High resolution δ17O-δ18O as a single mineral thermometer

Abstract:

The equilibrium relationship α¹⁷O/¹⁶O_a-b = (α¹⁸O/¹⁶O_a-b)^θ makes the analysis of δ¹⁷O 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 δ¹⁸O alone. If the δ¹⁸O 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 the δ¹⁸O 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, δ¹⁸O_{meteoric water} = -9‰. A glacial age diatom sample gives T=<10°C, δ¹⁸O_{meteoric water} = -20‰. These data could not be obtained from the δ¹⁸O 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., GRL, 2008) and is evidence for a lower relative humidity moisture source in glacial periods.