Fluid-melt immiscibility at ultra-high pressure conditions: a case study from diamond-bearing metapelites in the Scandinavian Caledonides

Volatiles, such as H₂O, CO₂, Cl, F, and S, play a critical role in the petrological processes that drive Earth's dynamic systems, particularly in melting, mineral stability, and mass transfer of elements.  While the fluid-melt immiscibility is well-documented in lower crustal settings (mostly granulites), its occurrence and implications at ultra-high pressure (UHP) conditions remain poorly understood. Here we present the formation of microdiamonds and primary melt inclusions in metapelitic gneiss from the Heia region of the Arctic Caledonides, Norway (see Janák et al., 2024 for more details).

The migmatitic Heia gneisses comprise garnet, kyanite, biotite, white mica, K-feldspar, plagioclase, and quartz, with accessory minerals including rutile, monazite, zircon, and apatite. Two types of inclusions coexisting in the same cluster were identified in garnet porphyroblasts: Type I (multiphase fluid inclusions) and Type II (primary melt inclusions). Type I inclusions contain microdiamond, rutile, apatite, Fe-Mg carbonates, and Al-phyllosilicates (muscovite-paragonite and pyrophyllite) as solid phases; the fluid phase is dominated by residual CO₂. Melt inclusions (Type II) contain muscovite, paragonite, phlogopite, K-feldspar, plagioclase, albite, quartz and kyanite. Excluding kyanite, the mineral assemblage suggests that the trapped melt was most likely granitic and derived from partial melting of the gneiss; kyanite, based on microstructural observations, is an accidentally trapped mineral. The coexistence of diamond-bearing fluid inclusions and melt inclusions in garnet provides evidence of partial melting and fluid-melt immiscibility under UHP conditions.

The occurrence of diamonds with carbonates and pyrophyllite as an OH-bearing phase suggests its crystallization from a C-O-H fluid, saturated by carbon potentially derived from an organic compound dissolved in the fluid. The fluid was likely derived internally through the devolatilization of hydrous silicates and decomposition of organic carbon during subduction and prograde metamorphism. Fluid-melt immiscibility at UHP conditions of 4.0–4.5 GPa and 840–900°C has been identified in both Åreskutan paragneisses from the Swedish Caledonides where microdiamonds were previously documented and melt inclusions were experimentally re-homogenized (Klonowska et al., 2017; Slupski, 2023), and in Heia. These findings are some of the first global discoveries, highlighting the role of subduction in transporting volatiles from the surface to the deep Earth, with significant tectonic implications.

Janák, M., Borghini, A., Klonowska, I., Yoshida, K., Dujnič, V., Kurylo, S., Froitzheim, N., Petrík, I., & Majka, J. (2024). Metamorphism and partial melting at UHP conditions revealed by microdiamonds and melt inclusions in metapelitic gneiss from Heia, Arctic Caledonides, Norway. Journal of Petrology, 65(11), egac114.

Klonowska, I., Janák, M., Majka, J., Petrik, I., Froitzheim, N., Gee, D. G. & Sasinková, V. (2017). Microdiamond on Åreskutan confirms regional UHP metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides. Journal of Metamorphic Geology, 35, 541–565.

Slupski, P. M. (2023). Former melt inclusions in garnet from UHP gneisses of the Seve Nappe Complex, Scandinavian Caledonides. PhD Thesis, Università degli Studi di Padova, Department of Geosciences, p. 113.