10:00 AM - 10:15 AM
[BCG06-05] Sulfur isotope record of the ca. 3.4 Ga Strelly Pool Formation, Pilbara Craton: A micro-scale S isotope investigation
★Invited Papers
Keywords:Archean, sulfur isotope, Strelly Pool Formation, NanoSIMS
The activities of early life and its environment remain uncertain. The Strelley Pool Formation (SPF) in the Pilbara Craton, Western Australia is one of the oldest sedimentary rocks containing microfossils, making it a suitable sample for elucidating early life activities and its environment. Spherical pyrite with inner concentric structures e (referred to as spherical pyrite) discovered in the SPF in this study provide clues to constrain the sulfur cycle of that time. Through morphological, microstructural, and isotopic analyses using microscopy, field emission scanning electron microscopy (FE-SEM), and secondary ion mass spectrometry (NanoSIMS), the origin of spherical pyrite occurring in microfossil-bearing sedimentary rocks of the SPF is estimated.
This spherical pyrite was found in a 15 cm thick black chert deposited at the top of the SPF. Previous studies have shown that the lower part of this black chert consists of silicified sandstone, and the upper part is unconformably overlain by Euro basalt. The black chert containing spherical pyrite is characterized by distributed organic matter and also observed microfossil-like structures. The spherical pyrite found in this black chert has an overall diameter of approximately 5-20 µm, with spherical structures (nuclei) of about 1 µm in diameter at the center. From there, layers containing organic matter and layers of pyrite repeat outward, forming a concentric structure. SEM observations revealed that the pyrite consists of dense layers of crystals and layers composed of needle-like crystals that grow outward with lower density. This reflects differences in the growth rates of pyrite due to environmental variations within the sediment.
Spherical pyrite typically appears as multilayered pyrite with 2-5 layers, either solely or clustered. Occasionally, a single and minute spherical pyrite without a layered structure is also observed. The cross-sections are nearly circular. Silica veins are well-developed throughout the rock, cutting across other materials (e.g. organic matter and pyrite) and disrupting its structure. Based on these characteristics, it is inferred that spherical pyrite formed an early diagenetic stage, at least before the solidification of the sediment.
To further understand its growth, sulfur isotope measurements were conducted layer by layer using NanoSIMS to observe local isotopic variations in the concentric structure. The measurement error of this analysis was approximately 1‰, and the variability in the standard pyrite was 1.1‰. As a result, a maximum variation of about 10‰ in δ34S values was observed between the innermost nucleus and the outermost layer within a single spherical pyrite. Similar analyses on multiple spherical pyrites showed that the δ34S values of the nucleus varied from low, -10.7‰, to high, +9.8‰. The outer layers showed a maximum of +15.8‰, with an average δ34S value higher than that of the nucleus, at approximately +4‰. However, the variation in δ34S values was smaller in the nucleus compared to the outer layers. These values, especially those obtained from the nucleus of spherical pyrite, suggest the involvement of microbial sulfate reduction in its formation. The trend of increasing δ34S values outward is considered a characteristic of Rayleigh fractionation.
This spherical pyrite was found in a 15 cm thick black chert deposited at the top of the SPF. Previous studies have shown that the lower part of this black chert consists of silicified sandstone, and the upper part is unconformably overlain by Euro basalt. The black chert containing spherical pyrite is characterized by distributed organic matter and also observed microfossil-like structures. The spherical pyrite found in this black chert has an overall diameter of approximately 5-20 µm, with spherical structures (nuclei) of about 1 µm in diameter at the center. From there, layers containing organic matter and layers of pyrite repeat outward, forming a concentric structure. SEM observations revealed that the pyrite consists of dense layers of crystals and layers composed of needle-like crystals that grow outward with lower density. This reflects differences in the growth rates of pyrite due to environmental variations within the sediment.
Spherical pyrite typically appears as multilayered pyrite with 2-5 layers, either solely or clustered. Occasionally, a single and minute spherical pyrite without a layered structure is also observed. The cross-sections are nearly circular. Silica veins are well-developed throughout the rock, cutting across other materials (e.g. organic matter and pyrite) and disrupting its structure. Based on these characteristics, it is inferred that spherical pyrite formed an early diagenetic stage, at least before the solidification of the sediment.
To further understand its growth, sulfur isotope measurements were conducted layer by layer using NanoSIMS to observe local isotopic variations in the concentric structure. The measurement error of this analysis was approximately 1‰, and the variability in the standard pyrite was 1.1‰. As a result, a maximum variation of about 10‰ in δ34S values was observed between the innermost nucleus and the outermost layer within a single spherical pyrite. Similar analyses on multiple spherical pyrites showed that the δ34S values of the nucleus varied from low, -10.7‰, to high, +9.8‰. The outer layers showed a maximum of +15.8‰, with an average δ34S value higher than that of the nucleus, at approximately +4‰. However, the variation in δ34S values was smaller in the nucleus compared to the outer layers. These values, especially those obtained from the nucleus of spherical pyrite, suggest the involvement of microbial sulfate reduction in its formation. The trend of increasing δ34S values outward is considered a characteristic of Rayleigh fractionation.