Stable Isotope Evidence for Closed System Fluid Evolution in KTB Rocks

D. Landwehr Geochemisches Institut der Universität, Goldschmidtstr.1, D-37077 Göttingen, Germany

Klaus Simon Geochemisches Institut der Universität, Goldschmidtstr.1, D-37077 Göttingen, Germany

J. Hoefs Geochemisches Institut der Universität, Goldschmidtstr.1, D-37077 Göttingen, Germany

In the 9101m deep main borehole of the KTB a sequence of metasedimentary gneisses as well as metabasic amphibolites and metagabbros of tholeiitic character have been drilled. The different lithologies show systematically different whole-rock oxygen isotope values. The metasedimentary gneisses (d18O from 10” to 13”) show a strong correlation between their d18O values and their chemical composition, indicating that the primary isotopic characteristics have been preserved during metamorphic evolution. In contrast, the amphibolites and metagabbros have been enriched in 18O (d18O from 5” to 10”) relative to the expected values for mantle derived material due to premetamorphic alteration of oceanic crust by seawater interaction. d18O fractionations of refractory minerals have preserved amphibolite facies temperatures whereas easily exchanging minerals show reequilibration under greenschist facies conditions. The d18O values of vein minerals are significantly lower than for the same minerals in the matrix. Mineralisations in 18O enriched metasedimentary rocks show significantly higher d18O values than for the same minerals having crystallized in veins in metabasic rocks. The hydrogen isotopic composition of hydroxl-bearing minerals show an apparent isotope homogenisation: especially chlorites are uniform at all depths and in all lithologies (dD from -50 to -40). Hydrogen exchange resistant minerals have partly preserved their primary isotopic composition (dD of muscovites = -40”, dD of hornblende = -70” to -80”). In conclusion, stable isotope data give no evidence for large scale water-rock interaction or fluid migration. Major isotopic variations are related to the premetamorphic fingerprint and the isotopic composition of the fluid was modified during uplift and cooling under closed system conditions at low water/rock-ratios, low fluid fluxes and slow cooling rates. Under these conditions oxygen in the rock controls the d18O of the fluid which decreases continuously with decreasing temperature due to decreasing fractionation factors between mineral and fluid. The d18O and dD values of the recent fluid are estimated to fall in the range of -5” to -10” for d18O and -10” to 0” for dD, respectively, fitting perfectly with the extrapolated values of the free fluid found in open cavities within the borehole. This continuous evoloution and equilibration seems to reflect a global phenomenon in crystalline rocks under conditions of low water /rock-ratios and long residence times.