Chemical and structural analysis of proposed ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) - a reappraisal

• DOI: 10.5194/egusphere-egu2020-6044
• Publication Date: 2020-03-09T17:38:24Z
• Authors: Mike Zawaski; Nigel Kelly; Omero Felipe Orla...; Claire Nichols; Abigail Allwood; Stephen Mojzsis
• Keywords: Outcrop

<p>The biogenicity of proposed stromatolites from deformed greenschist/amphibolite facies Eoarchean (ca. 3.71 Ga) rocks the Isua Supracrustal Belt (ISB) in West Greenland, is debated  [1,2; cf. 3]. To assess their promise as primary sedimentary structures – opposed to artefacts strain localization layered ductile we report new field mapping at discovery site Nutman et al. (2016) guide micro- and macro-structural investigations geochemical sampling. Discontinuous relations preclude confident assignment these outcrops being structurally overturned originally argued. The are not conical stromatolites, but instead linear inverted ridges aligned with azimuths local regional fold axes, parallel structures. Combined major element (e.g., Ca, Mg, Si) scanning μXRF maps, electron back-scattered diffraction (EBSD) patterns on fresh surfaces cut perpendicular show that lack any internal laminae. Seeming layering previously inferred for features arises variable weathering outcrop otherwise conceal structureless quartz ± dolomite granoblastic cores. These asymmetric boudins sit between semi-continuous competent layers enveloping quartzite a calc-silicate schist. Boudinage fabrics reflect viscosity contrasts different during deformation, thus origin. Collectively, our results such were probably never expected result tectonic fabric preserves no fine-scale structure.</p><p>[1] Nutman, A.P. 2016, Rapid emergence life shown by 3,700-million-year-old microbial structures: Nature, v. 537, p. 535–538; [2] al., 2019, Cross-examining Earth’s oldest stromatolites: Seeing through effects heterogeneous metamorphism metasomatism affecting (Greenland) ∼3700 Ma rocks: Precambrian Research, 331, 105347; [3] Allwood, A.C. 2018, Reassessing evidence Greenland: doi: 10.1038/s41586-018-0610-4.</p>