Download - Geosphere and Geochemistry

8/8/2019 Geosphere and Geochemistry

http://slidepdf.com/reader/full/geosphere-and-geochemistry 1/20

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, weshould 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 fromspectroscopy.

1



◊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 rings ◊Formation of the Earth

AccretionDifferentiation

Note: nebulous - cloudy, misty, or hazy

2



◊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

volume.

Formation of the Elements: H and He

1. The cosmic explosion produced, among other particles,

protons, neutrons and electrons that rapidly becameorganized into the elements.

2. First the subatomic particles (quarks, electrons, etc.) wereable to form,

• when temperature dropped further quarks were able to

organize into protons and neutrons,

• then these were able to form simple atomic nuclei and

• finally neutral atoms of hydrogen and helium could form

when temperatures had dropped even lower.

The formation of helium from hydrogen

3



◊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.

4



Formation of the Elements: Remaining Elements of the Periodic

Table

• The remaining elements of the periodic table wereproduced via successive nuclear fusion in 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 of silicon.

Increasing nuclear reactions, at higher temperatures lead to

the formation of elements with masses up to that of iron (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.

5



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 of the primitive light elements combined to form heavier

elements.

› 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.

6



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.

7



Neutron capture

Elements larger than Fe formed by neutron capture.

Neutron capture is a kind of nuclear 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.

8

http://en.wikipedia.org/wiki/Nuclear_reaction

http://en.wikipedia.org/wiki/Atomic_nucleus

http://en.wikipedia.org/wiki/Neutron

http://en.wikipedia.org/wiki/Nuclear_reaction

http://en.wikipedia.org/wiki/Atomic_nucleus

http://en.wikipedia.org/wiki/Neutron



9



◊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

gravity.

• 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.

10

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#fusion

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#gravity

http://imagine.gsfc.nasa.gov/docs/dict_qz.html#second

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#gravitational_collapse

http://imagine.gsfc.nasa.gov/docs/dict_qz.html#shock_wave

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#fusion

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#gravity

http://imagine.gsfc.nasa.gov/docs/dict_qz.html#second

http://imagine.gsfc.nasa.gov/docs/dict_ei.html#gravitational_collapse

http://imagine.gsfc.nasa.gov/docs/dict_qz.html#shock_wave



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

isotopes.

• 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 of neutrons -- 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

11

http://imagine.gsfc.nasa.gov/docs/dict_jp.html#matter

http://imagine.gsfc.nasa.gov/docs/dict_jp.html#neutron

http://imagine.gsfc.nasa.gov/docs/dict_ad.html#black_hole

http://imagine.gsfc.nasa.gov/docs/dict_jp.html#matter

http://imagine.gsfc.nasa.gov/docs/dict_jp.html#neutron

http://imagine.gsfc.nasa.gov/docs/dict_ad.html#black_hole



(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

slowly.

• Because of conservation of angular momentum, the cloud

spins faster as it contracts.

12

http://locscrollmenu%28%27http//antwrp.gsfc.nasa.gov/apod/ap951121.html','nebular',650,500)

http://locscrollmenu%28%27http//antwrp.gsfc.nasa.gov/apod/ap951121.html','nebular',650,500)



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.

13



(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 the protosun and protoplanets,

respectively.

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

rotating cloud

14



(b) Formation of Planetesimals

1. The heavy elements in the spinning disk have precipitated

into solid particles of dust and these particles and bits of dust began to coalesce into small bodies called planetesimals (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.

15



◊Formation of the Earth

Accretion

• The Earth accreted from Planetesimals.

• Homogeneous Accretion: Formation from undifferentiated

Planetesimals

Differentiation

• 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

16



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.

17

http://www.bhc.edu/academics/science/harwoodr/Geol101/study/images/Interior2.jpg

http://www.bhc.edu/academics/science/harwoodr/Geol101/study/images/Interior2.jpg



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.

18

http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/atoms.htm



http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/thermodynamics.htm

http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/atoms.htm#specific%20proportions

http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/atoms.htm#Minerals%20are%20the%20main%20building%20blocks%20of%20all%20rocks.




http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/thermodynamics.htm

http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/atoms.htm#specific%20proportions

http://www.uta.edu/geology/geol1425earth_system/images/gaia_chapter_5/atoms.htm#Minerals%20are%20the%20main%20building%20blocks%20of%20all%20rocks.



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

products.

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

19



According to the “big bang” theory for which there is now overwhelming

evidence, the universe as we know it (that is, all space, time, and matter) had its

origin in a point source or singularity 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 10 –32 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 heavier elements was delayed for about 106 years until nucleosynthesis commenced in

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

universe.