Were oxygen isotopes of hydrothermally altered minerals just elevated by the 18O-enriched water? Theoretical inversion with thermodynamics and kinetics

Abstract

For a long time, it has been implicitly believed that oxygen isotopes of hydrothermally altered rocks and/or minerals were only elevated by the heavy water enriched in ¹⁸O from the modern geothermal and/or fossil hydrothermal systems around the world. While it is logically likely, there is no any previous attempt to argue for the elevation of oxygen isotopes of hydrothermally altered rocks and/or minerals by a light water depleted in ¹⁸O under appropriate natural conditions. Based on a novel procedure recently proposed for dealing with thermodynamic reequilibration of oxygen isotopes between constituent minerals and water, the initial oxygen isotopes of water (i.e., \(\delta {^{18}{\text{O}}}_{\text{W}}^{\text{i}}\) value hereafter) prior to the hydrothermal alteration are theoretically inverted from the early Cretaceous postcollisional granitoid and Triassic gneissic country rock across the Dabie orogen in central-eastern China. The oxygen isotopes of hydrothermally altered rock-forming minerals were concurrently elevated by the magmatic water with moderate to high \({\delta ^{18}{\text{O}}}_{\text{W}}^{\text{i}}\) values ranging from 4.21 ± 0.04 (one standard deviation, 1SD) to 6.57 ± 0.05‰ in the course of postmagmatic processes. By contrast, oxygen isotopes of the susceptible alkali feldspar from a gneissic country rock could be preferentially elevated by the ancient meteoric water with low \(\delta {^{18}{\text{O}}}_{\text{W}}^{\text{i}}\) values down to -8.52 ± 0.56‰ during exhumation processes of the retrograde metamorphism. These fossil hydrothermal systems could kinetically sustain from a short duration of less than 12 thousand years (Kyr) via the surface-reaction oxygen exchange up to 1 million years (Myr) through the diffusive oxygen exchange, respectively, in this study. Cooling rates are further quantified for rock-forming minerals sequentially blocked and/or isolated from the magmatic water. Hereby, oxygen isotopes of constituent minerals can be hydrothermally elevated by diverse sources of water with paradoxical \(\delta {^{18}{\text{O}}}_{\text{W}}^{\text{i}}\) values, especially for the metamorphic rocks with anomalous oxygen isotopes. There is no doubt that more unexpected findings will be scientifically and methodologically decoded and/or unlocked worldwide in the coming decade(s).