# Evolution of ancient hydrothermal fluids theoretically inverted with initial oxygen isotopes of water

## TL;DR

The paper shows that ancient hydrothermal fluids' oxygen isotope values may have evolved over time, challenging previous assumptions about their original composition.

## Contribution

The study theoretically inverts the initial oxygen isotope values of meteoric and magmatic waters in ancient hydrothermal systems.

## Key findings

- Meteoric and magmatic water oxygen isotope endmembers are theoretically inverted from hydrothermally altered minerals.
- Fossil hydrothermal systems could have been active for durations ranging from 12 thousand to 1 million years.
- Using endmember oxygen isotope values for quantifying hydrothermal systems may be misleading due to thermodynamic and kinetic complexities.

## Abstract

While great successes have been achieved during past decades, oxygen isotopic endmembers of water were implicitly adopted by previous studies for quantifying fossil hydrothermal systems worldwide. Both endmember and evolved oxygen isotopes of meteoric (−11.01±0.43 vs. −3.82±0.01‰, one standard deviation of 1SD) and magmatic water (6.57±0.05 vs. 4.21±0.04‰) are, however, theoretically inverted from hydrothermally altered minerals herein. These waters could evolve through binary mixing under isothermal conditions or interacting with susceptible minerals prior to hydrothermal reequilibration. The studied fossil hydrothermal systems can energetically be alive from less than 12 thousand years (Kyr) to 1 million years (Myr). In the viewpoint of thermodynamics and kinetics, caution should be paid to straightforward quantifying hydrothermal systems with the endmember oxygen isotopes of water. Nevertheless, the complexity and diversity of ancient hydrothermal fluids scientifically and methodologically exemplified at the oxygen isotopic level would be secular scenarios during the geodynamic evolution of the continental lithosphere.

The online version contains supplementary material available at 10.1038/s41598-025-99653-x.