Sulfur Isotopic Fractionation in Vuv Photodissociation of Sulfur Dioxide: Implications for Meteorite Data

Introduction: Mass independent sulfur isotopic compositions have been observed in chondritic chondrules and organics [1, 2] and in bulk CM chondrites, achondrites and iron meteorites [3-6]. A significant excess of ∆33S (= δ33S – 0.515 δ34S) was observed in the Dhajala chondrite. Sulfonic acid extracts from the carbonaceous chondrite, Murchison also displayed a significant 33S anomaly and may be associated with deuterium enrichment and it was suggested that methanesulfonic acid could have resulted from gas-phase ultraviolet irradiation of a precursor, carbon disulfide [2]. It was shown through a step wise chemical extraction protocol that the mass independent sulfur component was mostly carried by chondrule rims [1]. Large 33S excesses in sulfides from achondrite meteoritic groups have been found [4]. The 33S excess present in oldhamite from the Norton County aubrite (∆33S = 0.161 ‰) suggests that refractory sulfide minerals condensed from a nebular gas with an enhanced carbon to oxygen ratio [3]. In a recent study of a large number of enstatite chondrites were analyzed for quadruple sulfur isotopic composition, but no significant massi n dependent signature (∆33S, ∆36S) was measured [7]. However, the oldhamite and aubrite fractions show clear massindependent character, however, no appare n t correlation between ∆33S and, ∆36S. The authors proposed decay of 36Cl (in FeCl2) as a source o f excess 36S. Stellar nucleosynthesis and cosmic ray spallation have been ruled out as the cause of the observed ∆33S anomaly [3]. Photochemical reactions in the early solar nebula was inferred to be a leading process to generate mass independent sulfur compositions as it has been shown in laboratory experiments [3, 8]. The other rational behind this proposition is that the fractionation trend (∆33S-∆36S space) observed for many meteorites, e.g., iron meteorites, CM chondrites follow the same as that measured for Ly-α photodissociation of H2 S [8]. Though H2 S is the major sulfur bearing species in the solar nebula, especially in the reducing environments, however, within the oxidizing pockets (with lower C/O ratio) in the solar nebula existence of SO2 could not be ruled out. The goal of the study reported here is to decipher the isotope effect during VUV photodissociation of SO2. SO2 photodissociation have been carried out in the past [9, 10], but this is the first ever experiments at the VUV energy regime. Experimental: We performed SO2 photolysis experiment using a differential pumping system [11] and a 120-cm long reaction chamber. The experiments were performed in a flow condition at a pressure on 200 to 300 mtorr. An appropriate sized high purity (99.99%) Aluminum foil was rolled and inserted inside the reaction chamber (as an inner jacket). The photolysis productelemental sulfur was collected inside the jacket. The entire jacket is treated as a sample and collected after each photolysis experiment and a fresh jacket for each experiment was utilized. After photolysis, elemental sulfur was extracted from the Al-jacket by CCl4 treatment. Elemental sulfur was converted to SF6 through a series of chemical procedures including fluorination of silver sulfide as described in [1]. Several VUV wavelengths from 98.8 through 150 nm was used from the ALS synchrotron. Results: Elemental sulfur is formed directly by photodissociation below 150 nm: