Ocean Drilling Program, United States

Ocean Drilling Program Leg 149 Holes 897C and 897D penetrated 67 m and 143 m, respectively, into a buried serpentinite basement ridge. Approximately the upper half of each basement section is extensively oxidized and carbonatized. The alteration is much more intense than that observed at the top of the normal basaltic oceanic crust. The chemical changes during carbonatization involved primarily the addition of CaCO (sub 3) , and the oxidation of the existing iron in the rocks. Smaller quantities of K (sub 2) O, total iron, P (sub 2) O (sub 5) , V, Ba, Sr, and perhaps Zn were added to the serpentinite during carbonatization. The abundances of Al (sub 2) O (sub 3) , TiO (sub 2) , and Cr reflect the primary concentration of spinel in the rocks. This mineral is little changed by either serpentinization or the later calcitization. This results in lower abundances of these three constituents in the altered rocks as a result of dilution. Chlorine appears to have been added to the host ultramafic rocks during early serpentinization; however Cl concentrations are reduced in the upper calcitized zone. The origin of a local Cl-rich zone with up to 1.0% Cl, marked by the occurrence of iowaite, is uncertain but may result from alteration by chloride-rich brines circulating through fractures in the serpentinite. Textural evidence suggests that, although veining and fracture filling predominate, serpentine minerals are extensively replaced by CaCO (sub 3) . Large amounts of MgO and SiO (sub 2) were probably removed during a protracted period of calcitization. These and the other changes associated with this process probably resulted from exposure to circulating cold seawater over an approximately 40 m.y. period following the cessation of tectonic activity at this site and prior to the burial of the rocks by the sediments of the Iberia Abyssal Plain.