Keywords:seawater, carbon cycling, serpentinite, serpentine, carbonate, carbonation
There are numerous gas and liquid, two-phase inclusions in dolomite crystals in a white-green-colored listvenite recovered through Oman Drilling Project Phase 1 (specimen name z68w). Most of the fluid inclusions are tiny, <6 micrometer across and seems to be too small to provide detectable Raman spectra of fluids. Fluid inclusions >6 micrometer across in dolomite provide Raman spectra of H2O, but they become dark possibly due to destruction during the cooling processes and usually prohibit from observation of melting ices. So far, Raman microscopy indicated that they are filled with H2O and do not detect any other gasses such as CH4 or CO2. Melting temperature of ice in the tiny fluid inclusions, on a Linkam THM600 heating stage with a Nikon 100X objective lens indicates that they are filled with saline fluids with 1.1 ± 0.5 wt% NaCl equivalent (n=42, Figure 1), which is much lower than that of seawater (3.5 wt% NaCl). Homogenization temperature of vapor-liquid suggests that the fluid inclusions were formed at temperature >139 ± 31°C (n=8, Figure 2). The fluid inclusions are mainly in dolomite veins, which cut magnesite and quartz (Figure 3, Raman mapping of magnesite, quartz, and dolomite). The present data indicate that low-salinity aqueous fluids were available during the crystallization of dolomite veins after the formation of magnesite-quartz assemblage. If seawater carries carbon to form listvenite from peridotite or serpentinite, the present low-salinity data suggest that chlorine needs to be absorbed by some phases during the fluid migration or be diluted by mixing with meteoric water.