An empirical calibration of the serpentine-water oxygen isotope fractionation at T=25-100 C

The ability to precisely constrain the physical-chemical conditions of serpentinization, such as temper-ature and fluid source, is limited by the accuracy of calibrations for oxygen isotope fractionation between serpentine and water - i.e., 1000 lna(serpentine-water) - which disagree by up to 20%degrees when extrapolated to T < 200 degrees C. In this study, we present a new empirical calibration of 1000 lna(serpentine-water) aiming to improve applications of oxygen isotope geochemistry to very low -T serpentinization (T < 100 degrees C). We used the high-spatial resolution capabilities of Secondary Ion Mass Spectrometry (SIMS) to analyze oxygen isotope ratios in mineral pairs of calcite + serpentine, quartz + serpentine and talc + serpentine co-crystallized at scales <= 50 lm in six serpentinite samples from the Samail ophiolite (Oman). SIMS analysis shows that the mineral pairs are relatively homogenous in oxygen isotope ratios with variability in d18O values <= 2%degrees (2 s). Clumped isotope thermometry and petrological constraints indicate crystalliza-tion temperatures from 25 to 100 degrees C for the investigated samples. These independent constraints on tem-perature allowed us to derive 1000 lna(serpentine-water) by combining D18O(quartz-serpentine), D18O(talc-serpentine) and D18O(calcite-serpentine) measured by SIMS in the investigated samples with D18O(quartz-water), D18O(talc-water) and D18O(calcite-water) calibrations available in the literature. Our empirical calibration of 1000 lna(serpentine-water) = 1.04 +/- 0.20 (2SE) x 106/T2 (T in K), from T = 25 to 100 degrees C, is within uncertainty of former high-temperature empirical calibrations extrapolated to T < 100 degrees C and matches experimental calibrations when extrapolated to T = 250 degrees C. The new serpentine-water calibration enables more accurate reconstructions of fluid-rock interactions occurring during low-temperature serpentinization processes in various tectonic settings. (c) 2023 Elsevier Ltd. All rights reserved.