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Deciphering primary signature from stable isotopes in Archean metamorphic rocks

Vincent Busigny Lab. Géochimie des Isotopes Stables & Lab. Géochimie et Cosmochimie IPG Paris, France. Deciphering primary signature from stable isotopes in Archean metamorphic rocks. Rock record of the early Earth. Oldest witness of earliest time

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Deciphering primary signature from stable isotopes in Archean metamorphic rocks

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  1. Vincent Busigny Lab. Géochimie des Isotopes Stables & Lab. Géochimie et Cosmochimie IPG Paris, France Deciphering primary signature from stable isotopes in Archean metamorphic rocks

  2. Rock record of the early Earth • Oldest witness of earliest time • = 4.4 Ga zircons in younger detrital sediments • (Froude et al., 1983) • Oldest “known” terrestrial rock • = 4.03 Ga orthogneiss from NW Canada • (Bowring et al., 1999) • -> No rocks formed before 4.03 survived crustal recycling! • Oldest supracrustal rocks • = 3.8 Ga rocks from Greenland and Canada • --> lithologies formed at the top of the crust • such as lava flow and sediments

  3. Interest of Eoarchean (3.8-3.6 Ga) supracrustal rocks To identify possible chemical sediments in Eoarchean because: 1 - They may host Earth’s earliest remnants of life - Was the Earth habitable? - Had life already emerged? - If so, what kind of organism lived at that time? 2 - They represent the only direct witness of the atmosphere/ocean system - How was the chemistry of the atmosphere/ocean system? - What was the temperature of the ocean and atmosphere?

  4. Study of Eoarchean rocks is not straightforward… • The quality of the record degrades with the antiquity of the rock • 1 - ancient rocks buried in the crust at high PT conditions => modification of their structure and mineralogy • 2 - interactions with crustal fluids => obliteration of chemical and isotopic fingerprints • --> difficult to interpret primary message = leads to controversies!

  5. Study of Eoarchean rocks is not straightforward… • The quality of the record degrades with the antiquity of the rock • 1 - ancient rocks buried in the crust at high PT conditions => modification of their structure and mineralogy • 2 - interactions with crustal fluids => obliteration of chemical and isotopic fingerprints • --> difficult to interpret primary message = leads to controversies! Stable isotopes can preserved precious information about protolith

  6. Framework of this talk… on Eoarchean metamorphic rocks… 1 - Recognition of Banded Iron Formation (BIF) 2 - Origin of graphite

  7. 1 - RECOGNITION OF BANDED IRON FORMATION IN EOARCHEAN METAMORPHIC ROCKS

  8. - Chemical sediment, laminated, Fe-rich • Mineralogy: mainly chert (SiO2), magnetite (Fe3O4), hematite (Fe2O3) + Fe-carbonate (FeCO3), pyrite (FeS2) High [Fe] in seawater --> anoxic conditions Banded iron-formations (BIF) Akilia? (Klein, Am. Mineral. 2005)

  9. Location of Eoarchean rocks in southern West Greenland 30 km

  10. Southwestern island of Akilia, southern West Greenland

  11. Origin of the quartz-pyroxene rock from Akilia (SW Greenland) • Two main hypotheses: • 1- Highly metamorphosed chemical sediment deposited in Archean seawater • (Mc Gregor and Mason, 1977; Mojsis et al., 1996; Nutman et al., 1997; Bolhar et al., 2004) • 2- Metasomatism of mafic and ultramafic rocks by quartz addition and • tectonic reworking • (Fedo and Whitehouse, 2002; André et al., 2006)

  12. Quartz-pyroxene rock from Akilia (SW Greenland) • -> Texture: layered banded rock • -> Age: > 3.85 Ga (Eoarchean) • -> Strong deformation • -> Metamorphism to amphibolite-granulite facies (>500°C, 5kbar) • -> Mineralogy: quartz, pyroxene (cpx, opx), magnetite, • amphibole (cum, hornb, grun), apatite, graphite --> Contain graphite interpreted as the oldest traces of life (Mojsis, Nature 1996)

  13. BIF samples from Isua Supracrustal Belt (SW Greenland) redrawn from Nutman et al. (1996)

  14. BIF samples from Isua Supracrustal Belt (SW Greenland) redrawn from Nutman et al. (1996)

  15. Iron has 4 stable isotopes: 54Fe 5.845% • 56Fe 91.754% • 57Fe 2.1191% • 58Fe 0.2819% • - Iron isotope composition is expressed using  notation in ‰ as, • or in ‰/amu as, • The standard used is IRMM-014 (Institute for Reference Material and Measurements) • FFe IRMM-014 ~ FFe Orgueil CI1 carbonaceous chondrite (Dauphas et al., Anal. Chem. 2004) Iron isotopes: analytical technique and notation

  16. Neighbor elements (efficiently ionized): • Cr+ at m/z = 54 Ni+ at m/z = 58 • Sample matrix element • 40Ca16O+ at m/z = 56 40Ca16OH+ at m/z = 57 • Doubly charged species interferences: • Pd2+, Cd2+ at m/z = 54 (=108/2) • Cd2+, Sn2+ at m/z = 56 (=112/2) • Cd2+, Sn2+ at m/z = 57 (=114/2) Main difficulty in Fe isotope analysis: isobaric interferences

  17. Neighbor elements (efficiently ionized): • Cr+ at m/z = 54 Ni+ at m/z = 58 • Sample matrix element • 40Ca16O+ at m/z = 56 40Ca16OH+ at m/z = 57 • Doubly charged species interferences: • Pd2+, Cd2+ at m/z = 54 (=108/2) • Cd2+, Sn2+ at m/z = 56 (=112/2) • Cd2+, Sn2+ at m/z = 57 (=114/2) Main difficulty in Fe isotope analysis: isobaric interferences => Ion-exchange chromatography But large isotopic fractionation may occur (Anbar et al., Sci 2000) => A good yield is essential!

  18. Chromatography on AG1-X8 anion exchange resin: • Sample solutions are loaded in 6M HCl • Matrix elements are eluted in 6M HCl • Fe is eluted in 0.4M HCl • MC-ICPMS analysis • => 6 Faraday cups are used: • 53Cr and 60Ni are measured to monitor and correct for interferences of 54Cr and 58Ni Main difficulty in Fe isotope analysis: isobaric interferences Fe 54 56 57 58 Cr 53 54 Ni 58 60

  19. Polyatomic argide interferences (from Ar plasma): • 40Ar14N+ at m/z = 54 40Ar16OH+ at m/z = 57 • 40Ar16O+ at m/z = 56 40Ar18O+ at m/z = 58 Main difficulty in Fe isotope analysis: isobaric interferences

  20. Polyatomic argide interferences (from Ar plasma): • 40Ar14N+ at m/z = 54 40Ar16OH+ at m/z = 57 • 40Ar16O+ at m/z = 56 40Ar18O+ at m/z = 58 This effect can be reduced by : - desolvating nebulizers => dry plasma (Belshaw et al., 2000) - collision cell (Beard et al., 2003; Rouxel et al., 2003) - cold plasma (Walczyk and von Blanckenburg, 2002; Kehm et al., 2003) - using highly concentrated solution (Zhu et al., 2002 ; Belshaw et al., 2000) - high mass resolution measurement (Weyer et al., 2003) Main difficulties: isobaric interferences

  21. Mass scan performed in high resolution with the Neptune (Weyer et al., 2003)

  22. Measurement ! Mass scan performed in high resolution with the Neptune (Weyer et al., 2003)

  23. Oxic small range Anoxic Large range Fe isotope composition in rocks from various environments (Beard and Johnson, 2004)

  24. - BIF from Transvaal Craton, South Africa - 56Fe values increase from: pyrite < Fe-carbonates < hematite < magnetite Fe isotope composition in Banded Iron Formation (Johnson et al., 2003; Beard & Johnson, 2004)

  25. +2.9 ‰ -0.5 ‰ Fe(II)aq Fe(III)aq -1.4 ‰ k2 Fe2O3 +1.0 ‰ k1 -k1 Fe oxide precipitation from Fe(II) is a two step process: 1 - oxidation: Fe(II)aq --> Fe(III)aq 2 - precipitation: Fe(III)aq --> Fe(OH)3 Overall, Fe(OH)3-Fe(II)aq = +1.5 ‰ (equilibrium+kinetic) Iron isotopes fractionation during Fe oxide precipitation Fe(II)aq --> Fe(III)aq (Welch et al., GCA2003) Fe(III)aq --> hematite (Skulan et al., GCA2002)

  26. Fe isotope composition of SW Greenland metamorphic rocks Dauphas et al. (Sci 2004, GCA 2007)

  27. Data from Rouxel et al. (GCA 2003) Fe isotope fractionation during basalt alteration Fe is depleted (relative to Ti) and Fe isotopes are enriched in heavy isotopes Could alteration produce the enrichment in heavy Fe isotopes in Akilia quartz-pyroxene rocks?

  28. Fe isotopic composition vs Fe/Ti in SW Greenland rocks • Enrichment in heavy Fe isotopes is not related to any Fe loss • purported metasediments are likely real BIFs ! Dauphas et al. (Sci 2004, GCA 2007)

  29. A new “window” on Eoarchean: Nuvvuagittuq Greenstone Belt • -> Recently discovered 3.8 Ga supracrustal belt in northern Quebec (Canada) • -> All units metamorphosed to upper amphibolite-lower granulite facies • --> Various lithologies including : • - mafic and ultramafic amphibolites • - quartz-biotite and pelitic schists • - orthogneisses • - banded quartz-magnetite-amphibole/pyroxene rocks I--> Chemical sedimentary origin ?

  30. Nuvvuagittuq, Innuksuac Complex, Northern Quebec (Canada) Nuvvuagittuq

  31. Fe isotope composition in 3.8 Ga rocks from Nuvvuagittuq Dauphas et al. (EPSL, 2007)

  32. --> Sample from Nuvvuagittuq (Canada) Py Cc Qtz+Cum Mgt Chemical and isotopic mapping of banded quartz-magnetite rock Dauphas et al. (EPSL, 2007)

  33. 2 - ORIGIN OF GRAPHITE IN EOARCHEAN METAMORPHIC ROCKS

  34. Carbon has 2 stable isotopes: 12C 98.93% • 13C 1.07% • Analytical techniques: • - bulk = gaseous CO2 extraction and IRMS analysis • - In situ measurement = ion microprobe analysis • - Carbon isotope composition is expressed in ‰ as, • The standard used is VPDB or PDB (Vienna Pee Dee Belemnite) Carbon stable isotopes: measurement and notation

  35. Processes fractionating carbon isotopes - Equilibrium isotope effects e.g. isotope equilibrium between two phases (e.g. CO2-CH4 at T=100°C) - Kinetic isotope effects e.g. incomplete, unidirectional processes (metabolism!) - Diffusion effects e.g. 12C16O16O moves 1% faster than 13C16O16O

  36. Carbon isotope composition in biological material and carbonates δ13C vs PDB (‰) -20 -10 0 10 -20 -10 0 10 Dissolved CO2 δ13C ~ -8 ‰ kinetic equilibrium Corganic HCO3- Ccarbonates δ13C ~ -25‰ δ13C ~ 0 ‰ CaCO3 Sediments

  37. Carbon isotope composition in biological material and carbonates Constant distribution over Earth geological time (from 3.5 Ga to present) --> evidence for life Schidlowski (1988, 2001)

  38. Carbon isotope composition in biological material and carbonates Constant distribution over Earth geological time (from 3.5 Ga to present) --> evidence for life ? ? Schidlowski (1988,2001)

  39. Carbon isotopes in Eoarchean metasediments from Greenland Bulk rock Graphite inclusions in apatite crystals Average graphite:13Cbulk rock ~ -13 ‰ 13Csingle inclusions ~ -37 ‰ (Schidlowski, 1988,2001; Mojsis et al., 1996)

  40. Carbon isotope fractionation during metamorphism Metamorphic rocks from Greenland have experienced amphibolite facies metamorphism: 1 - High temperature isotope exchange between organic and carbonate C --> Graphite 13C increases 2 - Devolatilization of the primary organic carbon - release of isotopically light CH4 --> graphite 13C increases - release of isotopically heavy CO2 --> graphite 13C decreases Small magnitude in isotopic shift (<3‰ for 90% of graphite loss) Overall, metamorphic processes increase graphite 13C --> the lowest values are likely the most pristine (Schidlowski, 1988, 2001; Mojsis et al., 1996)

  41. Equilibrium carbon isotope fractionation Equilibrium isotope fractionation Δ = 1000 ln α ΔA-CO2 = δ13CA-δ13CCO2 For Isua metamorphism temperature, graphite-carbonate~ 5 to 10 ‰ Modified after Chacko et al. 2001

  42. Carbon isotopes in Eoarchean metasediments from Greenland Bulk rock Graphite inclusions in apatite crystals Lowest graphite 13C values: - Bulk rock ~ -22 to -28 ‰ (similar to present organic matter) - Inclusions ~ -49 ‰ (methanotrophs bacteria?) (Schidlowski, 1988,2001; Mojsis et al., 1996)

  43. New C isotopes data in Eoarchean metasediments from Greenland (Van Zuilen et al., 2002, 2003)

  44. Step-heating combustion data in metasediments from Greenland In BIF and metacherts, the major fraction of C is released at T ≤ 450°C --> NOT graphite (= combustion at 700-800°C) --> BUT unmetamorphosed recent organic material ! Example of a metachert (Van Zuilen et al., 2002, 2003)

  45. Graphite occurrence in Eoarchean metasediments from Greenland Graphite is closely associated with siderite and magnetite suggesting that it was produced by thermal disproportionation of siderite, 6 FeCO3 --> 2 Fe3O4 +5 CO2 + C (Van Zuilen et al., 2002, 2003)

  46. Carbon isotopes in Eoarchean turbidite from Greenland Turbidite: Real graphite // not associated with siderite // 13C ~ -20‰ --> compatible with an organic origin

  47. 1 - Purported BIF can be identified from Fe isotopes (FFe > 0‰), a coupling with Fe/Ti ratio is necessary to test hypothesis of Fe leaching. 2 - Graphite from ISB rocks is mostly produced from siderite thermal decomposition --> Turbidite may carry a primary signature of early life… In metamorphic rocks, stable isotopes signature can provide constrain on the protolith but it must be taken with caution and has to be associated with petrological observation Conclusion

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