Recrystallized carbonates from the contact zone between sediments and apparently intrusive basalt, and from calcareous xenoliths in eruptive flows in several Deep-Sea Drilling Project cores, are depleted in O¹⁸ by not more than a few per mil relative to the estimated isotope composition of the original sediment. If this depletion were the result of isotope exchange to equilibrium with seawater in a closed-pore water-sediment system, then the upper limit of the temperature of recrystallization was 100°C, and probably less for most of the samples analyzed. If isotope exchange occurred in an open system, where seawater could readily circulate through the recrystallizing carbonate, the upper temperature limit would be even lower. The possibility that retrogres ive isotope exchange or kinetic isotope effects were responsible for the observed isotope fractionations is considered unlikely. The fact that bulk carbonates from the deepest parts of long Deep-Sea Drilling Project cores show only minor effects of diagenesis and apparently have retained their original oxygen-isotope composition suggests that burial and a concomitant moderate temperature increase are not the only factors involved in carbonate remobilization in the deep sea. It is concluded tentatively that chemical changes in the pore water-sediment system resulting from the interaction of seawater with volcanic material may play an important role in the alteration and lithification of carbonates associated with deep-sea basalts and pyroclastic material.

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