Sulfur isotopic fractionation in vacuum UV photodissociation of hydrogen sulfide and its potential relevance to meteorite analysis

Abstract

Select meteoritic classes possess mass-independent sulfur isotopic compositions in sulfide and organic phases. Photochemistry in the solar nebula has been attributed as a source of these anomalies. Hydrogen sulfide (H 2 S) is the most abundant gas-phase species in the solar nebula, and hence, photodissociation of H 2 S by solar vacuum UV (VUV) photons (especially by Lyman-α radiation) is a relevant process. Because of experimental difficulties associated with accessing VUV radiation, there is a paucity of data and a lack of theoretical basis to test the hypothesis of a photochemical origin of mass-independent sulfur. Here, we present multiisotopic measurements of elemental sulfur produced during the VUV photolysis of H 2 S. Mass-independent sulfur isotopic compositions are observed. The observed isotopic fractionation patterns are wavelengthdependent. VUV photodissociation of H 2 S takes place through several predissociative channels, and the measured mass-independent fractionation is most likely a manifestation of these processes. Meteorite sulfur data are discussed in light of the present experiments, and suggestions are made to guide future experiments and models.chondrites | achondrites | isotope

Early inner solar system origin for anomalous sulfur isotopes in differentiated protoplanets

“…1B). Of the currently known anomalous fractionations, it is Ly-α photolysis of H 2 S (27) that is most consistent with the negative correlation between Δ 36 S and Δ 33 S observed in both iron and achondrite meteorites (Fig. S3) and with the conditions present in the inner solar system.…”

“…Photolysis of H 2 S by UV radiation with wavelengths longer than ∼220 nm (26) yields a positive Δ 36 S/Δ 33 S array (∼+1.7) which does not match our observations. A series of recent photolysis experiments on H 2 S using Lyman-α (Ly-α) radiation, however, provide a possible match to the observed sulfur isotope compositions in iron meteorites, with an array between products and reactants delineating a Δ 36 S/Δ 33 S ratio of ∼−3 (27). Given our uncertainties on Δ 36 S and the limited spread of our data, this array supports a preaccretionary photochemical origin for the S-isotope signatures seen in troilite from magmatic irons (Fig.…”

“…S4), decreasing as the reaction's branching ratio increases toward dissociated troilite. However, the minor sulfur isotopes were not measured, and it is possible that this process deviates slightly from the canonical value; moreover, a recent (27). One additional S-isotope measurement on group IIIF (10) is also included.…”

Introduction to Chemistry and Applications in Nature of Mass Independent Isotope Effects Special Feature

“…The applications have subsequently expanded, particularly to oxygen isotopes, and in this special issue, many unique uses of both oxygen and sulfur isotopes are presented (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18).…”

“…In ref. 10, vacuum UV photolytic decomposition of H 2 S, a dominant sulfur species in the early solar system was performed using the Advanced Light Source synchrotron facility (Lawrence Berkeley Laboratory). Wavelengthdependent mass-independent isotopic fractionations are observed and are presumably associated with resonance-assisted curve crossing dynamics.…”