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[image] 000 0179. GEOCHRONOLOGY. M.E.A. MONDAL DEPARTMENT OF GEOLOGY AMU, ALIGARH – 202002 E. MAIL : [email protected].

References. Faure, Gunter. Principles of Isotope Geology 2nd edition (1986) Dickin, Alan P. Radiogenic Isotope Geology (1995) White, William M. Geochemistry (1997) Gunter Faure, T.M. Mensing Isotopes: Principles and Applications (2004) Allegre: isotope geology.

Why Geochronology so important?. Many dates for the same rock reported in literature Intricacies of geochronological methods What techniques for What Rocks? Techniques and targets ?.

AGE ?. Absolute age Relative age Radiometric age Isotopic age Chemical age Model age Age of igneous rocks: which event? Age of metamorphic rock : which event? Age of sedimentary rock : which event?.

The Basic questions:. What is an isotope ? Why are some isotopes stable, and some unstable? Why do we have unstable isotopes at all; how did they form? What exactly is radioactive decay? Applications: why and how are isotopes useful? (Dating, determining rates of processes, tracers) General principles: dating, mixing, fractionation Specific isotope systems, and how they are used.

Nomenclature. “Nuclide” = a particular atom An atom is made up of a nucleus and surrounding electrons The nucleus is made up of protons and neutrons (& other tiny, tiny particles).

Nuclear Stability. What makes a nucleus stable is something called its “Binding energy” “The whole is less than the sum of its parts” Mass Defect: = Δm = M – (∑mp + ∑mn + ∑me) Mass and energy are interchangeable (Einstein) E = mc2 EB = Δm c2 (Conversion factor for mass to energy: 1 amu = 931.5 MeV) EB is a measure of nuclear stability: those nuclei with the largest binding energy per nucleon are the most stable..

Isotope = line of equal Z; nuclides with the same # of protons (therefore they are the same element), but variable N; e.g. 12C, 13C, 14C are isotopes Isotone = line of equal N; nuclides with the same # of neutrons, but variable Z; e.g. 11B, 12C, 13N are isotones Isobar = line of equal mass; nuclides with the same mass number, but variable N and Z; e.g. 12C, 12B, 12Be are isobars.

Isotopes ? Atoms of elements → same number of electrons outside the nucleus and the same number of protons within the nucleus (Atm. No.) But the nuclei may contain differing number of “neutrons”. Thus atoms may differ in mass (P+n) and hence differ slightly in properties..

[Audio] May drop the subscript (redundant with symbol).

Types of Isotopes. Primordial Isotopes: formed before the formation of Earth. Primordial isotopes were formed by nucleosynthesis in Big Bang, and were present in the intersteller medium from which the solar system was formed Cosmogenic Isotopes: form constantly in the earth’s atmosphere or on the exposed surface of the earth by interaction of cosmic rays and target element Anthropogenic Isotopes: generated through nuclear reactions in nuclear reactors or nuclear weapons explosions. Radiogenic Isotopes: form as a product of radioactive decay Stable Isotopes: are not product of radioactive decay, but were formed during the formation of solar system.

Radioactive Decay. Unstable nuclides spontaneously decay to form stable nuclides ‘Radioactivity is the spontaneous transformation of an unstable nuclide, usually involving the emission of particles and energy’ A radioactive parent decays until a stable daughter nuclide is formed.

Types of Radioactive Decay. Alpha decay (α particle = 4He nucleus) Beta- decay (β- = electron) Positron emission (β+ = positron) Electron capture (ε = electron) Nuclear Fission Gamma emission (γ = high energy photon) If the daughter of any type of decay is left in an excited electron state, it will emit a γ ray to release the excess energy (Isomer = metastable state).

Radioactive vs Stable Isotopes. General rule : No. of Protons and Neutrons should be roughly same to promote stability When these numbers get out of balance – likely to be unstable (Radioactive) Stable isotope : stable in the sense that earth and other planets still contain the same number of atoms of each element as were present at the time of origin of solar system.

Radioactive Spontaneous radiation, change to nuclei of other elements Radiation α - particles (He nucleus) β – particles (e-) gamma radiation (high energy radiation) Daughter products (radiogenic) Stable : Do not decay.

A Pretty Picture of Radioactive Decay. Decay of Parent Atoms Growth of Daughter Atoms Number of Half-Lives Number of Half-Lives.

Measurement of Isotopes. Mass Spectrometer (MS) Conventionally by ID-TIM Isotope dilution thermal ionization MS During 70s & 80s : in situ analysis- development of high energy laser and ion beam technology In situ isotope measurements : SIMS, LA-ICPMS, SHRIMP (discovery of oldest zircon 4.4 Ga).

[Audio] With the mass spectrometer, as you see here we have the one located at San Diego State.

[Audio] There are many different kinds of mass spectrometers, but all use magnetic and/or electric fields to exert forces on the charged particles produced from the chemicals to be analyzed. A basic mass spectrometer consists of three parts: A source in which ions are produced from the chemical substances to be analyzed. An analyzer in which ions are separated according to mass. A detector which produces a signal from the separated ions. A magnetic field (in a "magnetic sector analyzer") separates ions according to their momentum (the product of their mass times their velocity). To understand how the force exerted by a magnetic field can be used to separate ions according to their mass, let us imagine that we have a bowling ball and a feather moving by us (both move at the same velocity). If we blow on the two objects in a direction perpendicular to the path of the objects, the feather will be deflected away from its path because it has a smaller mass (momentum), but the bowling ball, with its larger mass (momentum) will continue to move in its original path. Now, let's look at a more advanced….

[Audio] The sequence is : Stage 1: Ionisation The atom is ionised by knocking one or more electrons off to give a positive ion. This is true even for things which you would normally expect to form negative ions (chlorine, for example) or never form ions at all (argon, for example). Mass spectrometers always work with positive ions. Stage 2: Acceleration The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection The ions are then deflected by a magnetic field according to their masses. The lighter they are, the more they are deflected. The amount of deflection also depends on the number of positive charges on the ion - in other words, on how many electrons were knocked off in the first stage. The more the ion is charged, the more it gets deflected. Stage 4: Detection The beam of ions passing through the machine is detected electrically. Let's see how this works… Push button Walk through example Return to screen So because isotopes have different masses they can be distinguished through this process and therefore measured, but there are some problems with this measurement….

[image] SHRIMP diagram svg. SHRIMP.

Instrument Number Institution Location SHRIMP model Year of commissioning 1 Australian National University Canberra I 1980 2 Australian National University Canberra II 1992 3 Curtin University of Technology Perth II 1993 4 Geological Survey of Canada Ottawa II 1995 5 Hiroshima University Hiroshima II 1996 6 Australian National University Canberra RG 1998 7 USGS & Stanford University Stanford RG 1998 8 National Institute of Polar Research Tokyo II 1999 9 Chinese Academy of Geological Sciences Beijing II 2001 10 All Russian Geological Research Institute St. Petersburg II 2003 11 Curtin University of Technology Perth II 2003 12 University of São Paulo São Paulo II 2010 13 Chinese Academy of Geological Sciences Beijing IIe 14 Geoscience Australia Canberra IIe 2008 15 Korea Basic Science Institute Ochang IIe 2009 16 University of Grenada Grenada IIe 2011.

[image]. Ion counter. Magnet. Energy analyser. Primary ion source.

[image] Brazil SHRIMP II aerial small spun. [image].

Laser. Ablation cell. Mixing cell. Plasma. Ion optics.

[image] Electron Probe Electrons in X-rays out 10 pm.

Rationale of Radioactive Dating. - Radioactive decay goes on at a constant rate - Unaffected by T, P or chemical combinations hence, reliable clock for measuring geologic time Once imprisoned in a crystal structure, constant decay, ratio of daughter to parent increases constantly from which time elapsed since crystal was formed can be calculated. Assumption: Mineral has not altered since the formation.

Radioactive vs Radiogenic. Radioactive : decay over time Radiogenic : Products of radioactive decay Stable vs Radiogenically produced Stable isotope 12C 206Pb Which is what ?.

Importance of Radiogenic isotopes. They survive chemical frac. events, formation and evolution of magma Isotopes of heavier elements are not separated from each other through xl-liquid equilibria Thus, during partial melting, a magma will inherit the isotopic composition of its source and will remain constant during subsequent frac. xllon provided the magma is not contaminated by material with distinct isotopic ratio Hence isotopes have edge over trace element geochemistry Major elements vs Trace Elements vs Isotopes.

Mean Life vs Half-Life. Radioactivity is a nuclear property. For a given number of protons, there are certain number of neutrons which render the combination stable. Mean Life : the actual life of any particular radioactive atom can have any value between zero to infinity. The Mean Life of a large number of atoms is however a definite and important quantity. Half-life : the time required for one-half of the parent nuclei to transmute themselves into their nuclei.

D/P = eλt - 1. [image]. P. D. Geochronology: The concept.

Rate of decay of radioactive nuclei is proportional to the amount of the parent isotope (N) dN/dt α N.

ln N= –λt + C. At time t =0, N= N0. ln N0= –λ X 0 + C.

N= N0 e–λt. Fundamental equation of radioactivity NO PRACTICAL USE.

D measured = D initial + D*. Dm= Di + N (eλt -1).

Dm= Di + N (eλt -1) As methods of measurement improves, values of λ for the geologically important isotopes are refined and calculated ages that depend on λ must be revised. More easily visualized than λ is a related quantity called half-life.

Half Life. D* = N (eλt -1) D*/N = eλt -1 At t = t1/2 1= eλt1/2 -1 2 = eλt1/2 ln2 = λt1/2 t1/2 = ln 2/λ = 0.6932/λ.

Isotopic Ratio. Dm= Di + N (eλt -1) 238U 206Pb 206Pbmeasured = 206Pbinitial + 238U(eλt -1) It is very difficult to measure precisely the absolute abundance of a given nuclide. It is more convenient to convert this number to an isotopic ratio by dividing through a isotope not produced by radioactive decay and therefore remains constant with time (stable isotope, viz. 204Pb). (206Pb/204Pb)measured = (206Pb/204Pb)initial + 238U/204Pb (eλt -1).

Dating. We can use the radioactive decay equation to calculate the age of a sample. We can measure the present day ratios and λ, but we still have 2 unknowns: Di and t. What can we do?.

The most frequently asked question(s) relating to isotope geology:.

[image] Various (c tochron) chondrite Juvinas achondrite Lewis achondrite Chondrite suite Various 4 55 4 €.53 ± 0.004 4 50 ± 0.07 4.55 4.57 ± 0.06 K-Ar Re-Os.