Ion microprobe isotopic measurements of individual interplanetary dust particles

Abstract

Ion microprobe measurements of D/H ratios in individual fragments of eight stratospheric dust particles give δD values ranging from −386 to +2534‰ relative to SMOW. The δD values in five particles far exceed those in terrestrial samples and prove that the samples are interplanetary dust particles (IDPs). The hydrogen isotopic composition is heterogeneous on a scale of a few microns demonstrating that the dust is unequilibrated. Measurements of D/H ratios in conjunction with elemental and molecular ion signals in different fragments of individual IDPs show that a carbonaceous phase, not water, is the carrier of the D enrichments. Previous infrared transmission measurements have shown that IDPs fall into three main spectral classes. Particles from two of those three IR classes show large D/H ratios. Two particles studied from the third class do not. However, one of these contains solar flare tracks and is extraterrestrial. Thus, most, but not all, IDPs contain hydrogen with a non-terrestrial isotopic composition. Carbon isotopic measurements on fragments of three IDPs give ratios similar to terrestrial values and show a largely uniform isotopic composition for a given particle. Small, but significant, differences in δ 13C of ~40‰ between particles are seen. No correlations between the hydrogen and carbon isotopic compositions are observed. The magnesium and silicon isotopic compositions of fragments of three IDPs are found to be normal within measurement errors.