The Eclogite–Garnetite transformation in the MORB + H2O system

To decipher phase relations of oceanic crust in the coldest slab at the mantle transition zone, multi-anvil experiments were conducted in the MORB+H2O system at pressures of 10–19 GPa, and temperatures of 700–1500 °C. Garnet and stishovite were recognized in all run charges. Above 15 GPa, garnet drastically increases NaSi (Na2MSi5O12) component (M = Ca, Mg, Fe2+), jadeite occurs instead of omphacite. Na-, K-hollandite containing 7 mol% NaAlSi3O8 and Ca-perovskite with 60 mol% CaTiO3, were observed at P>17 GPa. With decomposition of omphacite and increase of modal ratio of garnet, there is a sharp increase of density at 440 km. The density increase due to appearance of Ca-perovskite at 570 km, is estimated approximately 100 km shallower than that of previous estimation.