Water-rock exchange of Sr isotopes evaluated through a reactive transport model: Application to the El Chichón hydrothermal system

Abstract

The strontium isotope ratio (87Sr/86Sr) is a natural tracer commonly used to determine water sources, water flow trajectories, and mixing relationships. In high-temperature hydrothermal systems, the low solubility of retrograde minerals such as calcite and anhydrite may significantly reduce the dissolved Sr concentration. Further contribution of Sr along the flow path from isotopically distinct sources will alter and potentially mask the original Sr isotopic signature of the fluid, limiting the potential of 87Sr/86Sr data for reconstructing fluid flow patterns. This study investigates the tracing potential of 87Sr/86Sr in terms of fluid circulation within the El Chichón volcano hydrothermal system. Although limestone aquifers are evidenced in the region, the 87Sr/86Sr signature of most thermal fluids from the volcano does not indicate any major Sr contribution from such lithologies, but rather reveals that Sr is inherited from volcanic rocks. An isotope-enabled reactive transport model is performed to investigate under which conditions a connection between the limestone and volcanic aquifers is compatible with chemical and Sr isotopic compositions of the thermal springs. Results indicate that the 87Sr/86Sr signature of a sedimentary fluid with moderate Sr concentration can be completely modified into a magmatic signature after interacting with Sr-rich primary minerals, such as plagioclases, within short period of times (<1 year). Primary minerals release magmatic Sr to the fluid while secondary minerals (such as calcite) progressively precipitate and remove the initial sedimentary Sr. The El Chichón hydrothermal system hence represents a case where the 87Sr/86Sr of most springs does not keep any memory of the circulation through the sedimentary basement, as magmatic Sr dominates the isotopic signature. A certain analogy exists with hydrothermal fluids vented at oceanic spreading centers, whose 87Sr/86Sr signature departs from seawater and reflects Sr inherited from water-basalt interaction.