Hydrogen and Oxygen Stable Isotope Compositions of Kaolinite Hydroxyl Water and their Paleoenvironmental Significance

Abstract

The phyllosilicate clay mineral kaolinite can preserve paleoclimate information on the hydrogen and oxygen stable isotope composition of the water the mineral formed from, as well as its formation temperature. Oxygen in kaolinite exists in three distinct bonded groups: Si-O-Si, Si-O-Al, and Al-OH; and it has been an outstanding analytical problem to feasibly and accurately measure the oxygen isotope compositions of the different groups. The ability to make δ¹⁸O_Al-OH measurements on kaolinite using thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS) is established herein. Using complementary fluorination and IRMS measurements, we add to the limited knowledge of intracrystalline oxygen isotope fractionation between that bound into Al-OH^- groups in kaolinite, and that of the bulk mineral, of which the average value is 1000 ln${\propto }_{\mathrm{Total}-OH}^O$ = 18.9 ‰. Together with δ²H_Al-OH measurements on kaolinite, we demonstrate the extent to which hydrogen in the Eocene-age paleo-Oxisol Ione Fm has isotopically exchanged with water after its initial formation. At Mesa Alta, New Mexico, kaolinite hydrogen appears to be pristine since its initial formation 147 Ma BP (early Cretaceous), and we use δ²H_Al-OH, δ¹⁸O_Al-OH, and δ¹⁸O_Total measurements to determine a paleo-environmental temperature of 26.9 °C, which is significantly warmer than the modern MAT of 8.9 °C. These examples illustrate the utility of TGA-IRIS to yield paleoclimatic information on the globally ubiquitous kaolinite mineral.