Mineralogy, isotopic composition of oxygen, hydrogen and carbon, and oxygen isotopic temperature of deep-sea silica and coexisting carbonates (Table 1)

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.

Delta18O-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 microcrystaUine 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.