Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation

preservation of primary signatures versus secondary disturbances

Robert Frei, Minik Rosing, Ole Stecher

    Publikation: Bidrag til tidsskriftTidsskriftartikelForskningpeer review

    Resumé

    Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fractions of a BIF sample from the northeastern part of the Isua supracrustal belt (West Greenland) yield an isochron of 3691 ± 22 Ma (MSWD = 0.4). In combination with previously published geochronological constraints for a minimum deposition age of 3.71 Ga for volcanic sequences (Nutman et al., 1997) comprising the studied BIFs, and supported by microtextural observations, this demonstrates an early Archean amphibolite facies metamorphic event in the supracrustal. PbSL data on magnetite from slightly discordant veins within the same iron stone, together with bulk Pb isotope values of paragenetically late pyrite, yield a 3.63 ± 0.07 Ga (MSWD = 8.6) errorchron, with significantly different Pb isotopic compositions from those obtained from the main magnetite layers, and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3.6 Ga granitic into 3.7 Ga tonalitic Amîtsoq gneisses enclosing the belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [Nd(3800) of +1.7 and +2.1], within error of published results from adjacent basic units in the supracrustal suite. In contrast, the magnetite-dominated layers yield unrealistically high Nd(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fractionated, continental sources. These high Nd(3800), together with radiogenic Sr leached from the magnetite-enriched separates, is ascribed to secondary hydroxyapatite, which predominantly forms as crystal overgrowths in the magnetite-rich bands. The timing of the hydrothermal event during which apatite was deposited within the BIF remains uncertain, but a TCHUR model age of 1.85 Ga from the apatite-dominated HCl leachate may point to a close genetic relationship with local Proterozoic metamorphism and granite formation by crustal remelting in the Isukasia area.

    Step leaching of magnetite from a similar silicate-oxide facies BIF from the western part of the Isua supracrustal belt yield a Pb-Pb isochron age of 2.84 ± 0.05 Ga (MSWD = 1.43). The agreement between the PbSL age and previously published field and isotopic evidence for a major late Archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2.75–2.83 Ga granodioritic Ikkattoq gneisses west and southwest of the western limb of the Isua supracrustal belt.

    We argue that the PbSL isochron ages date the peak of amphibolite facies metamorphic events which, respectively, affected the eastern and western sections of the Isua supracrustal belt during the early and late Archean. Our results give additional support to the suggestion that the scatter on published Nd(T) values from the Isua supracrustal belt and adjoining gneisses can be assigned to post-formational hydrothermal processes and underline the need for care in the interpretation of Sm-Nd bulk data from polymetamorphic rocks to constrain isotopic models of early Earth’s evolution.

    OriginalsprogEngelsk
    TidsskriftGeochimica et Cosmochimica Acta
    Vol/bind63
    Udgave nummer3-4
    Sider (fra-til)473-488
    Antal sider16
    ISSN0016-7037
    DOI
    StatusUdgivet - 1999

    Citer dette

    @article{936c336e36cb4affb8ea542b9aad59b3,
    title = "Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation: preservation of primary signatures versus secondary disturbances",
    abstract = "Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fractions of a BIF sample from the northeastern part of the Isua supracrustal belt (West Greenland) yield an isochron of 3691 ± 22 Ma (MSWD = 0.4). In combination with previously published geochronological constraints for a minimum deposition age of 3.71 Ga for volcanic sequences (Nutman et al., 1997) comprising the studied BIFs, and supported by microtextural observations, this demonstrates an early Archean amphibolite facies metamorphic event in the supracrustal. PbSL data on magnetite from slightly discordant veins within the same iron stone, together with bulk Pb isotope values of paragenetically late pyrite, yield a 3.63 ± 0.07 Ga (MSWD = 8.6) errorchron, with significantly different Pb isotopic compositions from those obtained from the main magnetite layers, and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3.6 Ga granitic into 3.7 Ga tonalitic Am{\^i}tsoq gneisses enclosing the belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [Nd(3800) of +1.7 and +2.1], within error of published results from adjacent basic units in the supracrustal suite. In contrast, the magnetite-dominated layers yield unrealistically high Nd(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fractionated, continental sources. These high Nd(3800), together with radiogenic Sr leached from the magnetite-enriched separates, is ascribed to secondary hydroxyapatite, which predominantly forms as crystal overgrowths in the magnetite-rich bands. The timing of the hydrothermal event during which apatite was deposited within the BIF remains uncertain, but a TCHUR model age of 1.85 Ga from the apatite-dominated HCl leachate may point to a close genetic relationship with local Proterozoic metamorphism and granite formation by crustal remelting in the Isukasia area. Step leaching of magnetite from a similar silicate-oxide facies BIF from the western part of the Isua supracrustal belt yield a Pb-Pb isochron age of 2.84 ± 0.05 Ga (MSWD = 1.43). The agreement between the PbSL age and previously published field and isotopic evidence for a major late Archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2.75–2.83 Ga granodioritic Ikkattoq gneisses west and southwest of the western limb of the Isua supracrustal belt. We argue that the PbSL isochron ages date the peak of amphibolite facies metamorphic events which, respectively, affected the eastern and western sections of the Isua supracrustal belt during the early and late Archean. Our results give additional support to the suggestion that the scatter on published Nd(T) values from the Isua supracrustal belt and adjoining gneisses can be assigned to post-formational hydrothermal processes and underline the need for care in the interpretation of Sm-Nd bulk data from polymetamorphic rocks to constrain isotopic models of early Earth’s evolution.",
    author = "Robert Frei and Minik Rosing and Ole Stecher",
    year = "1999",
    doi = "10.1016/S0016-7037(98)00290-7",
    language = "English",
    volume = "63",
    pages = "473--488",
    journal = "Geochimica et Cosmochimica Acta",
    issn = "0016-7037",
    publisher = "Pergamon Press",
    number = "3-4",

    }

    Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation : preservation of primary signatures versus secondary disturbances. / Frei, Robert; Rosing, Minik; Stecher, Ole.

    I: Geochimica et Cosmochimica Acta, Bind 63, Nr. 3-4, 1999, s. 473-488.

    Publikation: Bidrag til tidsskriftTidsskriftartikelForskningpeer review

    TY - JOUR

    T1 - Controversial Pb-Pb and Sm-Nd isotope results in the early Archean Isua (West Greenland) oxide iron formation

    T2 - preservation of primary signatures versus secondary disturbances

    AU - Frei, Robert

    AU - Rosing, Minik

    AU - Stecher, Ole

    PY - 1999

    Y1 - 1999

    N2 - Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fractions of a BIF sample from the northeastern part of the Isua supracrustal belt (West Greenland) yield an isochron of 3691 ± 22 Ma (MSWD = 0.4). In combination with previously published geochronological constraints for a minimum deposition age of 3.71 Ga for volcanic sequences (Nutman et al., 1997) comprising the studied BIFs, and supported by microtextural observations, this demonstrates an early Archean amphibolite facies metamorphic event in the supracrustal. PbSL data on magnetite from slightly discordant veins within the same iron stone, together with bulk Pb isotope values of paragenetically late pyrite, yield a 3.63 ± 0.07 Ga (MSWD = 8.6) errorchron, with significantly different Pb isotopic compositions from those obtained from the main magnetite layers, and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3.6 Ga granitic into 3.7 Ga tonalitic Amîtsoq gneisses enclosing the belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [Nd(3800) of +1.7 and +2.1], within error of published results from adjacent basic units in the supracrustal suite. In contrast, the magnetite-dominated layers yield unrealistically high Nd(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fractionated, continental sources. These high Nd(3800), together with radiogenic Sr leached from the magnetite-enriched separates, is ascribed to secondary hydroxyapatite, which predominantly forms as crystal overgrowths in the magnetite-rich bands. The timing of the hydrothermal event during which apatite was deposited within the BIF remains uncertain, but a TCHUR model age of 1.85 Ga from the apatite-dominated HCl leachate may point to a close genetic relationship with local Proterozoic metamorphism and granite formation by crustal remelting in the Isukasia area. Step leaching of magnetite from a similar silicate-oxide facies BIF from the western part of the Isua supracrustal belt yield a Pb-Pb isochron age of 2.84 ± 0.05 Ga (MSWD = 1.43). The agreement between the PbSL age and previously published field and isotopic evidence for a major late Archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2.75–2.83 Ga granodioritic Ikkattoq gneisses west and southwest of the western limb of the Isua supracrustal belt. We argue that the PbSL isochron ages date the peak of amphibolite facies metamorphic events which, respectively, affected the eastern and western sections of the Isua supracrustal belt during the early and late Archean. Our results give additional support to the suggestion that the scatter on published Nd(T) values from the Isua supracrustal belt and adjoining gneisses can be assigned to post-formational hydrothermal processes and underline the need for care in the interpretation of Sm-Nd bulk data from polymetamorphic rocks to constrain isotopic models of early Earth’s evolution.

    AB - Pb stepwise leaching (PbSL) determinations on two magnetite-enriched fractions of a BIF sample from the northeastern part of the Isua supracrustal belt (West Greenland) yield an isochron of 3691 ± 22 Ma (MSWD = 0.4). In combination with previously published geochronological constraints for a minimum deposition age of 3.71 Ga for volcanic sequences (Nutman et al., 1997) comprising the studied BIFs, and supported by microtextural observations, this demonstrates an early Archean amphibolite facies metamorphic event in the supracrustal. PbSL data on magnetite from slightly discordant veins within the same iron stone, together with bulk Pb isotope values of paragenetically late pyrite, yield a 3.63 ± 0.07 Ga (MSWD = 8.6) errorchron, with significantly different Pb isotopic compositions from those obtained from the main magnetite layers, and this suggests that the discordant layers of magnetite and sulfides crystallized when externally derived fluids passed through the formation a few tens of million of years later. The fluids controlling this redistribution of magnetite can be tentatively correlated with metasomatic alteration features produced during the widespread intrusion of 3.6 Ga granitic into 3.7 Ga tonalitic Amîtsoq gneisses enclosing the belt. Tremolite-rich layers in the BIF are characterized by Sm/Nd ratios close to chondritic [Nd(3800) of +1.7 and +2.1], within error of published results from adjacent basic units in the supracrustal suite. In contrast, the magnetite-dominated layers yield unrealistically high Nd(3800) of +14.8 and +14.4, indicative of Sm/Nd ratios resembling REE fractionated, continental sources. These high Nd(3800), together with radiogenic Sr leached from the magnetite-enriched separates, is ascribed to secondary hydroxyapatite, which predominantly forms as crystal overgrowths in the magnetite-rich bands. The timing of the hydrothermal event during which apatite was deposited within the BIF remains uncertain, but a TCHUR model age of 1.85 Ga from the apatite-dominated HCl leachate may point to a close genetic relationship with local Proterozoic metamorphism and granite formation by crustal remelting in the Isukasia area. Step leaching of magnetite from a similar silicate-oxide facies BIF from the western part of the Isua supracrustal belt yield a Pb-Pb isochron age of 2.84 ± 0.05 Ga (MSWD = 1.43). The agreement between the PbSL age and previously published field and isotopic evidence for a major late Archean metamorphic event affecting the western area suggests there was widespread equilibration accompanying the intrusion of the 2.75–2.83 Ga granodioritic Ikkattoq gneisses west and southwest of the western limb of the Isua supracrustal belt. We argue that the PbSL isochron ages date the peak of amphibolite facies metamorphic events which, respectively, affected the eastern and western sections of the Isua supracrustal belt during the early and late Archean. Our results give additional support to the suggestion that the scatter on published Nd(T) values from the Isua supracrustal belt and adjoining gneisses can be assigned to post-formational hydrothermal processes and underline the need for care in the interpretation of Sm-Nd bulk data from polymetamorphic rocks to constrain isotopic models of early Earth’s evolution.

    U2 - 10.1016/S0016-7037(98)00290-7

    DO - 10.1016/S0016-7037(98)00290-7

    M3 - Journal article

    VL - 63

    SP - 473

    EP - 488

    JO - Geochimica et Cosmochimica Acta

    JF - Geochimica et Cosmochimica Acta

    SN - 0016-7037

    IS - 3-4

    ER -