Bulk mineralogy, water abundance, and hydrogen isotope composition of unequilibrated ordinary chondrites

Abstract

The origin and transport of water in the early Solar System is an important topic in both astrophysics and planetary science, with applications to protosolar disk evolution, planetary formation, and astrobiology. Of particular interest for understanding primordial water transport are the unequilibrated ordinary chondrites (UOCs), which have been affected by very limited alteration since their formation. Using X‐ray diffraction and isotope ratio mass spectrometry, we determined the bulk mineralogy, H2O content, and D/H ratios of 21 UOCs spanning from petrologic subtypes 3.00–3.9. The studied UOC falls of the lowest subtypes contain approximately 1 wt% H2O, and water abundance globally decreases with increasing thermal metamorphism. In addition, UOC falls of the lowest subtypes have elevated D/H ratios as high as those determined for some outer Solar System comets. This does not easily fit with existing models of water in the protoplanetary disk, which suggest D/H ratios were low in the warm inner Solar System and increased radially. These new analyses confirm that OC parent bodies accreted a D‐rich component, possibly originating from either the outer protosolar nebula or from injection of molecular cloud streamers. The sharp decrease of D/H ratios with increasing metamorphism suggests that the phase(s) hosting this D‐rich component is readily destroyed through thermal alteration.

Alfred Otto Carl Nier: On the Shoulders of a Mass Spectrometry Giant

“…In line with Nier’s later research interests, IRMS-based analyses of chondrites have provided key evidence for the Giant Impact Theory of lunar origin . Additionally, they have shed light on primordial water transport phenomena in the solar system and have been crucial in analyzing amino acids in meteorites to search for the origin of life. − …”