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Geomorphology and Geochemistry

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An introduction to Geomorphology, Composition of Earth, Oddo Harkins rule, crust, core, mantle; P wave and S wave, Clarke of Concentration

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Geomorphology and Geochemistry

  1. 1. Introduction to Geomorphology Prof. P. K. Mani ,BCKV ACSS-751
  2. 2. At the beginning,,,At the beginning,,, Matter + antimatterMatter + antimatter Matter has the advantageMatter has the advantage baryonsbaryons  quarks, leptons,quarks, leptons, electrons, photons (noelectrons, photons (no protons or neutrons)protons or neutrons) HadronsHadrons  protons,protons, neutronsneutrons Hydrogen, helium (1:10 H:He)Hydrogen, helium (1:10 H:He) ORIGIN OFORIGIN OF ELEMENTSELEMENTS
  3. 3. What is the Solar System made of? What is the relative abundances of the various elements throughout the Universe? This turns out to be a difficult task for one obvious reason. Spectroscopic measurements of elements from the distant stars are strongly biased towards only those elements in excited states at or near the stellar surface. Those elements principally in the interior do not contribute to surface radiation in the same proportions as actually exist in a star. The situation is better for the Sun. When element distributions are stated as Cosmic Abundances, they actually are rough estimates made from Solar Abundances .
  4. 4. What is the Solar System made of? From the figure, we see four patterns:  An overwhelming abundance of light elements  A strong preference for even- numbered elements  A peak in abundance at iron, followed by a steady decrease.  Elements 3-5, Lithium, Beryllium and Boron, are very low in abundance. These patterns have to do with nucleosynthesis (element formation) in the stars.
  5. 5. Importance of Solar system in terms of geology Spectroscopic observation indicates that the relative abundances of different elements are about the same in every part of the Universe. The most striking feature is extreme abundance of H (5.1 x 108 per 104 atoms of Si) and He (1x 108 / 104 atom of Si) while Oxygen is also quite abundant (2.8 x 106 atoms/104 atom of Si) Composition of the Earth For the composition of interiors of planets we must fall back on analogy with our own planet and the meteorites are classified as (i) Siderites ( about 98% metal, mostly Ni, Fe) (ii) Siderolites(stony irons, about 50% metal and 50% silica) (iii) Aerolites (stones) (a)Chondrites- Due to presence of chondri which are small rounded bodies consisting of Olivine, pyroxenen and sometimes the glass (12% Ni , Fe combined, 46% Olivine, 28% Hypersthene, 4% Diopside, 11% Plagioclase) (b)Achondrites( Do not contain any chondri) (iv) Tektites: Glassy metorites about 70% Si
  6. 6. According to Goldschmidt, average composition of meteorites are Silicate-76.9% Triolite - 7.7% Metal Ni-Fe - 15.4% On the basis of the studies by Farington and other evidence, Daly concludes that meteorites originated by the disruption of a planet early in the evolution of solar system situated between Mars and Jupiter. In the region now occupied by Asteroids. Meteorites and terrestrial matter differ from stellar materials in the relative scarcity of gaseous elements but this can be explained by in terms of physical condition associated with the origin of the solar system. The variation in lighter elements like H, He, Li, B, N in different part parts of the universe is due to their participation in thermo nuclear fusion responsible for energy production. From the analysis of abundance data, first compiled by Goldschimdt and then Corrected by Urey. The following facts come out (i) the abundances show rapid exponential decrease for elements of the lower atomic number (Z=30) followed by an almost constant value for the heavier elements . Elements of even Z are more abundant than those of odd atomic No (Z) on either side - -----Oddo Harkins Rule
  7. 7. The Oddo-Harkin’s rule ElementsElements Wt. %Wt. % OO 62.562.5 SiSi 21.221.2 AlAl 6.56.5 FeFe 1.91.9 MgMg 1.841.84 CaCa 1.91.9 NaNa 2.62.6 KK 1.421.42 Why?Why? (1)(1) H, He, Li, Be andH, He, Li, Be and B (the lightest fiveB (the lightest five elements) do notelements) do not obey the ruleobey the rule (2)(2) Fe is higher andFe is higher and Sc is lower thanSc is lower than expectedexpected (3)(3) Check otherCheck other featuresfeatures Even atomic numbered elements are more dominant than the adjacent odd numbered ones
  8. 8. The relative abundances for elements of Z> 28 vary less than those of elements of lower Z. Only 10 Elements H, He, C, Na, O, Ne, Mg, Si, S, Fe all with Z less than 27 show appreciable abundance although H and He are far out way the other elements.  The above regularities suggest that absolute abundance of elements depends upon the nuclear rather chemical properties and are related to inherent stability of the nuclei.  Of more than 1000 isotopes only 270 are stable (non Radioactive). The elements with Z= 43, 61, 85,87, all having odd Z are completely absent in the earth. Due to complete instability of any nuclear arrangements for this proton nos. The light elements Li, Be, B are very rare due to the fact that the nuclei are most readily disintegrated by bombardment with α-particles as well as protons. The number of stable nucleii having (even Z and even N) are 163 much greater than stable species (even N + odd Z; 50) ≈ (odd N + even Z; 55) and very less than ((odd Z+ odd N); 4, H(1,1), Li(3,3), B(5, 5), N(7,7))
  9. 9. Structure and composition of the earth To obtain some understanding of earth’s internal structure we must use an indirect approach. Much information is derived from the analysis of earth quake waves. Among the waves of the various kind, two main types- the faster primary wave (P- wave) is transmitted by vibration in the direction of propagation and the slower secondary wave (S-wave) transmitted at right angles to the direction of propagation.
  10. 10. The earth is divided into three parts- (i) crust, (ii) mantle, and (iii) core. These divisions, which are concentric zones in depth, are established principally by their differing behavior with respect to the transmission of P (compressional) and S (shear) seismic waves and are separated from each other by distinct discontinuities in transmission velocity. (ii) The Mantle: From the base of 1st discontinuity to the Wiechert-Gutenberg discontinuity at a depth of approximately 2,900 km. (iii) The core : Zone upto the centre of the earth. The extinction of S waves at the base of the mantle suggest that material of the underlying core behaves as a fluid state. (i)The Crust : From the surface to the base of 1st discontinuity or Mohorovicic discontinuity at an average depth below sea level ranging from 37 km under parts of the continents to about 11 km beneath the deep sea basins (Poldervaart, 1955);
  11. 11. Recordings of seismic waves from earthquakes led to the discovery of the earth's core and eventual maps of the layers of the Earth's inside. Earthquakes generate three types of seismic waves: P (primary) waves, S (secondary) waves and surface waves, which arrive at seismic recording stations one after another. Both P and S waves penetrate the interior of the Earth while surface waves do not. Due to this, P and S waves are known as "body waves". Surface waves arrive last and are the least interesting to seismic tomographers because they don't penetrate deep inside the Earth Velocity of seismic waves in the Earth versus depth. The negligible S-wave velocity in the outer core occurs because it is liquid, while in the solid inner core the S- wave velocity is non-zero.
  12. 12. p-wave and s-wave from seismograp h P- & S-waves sharing with the propagation
  13. 13. The average density of the earth was determined by Cavendish in 1799 to be about 5.5 g per cm3 (grams per cubic centimeter) The velocity of the seismic wave changes with density of the medium by the following eqns: The velocity of P-waves in a homogeneous isotropic medium is given by where K is the bulk modulus (the modulus of incompressibility), µ is the shear modulus (modulus of rigidity, ρ is the density of the medium and .Of these, density shows the least variation, so the velocity is mostly controlled by K and μ. The density distribution inside the earth follow the curves of the primary wave velocity (P) with a sharp a break at 2900 km depth while the density of the surface rock is 2.8, the mean density of earth is 5.48 g/cc The measured thermal gradient is about 300 /km in the Crust, but in the Mantle, it is only 0.60 /km. At the core boundary, temp. is calculated to be 27000 C. The Mohorovicic discontinuity separates the heterogeneous crust from much more homogeneous mantle as marked by sudden increase in velocity of seismic waves.
  14. 14. At the upper part of the crust below the sea water (4 km) and sediments (2 km) the material is granitic called Si-Al(SIAL) while the lower layer is basaltic in composition often called Si-Mg (SIMA) layer. Total depth of the crust is…….  The mantle contains dunite (Olivine), Peridolite (Olivine +Pyroxene) and eclogite (Pyroxene+ Garnet) and the combined material has elastic properties to give the observed wave velocities. However, these material are polymorphic and the great pressure existing, they are transformed to denser polymorph and the change is almost complete at the depth of 900 km making a transition zone. It may also happen that the Silicates break up into simple oxides which crystallize in high density phases. The seismic discontinuity between the mantle and the core is a sharp one and its position 2900+2.5 km is known with considerable accuracy. Bullen suggested that the 2nd order discontinuity at a depth of about 5000 km is due to change in rigidity and that below this depth the core may be solid. Analysis of bodily tide of the earth and magnetic field and its secular (periodic) variation also indicate the presence of liquid Fe-Ni core. To complete the picture of earth’s structure three further zones may be added over the crust.
  15. 15. (i) Atmosphere: N2, O2 , water vapour, CO2, inert gases. (ii) Biosphere : Water, organic substances, skeletal material (fossil) - liquid and solid and often colloidal. (iii)Hydrosphere: Upto a depth of 4 km from the surface-fresh water, various salts as well as snow and ice, liquid as well as solid. (iv)Crust : upto 33 km. Composed of normal silicate rocks-granite and basalt- solid phase. (v) Mantle : Upto about 2900 km. There are Fe-Mg silicates with some FeS, solid phase. (vi)Core or Siderophore : Below 3500 km. Composed of Fe-Ni alloy, liquid in the upper part but the inner core may be solid (below 5000 km).
  16. 16. CRUST: The average composition corresponds to that of igneous rocks since total sedimentary and metamorphic rocks contribute little. According to Clarke and Washington, the upper 10 mile of the Crust is made up of 95% igneous rocks, 4% shell, 0.75% sand stone and 0.25% limestone. Si-Al shell is then or absent below the ocean basin and silica percentage is generally low for oceanic rocks. Poldervaart (1955): analysed the crust in terms of 4 geological divisions (i) deep oceanic region (ii) Continental shield region, (iii) Young folded belts region, and (iv) Suboceanic (continental platform with slope)
  17. 17. And finally gave the average composition of earth on water free basis (Silica…… 55.2%, Al2O3……15.3%, Fe2O3 ……2.8%. , FeO …. 5.8%, MnO …..0.2%, MgO ……….5.8%, CaO………8.8%, Na2O…..2.9%, K2O……….1.9%, TiO2…… 1.6%, P2O5…….0.3%) Clarke and Washington, from the analysis of 5159 samples and Goldschimdt independently from about 80 samples got about similar datas with minor variation in respect of the following compounds (Fe2O3 …3.14 %, FeO…3.88%, MgO ……….3.56%, CaO………5.17%, Na2O…..3.9%, K2O……….3.2%). Recently data for minor and trace elements has also been obtained. On the whole, eight elements……….. Al, O, Si, Fe, Ca, Na, K, and Mg make up nearly 99% of the crust and of those O2 alone make up 46.6% by wt or 93.8% by volume of the crust. Thus the crust is essentially a packing of O2 anions bonded by Si, Al and some of the common metals. Accordingly Goldschmidt remarked that the lithosphere may be called the Oxisphere. Some elements although present in the crust in considerable amount are
  18. 18. • Vernadsky called these dispersed elements, eg., Rb in K-minerals, Ga in Al minerals etc. the elements like Ti and Zr form specific mineral which are widely dispersed in small amounts in some of the common rocks. • Vernadsky introduced the term Clarke of concentration, a factor showing the concentration of the element in a particular deposit or minerals, e.g., Clarke of concentration of Mn in Pyrolusite is 639, in Rhodomite is 419, although the Clarke of Mn is 0.1%. • And Ore is simply a deposit in which the Clarke of concentration of the particular metal reaches a figure sufficient to make up its extraction profitable. The availability of an element depends largely on its ability to form individual mineral in which it is a major constituent. • The most unavailable elements are those which forms no minerals of their own but occurs in less than 1% in other minerals, e.g.; Rb, Ga, In, Re, Hf etc. Even for common elements, however, the dispersed amount exceeds vastly the amount available in so called ore deposits.
  19. 19. Fersman made sizable contributions to the solution of an important theoretical problem of geochemistry: the frequency of distribution of the chemical elements in the rocks of the earth’s crust (clarkes). The term “clarke” (the concentration of an element in the earth’s crust) was proposed by Fersman in honor of the American scientist F. W. Clarke, one of the first to consider this problem in his fundamental work, The Relative Abundance of the Chemical Elements (1889). Fersman calculated the clarkes for most of the elements. Before Fersman, clarkes were expressed in weight percentages. He showed that for geochemical purposes the atomic percentages were more important, thus introducing into science the concept of “atomic clarkes.” As a result he discovered the independence of geochemical abundances from the positions of the elements in the periodic system and the concentration and depletion of the various elements. He showed that abundances within the earth’s crust were determined by the effects of the migration of the elements, while abundances in space were related to the stability of the atomic nucleus. A E Fersman after his geological expedition to the Khibiny Mountains in 1922
  20. 20.                                                                                                                                                              The concentration of an element in a mineral or rock relative to its crustal abundance. The term is applied to specific as well as average occurrences Clarke of concentration According to Vernadsky, the clarke of concentration is the concentration of the chemical elements in a certain geochemical system relative to its crustal abundance. The use of this parameter (Kc) made it possible to highlight the particular and general geochemical properties of the rocks, including the sedimentary varieties, as well as the “geochemical affinity” of the rocks of different sedimentary formations, groups, and their large successions. V.I. Vernadsky Clarke of copper is about 55 ppm, or 0.006%. In the mineral chalcocite, Cu2S, the Cu conc. is 79.8% .Thus, the clarke of concn within this mineral is 79.8/0.006,= 13,300
  21. 21. MANTLE: Laboratory investigations indicate that of the common rock types, Dunite, Periodite and Eclogite satisfy the geochemical requirement such as elastic property and density for the mantle which is similar to that of chondritic meteorites. However, a mantle of Periodite composition since more in line with the evidence. Goldschmidt suggested layer at the lower part of the mantle. Since Triolite is a common phase in meteorites some quantity of FeS is probably dispersed throughout the mantle and core. CORE: The core has a mean density of 10.7 g/cc which can hardly be explained except by the presence of a heavy metal and being most abundant of them by Fe(then Ni). From the composition of meteorites, it is reasonable to suggest that the Fe is probably alloyed with appreciable quantity of Ni.
  22. 22. What is the Earth made of? Relative abundance by weight of elements in the whole Earth and in the Earth’s crust. Differentiation has created a light crust depleted in iron and enriched in oxygen, silicon, aluminum, calcium, potassium, and sodium.
  23. 23. MOHO - ~40km Wiechert- Gutenberg discontinuity CRUSTCRUST CORECORE 3 Major Zones, 2 Transitions Earth’s ChemicalEarth’s Chemical DifferentiationDifferentiation
  24. 24. Geochemical Classification of elements In the classification scheme of Goldschmidt, elements are divided according to how they partition between coexisting silicate liquid, sulfide liquid, metallic liquid, and gas phase…defined by examining ore smelting slags and meteorites Silicate Liquid Sulfide Liquid Metallic Liquid Gas PhaseGas Phase Siderophile Having affinity for metallic Fe, Ni Chalcophile Having affinity for sulphides Lithophile Having affinity for silicates Atmophile H, He, N, Noble gases Alkalis, Alkaline Earths, Halogens, B, O, Al, Si, Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Lanthanides, Hf, Ta, Th, U Cu, Zn, Ga, Ag, Cd, In, Hg, Tl, As, S, Sb, Se, Pb, Bi, Te Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Mo, Re, Au, C, P, Ge, Sn To first order, the distribution of elements between core and mantle resembles equilibrium partitioning between metal liquid and silicates…confirmed by iron and achondrite meteorites (but at high P, no separate sulfide phase) • Melting a chondrite gives 3 immiscible liquids plus vapor:

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