Abstract
The stable isotope ratios of sulfur (δ34S relative to Vienna Cañon Diablo Troilite) in sulfates and sulfides determined by elemental analysis and isotope ratio mass spectrometry (EA/IRMS) have been proven to be a remarkable tool for studies of the (bio)geochemical sulfur cycles in modern and ancient environments. However, the use of EA/IRMS to measure δ34S in arsenides and sulfarsenides may not be straightforward. This difficulty can lead to potential health and environmental hazards in the workplace and analytical problems such as instrument contamination, memory effects, and a non-matrix-matched standardization of δ34S measurements with suitable reference materials. To overcome these practical and analytical challenges, we developed a procedure for sulfur isotope analysis of arsenides, which can also be safely used for EA/IRMS analysis of arsenic sulfides (i.e., realgar, orpiment, arsenopyrite, and arsenian pyrite), and mercury sulfides (cinnabar). The sulfur dioxide produced from off-line EA combustion was trapped in an aqueous barium chloride solution in a leak-free system and precipitated as barium sulfate after quantitative oxidation of hydrogen sulfite by hydrogen peroxide. The derived barium sulfate was analyzed by conventional EA/IRMS, which bracketed the δ34S values of the samples with three international sulfate reference materials. The protocol (BaSO4-EA/IRMS) was validated by analyses of reference materials and laboratory standards of sulfate and sulfides and achieved accuracy and precision comparable with those of direct EA/IRMS. The δ34S values determined by BaSO4-EA/IRMS in sulfides (arsenopyrite, arsenic, and mercury sulfides) samples from different origins were comparable to those obtained by EA/IRMS, and no sulfur isotope fractionations were introduced during sample preparation. We report the first sulfur isotope data of arsenides obtained by BaSO4-EA/IRMS.Graphical abstract
Highlights
The sulfur stable isotopes (δ34S) in sulfates and sulfides have been proven to be a remarkable tool for studying geochemical and biogeochemical cycles in modern and ancient environments [1,2,3]
The δ34S and total sulfur (TS) values obtained by direct elemental analysis and isotope ratio mass spectrometry (EA/isotope ratio mass spectrometry (IRMS)) and by preconcentration as barium sulfate before elemental analysis (EA)/IRMS (BaSO4-EA/IRMS) are presented in Tables 3–5 and ESM
C Percent recovery calculated from the TS obtained by direct EA/IRMS or by gravimetric quantification of the barium sulfate d Oxidative overgrowth on the pyrite grains, checked by XRD and under the microscope, explain the lower TS wt.% value
Summary
The sulfur stable isotopes (δ34S) in sulfates and sulfides have been proven to be a remarkable tool for studying geochemical and biogeochemical cycles in modern and ancient environments [1,2,3]. In hydrothermal systems are arsenopyrite (ferrous arsenic sulfide, FeAsS) and arsenian pyrite ( FeAsxS2−x with x < 1), together with cobaltite (CoAsS), enargite (Cu3AsS4), gersdorffite (NiAsS), and glaucodot ((Co,Fe)AsS) [7, 8] These sulfarsenides are often found together with cobalt–nickel mono-, di-, and tri-arsenides, which may contain up to 3 wt.% sulfur [9, 10]. Sedimentary and volcanic rocks host HgS deposits in the lithosphere along convergent boundaries in recent and ancient mountain belts [13] The mining of these deposits initiated anthropogenic cycling of mercury [14]. The net formation (precipitation vs dissolution) of HgS is one of the major mercury sinks in the environment, as it removes this element from biogeochemical and anthropogenic cycling [17]. To the best of our knowledge, there are no published sulfur isotope data of fine-grained HgS in environmental samples
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