Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland

Mark A. van Zuilen; Kattathu Mathew; Brigitte Wopenka; Aivo Lepland; Kurt Marti; Gustaf Arrhenius

doi:10.1016/j.gca.2004.08.033

Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland

Mark A. van Zuilen, Kattathu Mathew, Brigitte Wopenka, Aivo Lepland, Kurt Marti, Gustaf Arrhenius

Research output: Contribution to journal › Article › peer-review

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Abstract

The problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early Archean rocks have prompted the search for additional chemical and isotopic biomarkers. Here we report an attempt to identify the origin of carbonaceous matter in the 3.8 Ga old Isua Supracrustal Belt in southern West Greenland by measuring the concentration and isotopic composition of a trapped nitrogen component. Stepped-combustion/pyrolysis-mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen: (1) nitrogen associated with a very small amount of reactive carbonaceous material, (2) nitrogen intercalated in graphite, correlated with intercalated radiogenic argon, (3) nitrogen strongly retained at defects or chemically bound in the graphite structure. The δ¹⁵N of nitrogen associated with reactive carbonaceous matter (ca. +6‰) overlaps with that of average Phanerozoic sedimentary organic matter, and is believed to be part of nonindigenous postmetamorphic biologic material. In situ Raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material, and this material is further referred to as graphite. Graphite interpreted as epigenetic (associated with Mg,Mn-siderite in metacarbonates) contains a very small strongly retained nitrogen component with a low δ¹⁵N ratio (-3 to -1‰). This range overlaps with values that are typically found in Archean kerogens, but also those of a metamorphically emplaced inorganic basaltic source. Geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks. Graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this Mg,Mn-siderite-derived graphite, based on degree of graphite crystallinity, amount of trapped radiogenic argon, low nitrogen concentration and δ¹⁵N signature. We conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed Isua Supracrustal Belt.

Original language	English
Pages (from-to)	1241-1252
Number of pages	12
Journal	Geochimica et Cosmochimica Acta
Volume	69
Issue number	5
DOIs	https://doi.org/10.1016/j.gca.2004.08.033
State	Published - Mar 1 2005
Externally published	Yes

Funding

We thank P. W. U. Appel for providing coordination, facilities and support for fieldwork as part of the Isua Multidisciplinary Research Project. NASA Exobiology grant NAGW-1035, NAG5-4563 and NAG5-12983 are gratefully acknowledged. We thank the reviewers J. Valley and G. Layne for their constructive comments.

Funders	Funder number
National Aeronautics and Space Administration	NAG5-12983, NAGW-1035, NAG5-4563

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10.1016/j.gca.2004.08.033

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title = "Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland",

abstract = "The problems involved with the interpretation of carbon isotopes as indicators for early life in highly metamorphosed early Archean rocks have prompted the search for additional chemical and isotopic biomarkers. Here we report an attempt to identify the origin of carbonaceous matter in the 3.8 Ga old Isua Supracrustal Belt in southern West Greenland by measuring the concentration and isotopic composition of a trapped nitrogen component. Stepped-combustion/pyrolysis-mass spectrometry of carbonaceous matter in several rock samples revealed three different reservoirs of trapped nitrogen: (1) nitrogen associated with a very small amount of reactive carbonaceous material, (2) nitrogen intercalated in graphite, correlated with intercalated radiogenic argon, (3) nitrogen strongly retained at defects or chemically bound in the graphite structure. The δ15N of nitrogen associated with reactive carbonaceous matter (ca. +6‰) overlaps with that of average Phanerozoic sedimentary organic matter, and is believed to be part of nonindigenous postmetamorphic biologic material. In situ Raman spectroscopy confirmed the high degree of crystallinity of the metamorphosed indigenous carbonaceous material, and this material is further referred to as graphite. Graphite interpreted as epigenetic (associated with Mg,Mn-siderite in metacarbonates) contains a very small strongly retained nitrogen component with a low δ15N ratio (-3 to -1‰). This range overlaps with values that are typically found in Archean kerogens, but also those of a metamorphically emplaced inorganic basaltic source. Geological constraints suggest that this graphite incorporated nitrogen from surrounding metabasaltic rocks. Graphite interpreted as syngenetic and biogenic found in a turbidite deposit is relatively similar to this Mg,Mn-siderite-derived graphite, based on degree of graphite crystallinity, amount of trapped radiogenic argon, low nitrogen concentration and δ15N signature. We conclude that nitrogen concentration and its isotope ratio in graphite cannot be used conclusively as a biomarker in these rocks from the highly metamorphosed Isua Supracrustal Belt.",

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Nitrogen and argon isotopic signatures in graphite from the 3.8-Ga-old Isua Supracrustal Belt, Southern West Greenland

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