CHEMISTRY AND PETROGRAPHY OF CALCITE IN THE KTB PILOT BOREHOLE, BAVARIAN OBERPFALZ, GERMANY

The KTB (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschland ) pilot borehole in northeast Bavaria, Germany, penetrates 4000 m of g neiss, amphibolite, and subordinate calc-silicate, lamprophyre and met agabbro. There are three types of calcite in the drilled section: (1) metamorphic calcite in calc-silicate and marble; (2) crack-filling cal cite in all lithologies; and (3) replacement calcite in altered minera ls. Crack-filling and replacement calcite postdate metamorphic calcite . Multiple calcite generations in individual cracks suggest that diffe rent generations of water repeatedly flowed through the same cracks. C racks that eventually filled with calcite carried water to sites where replacement calcite formed by plagioclase hydrolysis or other decarbo nation reactions. Whole-rock contents of all three calcite types range from similar to 35 wt% in near-surface calc-silicate to <0.1 wt% in a mphibolite with calcite-filled cracks at 3872 m. Calcite contains aver age Sr concentrations of 180 ppm and Mn concentrations of 3980 ppm. Cr ack-filling calcite microdrilled from thin sections has average delta( 13)C = -9.0 +/- 2.2 parts per thousand and delta(18)O = +10.5 +/- 1.5 parts per thousand (+/- values are 1 standard deviation). Crack-fillin g mineral assemblages that include calcite originally formed at temper atures of 150-350 degrees C. Presently, crack-filling calcite is in ch emical and isotopic equilibrium with saline to brackish water in the b orehole at temperatures of less than or equal to 120 degrees C. The sa line to brackish water contains a significant proportion of meteoric w ater. Re-equilibration of crack-filling calcite to lower temperatures means that calcite chemistry tells us little about water-rock interact ions in the crustal section at temperatures higher than similar to 120 degrees C.