U_Pb_Methods.pdf

Zircon U-Pb dating: comparison of methods

1) basics/ theory

2) Measurement procedures

3) Data reduction, necessary corrections of raw data

4 ) conclusions

M. Tichomirowa

U Pb radioaktiv radiogen

• Rutherford & Soddy: Gesetz des radioaktiven Zerfalls -dN/dt = λ x N λ– Zerfallskonstante (Wahrscheinlichkeit, mit der 1 Atom in bestimmter Zeit zerfällt N – Anzahl radioaktiver Atome t - Zeit

N = N0 e –λt

e – Integrationskonstante (2,718..)

N0 = N + D D = N (e λt – 1)

Zircon U-Pb dating: comparison of methods M. Tichomirowa

Isotopengeochemie und Geochronologie M. Tichomirowa

U/Pb-Datierung U-Th-Pb-Datierung: 3 Zerfallsreihen a) 238U → 206Pb + 8α + 6β-


U/Pb-Datierung

b) 235U → 207Pb + 7α + 4β-


U/Pb-Datierung

D = N · (e λt – 1) 206Pb = 238U · (eλt – 1) bei Pbi = 0 206Pb = 206Pbi + 238U · (eλt – 1)

206Pb 206Pb 238U 204Pb 204Pb 204Pb · (eλt – 1)

i

· (eλt – 1) i

207Pb 207Pb 235U 204Pb 204Pb 204Pb

· (eλt – 1) i

204Pb 204Pb 204Pb

208Pb 208Pb 232Th

(eλ2t – 1) 207Pb 235U 206Pb 238U (eλ1t – 1) ·

206Pb-Alter 207Pb-Alter 208Pb-Alter 207Pb/206Pb-Alter


U/Pb-Datierung 207Pb/206Pb-Alter


U/Pb-Datierung

t 207-206 > t 207 > t 206 > t 208


U/Pb-Datierung Diskordia


U/Pb-Datierung Pb-Verlust vor 1 Milliarde Jahre


Terra-Wasserburg diagram

207Pb/206Pb

M. Tichomirowa


2) Measurement procedures 1) Conventional U-Pb dating (ID-TIMS)

a) multi-grain dating b) single grain dating

- choose the best grains


U/Pb-Datierung Zirkone des Unteren Graugneises (Innerer Freiberger Gneis)

Tichomirowa (2003)


- Get rid of zircons or zircon domains with a disturbed U-Pb system: by „Annealing and Chemical Abrasion“ Annealing: heating at 850oC for 48 h

Tichomirowa et al. (2005)


- Get rid of zircons or zircon domains with a disturbed U-Pb system: by „Annealing and Chemical Abrasion“ Annealing: heating at 850oC for 48 h Chemical abrasion: leaching with HF + HNO3 washing the surface of zircons by clean acids and water

- Spiking (ET 535, ET 2535) - Dissolution with HF + HNO3 in bombs (48 – 72 h) - Separate U and Pb from zircons (column chemistry) - Measurement of Pb istope ratios (TIMS) - Measurement of U isotope ratios (TIMS)

M. Tichomirowa


2) U-Pb dating by SHRIMP/SIM

- Prepare polished zircons

- Make CL images

- Measurement on SHRIMP/SIMS


U/Pb-Datierung SHRIMP (Sensitive High Resolution Ion MicroProbe)


U/Pb-Datierung


U/Pb-Datierung

Datierung mittels SHRIMP

Tichomirowa et al (2001)

3) U-Pb dating by LA-ICPMS (laser ablation and inductively coupled mass spectrometry)

- Prepare polished zircons

- Make CL images

- Measurement (quadrupole or magnetic sector MS)

Kosler & Sylvester (2003)


4) Evaporation (Kober) method: 207Pb/206Pb model ages

- Take the best zircons

- Put into Re-filaments

- Measurement on TIMS with pre-cleaning (pre-heating)

Dickin (2005)


U/Pb-Datierung



M. Tichomirowa


3) Data reduction, corrections of raw data

The measured isotope ratios never correspond to those in zircons!

1. Fractionation of Pb and U isotopes during measurement: „mass bias“

4. Problem of „common Pb“ correction

5. Problem of „Pb loss“

6. Problem of „inherited ages“

2. Gain factor between different cups

3. Different measurement conditions for different samples

M. Tichomirowa


LA-ICPMS


M. Tichomirowa


1. Fractionation of Pb and U isotopes during measurement: „mass bias“

High precision U-Pb single grain dating (ID-TIMS)

precision: about ±0,1%

Ca. 60% of the uncertainty of the U-Pb-dating is due to instrumental fractionation

1) Using Spikes with 4 Pb isotopes: ET 2535 = 202Pb + 205Pb + 233U + 235U

Internal precision could be limited to about 0.1 %

2) Intercalibation of this spike by EARTHTIME group

4) Same calculation programme for all laboratories free from the Internet

5) Chemical Abrasion (annealing) – only areas with undisturbed U-Pb –System are dated

3) Very low total blanks (<1pg): important for the correction of common Pb

M. Tichomirowa


Chiaradia et al. (2013)

M. Tichomirowa

2. Gain factor between different cups

Correction usually done by gain calibration.

3. Different measurement conditions for different samples

- Different position of samples

- Changes in working conditions (beam intensity, gas flow, vacuum,…)


Impossible to correct, the only control are zircon standard values (outliers). Greater influence if measurement is done only with one cup (peak jumping)

U be anaylsis al. (1984) of

Dickin (2005)


after correction with mean/average fractionation

Ireland & Williams (2003)

M. Tichomirowa


4. Common Pb correction

Ratio 206Pb/204Pb (or 204Pb/206Pb) is indicator how much common Pb is obtained during zircon measurement

M. Tichomirowa

ID-TIMS U-Pb dating McLean et al. (2011)


Usual values for 204Pb/206Pb for zircons

M. Tichomirowa

Zircon U-Pb dating: comparison of methods Dickin (2005)

M. Tichomirowa


Diffusion of elements in zircon: Pb more mobile than Th, U

Cherniak & Watson (2003)



Consequences in dating diagrams



6. Problem of „inherited ages“


4) Conclusions



Bowring & Schmitz (2003)


4) Conclusions

M. Tichomirowa

Zircon U-Pb dating: comparison of methods Kosler and Sylvester (2003)

M. Tichomirowa


Thank you for your attention

M. Tichomirowa


References: Bowring & Schmitz (2003): High-precision U-Pb zircon geochronology and the stratigraphic record. In: Hanchar & Hoskin (Eds) Zircon, Reviews in Mineralogy and Geochemistry Vol 53, 305-326. Cherniak & Watson (2003): Diffusion in zircon. In: Hanchar & Hoskin (Eds) Zircon, Reviews in Mineralogy and Geochemistry Vol 53, 113-143. Chiaradia et al. (2013): How accurately can we date the duration of magmatic-hydrothermal events in porphyry systems? – An invited paper. Economic Geology 108, 565-583. Dickin (2005): Radiogenic isotope geology. Cambridge University press, 492 pp. Ireland & Williams (2003): Considerations in zircon geochronology by SIMS. In: Hanchar & Hoskin (Eds) Zircon, Reviews in Mineralogy and Geochemistry Vol 53, 215-241. Kosler & Sylvester (2003): Present trends and the future in geochronology: laser ablation ICPMS. In: Hanchar & Hoskin (Eds) Zircon, Reviews in Mineralogy and Geochemistry Vol 53, 243-275 McLean et al. (2011): An algorithm for U-Pb isotope dilution data reduction and uncertainty propagation. Geochemistry, Geophysics, Geosystems 12, Q0AA18, doi: 10.1029/2010GC003478. Nasdala et al. (2006): Effects of natural radiation damage on back-scattered electron images of single crystals of minerals. Am. Mineralogist 91, 1739-1746. Tichomirowa et al. (2001): Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides) – con- straints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt. Lithos 56, 303-332.


References: Tichomirowa (2003): Die Gneise des Erzgebirges – hochmetamorphe Äquivalente von neoproterozoisch-frühpaläozoischen Grauwacken und Granitoiden der Cadomiden. Freiberger Forschungshefte C 495, 222 S. Tichomirowa et al. (2005): Resorption, growth, solid state recrystallisation, and annealing of granulite facies zircon – a case study from the Central Erzgebirge, Bohemian Massif. Lithos 82, 25-50. Tichomirowa et al. (2012): Inferring protoliths of high-grade metamorphic gneisses of the Erzgebirge using zirconology, geochemistry and comparison with lower-grade rocks from Lusatia (Saxothuringia, Germany). Contrib. Mineral. Petrol. 164, 375-396.

U_Pb_Methods.pdf

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