DOI: 10.1016/j.epsl.2020.116409
论文题名: Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis
作者: Zawaski M.J. ; Kelly N.M. ; Orlandini O.F. ; Nichols C.I.O. ; Allwood A.C. ; Mojzsis S.J.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2020
卷: 545 语种: 英语
中文关键词: abiotic
; Archean
; biosignature
; deformation
; Isua Supracrustal Belt
; stromatolites
英文关键词: Chemical analysis
; Gallium
; Gallium compounds
; Metamorphic rocks
; Sedimentary rocks
; Silicates
; Structural analysis
; Weathering
; Discontinuous fields
; Electron back scatter diffraction
; Field characterization
; Isua supracrustal belts
; Primary sedimentary structures
; Strain localizations
; Structural investigation
; Viscosity contrast
; Sedimentology
; Archean
; chemical analysis
; mapping method
; sampling
; sedimentary structure
; stromatolite
; structural analysis
; supracrustal rock
; Arctic
; Greenland
; West Greenland
英文摘要: The biogenicity of proposed stromatolite structures from Eoarchean (ca. 3.71 Ga) rocks of the Isua Supracrustal Belt (ISB) in West Greenland is under debate. The structures in question are found within a suite of multiply deformed greenschist- to amphibolite-facies metamorphic rocks. To assess their premise as primary sedimentary features – as opposed to products of strain localization in layered, variably ductile rocks – we report new field mapping at the appropriate scale and resolution from the original discovery Sites reported by Nutman et al. (2016). Our new map was used to guide micro- and macro-structural investigations and comprehensive geochemical sampling. Here, we report detailed field characterization and structural analysis to show that the structures are linear inverted ridges aligned with azimuths of local and regional fold axes and parallel to linear structures; they are not deformed conical stromatolites. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, and electron backscatter diffraction (EBSD) patterns collected on fresh surfaces cut perpendicular and parallel to the ridges attest to the lack of any residual sedimentary laminae (e.g., compositional layering) within these structures' cores. Internal layering previously inferred for these features instead arises from variable weathering of outcrop surfaces that otherwise conceals granoblastic quartz ± dolomite cored boudins that sit between semi-continuous competent layers of enveloping quartzite in a calc-silicate schist. The morphology of boudins reflects viscosity contrasts of the different ductile layers during deformation. Therefore, these features are not of sedimentary origin. Furthermore, discontinuous field relationships and absence of primary sedimentary structures that could serve as way-up indicators preclude confident assignment of these outcrops as being structurally overturned, as originally argued. Collectively, our results show that the Isua structures are the expected result of a tectonic fabric that preserves no fine-scale primary sedimentary structures and were probably never stromatolites. © 2020 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165376
Appears in Collections: 气候变化与战略
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作者单位: Department of Geological Sciences, University of Colorado Boulder, 2200 Colorado Avenue, Boulder, CO 80309-0399, United States; Bruker Nano Analytics, Bruker Nano Analytics, 415 N. Quay Street, Kennewick, WA 99336, United States; Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, TX 78712–1722, United States; Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, United States; NASA Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States; Institute for Geological and Geochemical Research, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 45 Budaörsi Street, H-1112, Budapest, Hungary
Recommended Citation:
Zawaski M.J.,Kelly N.M.,Orlandini O.F.,et al. Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis[J]. Earth and Planetary Science Letters,2020-01-01,545