Detailed Analysis of Sulfur Isotopic Signatures via Atom Probe Tomography
EMSL Project ID
60536
Abstract
Pyrite is the end sink of sulfur in organic rich sedimentary environments. It is important both because sulfate is important for respiration of many organisms at low oxygen concentrations and because pyrite can be a sink for several trace elements in the sedimentary environments, some of which can be environmentally deleterious. Sulfur isotope analyses can aid in understanding the sulfur cycle in that it is more energetically favorable to utilize the light isotopes. Thus, in an open system, S-isotopes (reported as δ34S) are strongly negative. However, in a closed system S-isotopes of pyrite are progressively heavier as the lighter isotopes are preferentially used first. Thus, much can be learned about the relative degrees of openness of the system at different periods of diagenesis. The issue that arises is that pyrite forms in very small (typically 10 µm diameter or less) microcrystalline masses (framboids) that are difficult to analyze in situ so it is not well understood whether sulfur isotope zoning is present in the framboids. Recent studies have shown that there is trace element zonation and that zonation suggests a late, trace element rich overprint. What is not clear is whether that zonation is due to a late trace element rich fluid infiltrating the environment of the growing framboid or if the trace elements present are due to a progressive enrichment as the pore fluids naturally evolve. In this study we propose to develop the use of atom probe tomography to obtain nano-scale S-isotope analyses of pyrite. To do this we will first analyze a series of known S-isotope pyrite standards with nanoSIMS to ensure the material is homogenous at the nano-scale. This will be followed by preparation of APT tips with a focused ion beam SEM and analysis with the atom probe. This will test whether the atom probe can accurately and precisely analyse S-isotopes in pyrite. We will also use those analyses to determine the minimum number of counts needed to obtain an accurate analysis, which will be useful to determine how much subdivision of analyses can be completed on unknown samples. We will then analyze pyrite framboids from an euxinic environment (Effingham Inlet) that also has S isotope data acid volatile and pyrite sulfur in the bulk sediment and pore waters. This will be done by using STEM to select regions of interest (interface between high and low trace element pyrite). Then APT tips will be made from the pyrite samples with a focused ion beam SEM and the tips will be analyzed by APT. The analyses of the tips will be segmented to see if earlier (trace element poor) parts of the framboids formed in a more open system than the trace element rich later part of the framboid. This will allow for higher resolution understanding of the sulfur cycle in different sedimentary environments than achieved to date.
Project Details
Project type
Exploratory Research
Start Date
2022-12-01
End Date
2023-12-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members