A new methodology to study element speciation in magmatic fluids

The speciation of volatiles and metals in high-temperature fluids is unquenchable. Accordingly, it needs to be investigated in situ at high pressure (P) and temperature (T) conditions using spectroscopic techniques. One experimental apparatus frequently used for this purpose, the hydrothermal diamond anvil cell, has several limitations, including leaking, imprecise pressure control at crustal pressures and a practical upper temperature limit of ~ 700 ^oC. In addition, the control of redox conditions is challenging due to the reactivity of the diamonds with oxidizing fluids at high T. Furthermore, Raman spectroscopy, one of the key techniques used for in situ speciation studies at high T, suffers from elevated spectral background due to thermal incandescence of the sample above 700-800 ^oC when common lasers are used for excitation (e.g. 532 nm).

To alleviate these limitations, and with the particular goal of being able to study speciation in magmatic fluids at upper crustal P-T conditions at controlled fO₂, we developed a new methodology that comprises a new type of externally heated pressure vessel apparatus and a custom-configured Raman spectrometer optimized for in situ high-T spectroscopy on fluid samples. Magmatic fluid analogues are sampled at high P-T and controlled redox conditions in the form of synthetic fluid inclusions (SFI) in quartz, and are subsequently reheated under the Raman microscope in a Linkam TS1500 heating stage. Upon heating, the pressure increases within the SFI to approach the entrapment P, and on the timescale of the spectroscopic experiment, the fO₂ within the SFI can be maintained at a near constant value by ensuring that the surface of the quartz chip is parallel with the fast direction of hydrogen diffusion in quartz (crystallographic c-axis).

It is essential to ensure that redox-preequilibrated fluids are trapped as SFI, and therefore the quartz has to be fractured in situ during the pressure vessel experiment. To facilitate this simultaneously with redox control, we developed a new type of MHC pressure vessel apparatus, both ends open with a water-cooled pressure seals, and the capsule and a semi-permeable hydrogen membrane (Shaw-membrane) sitting in the hot spot in the middle. This included the development of a new type of Shaw membrane well-suited for operation within externally heated pressure vessels.

To facilitate the acquisition of Raman spectra with high signal-to-noise ratios at magmatic temperatures, free of the effect of thermal incandescence, we configured a high-resolution Raman spectrometer with a 405 nm laser source. This wavelength is sufficiently low to make sure that even the 3000 – 4000 cm^-1 region of the Raman spectra is not affected by overlap with blackbody radiation originating from samples at T far into the magmatic T range. At the same time, it is just high enough to be usable with visible light optics carrying numerous advantages over UV systems.

The methodology was successfully used to constrain redox-dependent sulfur speciation in magmatic fluids (Farsang and Zajacz, 2025).

 

Farsang S. and Zajacz Z. (2025) Nature Geoscience, 18, 98-104, https://doi.org/10.1038/s41561-024-01601-3