Metasomatization of the mantle by slab-derived silicic- and carbonate-rich fluids: a record from the world’s youngest UHP terrane, Papua New Guinea

Fluids from subducted slabs are thought to play a major role in mass transfer between the solid Earth and the atmosphere, yet their properties are typically inferred rather than observed. Direct evidence is rare because of their transient properties and later melting and tectonism overwriting their signatures. The active Woodlark rift in southeastern Papua New Guinea exposes the youngest known (ca. 5 Ma) ultrahigh-pressure (UHP) terrane on Earth. Structural data indicates that the PNG UHP terrane was exhumed as a diapir that rose through the former mantle wedge within the active continental rift. Multiple eclogites within the UHP terrane preserve evidence for metasomatic interaction with a fluid that crystallized apatite+Fe-rich dolomite+zircon+rutile+multiple sulfur phases (pyrite, anhydrite, barite) in a vein-like network within the matrix. The zircon associated with the fluid also contain abundant multi-phase solid inclusions, including nanogranite and carbonate-bearing assemblages, plus omphacite and anhydrite+pyrite inclusions that suggest crystallization at high-pressures (>1.6 GPa). To investigate the source and composition of the fluid, we collected major- and trace-element data and Sr-Nd isotopes from apatite and dolomite and trace-element data from rutile in the vein network. Apatite is more enriched in F and OH, compared to Cl, and also is enriched in SO₃ and Sr. Apatite yields uniform εNd_i = ~+3 and initial ⁸⁷Sr/⁸⁶Sr = ~0.70427. Dolomite is enriched in Sr and LREE and yields ⁸⁷Sr/⁸⁶Sr = ~0.70424. Finally, rutile yields Nb/Ta of 15–26, falling mostly within chondritic- to superchondritic values. The mineral assemblage and their trace-element signatures indicate the phases crystallized out of a fluid at eclogite-facies conditions, likely during early exhumation, and that overall, the fluid was volatile-rich (C-O-H-S-F) and transported abundant incompatible (Zr, Hf, Ti, Nb, Ta) and heat-producing (K, U, Th) elements. The fluid is interpreted to be sourced from subducted, carbonate-rich sediments from earlier subducted oceanic crust. The fluid ascended from the downgoing plate to metasomatize sub-arc mantle. Subsequently, the UHP terrane was subducted and then interacted with fluids derived from this metasomatized mantle, as both the UHP terrane and former mantle wedge underwent near isothermal-decompression within the active rift. The results have multiple implications. Fluids with this composition can lead to the formation of exotic lava/magma compositions, such as ultrapotassic and alkaline lavas. In addition, the presence of sulfate phases and the elevated SO₃ content in apatite indicates the fluid was oxidized, which enhances the potential to form porphyry copper-gold deposits commonly associated with arc systems. Finally, the superchrondritic Nb/Ta values observed in the rutile crystallized from the fluid indicate that some of the missing elements of the Nb-Ta paradox are likely stored within the metasomatized mantle. This study is the first to directly sample the composition of these fluids captured by subducted crustal rocks moving through a former mantle wedge, rather than relying on inferences from exhumed peridotites or volcanic rock compositions.