Isotope Geochemistry of Abyssal Peridotites and Related Rocks

摘要

The mantle origin of abyssal peridotites and their alteration by hydrothermalsolutions and seawater have been the object of intensive study in the scientific community in recent years. Abyssal peridotites from the American-Antarctic and southwest Indian ridges were studied using major elements, trace elements and the isotopic compositions of Nd, Sr and Os. These studies reveal the nature of the mantle these abyssal peridotites were once a part of, and the nature of alteration processes the peridotites have undergone during and after their emplacement on the sea floor. The major conclusions of this dissertation are as follows: Abyssal peridotites have lost significant Mg during their weathering on the seafloor. Abyssal peridotites are altered at low temperatures and high water/rock ratios during this process. The water/rock ratios during alteration are high enough to compromise the Sr, Nd and Os isotopic systems in bulk peridotite. 'Orphan' 87Sr detected in alteration products is due to the mechanical entrainment of sediment particles into the peridotite. Mantle Os and Nd isotopic compositions detected do not conform to current models for the formation of the depleted mantle. Peridotite, Alteration, Weathering, Isotopes, Mantle, Major elements, Trace elements.