Stable isotopic and trace-element analyses of vein-filling calcite in serpentinite at Site 897 and in serpentinite breccias at Site 899 suggest precipitation at low temperature (10 degrees to <20 degrees C) from seawater in the Early to middle Cretaceous. Trace amounts of Fe and Mn in the calcites are in excess of amounts expected from unmodified seawater and suggest mobilization of these elements from the serpentinites. Narrow variation of isotopic values (delta (super 18) O, delta (super 13) C, and (super 87) Sr/ (super 86) Sr) across texturally variable calcite types demonstrates that the isotopic composition of calcite-precipitating fluids was not strongly affected by rock dissolution or mineral precipitation. Given the degree of alteration observed in the noncarbonate fraction of these rocks, a high water/rock ratio during calcitization of the serpentinitic materials is implied. Mg and Sr concentrations in vein calcites at these sites are low (typically <4 mole% for Mg and below detection for Sr) in comparison to authigenic calcite in shallow-marine diagenetic settings.