Abstract
RECEIVED JANUARY 6, 2003; ACCEPTED SEPTEMBER 4, 2003A hypothetical model is proposed to explain the origin of composi-tional discontinuities in the layering observed in orogenic lherzolites.The observed collinearity of the whole-rock peridotite compositions isbest explained in terms of partial melting and melt segregation. Thepresence of chemical discontinuities implies that melt segregationincludes an abrupt and discontinuous process. A key concept in themodel is the topological transformation of melt geometry in partiallymolten rocks responding to the equality and inequality of the fluidpressure and solid pressure, which may be realized in a gravitationalfield. It is emphasized that the percolation threshold is a criticalboundary, beyond which a rapid microstructural change occurs inresponse to the change of local fluid pressure, thus causing a rapidincrease of permeability. The model implies that the mode of meltingis closer to batch melting than to fractional melting in the uppermantle.
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