Triple oxygen isotope systematics as a tracer of fluids in the crust: A study from modern geothermal systems of Iceland

Triple oxygen isotope systematics of ancient hydrothermally altered rocks has been previously used to constrain environmental conditions of the Precambrian. To validate those studies, we report high-precision triple oxygen isotope measurements (expressed as Δ′¹⁷O with reference slope 0.528) of quartz, epidote and well fluids from modern geothermal areas of Iceland, Krafla and Reykjanes, as well as measurements from the extinct 6 Ma Geitafell hydrothermal system. At these systems, basalts reacted with distinct fluid sources at temperatures ranging from 250 to 400 °C. Resolvable difference between isotope compositions of surface waters and rocks enables novel insights into boiling, isotope exchange at variable water-rock ratios, and remelting of altered rock. Our measurements of δD, Δ′¹⁷O, and δ′¹⁸O in well fluids show that the reactions proceeded at water-rock ratios of 0.1 to 2, and reveal the addition of meteoric water in the Reykjanes system, and near-surface boiling and steam-liquid separation at Krafla. The δ′¹⁸O and Δ′¹⁷O values of fluids shift due to exchange with rocks at high temperature following the slope 0.51 in the triple isotope space. Near-surface boiling and steam-liquid separation cause shifts in δD and δ′¹⁸O values of the fluids. Their Δ′¹⁷O values do not change significantly owning to the shallow slope of liquid-vapor equilibrium fractionation relative to the reference line slope. Epidote δ′¹⁸O and Δ′¹⁷O values in all three localities closely resemble the isotope composition of local fluid sources. The measured slope of triple oxygen isotope fractionation between quartz and epidote at 250–400 °C is 0.526 ± 0.001. We suggest that triple oxygen isotope composition of epidote can be used as a direct first-order proxy for the equilibrium fluid δ′¹⁸O and Δ′¹⁷O values. The calibrated quartz-water equilibrium fractionation for triple oxygen isotopes yields general agreement with the local fluid sources, within ±1.5‰ of their δ¹⁸O values, while the Δ′¹⁷O agree within ±0.02‰. We present in situ δ¹⁸O measurements in a quartz crystal from Krafla that show several ‰ heterogeneities which may affect the reconstructed equilibrium fluid values. We tested the effect of shallow crustal contamination on the Δ′¹⁷O values of rhyolitic glasses from Krafla, including those quenched and extracted by drilling, that likely formed by assimilation of low-δ¹⁸O hydrothermally altered crust. Our Δ′¹⁷O measurements constrain the degree of crustal assimilation to 10–20 %. Our study shows that the Δ′¹⁷O values measured in geothermal fluids, secondary minerals and low δ¹⁸O contaminated magmas can provide key information on the conditions of water-rock reaction and magma genesis, and contain additional details that were not accessible through conventional analyses of δD and δ¹⁸O.