Significance of stable-isotope variations in crustal rocks from the Kola Superdeep Borehole and their surface analogues

Abstract

The Kola Superdeep Borehole, the deepest ever drilled into the Earth's crust with a depth of 12.2 km, is located in the Kola Peninsula, Russia, and consists of high-grade metasedimentary and metaigneous lithologies ranging in age from Archean to Proterozoic. Metabasic rocks and amphibolites from the borehole have δ 18O values in the range from +4.4‰ to +9.2‰ and δD values in the range from −73‰ to −40‰, gneisses and a migmatite have δ 18O values in the range from +7.8‰ to +9.4‰ and δD values in the range from −63‰ to −44‰, and metasedimentary rocks show large variations in δ 18O values from +6.3‰ to +15‰, and have δD values in the range from −57‰ to −33‰. The isotopic compositions of the deep borehole samples are identical with their surface equivalents, indicating that the processes of emplacement and exposure of deep crustal rocks have not affected their H and O isotopic compositions in marked contrast to what has been observed for crust in tectonically active areas. Fluid compositions calculated from the δ 18O and δD values of coexisting amphibole, feldspar, biotite and garnet indicate that two isotopically distinct fluids have interacted with the Kola Superdeep Borehole rocks. One fluid has isotopic compositions consistent with locally rock-buffered regional metamorphic fluids, and the other fluid has compositions expected for Paleoproterozoic seawater. The δ 18O values of co-existing amphibole–feldspar and biotite–garnet pairs in amphibolites and quartzite have apparent equilibration temperatures from ca. 500 °C to 800 °C, which are in good agreement with temperatures determined by cation exchange geothermometry and co-existing assemblages. Only rocks associated with two major faults and the thrust zone that marks the Proterozoic–Archean boundary indicate isotopic exchange with regional metamorphic fluids circulating along these structures. The stable isotopic compositions of the Kola Superdeep Borehole rocks indicate that there was interaction of the rocks with channelized fluids moving along major faults, but there was no pervasive fluid flow during metamorphism.