Fluid and melt inclusions as archives of volatile speciation in high- to ultrahigh-pressure metasedimentary rocks

Fluids and melts are the principal carriers of volatiles during subduction and continental collision, yet their composition and mutual relationships at high- and ultrahigh-pressure (HP-UHP) conditions remain incompletely constrained. Here we synthesize published and new data on fluid and melt inclusions in diamond-bearing metasedimentary rocks of the Seve Nappe Complex (SNC) and equivalent units in the Scandinavian Caledonides.

In the southern part of the SNC, Tväråklumparna paragneisses contain primary multiphase fluid inclusions in garnet with microdiamond, carbonate, and CO₂, locally accompanied by graphitic carbon, indicating partial retrogression of diamond during exhumation (Majka et al. 2014, Geology). At Åreskutan, diamonds occur as single or as part of multiphase fluid inclusions (MFI) in garnet coexisting with abundant single inclusions of graphite (Klonowska et al. 2017, J. Metamorph. Geol.) and crystallized former melt inclusions (MI) (Slupski 2023, PhD thesis). Multiphase fluid inclusions contain carbonates, hydrous phyllosilicates, rutile, quartz, diamond and graphite, together with a residual fluid phase dominated by CO₂ with minor CH₄ and N₂. Nanogranitoids occurring in the same microstructural domains contain a mineral assemblage consistent with the trapping of felsic melts and preserve measurable H₂O and CO₂ contents. Together, these inclusions provide direct constraints on the nature of deep fluids, indicating that significant amounts of carbon and minor nitrogen were mobilized during deep subduction.

Farther north, diamond-bearing gneisses from Saxnäs (Petrík et al. 2019, J. Petrol.) preserve MFI containing diamonds, carbonates, rutile and hydrous phyllosilicates. New FIB-SEM data show that microdiamonds occur as multiple grains within the MFI located inside the host garnet, commonly attached to the walls of the cavity. Residual fluid is CO₂-rich; the presence of carbonates and phyllosilicates as step-daughter minerals suggests that the original fluid was C-O-H. Melt inclusions, possible nanogranitoids, are associated with diamond-bearing MFI. Graphite is widespread in these rocks, occurring both as inclusions and along grain boundaries, documenting extensive carbon re-equilibration during decompression and high-temperature overprint.

In the northernmost locality, the Heia (Nordmannvik) Nappe of northern Norway, garnet hosts diamond-bearing multiphase fluid inclusions coexisting with primary melt inclusions (Janák et al. 2024, J. Petrol.). Fluid inclusions contain CO₂, carbonates, hydrous phyllosilicates, and locally CH₄, whereas crystalized melt inclusions show a mineral assemblage consistent with a granitic composition. Their spatial association provides direct evidence for fluid–melt immiscibility at UHP conditions.

Taken together, these occurrences demonstrate that multiphase, diamond-bearing fluid inclusions and granitoid melts trapped by garnet in metasedimentary gneisses of the SNC consistently record carbon-dominated volatile systems, with carbon preserved as diamond, graphite, carbonate, CO₂, and reduced species such as CH₄, accompanied locally by N₂. Fluid and melt inclusions thus represent key archives for reconstructing volatile speciation, redox conditions, and mass transfer during deep subduction and UHP metamorphism of the orogen.