Metamorphic fluid flow in the northeastern part of the 3.8-3.7 Ga Isua Greenstone Belt (SW Greenland): A re-evalution of fluid inclusion evidence for early Archean seafloor-hydrothermal systems

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Heijlen, W, Appel, Pwu, Frezzotti, M, Horsewell, A, Touret, Jlr
• Other Authors: Appel, P, Touret, J
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2006
• Subjects: fluid inclusion metamorphic rocks; Greenland
• Online Access: http://hdl.handle.net/10281/174579; https://doi.org/10.1016/j.gca.2006.04.005

Fluid inclusions in quartz globules and quartz veins of a 3.8-3.7 Ga old, well-preserved pillow lava breccia in the northeastern Isua Greenstone Belt (IGB) were studied using microthermometry, Raman spectrometry and SEM Cathodoluminescence Imaging. Petrographic study of the different quartz segregations showed that they were affected by variable recrystallization which controlled their fluid inclusion content. The oldest unaltered fluid inclusions found are present in vein crystals that survived dynamic and static recrystallization. These crystals contain a cogenetic, immiscible assemblage of CO2-rich (+H2O, +graphite) and brine-rich (+CO2, +halite, +carbonate) inclusions. The gas-rich inclusions have molar volumes between 44.8 and 47.5 cm(3)/Mol, while the brine inclusions have a salinity of similar to 33 eq. wt% NaCl. Modeling equilibrium immiscibility using volumetric and compositional properties of the endmember fluids indicates that fluid unmixing occurred at or near peak-metamorphic conditions of similar to 460 degrees C and similar to 4 kbar. Carbonate and graphite were precipitated cogenetically from the physically separated endmember fluids and were trapped in fluid inclusions. In most quartz crystals, however, recrystallization obliterated such early fluid inclusion assemblages and left graphite and carbonate as solid inclusions in recrystallized grains. Intragranular fluid inclusion trails in the recrystallized grains of breccia cementing and crosscutting quartz veins have CO2-rich assemblages, with distinctly different molar volumes (either between 43.7 and 47.5 cm(3)/mol or between 53.5 and 74.1 cm(3)/Mol), and immiscible, halite-saturated H2O-CO2-NaCl(-other salt) inclusions. Later intergranular trails have CH4-H-2 (X-H2 up to similar to 0.3) inclusions of variable density (ranging from 48.0 to > 105.3 cm(3)/Mol) and metastable H2O NaCl(-other salt?) brines (similar to 28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain ...