Water extracted from opal-CT ("porcellanite", "cristobalite"), granular microcrystalline quartz (chert), and pure fibrous quartz (chalcedony) in cherts from the JOIDES Deep Sea Drilling Project is 56‰ to 87‰ depleted in deuterium relative to the water in which the silica formed. This large fractionation is similar in magnitude and sign to that observed for hydroxyl in clay minerals and suggests that water extracted from these forms of silica has been derived from hydroxyl groups within the silica. δ 18O-values for opal-CT at sites 61, 64, 70B and 149 vary from 34.3‰ to 37.2‰ and show no direct correlation with depth of burial. Granular microcrystalline quartz in these cores is 0.5‰ depleted in 18O relative to coexisting opal-CT at sediment depths of 100 m and the depletion increases to 2‰ for sediments buried below 384 m. These relationships suggest that opal-CT forms before significant burial while granular microcrystalline quartz forms during deeper burial at warmer temperatures. The temperature at which opal-CT forms is thus probably approximately equal to the temperature of the overlying bottom water. Isotopic temperatures deduced for opal-CT formation are preliminary and very approximate, but yield Eocene deep-water temperatures of 5-13°C, and 6°C for the upper Cretaceous sample. Pure euhedral quartz crystals lining a cavity in opal-CT at 388 m in core 8-70B-4cc have a δ 18O value of +29.8‰ and probably formed near maximum burial. The isotopic temperature is approximately 32°C.
ASJC Scopus subject areas
- Geochemistry and Petrology
- Earth and Planetary Sciences (miscellaneous)
- Space and Planetary Science