A Study Of Hydroxyl Content Of Nominally Anhydrous Minerals In UHP Metamorphic Rocks From The Chinese Continental Scientific Drilling Main Hole

The Chinese Continental Scientific Drilling Project (CCSD) is located in the Southern Sulu Ultrahigh-pressure (UHP) metamorphic belt. The CCSD main hole is 5158m deep. The depth of eclogite in the main hole reaches 1300m, and researchers pay close attention to it. UHP metamorphic rock such as eclogite is composed with garnet, omphacite, quartz and rutile. These minerals are usually called nominally anhydrous minerals (NMAs) for there is no H in their formulas. Researches in these decades show that almost all the NMAs could dissolve trace water.Analysis of Structure hydroxyl content in NAMs of UHP metamorphic rocks from CCSD and other area was carried out in situ, using the method of Fourier transform infrared spectroscopy (FTIR). The results show that all these NAMs contain trace water, mainly in the form of hydroxyl (OH-). FTIR analysis yield H2O concentrations of 23~398ppm for omphacite, indicate that omphacite might be the major hydrous phase in eclogite. Garnet is sorted of 3 types depend on the absorption characters of infrared spectroscopy. Water in A type garnet should undergo the UHP metamorphism. Water in type B and C could be affected by diffusion of the later fluid. The hydroxyl remained in rutile reaches 1494ppm at maximum, and a range of 20~22ppm for kyanite. Hydroxyl exists in quartz, but the concentrations are especially low. Heterogeneity is found in the same kind of minerals from different samples, in different grains from the same sample and different places in the same grain. The heterogeneity indicates that there are no fluid actives cosmically in the later process. Hydroxyl content in omphacite is correlated partly with its composition, the geochemistry of the whole rock and mineral assemblage, but it shows no correlation with the peak metamorphism conditions, isotope of hydrogen and the retrograde degree. Olivine in ultramafic rocks contains 63~77ppm water, 53~163ppm for clinopyroxene, and 4~6ppm for orthopyroxene.Water remains in the whole rock was calculated depend the bulk of minerals. The result shows that there is 14~220ppm in eclogite with an average of 76ppm, and 55.5ppm in the ultramafic rocks. It is assumed that 1225ppm water in eclogite with 50% garnet and 50% omphacite during the peak metamorphism process, and the water in the upper mantle should near 1000 ppm. About 1142ppm water exsolves from NAMs when the continental crust exhumation and the fluid is the main source of the HP hydrations of retrogression during the early stage. Descend crust into the deep mantle carries about 1225ppm, and this process is an important part of the circle of water between the earth crust and mantle.