Triple oxygen and hydrogen stable isotope composition of hydroxyl water in Orgueil (CI-type) and Tagish Lake (C2-type) carbonaceous chondrites
Erik J.H. Oerter, Christopher D.K. Herd, Conel M.O’D. Alexander
Geochimica et Cosmochimica Acta, In Press, Journal Pre-proof, Available online 27 February 2026
“The primitive carbonaceous chondrites are of interest to cosmochemical science because they contain relatively large amounts of ‘water’ (H2O and/or OH–) within phyllosilicate minerals. This water is evidence for the accretion of ices by their parent planetesimals, and thus represents an archive of the isotopic compositions of H2O in the protoplanetary environments. Here, we used thermogravimetry-enabled laser spectroscopy (TGA-IRIS) analyses of the Orgueil and Tagish Lake meteorites to make δ2H, δ18O, and Δ′17O measurements of the H2O and OH– contained in the different hydrous minerals that comprise each meteorite. In Orgueil, we measured mass-weighted averages of OH– in the saponite and serpentine phyllosilicate matrix to be δ2H = 192 ‰, δ18O = 1.5 ‰, which are unquestionably of extraterrestrial origin with Δ′17O = 1.0 ‰. For Tagish Lake, analogous values of OH– in the saponite and serpentine phyllosilicate matrix are δ2H = 704 ‰, δ18O = 11.3 ‰, and are similarly unambiguously extraterrestrial with Δ′17O = 0.82 ‰. We estimate that the parent H2O involved in aqueous alteration of Orgueil had δ18O value ≥ +23 ‰. In Orgueil, we interpret the phyllosilicate petrographic relationships, and the δ18O values of OH– in saponite and serpentine to indicate that saponite formed first, at a lower temperature by 35 to 53 °C than serpentine. This suggests that the Orgueil parent body experienced increasing temperature during the phase of active aqueous alteration (prograde) which set the δ18OOH values of the serpentine and saponite. In the case of Tagish Lake, serpentine formed at a lower temperature by 32 to 60 °C than saponite, for the simplest case with constant δ18OH2O values. If serpentine formed first, followed by saponite formation at 32 to 60 °C °C higher temperature, this suggests that Tagish Lake sample TL1 underwent prograde aqueous alteration as the parent body heated up. We find evidence that the parent H2O for Orgueil, Tagish Lake sample TL1, and Murchison (based on data from a previous study) may have had Δ′17O values of >0.64 ‰.”
