Carbonation of Cl-rich scapolite boudins in Skallen, East Antarctica: Evidence for changing fluid condition in the continental crust

Spectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lützow-Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (Scp_I) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine-grained recrystallized equigranular scapolite (Scp_II) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by ScpIII reaction zones along fractures with compositional gradients. Pure CO2 fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystalshaped fluid inclusions are moderately dense, and are believed to be coeval with ScpIII formation at temperatures > 600 °C and a minimum pressure of c. 3.8 kbar. Grain-scale LA-ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. ScpI developed in the presence of an internally buffered, brine-rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO₃)-bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from ScpI along fractures and grain boundaries. Movement of fluids was enhanced by micro-fracturing during the transformation of Scp_I to Scp_III. Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl-bearing fluids released from ScpI. The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.