Sulphur isotope effects in the dissociation and evaporation of troilite: A possible mechanism for 34S enrichment in lunar soils

Both the dissociation and evaporation of troilite. heated to its melting point in vacuo, are isotopically selective. The elemental sulphur from the dissociation of troilite has a ${}^{34}\text{S}{}^{32}\text{S}$ ratio which is 13.0‰ less than that of the undissociated material while the ${}^{34}\text{S}{}^{32}\text{S}$ ratio in evaporated troilite is 5.4‰ less than that of the residual material. The two processes occur simultaneously and the isotopic variations during the course of the reaction are in accord with those for a branched reaction where the unreacted material remains isotopically well-mixed, as in a Rayleigh distillation process. This isotopic selectivity must be taken into account, along with that in other lunar surface processes, when considering the heavy isotope enrichment of sulphur in lunar soils.