Chlorine isotopes in rhyolites

Understanding the details of crustal magmatic processes holds the key to explaining the variety of volcanic and plutonic rocks that exist, how volcanos erupt and the formation of many types of ore deposits. In the past few decades, the discourse on magma reservoirs has shifted from static melt-rich chambers (the type that are depicted as red circles below volcanoes) to long-lived, dynamic magma mushes where melt accumulations are transient phenomena.

Still, how magmatic volatile phases – that is, gases, liquids and supercritical fluids that exsolve from and coexist with melts and solids in a mush – interact with their environment is poorly known. Magmatic volatile phases influence crustal magma evolution and build-up to eruptions. They also redistribute material between melts, fluids and crust, for example contributing to the formation of the largest copper ore deposits on Earth. Very little direct information about fluid-melt interaction is preserved in the rock record, making MVPs hard to observe. Direct sampling of MVPs in magmatic-hydrothermal systems is also rare and often difficult to demonstrate.

This study aims to improve our understanding the role of chlorine-rich fluids during the formation of rhyolites. Chlorine-rich fluids unmix from rhyolitic melts during late-stage silicic magmatic evolution to form a magmatic brine (a salt-water fluid phase). But what happens to the brine? Is it stored in the mush? How does it interact with its environment?

To approach these questions, we analyzed chlorine isotope ratios (³⁷Cl/³⁵Cl) in Icelandic rhyolites. Chlorine isotopes are a potential tracer of fluid-melt interaction within magma mushes, as the isotopic ratio is not significantly affected by syn-eruptive degassing.

A surprising result is that the rhyolites are shifted to more negative δ³⁷Cl values compared to associated basalts and intermediate rocks from the same volcanic systems. These values are too negative to be caused by well known magmatic processes, such as fractional crystallization and degassing during eruptions. We argue that the negative δ³⁷Cl values in rhyolites are caused by assimilation of magmatic brines in long-lived silicic magma mushes.