129I/ 127I variations among enstatite chondrites

Abstract

This paper presents the 129I/ 127I composition for 11 different enstatite chondrites. There is a well-defined hiatus in iodine isotopic compositions separating the enstatite chondrites. The hiatus correlates with a similar hiatus defined by variations in chemical composition (both major and minor elements). EH(4,5) chondrites have 129I/ 127I ratios ∼20% higher (∼ 4 m.y. older) than EL(6) chondrites. The enstatite chondrites are the third meteorite group, joining silicates in IAB irons and C3V carbonaceous chondrites, in which 129I/ 127I correlates with chemical composition. Within each enstatite chondrite group, the variations in initial iodine isotopic composition is less than ~ ±4%, comparable to the experimental resolution. The ~20% difference between EH and EL may indicate a maximum iodine isotopic heterogeneity between the EH and EL nebular reservoirs. Alternatively, if EH and EL chondrites evolved from a homogeneous iodine isotopic reservoir, then the EL group must have evolved ~ 4.0 m.y. longer than the EH group and either in an environment with a much lower I/Xe ratio than presently observed or one which was open to the loss of radiogenic 129Xe. The latter case may imply a nebular environment and would therefore be consistent with the open system evolution required to produce the chemical hiatus. The duration of subsequent evolution in the high I/Xe environment must have been ≲1 m.y. and probably represents thermal metamorphism in two different parent bodies.