Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures | INSTITUT DE PHYSIQUE DU GLOBE DE PARIS

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  Carbonaceous cherts of the Barberton Greenstone Belt, South Africa: Isotopic, chemical and structural characteristics of individual microstructures

Type de publication:

Journal Article

Source:

Geochimica Et Cosmochimica Acta, Volume 71, Ticket 3, p.655-669 (2007)

ISBN:

0016-7037

URL:

http://www.sciencedirect.com/

Mots-clés:

PRECAMBRIAN CHERTS; GA METASEDIMENTS; LOW-TEMPERATURE; ORGANIC-MATTER

Résumé:

Carbonaceous matter occurring in chert deposits of the 3.4-3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the delta C-13 values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of delta C-13 that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Cheri of the Kromberg Formation have retained fairly well-preserved delta C-13 values, with ranges from -34 parts per thousand to -24 parts per thousand. and -40 parts per thousand to -32 parts per thousand, respectively. Abiologic reactions associated with hydrothermal serpent-inization of ultramafic crust (such as Fischer-Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material. (c) 2006 Elsevier Inc. All rights reserved.

Notes:

Geochim. Cosmochim. ActaISI Document Delivery No.: 132LQTimes Cited: 0Cited Reference Count: 64Cited References:ALEON J, 2003, GEOCHIM COSMOCHIM AC, V67, P3773BEAUMONT V, 1999, PRECAMBRIAN RES, V96, P63BERNDT ME, 1996, GEOLOGY, V24, P351BEYSSAC O, 2002, J METAMORPH GEOL, V20, P859BEYSSAC O, 2003, SPECTROCHIM ACTA A, V59, P2267BRASIER MD, 2002, NATURE, V416, P76BRASIER MD, 2005, PRECAMBRIAN RES, V140, P55BYERLY GR, 1986, NATURE, V319, P489CHACKO T, 2001, STABLE ISOTOPE GEOCH, V43, P1CHANG S, 1983, EARTHS EARLIEST BIOS, P53DESMARAIS DJ, 1984, EARTH PLANET SC LETT, V69, P43DESMARAIS DJ, 2001, STABLE ISOTOPE GEOCH, V43, P555DEWIT MJ, 1982, ECON GEOL, V77, P1783DEWIT MJ, 1993, LITHOS, V30, P309GARCIARUIZ JM, 2003, SCIENCE, V302, P1194GRASSINEAU NV, 2001, P ROY SOC LOND B BIO, V268, P113HAYES JM, 1983, EARTHS EARLIEST BIOS, P93HOLM NG, 2001, EARTH PLANET SC LETT, V191, P1HOUSE CH, 2000, GEOLOGY, V28, P707HOUSE CH, 2003, ORG GEOCHEM, V34, P345KELLEY DS, 2001, NATURE, V412, P145KELLEY DS, 2005, SCIENCE, V307, P1428KNAUTH LP, 1978, EARTH PLANET SC LETT, V41, P209KNAUTH LP, 2003, GEOL SOC AM BULL, V115, P566KUNG CC, 1979, EARTH PLANET SC LETT, V46, P141LINDSAY JF, 2005, PRECAMBRIAN RES, V143, P1LOWE DR, 1986, NATURE, V324, P245LOWE DR, 1999, GEOL SOC AM SPEC PAP, V329, P83LOWE DR, 1999, GEOLOGIC EVOLUTION BLOWE DR, 2003, FIELD GUIDE GEOLOGYLUQUE FJ, 1998, AM J SCI, V298, P471MCCOLLOM TM, 2003, GEOCHIM COSMOCHIM AC, V67, P311MCCOLLOM TM, 2006, EARTH PLANET SC LETT, V243, P74MOJZSIS SJ, 1996, NATURE, V384, P55NARAOKA H, 1996, CHEM GEOL, V133, P251PARIS I, 1985, J GEOL, V93, P111PASTERIS JD, 1989, APPL SPECTROSC, V43, P567PASTERIS JD, 2002, NATURE, V420, P476PASTERIS JD, 2003, ASTROBIOLOGY, V3, P727PINTI DL, 2001, GEOCHIM COSMOCHIM AC, V65, P2301ROBERT F, 1988, GEOCHIM COSMOCHIM AC, V52, P1473ROSE NM, 1996, AM J SCI, V296, P1004SANGELY L, 2005, CHEM GEOL, V223, P179SCHIDLOWSKI M, 2001, PRECAMBRIAN RES, V106, P117SCHOPF JW, 2002, NATURE, V416, P73SCHOPF JW, 2005, GEOBIOLOGY, V3, P1STRAUSS H, 1992, PROTEROZOIC BIOSPHER, P117TICE MM, 2004, GEOLOGY, V32, P37TICE MM, 2004, NATURE, V431, P549TICE MM, 2006, GEOLOGY, V34, P37TOULKERIDIS T, 1998, PRECAMBRIAN RES, V92, P129UENO Y, 2001, INT GEOL REV, V43, P196UENO Y, 2002, GEOCHIM COSMOCHIM AC, V66, P1257VANZUILEN MA, 2002, NATURE, V418, P627VANZUILEN MA, 2003, PRECAMBRIAN RES, V126, P331VANZUILEN MA, 2005, GEOCHIM COSMOCHIM AC, V69, P1241WALSH MM, 1992, PRECAMBRIAN RES, V54, P271WALSH MM, 1999, GEOL SOC AM SPEC PAP, V329, P115WATANABE Y, 1997, GEOCHIM COSMOCHIM AC, V61, P3441WEIS D, 1987, GEOCHIM COSMOCHIM AC, V51, P973WESTALL F, 2001, PRECAMBRIAN RES, V106, P93WESTALL F, 2005, SCIENCE, V308, P366WOPENKA B, 1993, AM MINERAL, V78, P533XIE XG, 1997, CONTRIB MINERAL PETR, V126, P275