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In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust

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Title: In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust
Authors: Hiess, Joe1 joehiess@kbsi.re.kr, Bennett, Vickie C.1, Nutman, Allen P.2, Williams, Ian S.1
Source: Geochimica et Cosmochimica Acta. Aug2009, Vol. 73 Issue 15, p4489-4516. 28p.
Subjects: Rock-forming minerals, Zircon, Olivine, Archaean stratigraphic geology, Isotope geology, Petrogenesis, Volcanic ash, tuff, etc., Tonalite, Hafnium isotopes, Crust of the earth, Earth (Planet)
Geographic Terms: Greenland
Abstract: Abstract: The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U–Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. 207Pb/206Pb dating of oscillatory-zoned zircon using SHRIMP RG (n =142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69Ga. Analyses of 18O/16O compositions of olivine from a harzburgite/dunite (n =8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (δ 18OOl =6.0±0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ 18O measurements from the six felsic rocks (n =93) record mean or weighted mean compositions ranging from 4.9±0.7‰ to 5.1±0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ 18OZr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ 18OWR compositions (6.7–6.9‰) resemble those of modern adakites. LA-MC-ICPMS zircon 176Hf/177Hf analyses were obtained from six samples (n =122). Five samples record weighted mean initial ε Hf compositions ranging from to 0.5±0.6 to −0.1±0.7 (calculated using λ 176Lu=1.867×10−11 yr−1), while one sample records a composition of 1.3±0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf. Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (δ 18OWR =5.5‰) with ∼20% hydrothermally altered gabbro/basalt (δ 18OWR =4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab. [Copyright &y& Elsevier]
Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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DbLabel: Engineering Source
An: 42965800
AccessLevel: 6
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  Label: Title
  Group: Ti
  Data: In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust
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  Data: <searchLink fieldCode="AR" term="%22Hiess%2C+Joe%22">Hiess, Joe</searchLink><relatesTo>1</relatesTo><i> joehiess@kbsi.re.kr</i><br /><searchLink fieldCode="AR" term="%22Bennett%2C+Vickie+C%2E%22">Bennett, Vickie C.</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Nutman%2C+Allen+P%2E%22">Nutman, Allen P.</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Williams%2C+Ian+S%2E%22">Williams, Ian S.</searchLink><relatesTo>1</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Aug2009, Vol. 73 Issue 15, p4489-4516. 28p.
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  Data: <searchLink fieldCode="DE" term="%22Rock-forming+minerals%22">Rock-forming minerals</searchLink><br /><searchLink fieldCode="DE" term="%22Zircon%22">Zircon</searchLink><br /><searchLink fieldCode="DE" term="%22Olivine%22">Olivine</searchLink><br /><searchLink fieldCode="DE" term="%22Archaean+stratigraphic+geology%22">Archaean stratigraphic geology</searchLink><br /><searchLink fieldCode="DE" term="%22Isotope+geology%22">Isotope geology</searchLink><br /><searchLink fieldCode="DE" term="%22Petrogenesis%22">Petrogenesis</searchLink><br /><searchLink fieldCode="DE" term="%22Volcanic+ash%2C+tuff%2C+etc%2E%22">Volcanic ash, tuff, etc.</searchLink><br /><searchLink fieldCode="DE" term="%22Tonalite%22">Tonalite</searchLink><br /><searchLink fieldCode="DE" term="%22Hafnium+isotopes%22">Hafnium isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Crust+of+the+earth%22">Crust of the earth</searchLink><br /><searchLink fieldCode="DE" term="%22Earth+%28Planet%29%22">Earth (Planet)</searchLink>
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  Label: Geographic Terms
  Group: Su
  Data: <searchLink fieldCode="DE" term="%22Greenland%22">Greenland</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: Abstract: The sources and petrogenetic processes that generated some of the Earth’s oldest continental crust have been more tightly constrained via an integrated, in situ (U–Pb, O and Hf) isotopic approach. The minerals analysed were representative zircon from four Eoarchaean TTG tonalites and two felsic volcanic rocks, and olivine from one harzburgite/dunite of the Itsaq Gneiss Complex (IGC), southern West Greenland. The samples were carefully chosen from localities with least migmatisation, metasomatism and strain. Zircon was thoroughly characterized prior to analysis using cathodoluminescence, scanning electron, reflected and transmitted light imaging. The zircon from all but one sample showed only minor post-magmatic recrystallisation. 207Pb/206Pb dating of oscillatory-zoned zircon using SHRIMP RG (n =142) indicates derivation of the felsic igneous rocks from different batches of magma at 3.88, 3.85, 3.81, 3.80 and 3.69Ga. Analyses of 18O/16O compositions of olivine from a harzburgite/dunite (n =8) using SHRIMP II in multi-collector mode, indicate that the oxygen isotopic composition of this sample of Eoarchaean mantle (δ 18OOl =6.0±0.4‰) was slightly enriched in 18O, but not significantly different from that of the modern mantle. Zircon δ 18O measurements from the six felsic rocks (n =93) record mean or weighted mean compositions ranging from 4.9±0.7‰ to 5.1±0.4‰, with recrystallised domains showing no indication of oxygen isotopic exchange during younger tectonothermal events. δ 18OZr compositions indicate that the primary magmas were largely in equilibrium with the mantle or mantle-derived melts generated at similar high temperatures, while calculated tonalite δ 18OWR compositions (6.7–6.9‰) resemble those of modern adakites. LA-MC-ICPMS zircon 176Hf/177Hf analyses were obtained from six samples (n =122). Five samples record weighted mean initial ε Hf compositions ranging from to 0.5±0.6 to −0.1±0.7 (calculated using λ 176Lu=1.867×10−11 yr−1), while one sample records a composition of 1.3±0.7, indicating the magmas were generated from a reservoir with a time averaged, near chondritic Lu/Hf. The derivation of TTG magmas from a chondritic Lu/Hf source implies either that there was not voluminous continental crustal growth nor major mantle differentiation leading to Lu/Hf fractionation during the Hadean or Eoarchaean, or alternatively that rapid recycling of an early formed crust allowed the early mantle to maintain a chondritic Lu/Hf. Previous studies have demonstrated that ancient TTG rocks were mostly produced by dehydration melting of mafic rocks within the stability field of garnet, probably in flatly-subducted or buried oceanic crust. The oxygen isotopic signatures measured here at high spatial resolution allow the source materials to be better defined. Melting of a mixed mafic source consisting of ∼80% unaltered gabbro (δ 18OWR =5.5‰) with ∼20% hydrothermally altered gabbro/basalt (δ 18OWR =4.0‰) would produce tonalite magmas within the average compositional range observed. 18O-enriched components such as altered shallow basaltic oceanic crust and pelagic or continental sediments were not present in the sources of these TTG melts. The absence of high 18O signatures may indicate either the rarity of low temperature altered sediments, or their effective removal from the down-going slab. [Copyright &y& Elsevier]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Geochimica et Cosmochimica Acta is the property of Pergamon Press - An Imprint of Elsevier Science and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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RecordInfo BibRecord:
  BibEntity:
    Identifiers:
      – Type: doi
        Value: 10.1016/j.gca.2009.04.019
    Languages:
      – Code: eng
        Text: English
    PhysicalDescription:
      Pagination:
        PageCount: 28
        StartPage: 4489
    Subjects:
      – SubjectFull: Rock-forming minerals
        Type: general
      – SubjectFull: Zircon
        Type: general
      – SubjectFull: Olivine
        Type: general
      – SubjectFull: Archaean stratigraphic geology
        Type: general
      – SubjectFull: Isotope geology
        Type: general
      – SubjectFull: Petrogenesis
        Type: general
      – SubjectFull: Volcanic ash, tuff, etc.
        Type: general
      – SubjectFull: Tonalite
        Type: general
      – SubjectFull: Hafnium isotopes
        Type: general
      – SubjectFull: Crust of the earth
        Type: general
      – SubjectFull: Earth (Planet)
        Type: general
      – SubjectFull: Greenland
        Type: general
    Titles:
      – TitleFull: In situ U–Pb, O and Hf isotopic compositions of zircon and olivine from Eoarchaean rocks, West Greenland: New insights to making old crust
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            NameFull: Hiess, Joe
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            NameFull: Bennett, Vickie C.
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            NameFull: Nutman, Allen P.
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            – D: 01
              M: 08
              Text: Aug2009
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              Y: 2009
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              Value: 73
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