1. Introduction to Geochronology Part 4: Rocks into Ratios Geochronology & Tracers Facility NERC Isotope Geosciences Laboratory British Geological Survey

2. Rocks into ratios The need to determine P/D What can mass spectrometers do? Determining P/D via Isotope dilution Calibration/traceability of P/D Absolute relative dating methods: In-situ techniques (Laser ablation and Ionprobe) 40 Ar/ 39 Ar Decay constants

3. Where do ‘dates’ come from? Mineral dates from mass spectrometry

4. The sector-type instrument designed by Alfred Nier was such an advance in mass spectrometer design that this type of instrument is often called the 'Nier type'. In the most general terms the instrument operates by ionizing the sample of interest, accelerating it over a potential in the kilo-volt range, and separating the resulting stream of ions according to their mass-to-charge ratio (m/z). Beams with lighter ions bend at a smaller radius than beams with heavier ions. Isotope Ratio Mass Spectrometry (IRMS) Mass Spectrometers: very good at measuring isotope ratios for a given element

5. Mass Spectrometers: very good at measuring isotope ratios for a given element

6. ?????????? How do we derive the ratio of isotopes from different elements?

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8. U-Pb (zircon) geochronology commonly referred to as the ‘gold-standard’, but how gold is it? At what level can we quantify the accuracy of U-Pb geochronology? Known knowns but are there and known unknowns?

9. Looking down onto two box of ball of unknown depth… Left box 100 red balls ( 206 Pb) and 60 gold balls ( 207 Pb) Red/gold = 100/60 = 1.667 206 Pb/ 207 Pb = 1.677 Right Box 120 blue balls ( 238 U) and 40 pink balls ( 235 U) Blue/pink = 120/40 = 3 238 U/ 235 U = 3 What is the ratio of red ( 206 Pb) to blue ( 238 U) balls? What is the 206 Pb/ 238 U ratio? How do we go from IRMS to isotope P/D???

10. Now we add some ‘tracer balls’, 1,000 teal balls ( 205 Pb) to the left box and 20,000 green balls (233U) to the right box, mix it up… Left box 12:48:80 teal:gold:red balls, therefore the 206 Pb/ 205 Pb ratio = 80/12 = 6.667 = 6,667 red 206 Pb balls Right Box 16:36:108 green:pink:blue balls, therefore 233 U/ 238 U ratio = 108/16 = 6.75 = 135,000 blue 238 U balls How do we go from IRMS to isotope P/D??? Isotope dilution (ID)

11. Now we add some ‘tracer balls’, 1,000 teal balls ( 205 Pb) to the left box and 20,000 green balls (233U) to the right box, mix it up… Left box 12:48:80 teal:gold:red balls, therefore the 206 Pb/ 205 Pb ratio = 80/12 = 6.667 = 6,667 red 206 Pb balls Right Box 16:36:108 green:pink:blue balls, therefore 233 U/ 238 U ratio = 108/16 = 6.75 = 135,000 blue 238 U balls 206 Pb/ 238 U = 6667/135000 = 0.0494 How do we go from IRMS to isotope P/D??? Isotope dilution (ID)

12. Need to correct for: 1.mass dependent fractionation 2.tracer ( 204 Pb etc. w.r.t 205 Pb) 3.subtract ‘initial’ (w.r.t 204 Pb)

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17. RATIOS TO DATES (TRACER CALIBRATION) Do ID again, but this time treat the mixed U-Pb tracer as the unknown and calibrate against a solution with isotopes natural Pb and U (i.e., no 205 Pb or 233 U). Requires this solution you calibrate to have a known U/Pb… How do we make such a solution….

18. How to make a gravimetric reference solution. 1. Buy some (high-purity) metals

19. How to make a gravimetric reference solution. 2. Gather some buddies (to share the blame)

20. How to make a gravimetric reference solution. 3. Clean your metals 20

21. How to make a gravimetric reference solution. 4. Weigh your metals

22. How to make a gravimetric reference solution. 5. dissolve your metals

23. How to make a gravimetric reference solution. 6. Write a really interesting paper about it (with your buddies). 23

26. Microbeam methods ●No spike involved (typically) ●Involves measurement of isotopic ratios, and normalisation of samples to reference materials ●Reference materials are characterised by ID methods ●‘High’ spatial resolution technique

29. Where do ‘dates’ come from? Mineral dates from mass spectrometry, plus a decay constant

30. The (simple form) age equation: t = 1/ λ * ln*(D/P + 1) T is the age relative to T o, the starting point of decay in past; D/P is a measured atom ratio of radiogenic daughter isotope to parent isotope λ Is the decay constant where t 1/2 = ln2/λ

31. 1. Countinge.g., 238 U alpha emission 2. In-growthget some high-purity material (e.g., K or Rb) and let is sit for some years… 3. Inter-calibrationImprove the relative accuracy of one (e.g., λ 40 K) to another (e.g., λ 238 U) by dating two things that should be the same age… How do we determined the half-lives/decay constants for the various long-lived/geologically useful radionuclides?

32. 32 Accurate determination of 238 U decay constant by counting Jaffey et al (1971)

33. Inter-calibration, transferring information from one decay scheme ( 238 U/ 206 Pb) to another… For a closed system: 206 Pb/ 238 U date = 207 Pb/ 235 U date (1) [ ln (1+ 206 Pb*/ 238 U ) ] / λ 238 = [ ln (1+ 207 Pb*/ 235 U )] / λ 235 (2) [ ln (1+ 207 Pb*/ 235 U ) ] / [[ ln (1+ 206 Pb*/ 238 U ) ] /λ 238 ] = λ 235 (3)

34. Inter-calibration, transferring information from one decay scheme ( 238 U/ 206 Pb) to another…

37. λ 238 U (Jaffey et al., 1971) U/Pb calibration (EARTHTIME) 40 Ar/ 39 Ar – U-Pb ‘pairs’ (Renne et al., 2010) Re-Os (Selby et al., 2006) λ 235 U closed system (Mattinson., 2010) λ 234 U closed system (Cheng et al., 2000) 14 C (INTCAL) Lu- Hf

38. Inter-calibration, transferring information from one decay scheme (astrochronology) to another…

40. The Grand Challenge – integrating different* datasets without loss of resolving power, how do different chronometers inter-relate? We need to be concerned with (1) traceability of dates; (2) distinguishing between data and interpretations. * different laboratories, different decay schemes…