ISOTOPE GEOLOGY

ABUNDANCE OF ELEMENTS

  • H,He are most abundant in solar system and H/He=12.5

  • First 50 elements: Exponentially decreases with increasing Z

    (We can see progressive decrease in abundance with increase in atomic number but there is not uniformity in that, generally Zig-Zag pattern is observed)

  • After 50 elements: Do not vary appreciably with increasing Z

  • Even atomic number are more abundant than neighbour odd ones

  • Abundance Li,Be and B is anomalously low

  • Abundance of Fe is anomalously higher

  • Tc and Pm do not occur naturally in our solar system (because all

    of their isotopes are unstable and decay rapidly)

  • element with atomic number > 83 (Bi) have no stable isotopes but

    occur naturally at low abundances

CHONDRITES AND METEORITES

  • Chondrite corresponds to bulk composition of silicate earth

  • Fe-meteorite corresponds to core of earth

  • Chondrites are undifferentiated, unmelted having smaller masses

NUCLEAR BINDING ENERGY CURVE

  • Fe is the most abundant element of earth

  • Fe is the highest atomic number that can be synthesized

  • For synthesizing element having higher atomic number greater than Fe, we need greater energy like Supernova explosion

HERTZSPRUNG-RUSSEL DIAGRAM

  • Sun is a star which is in main sequence stage right now, but it will not remain there forever because it has an evolutionary path, it will end up as white dwarf

  • Sun - Red giant - White dwarf

  • Chandrasekhar limit : limits the mass of star which can undergo explosive events like supernova

  • Sun has a mass which is below chandrasekhar limit

NUCLEOSYNTHESIS

  • Process of creation of different chemical Elements

  • Process of creating new atomic nuclei from pre-existing nucleons (protons + neutrons)

  • H-burning and He-burning are very long duration processes

  • S-process : slow process but rapid than H and He-burning

  • r-process: Extremely rapid process

  • Other processes

    • e-process : silicon burning

    • p-process : proton capture

    • r and s-processes : neutron capture

    • x-process : light element synthesis

NUCLEAR STABILITY AND DECAY

1.Neutron - Have mass but no charge

2.Proton - Posses mass and positively charged

  • Mass of both proton and neutron = 1.67*10 -23

  • Mass of Electron = 1/1836.1 of mass of Proton

  • Isotopes: Nuclides with Same Z, different N

  • Isotones : Nuclides with Same N, different Z.

  • Isobars: Nuclides with Same A, different Z and N

  • Out of 2500 known nuclides only 270 are stable

  • Nuclides having magic proton number or magic neutron numbers are unsually stable

FUNDAMENTAL PARTICLES

  • Proton and Neutrons are composite particles i.e they can be broken down further

  • Proton : Consist of two up quarks and one down quark

  • Neutron : Consist of two down quark and one up quark

  • W-boson is extremely unstable and immediately decays to electron as beta-particles

RADIOACTIVE DECAY

  • Process by which unstable nuclides are converted into stable nuclides

  • Nuclear transmutation process

TYPES OF DECAY MECHANISM

  • Beta Negatron : neutron rich

    • N —> P + e (Transformation of neutron to a proton and electron)

    • Z+1 and N-1

    • Atomic no. increased by 1 and Neutron no. reduced by 1

      eg : 37 Rb ——> 38 Sr

  • Beta Positron : proton rich

    • P ——> N + Positron + Neutrino (Transformation of a proton to neutron, positron and neutrino)

    • Z-1 and N+1

      eg: 9 F ——→ 8 O

    • Mass is conserved in case of beta decay

  • Electron-capture decay

    • Reaction between extranuclear electron and proton gives rise to neutron and neutrino

    • e + P ———→ N + neutrino + X-ray

    • X-ray is the product

  • Alpha decay : proton rich

    • Alpha particle is the He-nuclei ( 24He)

    • Z-2 and N-2

    • Overall atomic mass reduced by 4

      eg: 92 U ——→ 82 Th + 24He + Q

  • Nuclear Fission

    • Heaviest isotopes are decay by this Nuclear fission

    • Elements with greater atomic number than uranium and transuranic elements

    • the fission products have excess neutrons and therefore

      undergo further decay by β- and gamma-ray emission until a

      stable nuclide is formed

FORMULAE

1) D= D0+ N (e λt-1)

2) M= eλt-1 , (M=Slope)

3) N/No = e λt-1

4) N/No = 1/2 n , (n= No. of half lives) , n=t/t1/2

5) t1/2= 0.693/λ, (t1/2= Half life, λ= Decay constant)

6) t1/2 = 0.693* Mean life

7) N1/t1/2(P) = N2/t1/2 (D), N1λ1=N2λ2 (Secular Equilibrium)

8) D= No-N

  • DECAY SCHEME AND THEIR HALF LIVES

  • K-Ar method:

    • used for potteris

    • date cenozoic/mesozoic volcanic rocks (younger volcanic rocks)

    • used to date metamorphic events

  • Rb-Sr method:

    • dating acidic and intermediate igneous rocks such as rhyolite, dacite, granite etc.

  • U-Th-Pb method:

    • Zircon dating

  • C-N method

    • date recent events, Holocene and Pleistocene rocks

MASS SPECTROMETRY

  • Instrument that we used to measure age of earth

  • Mass spectrometer separate ions of different mass

  • All these parts are evacuated to the pressure order of 10-6 to 10-9

SOURCES OF MASS SPECTROMETER

  • TIMS (Thermal Inonization MS)

    • Pure form of the element is deposited in metal filament made up of Re,Ta, W

    • Which is electrically heated to volatilize and ionize the element into positively charged ion by stripping outer most electron

  • PLASMA SOURCE ( ICP-MS)

    • also a form of thermal ionisation

    • restricted to filament under vaccum - technique

    • A plasma is generated by an electrically excited charge

  • SECONDARY ION (SIMS)

    • a secondary target mineral surface is bombarded insitu

    • with a high energy beams of ion to split or remove secondary ions

    • extracted positive ions are accelerated and analyse in a double focusing mass spectrometer

    • Here we don’t need to digest the sample

  • GAS SOURCE ( NOBLE GAS MS)

    • Extracted under ultra high vaccum

    • gas extraction is done using a furnace or infra-red laser

  • ACCELERATED MS (AMS)

    • Particularly used for samples which are very low in abundances

SAMPLE PROCESSING

  • Non-invasive technique : (SIMS, LA-ICP-MS)

    • Suitable polished section should be prepared

    • Detailed petrographic study, Backscattered electron image (BSE) and Cathodoluminiscene image (CL) are needed

    • Minerals such as Zircon, Titanite, etc can be separated from > 5 kg rock sample crushed to 180 micro m

    • Mozley superpanner or Wilfley table may be used

  • Invasive technique (TIMS, ICP-MS)

    • Rock needs to be digested by attacking HF+HNO3+Hcl acids

    • obtaining 70 micro m powder for digestion

    • Most rock forming mineral dissolve above 100 C

    • Resistant mineral like Zircon needs temp upto 220 C

    • Low procedural blank

ISOTOPE DILUTION

  • Digested sample split into two weighted aliquots

  • One of which is spiked with an enriched isotope and homogenized for isotope dilution analyis

  • Other unspiked aliquot is used for determination of isotropic compostion or isotropic ratio

K-Ar Method

  • daughter element Ar is an inert gas

  • parent element potassium is an alakali metal

  • K had been the dominant contributor to the heat budget

  • DISTRUBUTION OF K

    • Highest conc in crust being an alkali metal

    • It is incmpatible

    • In bulk mantle, K is an accessory or trace element

    • K has 3 isotope among which 40 K is radiogenic

    • It has a brached decay

    • 40 K decays to 40 Ar (EC)and 40 Ca (beta)

    • Braching ratio i.e EC/Beta = 0.117

  • APPLICATION OF K-Ar METHOD

    • Contact metamorphic zones

    • Retention sequence :

      K-Feldspar < Biotite (350) < Muscovite (450) < Hornblene (550)

    • Hornblende has better retention of Ar

    • Biotite has lower retention of Ar

    • Date cenozoic/mesozoic volcanic rocks

    • Date ocean mounds and ocean islands