Reports: AC2
48343-AC2 Experimental Sulfate Triple-Oxygen Isotope Geochemistry
PRF-Narrative report-2009
1.We have narrowed the window of predictable H2O:O2 oxygen source ratios for sulfate produced from abiotic pyrite oxidation.
Previously published data indicated wide ranges of O2 oxygen incorporation from various pyrite oxidation experiments (9-60%). This was attributed to both: 1) multiple oxidation pathways involving different oxidants (O2, Fe3+, Mn3+, etc…) and 2) the inability of the traditional δ18O label to differentiate kinetic effects on the degree of oxygen exchange occurring between water and an intermediate oxidation product, SO32-. We utilize a D17O isotope label in the solutions, enabling a quantitative determination of oxygen source ratios (O2 vs. H2O) in the produced sulfate. We investigated the reaction products in a number of geologically relevant pH conditions (pH = 2,7,9,10,11). The effect of Fe3+ (a common oxidant in AMD type environments and the product of pyrite oxidation) was also investigated by spiking a duplicate set of solutions with FeCl3.
Results indicate that sulfite-water oxygen exchange determines the O2 signal in sulfate produced from oxidation of pyrite for all pH conditions examined. In alkaline conditions, although the exchange rate of sulfite with water is slow, their stability in solution offsets the low exchange rate. The final oxidation of sulfite to sulfate results in a consistent O2 (21-29%) incorporation in the produced sulfate. Our results suggest that abiotic oxidative weathering of pyrite produces sulfate with 25±4% air O2 oxygen, a much smaller range than previously proposed with the use of δ18O labels. This provides important constraints on pyrite oxidation mechanisms and interpreting the anomalous 17O signals found in Marinoan barite (BaSO4) deposits, which are believed to come from atmospheric O2 at a time when global glaciation resulted in unique atmospheric conditions.
This study is being submitted to Geochimica et Cosmochimica Acta and an oral presentation will be given at Fall AGU 2009.
2.The oxidation of other sulfide minerals (sphalerite, galena, and FeS) as well as hydrogen sulfide gas is being investigated using the same techniques and results are expected before the end of the year (2009).
3.The in-situ (syn/post diagenetic) oxidation of pyrite within carbonate rocks is being investigated as a possible contaminant for CAS isotope compositions. The samples were obtained from a Marinoan age Cap Carbonate sequence in Southern China. They are characterized by varying pyrite and CAS concentrations as well as distinct CAS isotopic compositions. Some contain distinct negative D17O anomalies indicating weathering in a terrestrial environment and contain no anomaly and could have been produced via oxidation in diagenetic fluid. This study is scheduled to be completed during the month of October, at LSU’s CAMD, synchotron radiation facility, using sulfur K-edge micro-XANES.
4. An undergraduate student worker, Mr. Bryan Killingsworth, has been working under PhD candidate Issaku Kohl on a project to track the isotopic composition of sulfate being supplied to the Gulf of Mexico via the Mississippi River. Modeling of seawater sulfate isotopic variation during geologic time is important for understanding small variations preserved in marine barite deposits. Values for δ18OSO4 from riverine input used in models range from 4-6‰ but this value is poorly constrained and very little supporting data exists. Mr. Killingsworth has collected, processed, and precipitated barite samples from Mississippi River sulfate weekly since March and preliminary isotope data indicates the current δ18O values are very similar to those being used in models representing pre-industrial time. This presents a problem in that anthropogenic sulfate sources tend to be more positive than those derived from weathering of sulfides and evaporites. Our data suggests that geologically relevant values are likely to be more negative than those being used to model ancient riverine sulfate.
