33. Chemical Evidence for Near-Seafloor Precipitation of Cacite in Serpentinites (Site 897) and Serpentinite Breccias (Site 899), Iberia Abyssal Plain¹

K.L. Milliken² and J.K. Morgan³

ABSTRACT

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° to <20°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 (¹⁸O, ¹³C, and ⁸⁷Sr/⁸⁶Sr) 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.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume. 
²Department of Geological Sciences, University of Texas, Austin, TX 78712, U.S.A. kitty@maestro.geo.utexas.edu
³Department of Geological Science, University of Washington, Seattle, WA 98195, U.S.A.

Date of initial receipt: 1 December 1994
Date of acceptance: 22 May 1995
Reproduced online: 23 July 2004
Ms 149SR-229