CONCLUSIONS

Relatively uniform ¹³C, ¹⁸O, and ⁸⁷Sr/⁸⁶Sr in the fracture-filling calcites at Sites 897 and 899 are most plausibly interpreted as the result of precipitation in cracks that were open to circulation of little-modified seawater at ambient ocean-bottom conditions (Neptunian fractures) in the Early to middle Cretaceous. This lends strong support to the notion that alteration (and especially calcitization) in the upper portions of the (acoustic) basement rocks at both sites (Shipboard Scientific Party, 1994a, 1994b; Gibson et al., this volume) relates to a period of submarine exposure of these ultramafic rocks in the period between initial rifting and the onset of significant sedimentation. Variations in Fe and Mn in the calcites are likely the result of small and possibly very local fluctuations in the oxidation state of the precipitating fluids. The low concentrations of Mg and Sr, as compared to typical authigenic calcites in shallow-marine settings, are best interpreted as indicative of a low temperature of precipitation (cf., Morse and Mackenzie, 1990).