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 gneiss, amphibolite, and subordinate calc-silicate, lamprophyre and metagabbro. There are three types of calcite in the drilled section: (1) metamorphic calcite in calc-silicate and marble; (2) crack-filling calcite in all lithologies; and (3) replacement calcite in altered minerals. Crack-filling and replacement calcite postdate metamorphic calcite. Multiple calcite generations in individual cracks suggest that different generations of water repeatedly flowed through the same cracks. Cracks that eventually filled with calcite carried water to sites where replacement calcite formed by plagioclase hydrolysis or other decarbonation reactions. Whole-rock contents of all three calcite types range from ∼ 35 wt% in near-surface calc-silicate to < 0.1 wt% in amphibolite with calcite-filled cracks at 3872 m. Calcite contains average Sr concentrations of 180 ppm and Mn concentrations of 3980 ppm. Crack-filling calcite microdrilled from thin sections has average δ13C = −9.0±2.2‰ and δ18O = +10.5±1.5‰ (± values are 1 standard deviation). Crack-filling mineral assemblages that include calcite originally formed at temperatures of 150–350°C. Presently, crack-filling calcite is in chemical and isotopic equilibrium with saline to brackish water in the borehole at temperatures of ⩽120°C. The saline to brackish water contains a significant proportion of meteoric water. Re-equilibration of crack-filling calcite to lower temperatures means that calcite chemistry tells us little about water-rock interactions in the crustal section at temperatures higher than ∼ 120°C.