MELTING PHASE-RELATIONS OF AN ANHYDROUS MIDOCEAN RIDGE BASALT FROM 3 TO 20 GPA - IMPLICATIONS FOR THE BEHAVIOR OF SUBDUCTED OCEANIC-CRUST IN THE MANTLE

High pressure melting experiments on an anhydrous abyssal tholeiite collected from the Mid-Atlantic Ridge have been conducted over the pressure interval 3 to 20 GPa to explore the fate of subducted oceanic crust in the mantle. The composition of the mid-ocean ridge basalt (MORB) is almost identical to the average basaltic layer of the oceanic lithosphere. The melting phase relations of the MORB are summarized as follows: (1) the liquidus temperature is about 1425-degrees-C at 3 GPa, and it rises almost linearly to above 1900-degrees-C at 10 GPa. The slope of the liquidus curve decreases slightly above 10 GPa. Nevertheless, it still has a positive slope with increasing pressure. At 20 GPa, the liquidus temperature is about 2200-degrees-C. (2) The liquidus phase changes from clinopyroxene to garnet above 3.5 GPa. (3) The solidus temperature rises almost linearly to 2100-degrees-C at 20 GPa; consequently, the melting interval is slightly narrower at high pressures (<140-degrees-C at 5 GPa, <100-degrees-C at 20 GPa). (4) Silica minerals (coesite, stishovite) are stable near and below the solidus. (5) At shallow mantle conditions (2-7 GPa), the liquidus temperature of the MORB is slightly lower than the solidus of the mantle material (peridotite, KLB-1); however, the solidus temperature of the MORB nearly equals or exceeds the liquidus temperature of the mantle material at about 16 GPa. The solidus temperature of the mantle material is expected to exceed the liquidus temperature of the MORB again at higher pressures where the liquidus phase in the peridotite system is majorite and/or modified spinel rather than olivine. In the ordinary thermal structure of the present mantle, subducted MORB is difficult to melt in the absence of volatiles because its dry solidus is much higher than the estimated mantle geotherm. However, in the ancient Earth, when the mantle was supposedly much hotter than today, complete or partial melting of subducting MORB was quite probable and this melting process could have played a significant role in controlling the movements of subducted slab and the evolution of the mantle and continental crust.