Volcanic Influence on Atmospheric Sulfur Cycle in 2.7 Ga inferred from Multiple Sulfur Isotope Record of Pyrite Nodules in Black Shales from Nimbus, Western Australia

  • Author / Creator
    Zhang, Zhe
  • Sulfur isotope compositions of Archean sulfide and sulfate minerals show mass-independent fractionation (S-MIF) signatures, which are characterized by non-zero Δ33S and Δ36S values. Producing the Archean S-MIF signal requires an O2-free atmosphere where broad-band UV radiation can penetrate to photolyze volcanic SO2. Transfering and preserving the S-MIF signal to sediments also requires a reducing surface environment in which the dissociated SO2 products (i.e., sulfide and sulfate) will not be oxidized and re-homogenized. In general, the Archean S-MIF indicates an extremely low oxygen level in the Archean atmosphere. Most Archean sulfide minerals have δ34S and Δ33S values falling in a well-defined linear relationship with a positive Δ33S/δ34S slope (around 0.9), which is named as the “Archean Reference Array” (ARA). However, a negative correlation between δ34S and Δ33S values has also been recently reported from the sulfide minerals in the 3.2 Ga Mapepe Formation, South Africa (Philippot et al., 2012). This anomaly was hypothesized to be the result of some different photolytic mechanism attributed to the shielding effect of volcanic aerosols during periods of intensive sub-aerial volcanic emissions, and was consequently named as the “Felsic Volcanic Array” (FVA). To test the volcanic effect on atmospheric sulfur cycle in the Archean, Li et al. (2017) examined the multiple sulfur isotope compositions of pyrite nodules from the Joy Lake Sequence in the Superior Province with an age of 2.7 Ga, which was a peak time in Earth’s Archean Eon for continental crust growth associated with intensive volcanic eruptions. The isotopic results of the pyrite nodules in the Joy Lake Sequence show a pattern very similar to FVA, suggesting a potential volcanic effect on the sulfur cycle in 2.7 Ga. However, the spatial scale of the volcanic effect is still not well constrained. To address this question, in particular to examine whether the multiple sulfur isotopic signature with volcanic effect was only limited to local or expanded to a global scale of occurrence, we carried out high-resolution, in-situ multiple sulfur isotopic analysis of pyrite nodules from the 2.7 Ga Nimbus volcanic-hosted massive sulfide (VHMS) deposits in West Australia. We used EPMA (for major and minor elements) and SIMS (for multiple sulfur isotopes) techniques to carefully characterize large diagenetic pyrite nodules, as well as fine-grained secondary hydrothermal pyrite grains disseminated in quartz veins, in the same samples from the Nimbus deposit. The results indicate that diagenetic pyrite nodules display elemental and isotopic patterns distinct from hydrothermal pyrite, indicating that the primary sulfur isotopic signatures of diagenetic pyrite nodules have not been geochemically altered by secondary hydrothermal alteration. The Nimbus pyrite nodules show obvious isotopic zonation patterns with abrupt isotopic shift from core to rim regions. The cores are characterized by relatively low Δ33S values (-0.6‰ to 1.0‰) and relatively high δ34S values (-0.2‰ to 2.7‰), whereas the rims are characterized by higher Δ33S values (2.1‰ to 3.4‰) and lower δ34S values (-1.4‰ to 0.8‰). The multiple sulfur isotopic signatures of pyrite nodules suggest variable sulfur sources contributed to pyrite nodule formation. The cores of the pyrite nodules were derived from a mixture of seawater sulfate, submarine volcanic/hydrothermal sulfur, and elemental sulfur with ARA-like isotopic features from the commonly seen photochemical processes in the Archean. In contrast, the inner rims of pyrite nodules show a significant mass contribution from elemental sulfur derived from FVA-like elemental sulfur. This suggests a strong impact of volcanic activities on the atmospheric chemistry in the region, which resulted in a different S-MIF signature. The outer rims of the pyrite nodules show a progressive change from FVA-like to ARA-like isotopic patterns, again implying a recovery of atmospheric chemistry from volcanic disturbance to the Archean background during the late stage of the pyrite growth. A compilation of multiple sulfur isotope data of 2.7 Ga pyrite nodules from localities close to Nimbus shows that similar isotopic shift toward FVA-like elemental sulfur source can be clearly identified during the growth of pyrite nodules in sites extending for 300 km from the deposit. This suggests that Archean atmospheric environment could be very sensitive (both in effect and recovery) to volcanic activities at least at a regional scale.

  • Subjects / Keywords
  • Graduation date
    Spring 2018
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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