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A new occurrence of ambient inclusion trails from the ~1900‐million‐year‐old Gunflint Formation, Ontario: nanocharacterization and testing of potential formation mechanisms
Author(s) -
Wacey D.,
Saunders M.,
Kong C.,
Kilburn M. R.
Publication year - 2016
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12186
Subject(s) - pyrite , chlorite , diagenesis , mineralogy , geology , geochemistry , seawater , mineral , calcite , chemistry , paleontology , quartz , oceanography , organic chemistry
Abstract Ambient inclusion trails ( AIT s) are tubular microstructures thought to form when a microscopic mineral crystal is propelled through a fine‐grained rock matrix. Here, we report a new occurrence of AIT s from a fossilized microbial mat within the 1878‐Ma Gunflint Formation, at Current River, Ontario. The AIT s are 1–15 μm in diameter, have pyrite as the propelled crystal, are infilled with chlorite and have been propelled through a microquartz (chert) or chlorite matrix. AIT s most commonly originate at the boundary between pyrite‐ and chlorite‐rich laminae and chert‐filled fenestrae, with pyrite crystals propelled into the fenestrae. A subset of AIT s originate within the fenestrae, rooted either within the chert or within patches of chlorite. Sulphur isotope data ( 34 S/ 32 S) obtained in situ from AIT pyrite have a δ 34 S of −8.5 to +8.0 ‰, indicating a maximum of ~30 ‰ fractionation from Palaeoproterozoic seawater sulphate (δ 34 S ≈ +20 ‰). Organic carbon is common both at the outer margins of the fenestrae and in patches of chlorite where most AIT s originate, and can be found in smaller quantities further along some AIT s towards the terminal pyrite grain. We infer that pyrite crystals now found within the AIT s formed via the action of heterotrophic sulphate‐reducing bacteria during early diagenesis within the microbial mat, as pore waters were becoming depleted in seawater sulphate. Gases derived from this process such as CO 2 and H 2 S were partially trapped within the microbial mat, helping produce birds‐eye fenestrae, while rapid microquartz precipitation closed porosity. We propose that propulsion of the pyrite crystals to form AIT s was driven by two complementary mechanisms during burial and low‐grade metamorphism: firstly, thermal decomposition of residual organic material providing CO 2 , and potentially CH 4 , as propulsive gases, plus organic acids to locally dissolve the microquartz matrix; and secondly, reactions involving clay minerals that potentially led to enhanced quartz solubility, plus increases in fluid and/or gas pressure during chlorite formation, with chlorite then infilling the AIT s. This latter mechanism is novel and represents a possible way to generate AIT s in environments lacking organic material.

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