Post-magmatic hydrothermal activities in the Bursa ophiolite (NW Turkey): Implications for unusual minerals recovered from ophiolites

Abstract

Low-temperature hydrothermal activities in ophiolites and their modification of chromite have been less studied compared to the magmatic processes. Here we present evidence of hydrothermal modification of chromitites from the mantle sequence of the Bursa ophiolite (NW Turkey). The chromitites are composed mainly of chromite with variable amounts of magnesite and quartz in the matrix. Many chromite grains contain various inclusions such as monophase magnesite or quartz, as well as multi-phase inclusions of quartz + chlorite ± magnesite associated with or without ferrian chromite, which are unusual in ophiolitic chromitites worldwide. Ferrian chromite intergrown with chlorite and quartz inclusions, has higher Cr# values (88.3–94.1) but lower MgO (6.11–9.74 wt%), and Fe3+/(Fe3++Fe2+) ratios (0.13–0.25) than primary chromite (Cr#: 78.3–79.1); MgO: 15.4–15.6 wt%; Fe3+/(Fe3++Fe2+): 0.36–0.40), indicating the infiltration of reducing and SiO2-rich serpentinizing fluids. The pyroxene and olivine were likely broken down by the pre-existing fluids and contributed to the SiO2-rich characteristic of serpentinizing fluid. Fluxing by CO2-rich fluids produced carbonation of the previous serpentine matrix and formed interstitial magnesite and quartz. Lower MgO contents and overall higher Cr# values of small chromite grains hosted in magnesite matrix compared to those hosted in quartz matrix (MgO: 13.2–15.0 wt% vs. 14.8–15.4 wt%; Cr#: 78.7–79.8 vs. 78.2–78.8) suggest that the carbonated fluids redistributed Mg2+ into magnesite and subtly increased Cr solubility in the fluids. Recrystallization may have prompted fracture healing in chromite grains and preservation of quartz when not associated with ferrian chromite. The carbon and oxygen isotope compositions of magnesite (δ13C: −3.35‰; δ18O: 17.29‰) indicate that the CO2-rich fluids were likely derived by decarbonation of marble and metaclastic rocks from the Tavşanlı zone metamorphic rocks in the Bursa ophiolite.