Mass Independent Sulfur in Achondrites: Possible Evidence of Photochemistry in the Solar Nebula

Introduction: In our continuing effort to resolve and interpret the sulfur isotopic heterogeneity of meteorites[1-5], we analyzed a number of achondritic meteorites. Like oxygen, sulfur has more than two stable is otopes and therefore it is feasible to deconvolute the isotopic anomaly from the mass dependent isotopic fractionation that occurred during parent body processes . Any non-mass dependent fractionation process produced due to mass independent photochemical or physicochemical processes in the presolor nebula may be characterized by a nonzero (or variable) ? S or ? S. It has been shown that the gas phase photochemical reactions, involving CS2, SO2 and H2S, either in the Earth’s Archaean as well as present day atmosphere, Mars, or in laboratory experiments, fractionate S is otopes mass independently [6-12]. Lab studies of photochemical reactions have shown that the sign of mass independent fractionation (MIF) depends upon the wavelength of UV light [7], presumably due to the energetics of intersystem, electronic state crossings. In light of these observations, it is important to investigate S in various meteorites as other than oxygen, sulfur is the only multiisotopic element that could potentially record early mass independent processes, both self shielding, as first hypothesized by [13], photochemical, or mass independent chemical reactions. Sulfur exists in a variety of valence states in meteorites its composition is remarkably constant [1,2] with exceptions for isolated phases in Allende and Murchison [14,5]. Unlike primitive chondrites where the anomalous isotopic composition of various elements has been observed, in achondrites, these primitive signatures were lost during parent body differentiation. For sulfur, this scenario differs as S enriched components have been observed in ureilites and oldhamite from the Norton County aubrite [16,4]. In order to resolve the origin of the S anomaly and to investigate whether it is present in other achondrite, we have analyzed sulfur form six HEDs, five aubirtes, two acapulcoite-lodranite and anomalous Weatherford, Bencubbin meteorites for bulk S isotopic composition. For HEDs, sulfur data has been reported for the first time whereas we have analyzed two more samples from the unique acapulcoite-lodranite group. Bencubbin and Weatherford are interesting as they have been shown to possess very heavy nitrogen [17]. Despite several detailed studies , the origin of heavy nitrogen is still unexplained. The study of S in these meteorites may be helpful in understanding the origin of anomalous nitrogen. δ34SCDT -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8 δ 3 3 S C D T