2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 8
Presentation Time: 3:30 PM

MULTIPLE SULFUR AND OXYGEN ISOTOPE FRACTIONATION FACTORS DURING UV PHOTOLYSIS OF SO 2


POULSON, Simon R., Department of Geological Sciences and Engineering, University of Nevada-Reno, MS-172, 1664 N Virginia St, Reno, NV 89557, NARAOKA, Hiroshi, Department of Earth & Planetary Sciences, Kyushu University, 6-10-1 Hakozaki, Higashi-kui, Fukuoka, 812-8581, Japan and CHIBA, Hitoshi, Department of Earth Sciences, Okayama University, 1-1, Naka-3-chome, Okayama, 700-8530, Japan, poulson@mines.unr.edu

The mass-independent fractionation (MIF) of sulfur isotopes during UV photolysis of SO2 is a key result that has been used to relate the MIF-S found in many Archean rocks to the concentration of O2 in the Archean atmosphere. MIF-S during SO2 photolysis only occurs at UV wavelengths that would be adsorbed by significant concentrations of O2 and O3, so the presence of MIF-S implies that O2 concentrations were very low in the Archean atmosphere. Since the study of Farquhar et al. (2001), relatively little further direct experimental research has been devoted to studying MIF during UV photolysis of SO2.

This study has performed UV photolysis experiments using a Nd-YAG laser (213 nm wavelength) and a low pressure Hg lamp (wavelengths of 185 & 254 nm), for a 5% SO2–95% He gas mixture within synthetic quartz reactor vessels equipped with a gas syringe sampling valve. Stable isotope analyses were performed using the SO+ fragment of SO2 gas, and the concentration of SO2 gas was also measured. Analyses were performed both with and without passing the SO2 gas through a tube packed with quartz at 890°C, which buffers δ18O of the SO2 to a constant composition.

MIF-S results obtained with the low pressure Hg lamp show excellent agreement with the results of Farquhar et al. (2001), yield very large values for 33αS and 34αS, and demonstrate a relatively efficient mechanism for generating MIF-S, as large negative values of Δ33S are produced at relatively modest values of ln F and δ34S. Values of δ49 and δ50 differ between the O-isotope buffered vs. non-buffered experiments, indicating that O isotope fractionation also occurs during SO2 photolysis. Furthermore, using 33αS and 34αS values determined for the O-isotope buffered experiment, the results for the non-buffered experiment allow for calculation of the values of 17αO and 18αO, and suggest that extensive MIF of O isotopes is also taking place. In comparison, results using the Nd-YAG laser show that both S and O isotope fractionation occurs, but indicate that little or no MIF-S or MIF-O takes place.

Results illustrate the utility of the experimental design for studying multiple S and O isotope effects during UV photolysis of SO2, and suggest the possibility that MIF of oxygen may have been produced during the Archean, in addition to MIF of sulfur.

Farquhar et al. (2001) JGR 106: 32829-39.