning star.


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    The peripheral stellar layers produce helium,

    The intermediate layers synthesize a variety of heavier-than-helium nuclei and

    The innermost core layers house the sites that creates heavy

    nuclei up to and including iron.


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    Neutron capture

    Elements larger than Fe formed by neutron capture.

    Neutron capture is a kind ofnuclear reaction in which an

    atomic nucleus collides with a neutron and they merge to forma heavier nucleus.

    The process is also known as thermal capture.


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    Formation of the Solar System:

    (3) Supernova explosion: Explosion of a massive star

    These are stars 8 times or more massive than our Sun.

    A supernova explosion occurs when there is no longer enough fuelfor the fusion process in the core of the star to create an outwardpressure which combats the inward gravitational pull of the star's

    great mass.

    When the nuclear reactions have used up all of the hydrogen at thecenter with no hydrogen left anywhere this abrupt cessation of

    hydrogen burning means that the star now has no nuclear energysource.

    First, the star will swell into a supergiant...at least on the outside.

    On the inside, the core yields to gravity and begins shrinking.

    As it shrinks, it grows hotter and denser.

    A new series of nuclear reactions begin to occur....temporarilyhalting the collapse of the core... but alas, it is only temporary.

    When the core contains essentially just iron, it has nothing left tofuse (because of iron's nuclear structure, it does not permit its

    atoms to fuse into heavier elements).

    Fusion in the core ceases.

    In less than a second, the star begins the final phase of

    gravitational collapse.

    The core temperature rises to over 100 billion degrees as the

    iron atoms are crushed together.

    The repulsive force between the nuclei overcomes the force of


    So the core compresses, but then recoils.

    The energy of the recoil is transferred to the envelope of the

    star, which then expodes and produces a shock wave.


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    As the shock encounters material in the star's outer layers, thematerial is heated, fusing to form new elements and radioactive


    The shock then propels the matter out into space.

    The material that is exploded away from the star is now known

    as a supernova remnant.

    All that remains of the original star is a small, super-dense core

    composed almost entirely ofneutrons -- a neutron star. Or, if theoriginal star was very massive indeed (say 15 or more times the

    mass of our Sun), even the neutrons cannot survive the corecollapse...and a black hole forms.

    The Life Cycle of a Massive Star


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    (4) Solar Nebula Condensation:The solar system formed from the collapse of a huge cloud

    of material (gas and dust) known as the Solar Nebula.

    (a) As the gas collapsed from gravitation it formed a rotating disk of gasthat first heated and then cooled.

    Collapsing Clouds of Gas and Dust:

    A great cloud of gas and dust (called a nebula) begins to

    collapse because the gravitational forces that would like tocollapse it overcome the forces associated with gas pressure

    that would like to expand

    The Spinning Nebula:

    It is unlikely that such a nebula can be created with noangular momentum, so it is probably initially spinning


    Because of conservation of angular momentum, the cloud

    spins faster as it contracts.


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    The Spinning Nebula Flattens:

    Because of the competing forces associated with gravity, gas

    pressure, and rotation, the contracting nebula begins toflatten into a spinning pancake shape with a bulge at the

    center, as illustrated in the following figure.

    (b) Meanwhile out in the spinning disk as the gas cooled heavy elementsbegan to precipitate into solid particles of dust.


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    (5) Formation of the Sun and Planetary rings

    (a) Condensation of Protosun and Protoplanets

    As the nebula collapses further, instabilities in the

    collapsing, rotating cloud cause local regions to begin tocontract gravitationally.

    These local regions of condensation will become the Sunand the planets, as well as their moons and other debris in

    the Solar System.

    While they are still condensing, the incipient Sun and

    planets are called theprotosun andprotoplanets,


    As the nebula collapses further, local regions begin to contractgravitationally on their own because of instabilities in the collapsing,

    rotating cloud


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    (b) Formation of Planetesimals

    1. The heavy elements in the spinning disk have precipitated

    into solid particles of dust and these particles and bits ofdust began to coalesce into small bodies calledplanetesimals(sort of mini planets).

    The precipitates were crystals (minerals) of platinum-groupmetals, Os, Ru and Ir followed by Al oxides, metallic nickel-

    iron and Mg silicates followed by more complex silicates

    then various sulfides of heavy metals.This precipitation sequence is preserved in primitive

    meteorites. Chondrules, small rounded objects found insome meteorites, are formed.

    2. Over time, the planetesimals collide and group together intolarger and larger bodies, eventually reaching planetary size.

    Eventually, the large planets sweep up most of theremaining planetesimals (and smaller bodies) and the solar

    system reaches a relatively stable configuration.Overall, this process from collapse to solar system takes

    something like 0.1-1 billion years.

    3. Before temperatures cooled sufficiently in the inner solar

    system so that the most volatile elements (H, C, N and the

    noble gases) could condense, H fusion in the sun ignited andblew these elements to the outer solar system where they

    are enriched in the outer planets, Jupiter, Saturn, Uranus,and Neptune.


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    Formation of the Earth


    The Earth accreted from Planetesimals.

    Homogeneous Accretion: Formation from undifferentiated



    As the proto Earth grew from the influx of solid particles itgot hot enough to melt so that the dense Ni-Fe metal

    together with elements soluble in the metal sank to the


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    center and formed the core, and the lighter oxygen-bearingminerals (mostly silicates) formed the mantle.

    Differentiation is the separation of materials due to densitydifferences. Planet Earth develops with a core of iron and

    nickel, and a crust of silicates due to differentiation.

    Today the mantle is entirely solid and has been throughoutmost of Earth's history, whereas the core comprises a liquid

    metal outer core and a solid metal inner core.


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    How did this differentiation happen?

    What happened to that mixture of all the elements constitutingthe Earth once the Earth started to heat up and differentiate?

    1. Basically, whenever chemical elements (atoms) are brought

    together there is a tendency for them to react with each

    other and to form compounds.

    [How this works exactly is the subject of thermodynamics or

    physical chemistry. Thermodynamics allows us to calculate theoutcome of chemical reactions when we bring certain

    substances together.]

    The atoms and molecules in these compounds are present in

    compound-specific proportions, and they are not randomlydistributed.

    Instead, they show very specific geometric arrangements.We know these compounds making up the crust and mantle

    are commonly known as minerals.

    2. The first precipitates were crystals (minerals) of platinum-

    group metals, Os, Ru, and Ir, followed by aluminum oxides,metallic Ni-Fe, and Mg silicates.

    This was followed by more complex silicates and then by

    various sulfides of heavy metals.We can see this precipitation sequence preserved in

    primitive meteorites.

    3. As the proto Earth grew from the influx of solid particles itgot hot enough to melt so that the dense Ni-Fe metal

    together with elements soluble in the metal sank to the

    center and formed the core, and the lighter oxygen-bearingminerals (mostly silicates) formed the mantle.

    The material that was displaced into the mantle duringformation of the iron core contained abundant O, Si, Mg, Fe,

    Al, and Ca (plus smaller quantities of a range of other

    elements) and under the pressures and temperatures thatprevail there, chemical reactions (following the laws of

    thermodynamics) produce compounds (minerals) that areknown as olivine and pyroxene.


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    During formation of the crust, other compounds (minerals),in particular feldspars and quartz were common reaction


    4. So, except for the oceans and atmosphere, the Earth today is

    made up of solid minerals to a depth of about 2900 km.

    The physics and chemistry of the solid phases (minerals) of theEarth control much of the physics and chemistry of our

    environment. Unlike fluids, minerals preserve the records of

    Earth's history. Further minerals contain the wealth of naturalresources of the planet. Therefore understanding the physics

    and chemistry of the solid materials of the planet (mineralogy)is central to geochemistry


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    According to the big bang theoryfor which there is now overwhelmingevidence, the universe as we know it (that is, all space, time, and matter) had its

    origin in a point source orsingularity that began an explosive expansion about

    12-15 billion years ago, and which is still continuing.

    Following a brief period of extremely rapid expansion called inflation, protons

    and neutrons condensed out of the initial quantum soup after about 1032 s. The

    first chemical species, 1H, became stable during the first few minutes, along

    with some of the very lightest nuclides up to 7Li, which were formed through

    various fusion and neutron-absorption processes. Formation of most heavierelements was delayed for about 106 years until nucleosynthesis commenced in

    the first stars. Hydrogen still accounts for about 93% of the atoms in the


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LU 2 GEOSPHERE AND GEOCHEMISTRY ELEMENTS How did the elements on earth originate? These originated with the creation of the universe. Therefore, before considering the elements on Earth, we should consider the origin of the universe and the solar system. Information on the composition of this nebula has come from meteorites and information on solar composition from spectroscopy. 1
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