High-pressure homogenization of olivine-hosted CO<sub>2</sub>-rich melt inclusions in a piston cylinder: insight into the volatile content of primary mantle melts

Abstract

Abstract. Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO2 contents of olivine-hosted melt inclusions may reach values of the order of a single to several weight percent, especially in intraplate continental basalts. To be able to homogenize melt inclusions with such high CO2 contents, we developed a technique allowing for heat treating of the melt inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder, using thick-walled Au80–Pd20 containers and molten NaCl as the surrounding medium for the inclusion-bearing olivines. We applied this technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais volcanic province, French Massif Central. Thueyts melt inclusions were chosen because of their high CO2 contents, as indicated by up to 1.19 wt % dissolved CO2 in the glasses and by the presence of shrinkage bubbles containing abundant carbonate microcrystals in addition to a CO2 fluid phase. The homogenization experiments were conducted at pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 ∘C, and run durations of 30 min. In all the melt inclusions treated at 2.5 GPa–1300 ∘C and half of those treated at 2 GPa–1300 ∘C, we were able to completely homogenize the inclusions, as indicated by the disappearance of the starting bubbles, and we obtained total CO2 contents ranging from 3.2 wt % to 4.3 wt % (3.7 wt % on average). In all the other melt inclusions (equilibrated at 1.5 or 2 GPa and 1300 ∘C or at 2.5 GPa–1275 ∘C), we obtained lower and more variable total CO2 contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest total CO2 contents, the size of the shrinkage bubble was in most cases small (<5 vol %) to medium (<10 vol %): this is a strong argument in favor of an origin of these melt inclusions by homogeneous entrapment of very CO2-rich basanitic liquids (∼ 4 wt %) at pressures of 2 to 2.5 GPa. The lower total CO2 contents measured in some inclusions could reflect a natural variability in the initial CO2 contents, due for instance to melt entrapment at different pressures, or CO2 loss by decrepitation. An alternative scenario is heterogeneous entrapment of basanitic liquid plus dense CO2 fluid at lower pressures but still at least on the order of 1 GPa as indicated by dissolved CO2 contents up to 1.19 wt % in the glasses of unheated melt inclusions. Whatever the scenario, the basanites from the Bas-Vivarais volcanic province were generated in a mantle environment extremely rich in carbon dioxide.

Determining the Pressure – Temperature – Composition (P-T-X) conditions of magma storage

“…Once CO 2 in the vapour bubble is included, pressures rise to ~2-11 kbar. Similarly high glass CO 2 contents (1.2 wt%) are reported by Buso et al (2022) in melt inclusions from the French Massive Central, corresponding to entrapment depths of >10 kbar. Once bubbles are dissolved using homogenization methods, saturation pressures in these samples extend to 15-25 kbar.…”

“…One limitation of these hydrated experimental reheating methods is that the initial H 2 O content of the system must be estimated when preparing the hydrated matrix, and melt inclusion water contents will be reset to this value (Buso et al, 2022). While Rasmussen et al (2020) use unheated melt inclusion volatile contents as a guide, there is still the risk of adding too much or too little H 2 O to any specific inclusion.…”

Magma storage and degassing beneath the youngest volcanoes of the Massif Central (France): Lessons for the monitoring of a dormant volcanic province

“…Indeed, the extensive CO 2 degassing we documented in this study (more than 80 % of the initial CO 2 would be lost for melts of the Pavin's Group of Volcanoes) suggests the presence of a deeper CO 2 vapour phase. This inference is supported by recent results in experimental petrology that show a significative enrichment in volatile elements in primary mantle melts beneath the FMC (Buso et al, 2022). This CO 2 phase, exsolved early and thus characterised by a less negative  13 C of CO 2 , may contribute to flux overlying magma reservoirs (Edmonds, 2008) and consequently to modify the pristine signature of the melt-related CO 2 .…”

Geochemical processes in the roots of the Azores magmatic systems

“…At these depths CO 2 values required to saturate magma batches range from 0.47 to 0.99 wt.%, varying across different volcanic systems. These values are significantly lower than those (4.5-4.7 wt.%) recently found in basanites erupted at Fogo volcano at Cape Verde and La Palma at Canary Islands (Burton et al 2023;Lo Forte et al 2024), but also in French continental basanites (4.3 wt.%; Buso et al 2022). Pre-eruptive degassing significantly reduced the CO 2 content in melts, prior to SMIs trapping and intracrustal ascent, as suggested by the CO 2 values retrieved from ratios with Ba, Rb and Nb incompatible elements.…”