Theoretical inversion of the fossil hydrothermal systems with oxygen isotopes of constituent minerals partially re-equilibrated with externally infiltrated fluids

While the external infiltration of water has been identified from modern geothermal and/or fossil hydrothermal systems through stable isotopes, the physicochemical boundary conditions like the initial oxygen isotopes of water (delta O-18(w)i) and rock as well as alteration temperature were implicitly presumed or empirically estimated by the conventional forward modelling. In terms of a novel procedure proposed to deal with partial re-equilibration of oxygen isotopes between constituent minerals and water, the externally infiltrated meteoric and magmatic water are theoretically inverted from the early Cretaceous post-collisional granitoid and intruded Triassic gneissic country rock across the Dabie orogen in central-eastern China. The meteoric water with a delta O-18(w)i value of -11.01 parts per thousand was externally infiltrated with a granitoid and thermodynamically re-equilibrated with rock-forming minerals at 140 degrees C with a minimum water/rock (W/R)(o) ratio around 1.10 for an open system. The lifetime of this meteoric hydrothermal system is kinetically constrained less than 0.7 million years (Myr) via modelling of surface reaction oxygen exchange. A gneissic country rock, however, was externally infiltrated by a magmatic water with delta O-18(w)i value of 4.21 parts per thousand at 340 degrees C with a (W/R)(o) ratio of 1.23, and this magmatic hydrothermal system could last no more than 12 thousand years (Kyr) to rapidly re-equilibrate with rock-forming minerals. Nevertheless, the external infiltration of water can be theoretically inverted with oxygen isotopes of re-equilibrated rock-forming minerals, and the ancient hydrothermal systems driven by magmatism or metamorphism within continental orogens worldwide can be reliably quantified.

Automated summary

This study explores the external infiltration of meteoric and magmatic water in the Dabie orogen in central-eastern China, utilizing a novel procedure to deal with oxygen isotopes re-equilibration between minerals and water. The results reveal that the lifetime of the meteoric hydrothermal system is kinetically constrained, while the magmatic hydrothermal system could rapidly re-equilibrate with rock-forming minerals within a shorter period.