Hydrogen isotope profile of the serpentinization of ultramafic rocks as a reflection of the paleoclimate and neotectonics

The isotopic composition of hydrogen in serpentine and serpentinized peridotite was determined in a drill core from the Kempirsai massif (southern Urals) to a depth of 1.4 km. It was established that in the upper 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 off). Such a two-step hydrogen isotope profile develops under the influence of meteoric waters, when the isotope exchange of the already formed serpentine with new portions of water is superimposed on the penetrative serpentinization proceeding at low temperatures (<50-25 degrees C) with Raleigh depletion of the aqueous fluid. The absence on the continents of a strict global correlation between the isotopic composition of the hydrogen of rock-forming serpentines and local meteoric waters may be explained above all by the differing geologic age of serpentinization and the lack of correspondence between paleoclimates and present-day climate zoning, and also by the fact that, owing to the different rates of neotectonic up-lifting, different horizons of the hydrogen isotope profile become exposed by erosion in different areas.