The high-temperature origin of hydrogen in enstatite chondrite chondrules and implications for the origin of terrestrial water

Due to their numerous isotopic similarities to terrestrial rocks, enstatite chondrites (ECs) are commonly proposed as Earth's main building blocks. Although ECs contain sufficient H concentrations to account for the mass of Earth's oceans, the physicochemical process(es) behind their H incorporation remain under constrained. Here, we combined secondary ion mass spectrometry analyses of volatile contents (H, C, F, Cl, S) and H isotopic compositions with Raman spectroscopy analyses of H speciation in the glassy mesostases of EC chondrules. EC chondrule mesostases (68-830 wt. ppm H) contain much more H than chondrule silicates (5-25 wt. ppm) and are characterized by H isotopic compositions of δD = -109 ± 27‰. Hydrogen and sulfur contents are positively correlated in EC chondrule mesostases, and we commonly observed well-resolved Raman peaks at 2580 cm^-1, corresponding to HS^- or H₂S bonding. These results illustrate that the high H abundances in EC chondrule mesostases do not result from terrestrial contamination or secondary asteroidal processes, nor were their high volatile contents inherited from chondrule precursors. Instead, they were established at high temperature during chondrule formation via interactions between Fe-poor melts and S-rich gas under extremely reducing conditions. Our data confirm that ECs contain sufficient primordial hydrogen to explain the terrestrial water budget, and likely contributed important amounts of other volatile elements such as carbon, which was fundamental to the formation of life.