Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging : Extracting local paleo ‐ environmental information from modern and ancient sediments

Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging : Extracting local paleo ‐ environmental information from modern and ancient sediments. Bryant, R. N, Jones, C., Raven, M. R, Gomes, M. L, Berelson, W. M, Bradley, A. S, & Fike, D. A Rapid Communications in Mass Spectrometry, 33:491–502, 2019.
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Rationale: Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10μm) grains or for detecting intra‐grain variability. Methods: Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20–130μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 ×128 pixel grids) for between 20 and 960 cycles. Results: The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32Sin1–4μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was $±$2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤$±$2‰,1σ). In contrast, grain‐specific 34S/32S ratios inmodern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions: SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. These data allow past local environmental conditions to be inferred.

@article{Bryant2019,
	Abstract = {Rationale: Sulfur isotope ratio measurements of bulk sulfide from marine sediments have often been used to reconstruct environmental conditions associated with their formation. In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10μm) grains or for detecting intra‐grain variability. Methods: Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of
20--130μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 ×128 pixel grids) for between 20 and 960 cycles. Results: The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32Sin1--4μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ $\pm$2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤$\pm$2‰,1σ). In contrast, grain‐specific 34S/32S ratios inmodern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions: SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. These data allow past local environmental conditions to be inferred.},
	Author = {Bryant, Roger N and Jones, Clive and Raven, Morgan R and Gomes, Maya L and Berelson, William M and Bradley, Alexander S and Fike, David A},
	Date-Modified = {2020-10-26 14:53:26 -0500},
	Doi = {10.1002/rcm.8375},
	File = {:Users/abradley/Documents/Mendeley{\_}Library/Bryant et al/2019/Bryant et al.{\_}2019{\_}Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging Extracting.pdf:pdf},
	Journal = {Rapid Communications in Mass Spectrometry},
	Pages = {491--502},
	Title = {{Sulfur isotope analysis of microcrystalline iron sulfides using secondary ion mass spectrometry imaging : Extracting local paleo ‐ environmental information from modern and ancient sediments}},
	Volume = {33},
	Year = {2019},
	Bdsk-Url-1 = {https://doi.org/10.1002/rcm.8375}}

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However, these techniques are often not suitable for studying small (≤10μm) grains or for detecting intra‐grain variability. Methods: Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of 20–130μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 ×128 pixel grids) for between 20 and 960 cycles. Results: The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32Sin1–4μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was $±$2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤$±$2‰,1σ). In contrast, grain‐specific 34S/32S ratios inmodern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions: SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. 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In situ microscale spot analyses by secondary ion mass spectrometry (SIMS) and laser ablation multiple‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) have been utilized for the same purpose. However, these techniques are often not suitable for studying small (≤10μm) grains or for detecting intra‐grain variability. Methods: Here, we present a method for the physical extraction (using lithium polytungstate heavy liquid) and subsequent sulfur isotope analysis (using SIMS; CAMECA IMS 7f‐GEO) of microcrystalline iron sulfides. SIMS sulfur isotope ratio measurements were made via Cs+ bombardment of raster squares with sides of\n20--130μm, using an electron multiplier (EM) detector to collect counts of 32S− and 34S− for each pixel (128 ×128 pixel grids) for between 20 and 960 cycles. Results: The extraction procedure did not discernibly alter pyrite grain‐size distributions. The apparent inter‐grain variability in 34S/32Sin1--4μm‐sized pyrite and marcasite fragments from isotopically homogeneous hydrothermal crystals was ~ $\\pm$2‰ (1σ), comparable with the standard error of the mean for individual measurements (≤$\\pm$2‰,1σ). In contrast, grain‐specific 34S/32S ratios inmodern and ancient sedimentary pyrites and marcasites can have inter‐ and intra‐grain variability >60‰. The distributions of intra‐sample isotopic variability are consistent with bulk 34S/32S values. Conclusions: SIMS analyses of isolated iron sulfide grains yielded distributions that are isotopically representative of bulk 34S/32S values. Populations of iron sulfide grains from sedimentary samples record the evolution of the S‐isotopic composition of pore water sulfide in their S‐isotopic compositions. 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