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General Isotope Systematics

General Isotope Systematics The change in the radiogenic daughter isotopic ratio due to the decay of the long-lived radiogenic parent over time can be described by the equation. where (D) = the total number of daughter atoms measured in the system,

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General Isotope Systematics

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  1. General Isotope Systematics • The change in the radiogenic daughter isotopic ratio due to the decay of the long-lived radiogenic parent over time can be described by the equation where (D) = the total number of daughter atoms measured in the system, (Di) is the total number of daughter atoms present initially, (P) is the number of parent atoms remaining, (t) equals time and () is the decay constant for the parent isotope.

  2. The absolute abundance of a given element is difficult to measure, consequently the decay equation is modified by dividing each term by a stable isotope of the daughter element. For example, the equation is written for the Rb-Sr system:

  3. Re-Os System 187Os 187Os 187Re (eλt - 1) 188Os 188Os 188Os m i y = b + x m 187Re = 1.666 x 10-11 y-1 Half-Life = 4.16 x 1010 y

  4. Isochron Diagram

  5. The Re-Os isotopic system • Os is compatible during mantle melting • Re is slightly incompatible • Therefore the mantle will be resistant to later metasomatic events. • Crust will have low Os and high Re/Os.

  6. Isotopic System Basics

  7. Re-Os and Sm-Nd in Mantle Reservoirs. • Sm/Nd and 143Nd/144Nd display wide variations. • Re/Os and 187Os/188Os display very limited variations.

  8. Isotopic “Fingerprints” • Isotopic ratio of melt is equal to that of its parent. • This ratio only changes because of time • or by mixing with a material of a different signature.

  9. It’s Simple, Right?

  10. Reproducibility

  11. Separation (by hand, magnetic and/or Heavy liquids)

  12. Advantages • Os is one of the six platinum-group elements. • Determine age directly on ore minerals. • Determine Source of Ore Metals • Chalcophile and Siderophile • Role of Crust and mantle in ore deposition and igneous petrogenesis.

  13. An Date is a Date…is a Date? • Model Ages. • Re Depletion Ages. • Isochron Ages. • Single mineral Ages • Assumed Initial • Checks and Balances?

  14. Multiple Mineral Isochron Diagram Determines Age Initial Ratio

  15. Ore Deposits Direct Dating of Ore Minerals

  16. Indirect Dating Examples... • 1) Rb-Sr K-Ar, Ar-Ar wall rock or clays. • 2) Pb-Pb Galenas. • 3) Paleomagnetic dating.

  17. Direct Dating Examples... • 1) Rb-Sr Sphalerite • 2) Sm-Nd Fluorite • 3) U-Th-Pb in Calcite and U-bearing Minerals • 4) Re-Os in Molybdenite and other Sulfides • 5) others in specific cases.

  18. Differences with other isotopic systems for dating ore deposits. • Useful for Lithophile-Rich Minerals • U-Th-Pb : • High Closure T, • Dates the Time of Intrusion • Specific U or REE rich ore deposits • K-Ar and Ar-Ar : • Lower Closure T • Dates Cooling • Useful for alteration minerals • Others : Rb-Sr, Sm-Nd : • Used in gangue minerals • Re-Os •  CHALCOPHILE and SIDEROPHILE • Widespread Usage

  19. Re-Os in Molybdenites 187Os(m) = 187Os(i) + 187Re (eλt - 1) • Advantage: • No Initial or common Os. • High concentrations of Re (ppm). • Disadvantage: • No metal source can be determined. • Hydrothermally altered.

  20. Closure Temperatures

  21. Urad-Henderson

  22. Urad-Henderson Isochron

  23. EL TENIENTE (Chile)

  24. Arc Volcanoes.

  25. Why are Arcs important?

  26. It’s Just One Little Volcano, Right?

  27. Subduction Zones and Arc VolcanoesWhy should we study arcs? • Potential eruptions • Areas of high seismicity. • Building block for continents. • Chemical recycling. • Source of juvenile additions to the lithosphere. • Crustal recycling?

  28. Is the Crust Important? • It is impossible to address the question of chemical recycling and mass balance without determining the problem of when and where crustal material contributes to arc magmas .

  29. The absolute abundance of a given element is difficult to measure, consequently the decay equation is modified by dividing each term by a stable isotope of the daughter element.

  30. Re-Os System 187Os 187Os 187Re (eλt - 1) 188Os 188Os 188Os m i y = b + x m 187Re = 1.666 x 10-11 y-1 Half-Life = 4.16 x 1010 y

  31. Isochron Diagram

  32. Isotopic “Fingerprints” • Isotopic ratio of melt is equal to that of its parent. • This ratio only changes because of time • or by mixing with a material of a different signature. • Therefore in variations of the initial isotopic ratio of a “zero age” sample must be the result of a physical process. • Metasomatism • Assimilation

  33. Questions?

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