INTRODUCTION

Calcitized serpentinites recovered at Ocean Drilling Program (ODP) Sites 897 and 899 provide a useful model for understanding the textural and chemical evolution of ophicarbonates. Textural and chemical attributes established early in the history of rifting can be examined in these formerly exposed mantle materials, without the subsequent tectonic overprinting that characterizes ophiolitic materials exposed in mountain belts (e.g., Weissert and Bernoulli, 1984; Früh-Green et al., 1990). The relatively simple postrifting burial history at Sites 897 and 899 (Shipboard Scientific Party, 1994a, 1994b) provides a relatively well-constrained context for interpretation of chemical data. A close analogy can be drawn between the calcitized serpentinites at Sites 897 and 899 and previously described carbonate-enriched ultramafic rocks at mid-ocean ridges (e.g., Bonatti et al., 1974, 1980) and seaward of the Galicia margin (Agrinier et al., 1988; Kimball and Evans, 1988). Samples recovered during Leg 149 span the sediment/basement contact and extend nearly 200 m into serpentinitic materials at both sites, allowing examination of ophicarbonate generation in unprecedented detail.

This study undertakes to evaluate the timing and conditions of precipitation of fracture-filling calcite in serpentinitic basement samples at Sites 897 and 899 using trace-element (Mg, Fe, Mn, and Sr) and isotopic (¹⁸O, ¹³C, and ⁸⁷Sr/⁸⁶Sr) analyses. Petrographic evidence on the paragenesis of fracturing and calcite precipitation is presented in a companion paper by Morgan and Milliken (this volume); stratigraphic trends in calcite distribution and the effects of calcite precipitation on bulk composition are discussed in the Initial Reports site chapters (Shipboard Scientific Party, 1994a, 1994b) and by Gibson et al. (this volume) and Milliken et al. (this volume).

Constraints drawn from the above-cited works on the serpentinites of the Iberia Abyssal Plain, together with the geochemical data presented here, strongly suggest that the vein-filling calcite at both sites was precipitated at 10° to <20°C from a fluid that was isotopically similar to Early to middle Cretaceous seawater. Mg and Sr contents are unusually low for marine precipitates and may reflect the rather low temperatures of precipitation. Trace amounts of Fe and Mn in the calcite were mobilized from the serpentinites through dissolution. Several lines of evidence suggest calcite precipitation in a system characterized by very high water/rock ratios.