High-resolution δ17O-δ18O as a single mineral thermometer
The equilibrium relationship α17O/16Oa-b = (α18O/16Oa-b)θ makes the analysis of d17O redundant for most terrestrial applications. However the θ term varies with temperature, so that ultra-high precision δ17O data provide additional information not available from d18O alone. If the d18O and δ17O values of formation water covary in a known way (e.g., meteoric water, ocean water), then a unique solution for both temperature and thed18O of the formation fluids can be obtained from the combined δ18O-δ17O mineral values. The paired δ18O-δ17O values are in essence a single mineral thermometer. Unlike clumped isotopes or combined δ18O-δD data, the δ18O and δ17O values of a mineral have identical 'diagenetic potential', and will only be altered with a high F/R ratio. We have made an empirical determination of the temperature dependence on θ = -710/T2 + 0.5305 using Pleistocene diatom data from ODP Leg 177, Site 1093 (δ18O = 39.610, δ17O = 20.536‰), which is almost identical to Pack and Herwartz (EPSL 2014). Application to ancient cherts gives the following results: The δ18O-δ17O values of cherts vary systematically with age, from Archean to Proterozoic to Phanerozoic. The Archean cherts are incompatible with modern seawater under any temperature conditions. Instead they have equilibrated with water of δ18O= ‑10±3 (‰ vs SMOW) at 50 to 70°C. These data support a lighter ocean in the Archean by ~5‰. Proterozoic cherts equilibrated at 35-50°C with meteoric water of -8±3‰ and Phanerozoic cherts equilibrated with mixed meteoric water/ocean water at similar temperatures and higher δ18O values (‑3±3‰). The δ18O values of lacustrine diatoms from the Valles Caldera, NM, vary by over 20‰ between glacial and interglacial times. The combined δ18O-δ17O values of interglacial diatoms give T= ~12°C, δ18Ometeoric water = -9‰. A glacial age diatom sample gives T=<10°C, δ18Ometeoric water = -20‰. These data could not be obtained from the d18O values alone. The Δ'17O (θ=0.528) values of the glacial and interglacial diatoms are -0.22 and ‑0.19‰, respectively. This change is in excellent agreement with changes seen in Δ'17O values of Vostok Antarctic ice between glacial and interglacial periods (Landais et al. 2008) and is evidence for a lower relative humidity moisture source in glacial periods.
Zachary Sharp is the director of the UNM Center for Stable Isotopes and a Distinguished Professor in the Earth & Planetary Sciences Department. His isotope research includes high temperature igneous and metamorphic petrology, planetary sciences, including studies of Moon and Mars, as well as low temperature studies of soil carbonates, water and atmospheric water vapor. He has developed systems for laser extraction, chlorine isotope geochemistry and most recently, high precision isotope measurements of the three isotopes of oxygen.