HYDROGEN ISOTOPE PROFILE OF THE SERPENTIN IZATION OF ULTRAMAFIC ROCKSAS A REFLECTION OF THE PALEOCLIMATE AND NEOTECTONICS

The isotopic composition of hydrogen in serpentine and serpentinized p eridotite was determined in a drill core from the Kempirsai massif (so uthern Urals) to a depth of 1.4 km. It was established that in the upp er ca. 650 m of the sequence the delta D value remains constant (-116 parts per thousand), but at greater depths it gradually increases (to -32 parts per thousand at the depth at which serpentinization tapers o ff). Such a two-step hydrogen isotope profile develops under the influ ence of meteoric waters, when the isotope exchange of the already form ed serpentine with new portions of water is superimposed on the penetr ative serpentinization proceeding at low temperatures (<50-25 degrees C) with Raleigh depletion of the aqueous fluid. The absence on the con tinents of a strict global correlation between the isotopic compositio n of the hydrogen of rock-forming serpentines and local meteoric water s may be explained above all by the differing geologic age of serpenti nization and the lack of correspondence between paleoclimates and pres ent-day climate zoning, and also by the fact that, owing to the differ ent rates of neotectonic up-lifting, different horizons of the hydroge n isotope profile become exposed by erosion in different areas.