Paradoxically lowered oxygen isotopes of hydrothermally altered minerals by an evolved magmatic water

Abstract

It has been well known that the influxing meteoric water can hydrothermally lower oxygen and hydrogen isotopes of rocks and/or minerals during continental magmatic or metamorphic processes in certain appropriate cases. Its opposite, however, is not implicitly true and needs independent testing. In terms of a novel procedure recently proposed for dealing with thermodynamic re-equilibration of oxygen isotopes between constituent minerals and water from fossil hydrothermal systems, the initial oxygen isotopes of water ($${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$ δ 18 O W i ) are theoretically inverted from the early Cretaceous post-collisional granitoids and Triassic gneissic country rock across the Dabie orogen in central-eastern China. Despite ancient meteoric waters with low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$ δ 18 O W i value down to − 11.01 ± 0.43‰ (one standard deviation, 1SD), oxygen isotopes of hydrothermally altered rock-forming minerals from a granitoid were unexpectedly but concurrently lowered by an evolved magmatic water with mildly high $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$ δ 18 O W i value of 2.81 ± 0.05‰ at 375 °C with a water/rock (W/R)c ratio of 1.78 ± 0.20 for the closed system. The lifetime of fossil hydrothermal systems studied herein is kinetically constrained to no more than 1.2 million years (Myr) via surface-reaction oxygen exchange in the late-stage of continental magmatism or metamorphism. Thereby, caution should be paid when lowered oxygen isotopes of hydrothermally altered rocks and/or minerals were intuitively and/or empirically inferred from the external infiltration of the purely meteoric water with a low $${\updelta }^{18}{\text{O}}_{\text{W}}^{\text{i}}$$ δ 18 O W i value alone.

Palaeo-climate and -topography of the continental orogen: Theoretical inversion with initial oxygen isotopes of ancient meteoric water

“…As recently pointed out by Wei & Zhao (2022), 18 O-depleted metamorphic rocks were regionally documented across the Dabie orogen (Zheng et al 2004;Fu et al 2013;Wei & Zhao 2017). Thereby, oxygen isotopes of the studied granitoids could be lowered through the magmatic assimilation by metamorphic country rocks (in particular gneisses in most cases).…”

“…As recently pointed out by Wei & Zhao (2022), 18 O-depleted metamorphic rocks were regionally documented across the Dabie orogen (Zheng et al . 2004; Fu et al .…”

“…These uncertainties not well resolved thereby largely limit the applicability of ancient meteoric water as one of the ideal candidates for reconstructing palaeoenvironments. However, the values of ancient meteoric water can be theoretically inverted from constituent minerals when their oxygen isotopes thermodynamically re-equilibrated each other (Wei & Zhao 2021, 2022). In other words, the palaeoenvironmental imprint can be uniquely unravelled through the values of ancient meteoric water theoretically inverted from hydrothermally altered minerals (in particular rock-forming minerals in most cases).…”