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.