Compositional continuity and discontinuity in the Horoman peridotite, Japan, and its implication for melt extraction processes in partially molten upper mantle

A hypothetical model is proposed to explain the origin of compositional discontinuities in the layering observed in orogenic lherzolites. The observed collinearity of the whole-rock peridotite compositions is best explained in terms of partial melting and melt segregation. The presence of chemical discontinuities implies that melt segregation includes an abrupt and discontinuous process. A kg concept in the model is the topological transformation of melt geometry in partially molten rocks responding to the equality and inequality of the fluid pressure and solid pressure, which may be realized in a gravitational field. It is emphasized that the percolation threshold is a critical boundary, beyond which a rapid microstructural change occurs in response to the change of local fluid pressure, thus causing a rapid increase of permeability. The model implies that the mode of melting is closer to batch melting than to fractional melting in the upper mantle.