Identify Metamorphic and Primary Multiple Sulfur Isotopic Signatures in the 2.7 Ga Pyrite Nodules from the Southwestern Superior Province (Minnesota, USA)

• Author / Creator

  Li, Jianghanyang

• Sulfur isotopes of some Archean rocks show unique mass independent fractionation (S-MIF) signatures, which are generally considered to have been produced by UV radiolysis on volcanic SO2 in the oxygen-free Archean atmosphere. Based on the distinct S-MIF signals of different sulfur sources (i.e., atmospheric and non-atmospheric) in the Archean ocean and 34S fractionation associated with biological processes, the multiple sulfur isotope system has been widely used to infer the energy sources of biological activities and the pathways of sulfur cycling from the atmosphere to the ocean, and further to sediments in the Archean. The premise of such applications is that the sulfur isotopic compositions of sulfur-bearing minerals in the samples are not affected by post-depositional processes. However, all the Archean rocks discovered to date have experienced at least greenschist-facies metamorphism. Although most previous studies employing multiple sulfur isotopes to infer Archean atmospheric, oceanic or lacustrine conditions have cautiously targeted the least metamorphosed samples (e.g., lower greenschist- to greenschist-facies), quantitative assessment of metamorphism and fluid alteration on the sulfur isotopic compositions of minerals (e.g., pyrite) is lacking. In this study, we employed a Secondary Ion Mass spectrometry (SIMS) and an electron microprobe analyzer (EPMA) instruments to carry out in-situ multiple sulfur isotope and element analysis (Fe, S, Ni and Cu) on three types of pyrite grains in the 2.7 Ga shale samples from the Deer Lake Greenstone Belt, Superior Province: (1) large diagenetic pyrite nodules, (2) metamorphic fluid related subhedral pyrite grains disseminated in quartz veins surrounding the pyrite nodules; and (3) a thin lamina of ! ii! pyrite parallel to quartz veins. The purpose of this thesis work is to (1) quantitatively assess the metamorphic effect on sulfur isotopic compositions of pyrite, and (2) infer the photochemical and biological processes involved in the sulfur cycle in 2.7 Ga from the unaltered pyrite. Our new data are expected to contribute to a better understanding of the microbial activities related to sulfur cycling in the 2.7 Ga ocean. Our results show that the subhedral pyrite grains contain lower Cu, more variable Ni concentrations and higher δ34S (>3.2‰) and Δ33S (>4.1‰) values than the adjacent pyrite nodules. Measurements on the pyrite grains in the lamina yield two groups of data. One group shows high Cu concentrations, low Ni concentrations, low δ34S (-2.7‰ to - 1.1‰) and Δ33S (2.5‰ to 2.9‰) values, all of which are identical to those of the pyrite nodule rims; the other group displays undetectable Cu concentrations, high Ni concentrations, and high δ34S (>2.5‰) and Δ33S (>3.6‰) values, close to those of the subhedral pyrite grains, implying (metamorphic) hydrothermal origin, which was characterized by little Cu but very high Ni concentrations and high δ34S (15.2‰) and ∆33S (5.9‰) values. Three diagenetic pyrite nodules examined in this study show trace element and sulfur isotope compositions similar to each other, suggesting a common growth history. All nodules show cross-grain variations in both δ34S (-2.9‰ to +2.0‰) and Δ33S (+0.3‰ to +2.5‰) values, with decrease in δ34S and increase in Δ33S from cores to rims. This correlation, together with the obvious isotopic and elemental difference between the subhedral pyrite grains and nearby nodule rim, suggests that the isotopic compositions of pyrite nodules were not shifted by metamorphic fluid. The negative correlation between δ34S and ∆33S values observed in the three ! iii! diagenetic pyrite nodules can be explained by a mixing between two sulfur sources: one is sulfate with positive δ34S and negative ∆33S values; the other has negative δ34S and positive ∆33S values, pointing to elemental sulfur. The negative correlation between δ34S and ∆33S observed in this study is close to the “felsic volcanic array” and the 193 nm photolytic array except our data sit slightly above these arrays and show a gentler slope. This distribution pattern can be best explained by a mixing between elemental sulfur on these arrays with sulfate shifted from these arrays to slightly higher δ34S values by low- extent removal of the sulfate via bacterial sulfate reduction. Our results support the hypothesis that the photochemical reactions during intensive volcanic periods could be different and result in a specific distribution pattern as the “felsic volcanic array” or the 193 nm experimental photolytic array.

• Subjects / Keywords

  • Pyrite nodules
  • Archean
  • Sulfur isotope
  • SIMS

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/R3599ZC5V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.