HR: 10:35h
AN: V42A-02    [Abstracts]
TI: A Photochemical Mechanism for Sulfur Mass-independent Fractionation in Ancient Rocks
AU: * Lyons, J R
EM: jimlyons@ucla.edu
AF: IGPP UCLA, 595 Charles Young Dr East, Los Angeles, CA 90095-1567, United States
AB: A quantitative understanding of the origin of sulfur isotope mass-independent fractionation (MIF) is essential to a full interpretation of the sulfur geochemical record in Archean and Paleoproterozoic rocks. However the chemical mechanism responsible for the MIF remains unknown. Several possible sources of sulfur MIF can be identified: 1) symmetry-dependent non-statistical reactions during formation of poly-sulfur compounds (analogous to oxygen MIF during ozone formation); 2) MIF during photodissociation of sulfur gases; 3) hyperfine effects (nuclear- spin electron-spin interactions) which preferentially produce MIF in ³³S; and 4) nuclear field shift effects. I will briefly review the implications of laboratory experiments in distinguishing between these possible MIF mechanisms. I will then report the results of atmospheric chemistry modeling of isotope-selective photodissociation of SO₂ in the key vibronic band system from 190 to 220 nm. This band system is dominated by a bending mode progression that produces shifts in the absorption spectrum upon sulfur isotope substitution. Utilizing the results of ab initio methods to determine the band shifts for SO₂ isotopologues, I will show that photodissociation of SO₂ in the atmosphere yields MIF signatures (both Δ33S and Δ36S) that are comparable to values measured in ancient rocks. The sulfur MIF arises in part from self- shielding due to photon absorption in the rotationally-resolved lines of ³²SO₂. In addition to SO₂ photolysis, dissociation of SO may also contribute to the sulfur MIF record. Field shift effects are too small to be a significant source of sulfur MIF. Hyperfine effects remain to be evaluated. A photolytic origin for sulfur MIF demonstrates that MIF in terrestrial rocks (and meteorites) can be derived from photochemistry independent of molecular symmetry.
DE: 0317 Chemical kinetic and photochemical properties
DE: 0325 Evolution of the atmosphere (1610, 8125)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 5225 Early environment of Earth
DE: 8409 Atmospheric effects (0370)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting