Carbon-saturated COH fluids in the upper mantle: what ex situ experiments tell us about carbon at high-pressure and high-temperature conditions

High-pressure COH fluids have a fundamental role in a variety of geological processes. Their composition in terms of volatile species can control the solidus temperature, carbonation/decarbonation reactions, and influences the amount of solutes generated during fluid-rock interaction at depth. Over the last decades, several systems have been experimentally investigated to unravel the effect of COH fluids at upper mantle conditions. However, fluid composition is rarely tackled as a quantitative issue, and rather infrequently fluids are analyzed in the same way as the associated solid phases in the experimental assemblage. A comprehensive characterization of carbon-bearing aqueous fluids in terms of composition is hampered by experimental difficulties in synthetizing and analyzing high-pressure fluids, without altering their composition upon quench.

Recently, improved ex situ techniques have been proposed for the analyses of experimental COH fluids, leading to significant advancement in synthetic fluids characterization. The development of customized techniques in order to investigate these fluids, in terms of volatile speciation and dissolved solute load, allowed to elucidate some of the processes involving carbon at high-pressure conditions and to assess its influence in the mantle wedge.

Some of the recently developed techniques employed for ex situ quantitative analyses of carbon-saturated COH fluids will be presented, such as the capsule piercing QMS technique (Tiraboschi et al., 2016) and the cryogenic LA-ICP-MS technique (Kessel et al., 2004; Tiraboschi et al., 2018). The capsule piercing QMS technique allow to measure the main uncharged volatile species in the COH system (i.e., H₂O, CO₂, CH₄, H₂, O₂, CO), while the cryogenic LA-ICP-MS technique permits to measure the amount solutes generated by mineral dissolution in COH fluids, in terms of mol/kg.

The results obtained by employing these analytical strategies indicate that a quantitative approach to COH fluid analyses is a fundamental step to understand the effect of carbon-bearing fluids at upper mantle conditions and to ultimately unravel the deep cycling of carbon.

 

Kessel, R., Ulmer, P., Pettke, T., Schmidt, M. W. and Thompson, A. B. (2004) A novel approach to determine high-pressure high-temperature fluid and melt compositions using diamond-trap experiments, Am. Mineral., 89(7), 1078–1086.

Tiraboschi, C., Tumiati, S., Recchia, S., Miozzi, F. and Poli, S. (2016) Quantitative analysis of COH fluids synthesized at HP–HT conditions: an optimized methodology to measure volatiles in experimental capsules, Geofluids, 16(5), 841–855.

Tiraboschi, C., Tumiati, S., Sverjensky, D., Pettke, T., Ulmer, P. and Poli, S. (2018) Experimental determination of magnesia and silica solubilities in graphite-saturated and redox-buffered high-pressure COH fluids in equilibrium with forsterite + enstatite and magnesite + enstatite, Contrib. to Mineral. Petrol., 173(1), 1–17.