Oxidation of pyrite during barite extraction

Marine barite is widely considered a reliable recorder of sulfur and oxygen isotope compositions of seawater sulfate. The traditional barite extraction method involves multiple oxidative cleaning steps to sequentially remove other minerals from sediments and additional purification steps to remove residual insoluble O-bearing minerals like rutile. During these processes pyrite is likely oxidized, thereby introducing sulfur from pyrite and oxygen from water and/or air to sulfate. We systematically investigate the effects of pyrite oxidation during barite extraction using two sets of synthetic marine sediments spiked with varying amounts of pyrite. One is subjected to acid-based leaching and Na2CO3-based purification, while the other is only treated with Na2CO3. Our results show a negative correlation between the pyrite-to-barite mass ratio and the apparent S- and O-isotope values of extracted barite. The difference in S-isotope ratios between two sample sets suggests that acid leaching with subsequent Na2CO3 purification results in the conversion of approximately 9% of the added pyrite to barite, whereas purification alone converts about 4.8% of the added pyrite. Based on these findings, we estimate that for marine sediments with a pyrite-to-barite ratio lower than 3 wt% and an S-isotope difference smaller than 50%o between marine barite and sedimentary pyrite, the S-isotope offset imparted by pyrite oxidation during sequential leaching is within a 0.3%o error.

Automated summary

Marine barite is commonly used to analyze sulfur and oxygen isotope compositions of seawater sulfate. However, traditional extraction methods can introduce sulfur and oxygen from pyrite oxidation, affecting the isotopic values of extracted barite. This study found that acid leaching followed by Na2CO3 purification can convert approximately 9% of added pyrite to barite, while purification alone converts about 4.8% of added pyrite. The S-isotope offset caused by pyrite oxidation during sequential leaching is within a 0.3%o error for sediment samples with pyrite-to-barite ratios lower than 3 wt% and S-isotope differences smaller than 50%o.