The MOHO as a phase transition


Fig. 4 shows various suggestions (made at one time or other) for how the MOHO beneath oceans and continents could be a phase transition (change of mineralogy, but not a major change in composition). Serpentine is the hydrated variety of peridotite (with ca 12% water, thus lower density). Eclogite is the high-pressure form of basalt or gabbro. But are these models realistic? The serpentine-peridotite model is now discounted.

 

The MOHO and the gabbro-eclogite transformation: Basalt transforms to eclogite at high pressures according to the equation:

Olivine + pyroxene + plagioclase > jadeitic pyroxene + garnet

 

For this to be capable of explaining the MOHO it must be a relatively sharp transition, of no more than a few km. Green and Ringwood (1967) studied this experimentally to 30 kilobars (= ca 100 km) using a quartz tholeiite and an alkali basalt (Fig 5a, b).

Fig. 5a. Experimental studies of Green & Ringwood (1967) on quartz tholeiite basalt showed that transformation to eclogite occurred over a considerable depth interval. Note that eclogite has a lot more quartz than the equivalent basalt. But mantle eclogites have no quartz. Where does the silica go?

With both compositions the transformation was found to be gradual. The disappearance of the low density phase (plagioclase) and its replacement by the high density phases (garnet and jadeite) occurred over a pressure range of ca 10 kb (= ca 25 km). Thus the MOHO cannot be a phase transformation and must be a compositional transition.