Water contents and hydrogen isotopes in nominally anhydrous minerals from UHP metamorphic rocks in the Dabie-Sulu orogenic belt

A continuous flow method, by a combination of thermal conversion elemental analyzer (TC/EA) with isotope ratio mass spectrometry (MS), was developed to determine both H isotope composition and H₂O concentration of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu orogenic belt. By using the developed step-heating technique, we have studied H₂O concentration and H isotope composition of the different forms of water (structural OH and molecular H₂O) in garnet. The quantitative measurements of H₂O concentration and H isotope composition of minerals in UHP metamorphic rocks from several typical outcrops indicate that the gneisses can release more amounts of water than the eclogites during exhumation of the deeply subducted continental crust. Therefore, by decompression dehydration at the contact between eclogite and gneiss, the released water could flow from the gneiss to the eclogite and result in significant hydration of the eclogite adjacent to the gneiss. The measured maximum water contents of minerals in eclogites indicate that garnet and omphacite have the maximum water solubilities of 2500 and 3500 ppm, respectively, under the peak UHP metamorphic conditions.