Introduction to radiogenic isotopes as tracers of provenance and petrology

1. EURISPET Feb. 2008 Introduction to radiogenic isotopes as tracers of provenance and petrology Vickie Bennett Research School of Earth Sciences Australian National University

2. Radiogenic Isotopes • Geochronology- determining ages of rocks and minerals • Tracing • Determining sources • Unraveling petrologic processes

3. Overview • Background • Mixing • Model Ages • An example: 176Hf- 142Nd in early Earth

5. Evolution of daughter isotopes • Parent and daughter isotopes are frequently measured with mass spectrometers, which only measure ratios accurately, so we choose a third stable, nonradiogenic nuclide S such that in a closed system S(t) = So: * Concentration ratios

6. In general we want the initial ratio, = (87Sr/86Sr) measured [87Sr/86Sr]initial (et - 1) - 87Rb/86Sr Where t is the crystallization age.

7. The toolbox 1.867 *

8. Creation of isotopic diversity • Change parent/daughter ratios • Petrologic processes e.g. partial melting • Mixing of different materials • Evolve isotopic differences over time

11. Sm-Nd systematics • Parent isotope is 147Sm, alpha decay half-life 106 Ga. • Daughter isotope is 143Nd, 12% of natural Nd. • Stable nonradiogenic reference isotope is 144Nd. • Strong emphasis on small deviations from chondritic leads to epsilon notation. where CHUR is the chondritic uniform reservoir, the evolution of a reservoir with bulk earth or bulk solar system Sm/Nd ratio and initial 143Nd/144Nd.

12. More than for an other isotopic system, we know the BSE baseline for Sm-Nd (refractory lithophile elements) Patchett et al., 2004

13. Distribution coeffcients- garnet/melt Lu Sm Hf Nd =high Sm/Nd =extreme Lu/Hf Johnson, 1998 CMP

14. Nd Sm Cont. crust MORB From Hofmann, 1997

15. Sample CI chondrite Sm-Nd systematics One-stage Nd evolution • Since the rock crystallized from the extracted melt phase has a lower Sm/Nd ratio than the source, it evolves with time to a less radiogenic isotope ratio. • Since the residual solids have a higher Sm/Nd ratio than the source they evolve with time to a more radiogenic isotope ratio.

17. Rb and Sr variably volatile during accretion Mobile during metamorphism. Sr isotope evolution of Archean Is largely unknown.

20. Model Ages Initial ratios

21. Two stage Model ages

22. When does a model age= crystallization age? Only in the case of primitive (juvenile) materials, formed from mantle source with short residence time relative to the decay constant. In most case model age will be represent weighted average of crustal sources.

23. Mantle evolution is difficult to constrain. Age and geologic uncertainties are present. Empirical attempts had varying success. Commonly used linear curve has fewest assumptions, but not correct in detail. Bowring and Housch, 2002

25. Regional Isotopic mapping Bennett and DePaolo, 1987

29. Hf-Nd array

30. Hf in zircons avoids age and alteration uncertainties. Hf model ages have their own complexities/ For example, must estimate Lu/Hf of parental rock. Nebel et al, 2007 Gawler craton

31. (nucleosynthesis, mixing) Summary of Earth Differentiation Solar Nebula (volatiles) (gas-solid equilibria) (refractories) Condensation and Accretion (late veneer) (continuing cometary flux?) (siderophile & chalcophile) (melting; gravity and geochemical affinity) (atmophile) (lithophile) (lost due to impacts) Core Silicate Earth Primitive Atmosphere (freezing) (catastrophic impact) Moon Primitive Mantle Inner Core Outer Core (partial melting; liquid-crystal partitioning) degassing Upper Mantle Lower Mantle Continental Crust (plate tectonics: partial melting, recycling) (hotspot plumes) degassing Modern Ocean & Atmosphere Oceanic Crust

32. More reservoirs than crust and mantle.

34. Complexity of magma sources Injection of basaltic melts into granitic magma chambers (and vice versa) Replenishment of an evolved chamber with more primitive melt hybrid magma Mixing of melts from different levels of an evolving magma chamber Melting of country rock and mixing of these melts into magma chamber Melting of sources of different compositions (mantle and/or crust)

35. Ideal hybridization A B M Mix melts A and B to create a hybrid melt M ALL LIQUIDS!

36. Two component mixing Proterozoic granite Fractional xtal.

37. Curvature controlled by ’K’

38. Example: Kimberlites, W. Australia From: McCulloch et al. (1983) Nd and Sr isotopes in kimberlites and lamproites from Western Australia: an enriched mantle origin. Nature 302: 401-403

39. Example: Hawaii

40. Single crystals may record magma mixing events through ’Crystal Isotope Stratigraphy’ Figure from: Recharge in Volcanic Systems: Evidence from Isotope Profiles of Phenocrysts Jon P. Davidson andFrank J. Tepley III Science, Vol 275, Issue 5301, 826-829 , 7 February 1997

42. Swan nebula

43. 146Sm-142Nd “extinct” system Generation of 142Nd variations requires parent/daughter fractionation (Sm/Nd) within 400 myr of solar system formation. No other known process e.g. metamorphism to generate 142Nd/144Nd variations. = robust signature of earliest planetary processes

44. “Extinct” in 4-5 half lives

45. 146Sm-142Nd Generation of 142Nd variations requires parent/daughter fractionation (Sm/Nd) within 400 myr of solar system formation. No corrections for in situ decay.

47. History -search for terrestrial 142 Nd Variations. Anomaly? Precision 1992 Harper and Jacobsen 33ppm 10 ppm 1992 Goldstein and Galer No 12 ppm 1993 McCulloch and Bennett No 12 ppm 1996 Regelous and Collerson No 10 ppm 1996 Sharma et al. Maybe 20 ppm 2003 Boyet et al. 30ppm 20 ppm 2003 Caro et al. 15 ppm 5 ppm 2006 Caro et al. 8-15ppm 2ppm 2006 Bennett and Brandon 8-19ppm 3ppm

48. Johnson Space Center Triton TIMS Multidynamic data Collection ± 3.3 ppm 2s S.D.

49. Isua region, southwest Greenland

50. Itsaq gneiss complex (Nutman et al, 1996) Contains multiple generations of rocks with ages of 3.63-3.87 Ga. Comprises two major terranes with >3600 Ma ages