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

Ambient inclusion trails (AITs) are tubular microstructures thought to form when amicroscopic 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 micro-quartz (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.

Sulfur isotope data (³⁴S/³²S) obtained in situ from AIT pyrite have a δ34S of -8.5 to +8.0 ‰, indicating a maximum of ~30 ‰ fractionation from Palaeoproterozoic seawater sulfate (δ³⁴S ≈ +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 sulfate-reducing bacteria during early diagenesis within the microbial mat, as porewaters were becoming depleted in seawater sulfate. Gases derived from this process such as CO₂ and H₂S were partially trapped within the microbial mat, helping produce birds-eye fenestrae, while rapid micro-quartz 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. 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 AITs. This latter mechanism is novel and represents a possible way to generate AITs in environments lacking organic material.