Abstract
The core of the former atoll on Niue island, South Pacific, is extensively dolomitized. A detailed stable isotope study reveals that the dolomites are uniformly enriched in 18O and 13C(δ18O = 2.8‰ ± 0.5; δ13C = 2.0‰ ± 0.3) relative to their carbonate precursor. The Sr isotope ratios are equally constant with depth yielding 87Sr/86Sr values between 0.70912 and 0.70916. The observed isotopic compositions point to sea water as the dominant dolomitizing fluid. The "Sr age" of the dolomites is younger than the biostratigraphic age (Mid to Late Miocene) placing the dolomitization event at Plio-Pleistocene time. A model of sea water convection is proposed for atoll dolomitization on the basis of a thermal gradient between the atoll and the ambient ocean water. Sea water is drawn through the atoll margin and transferred upward by convective flow delivering Mg to the sites of dolomite precipitation. Transported with the sea water are volcano-derived metals (Fe, Cu, Zn, Mn) as evidenced by the chemical gradient in the dolomite unit. It is suggested that the thermal convection model can find general application in atolls and other carbonate platforms containing dolomite where thermal instability exists.
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