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Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition

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Title: Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition
Authors: Ueno, Yuichiro1 ichiro@geo.titech.ac.jp, Yurimoto, Hisayoshi1, Yoshioka, Hideyoshi2, Komiya, Tsuyoshi1, Maruyama, Shigenori1
Source: Geochimica et Cosmochimica Acta. Apr2002, Vol. 66 Issue 7, p1257. 12p.
Subjects: Field ion microscopes, Microprobe analysis, Isotopes, Graphite
Abstract: In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ13C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from –18 to +2‰ and from –7 to –3‰, respectively. The maximum δ13C value of graphite globules in the metasediment rises from –14 to –5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ13C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ13C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively 13C-depleted graphite indicates that the regional metamorphism increased the δ13C values of the Isua graphite. This is consistent with the regional trend of 13C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused 13C-enrichment of Isua graphite. The δ13C values of graphite reported here (δ13C > –18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ13C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13C-enrichment of Isua graphite, previously reported large 13C-depletion of graphite, especially armored by apatite was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga). [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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  Data: Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition
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  Data: <searchLink fieldCode="AR" term="%22Ueno%2C+Yuichiro%22">Ueno, Yuichiro</searchLink><relatesTo>1</relatesTo><i> ichiro@geo.titech.ac.jp</i><br /><searchLink fieldCode="AR" term="%22Yurimoto%2C+Hisayoshi%22">Yurimoto, Hisayoshi</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Yoshioka%2C+Hideyoshi%22">Yoshioka, Hideyoshi</searchLink><relatesTo>2</relatesTo><br /><searchLink fieldCode="AR" term="%22Komiya%2C+Tsuyoshi%22">Komiya, Tsuyoshi</searchLink><relatesTo>1</relatesTo><br /><searchLink fieldCode="AR" term="%22Maruyama%2C+Shigenori%22">Maruyama, Shigenori</searchLink><relatesTo>1</relatesTo>
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  Data: <searchLink fieldCode="JN" term="%22Geochimica+et+Cosmochimica+Acta%22">Geochimica et Cosmochimica Acta</searchLink>. Apr2002, Vol. 66 Issue 7, p1257. 12p.
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  Data: <searchLink fieldCode="DE" term="%22Field+ion+microscopes%22">Field ion microscopes</searchLink><br /><searchLink fieldCode="DE" term="%22Microprobe+analysis%22">Microprobe analysis</searchLink><br /><searchLink fieldCode="DE" term="%22Isotopes%22">Isotopes</searchLink><br /><searchLink fieldCode="DE" term="%22Graphite%22">Graphite</searchLink>
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  Label: Abstract
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  Data: In-situ ion microprobe measurements of carbon isotopic compositions of graphite were made in seven metasediments and two carbonate rocks from the ca. 3.8 Ga Isua supracrustal belt, West Greenland. The δ13C values of micron-scale graphite globules in the metasediments and the carbonate rocks vary from –18 to +2‰ and from –7 to –3‰, respectively. The maximum δ13C value of graphite globules in the metasediment rises from –14 to –5‰, as the metamorphic grade increases from epidote-amphibolite to upper amphibolite facies. In a single hand specimen, the δ13C values of graphite inclusions in garnet are ∼7‰ lower on average than those outside garnet. Similarly, graphite armored by quartz apparently shows a few permil lower δ13C values than those on grain boundaries between noncarbonate minerals. The fact that early crystallized minerals include relatively 13C-depleted graphite indicates that the regional metamorphism increased the δ13C values of the Isua graphite. This is consistent with the regional trend of 13C-enrichment accompanied by the increase of metamorphic grade. The minimum fractionation between graphite and carbonate is consistent with the equilibrium fractionation at about 400 to 550 °C. These observations indicate that isotopic exchange with isotopically heavy carbonate caused 13C-enrichment of Isua graphite. The δ13C values of graphite reported here (δ13C > –18‰) were produced either as a metamorphic modification of organic carbon with initially much lower δ13C values, or as an abiological reaction such as decomposition of carbonate. If the isotopic exchange between carbonate and graphite during regional metamorphism controlled the 13C-enrichment of Isua graphite, previously reported large 13C-depletion of graphite, especially armored by apatite was probably premetamorphic in origin. This supports the existence of life at Isua time (ca. 3.8 Ga). [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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        Value: 10.1016/S0016-7037(01)00840-7
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      – Code: eng
        Text: English
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        PageCount: 12
        StartPage: 1257
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      – SubjectFull: Field ion microscopes
        Type: general
      – SubjectFull: Microprobe analysis
        Type: general
      – SubjectFull: Isotopes
        Type: general
      – SubjectFull: Graphite
        Type: general
    Titles:
      – TitleFull: Ion microprobe analysis of graphite from ca. 3.8 Ga metasediments, Isua supracrustal belt, West Greenland: Relationship between metamorphism and carbon isotopic composition
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            NameFull: Ueno, Yuichiro
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              Text: Apr2002
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              Y: 2002
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