JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102, NO. B8, PAGES 18,233-18,254, AUGUST 10, 1997 Paleofiuids and Recent fluids in the upper continental crust: Resultsfrom the German Continental Deep Drilling Program (KTB) P. M611er, • S M. ' • ß We•se, E. Althaus, 3 W. Bach, 4H. J. Behr, • R. Borchardtf K. Br•iuer, 7 J.Drescher, • J.Erzinger, • E. Faber, 9 B.T.Hansen, • E. E. Horn, • E. Huenges, • H. K•impf, • W. Kessels, •ø T. Kirsten, • D. Landwehr, • M. Lodemann, • L. Machon, •ø A. Pekdeger, •'- H.-U. Pielow, • C. Reutel, • K. Simon, • J. Walther, • F. H. Weinlich, 7and M. Zimmer • Abstract. The two German Continental Deep Drilling Program (KTB) boreholes provided samples for studies of fluid inclusions (paleofluids), "free fluids" of the crystalline basement, and their fingerprints on the chemicaland isotopic composition of minerals and rocksto 910t m depth, whichallowed reconstruction of the evolution of fluids,theirmigration pathways, and their sources. Aqueous fluids were largelylost duringthe Devonian amphibolite facies metamorphism. Thereafter, radiogenic, nucleogenic, andfissiogenic gases, together with NH a- fixed nitrogen, were released from host rocks andpartly enclosed in secondary inclusions. During the Hercynian uplift, Na-Cl fluids (formation water) infiltrated and dissolved noble gases andN2largelyoriginating from thehost rocks. In the course of theCretaceous denudation, high- salinityCa-Na-CI brines, possibly derivedfrom Permo-Carboniferous sediments but altered by fluid/rockinteraction, migrated into their present position. This fluid introduced low-maturity hydrocarbons released together with nitrogen from early metamorphic organic-rich sediments. The 4000-m fluid from the KTB pilot holepumping test,whichwasanalyzed chemically and isotopically, seems to be a mixtureof an ascending basement brineand a descending paleometeoric water,from whichthe late alteration minerals calcite and laumontite precipitated in fractures. The calciteis neither chemically nor isotopically in equilibrium with the recovered "free fluid." Hydraulic tests indicate a communicating system of fractures between the boreholes with a distinct matrix and fracture porosity. Introduction Study of fluidsin the Earth's crust hasbecome oneof the majorgoals in geochemistry because fluidsplay an important role in the evolution and dynamics of the crust.They transport •GeoForschungsZentrum Potsdam, Potsdam, Germany 2GSF-lnstitut farHydrologie, Oberschleil•heim, Germany 31nstitut farMineralogie, Universit•it Karlsruhe, Karlsruhe, Germany. 41nstitut tilt Geowissenschaften, Universit•it Potsdam, Potsdam, Germany •Institut ftir Geologie undDynamik der Lithosph;,ire, Gtttingen, Germany. 61nstitut fiir Geowissenschaften undLithosph•,irenforschung, GielSen, Germany. 71nstitut fiir Geophysik und Geologie, Abteilung Geochemie, Universit•it Leipzig, Leipzig, Germany. gMax-Planck-lnstitut ftir Kernphysik, Heidelberg, Germany. 9Bundesanstalt fiir Geowissenschaften und Rohstoffe,Hannover, Germany. 'øNieders•ichsisches Landesamt far Bodenforschung, Geolo-gische Gemeinschaftsaufgaben, Hannover, Germany. •Geochemisches lnstitut, Universitfit Gtttingen, Gtttingen, Germany. •2Geowissenschaften, FB Rohstoff- und Umweltgeologie, Freie Universit•it Berlin, Berlin, Germany. Copyright 1997 bythe American Geophysical Union. Paper number 96JB02899. 0148-0227/97/96JB-02899509.00 heat and matter; induceendothermic and exothermic mineral reactions; vary the rate of intercrystalline exchange of matter; control diagenesis, metamorphism, and formation of hydrothermal ore deposit.s; influence the effective strain in the intracry.stalline rangeas well as the strength of rocksand their brittle-ductile transition; and hydrofracture low to medium grade metamorphic rocks. The migration of basinal and basementbrines is controlled by the activation of tectonic structures in space and time [Irwin and Barnes, 1975; Fyfe et al., 1978; Fritz andFr•ape, 1987; Fyfe, 1987; Nesbitt, 1990]. From1987 to 1989, theGerman Continental Deep Drilling Program (KTB) pilot hole(VB) wasdrilledto a depth of 4000 m. The 9101-m-deep main hole (HB) followed in 1991 to 1994 at a location 250 m east of KTB-VB. It was one of the major goals of KTB to study the chemical composition and sources of fluids, as well as their transport processes, and the alteration of rocks associated with fluid movement [Ernrnerrnann and Lauterjung, this issue]. Both drilling campaigns were a challenge to earth scientists because of the uniqueopportunity to sample unweathered rocks and indigenous gases continuously from a considerablesection of the upper crust. From these samples, informationabout the evolutionof the fluids in space and time are derived from studies of (1) the chemical and isotopic fingerprints of rocks and minerals, which were influenced by fluid/rock interaction, (2) fluid inclusions trapped as remnants of past fluids within the crustalsegment, and (3) the chemical and isotopiccomposition of "free fluids" present in opencavitiesand fractures. 18,233