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

Abstract

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 CO 2-rich (+H 2O, +graphite) and brine-rich (+CO 2, +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 ∼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 ∼460 °C and ∼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 CO 2-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 H 2O–CO 2–NaCl(–other salt) inclusions. Later intergranular trails have CH 4–H 2 ( X H 2 up to ∼0.3) inclusions of variable density (ranging from 48.0 to >105.3 cm 3/mol) and metastable H 2O–NaCl(–other salt?) brines (∼28 eq. wt% NaCl). Finally, the youngest fluid inclusion assemblages are found in non-luminescent secondary quartz and contain low-density CH 4 (molar volume > 105.33 cm 3/mol) and low-salinity H 2O–NaCl (0.2–3.7 eq. wt% NaCl). These successive fluid inclusion assemblages record a retrograde P–T evolution close to a geothermal gradient of ∼30 °C/km, but also indicate fluid pressure variations and the introduction of highly reducing fluids at ∼200–300 °C and 0.5–2 kbar. The quartz globules in the pillow fragments only contain sporadic CH 4(+H 2) and brine inclusions, corresponding with the late generations present in the cementing and crosscutting veins. We argue that due to the large extent of static recrystallization in quartz globules in the pillow breccia fragments, only these relatively late fluid inclusions have been preserved, and that they do not represent remnants of an early, seafloor-hydrothermal system as was previously proposed. Modeling the oxidation state of the fluids indicates a rock buffered system at peak-metamorphic conditions, but suggests a change towards fluid–graphite disequilibrium and a log f H 2 / f H 2 O above the Quartz–Fayalite–Magnetite buffer during retrograde evolution. Most likely, this indicates a control on redox conditions and on fluid speciation by ultramafic rocks in the IGB. Finally, this study shows that microscopic solid graphite in recrystallized metamorphic rocks from Isua can be deposited inorganically from a fluid phase, adding to the complexity of processes that formed reduced carbon in the oldest, well-preserved supracrustal rocks on Earth.