O,H, C isotope study of rocks from the KTB pilot hole: crustal profile and constraints on fluid evolution

The pilot hole of the Continental Deep Borehole (KTB) drilling project is located in the Bavarian Oberpfalz at the western margin of the Bohemian Massif. The 4-km deep borehole penetrated various paragneisses and minor orthogneisses with intercalations of amphibolites and metagabbros. The different lithologies have systematically different whole-rock oxygen isotope values and give little evidence for large scale water-rock interaction. Minor fluid interaction is well documented during retrograde metamorphism by non-equilibrium fractionations between refractory minerals (quartz, garnet and hornblende) and altered minerals (chlorite/biolite and feldspar). Ubiquitous vein mineralisation indicates fluid-induced retrogression at temperatures between 150°C and 400°C. The D values of hydroxylbearing minerals are very uniform in all lithologic units. The calculated hydrogen isotope composition of the fluid in equilibrium with matrix and vein minerals increases from -45 for metabasic rocks, to -20 for gneisses, to about -5 for vein minerals. The oxygen isotope composition of the fluid has been buffered by the rock and decreases with decreasing temperature because of increasing fractionations at low temperatures and low water-rock ratios. Modern fluids sampled from open cavities within the borehole have isotopic compositions that suggest a continuous fluid evolution during retrogression in a closed system. The ¹³C values of calcite and graphite also indicate closed system mixing processes.