A new occurrence of ambient inclusion trails from the ~1900-million-year-old Gunflint Formation, Ontario: nanocharacterization and testing of potential formation mechanisms

© 2016 John Wiley & Sons Ltd

Ambient inclusion trails (AITs) 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 AITs from a fossilized microbial mat within the 1878-Ma Gunflint Formation, at Current River, Ontario. The AITs 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. AITs 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 AITs originate within the fenestrae, rooted either within the chert or within patches of chlorite. Sulphur isotope data (34S/32S) obtained in situ from AIT pyrite have a d34S of -8.5 to +8.0 ‰, indicating a maximum of ~30 ‰ fractionation from Palaeoproterozoic seawater sulphate (d34S ˜ +20 ‰). Organic carbon is common both at the outer margins of the fenestrae and in patches of chlorite where most AITs originate, and can be found in smaller quantities further along some AITs towards the terminal pyrite grain. We infer that pyrite crystals now found within the AITs 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 CO2 and H2S 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 AITs was driven by two complementary mechanisms during burial and low-grade metamorphism: firstly, thermal decomposition of residual organic material providing CO2, and potentially CH4, 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, pl