The deformation and compaction of partial molten zones

Abstract

Summary. The segregation of melt from a partially molten source region requires a corresponding deformation of the unmelted residue (‘matrix’). The role of matrix deformation during melt segregation is examined using simple one‐dimensional models, for which the deformation consists only of bulk compression or ‘compaction’. In model I, a volume fraction φ0 of ascending mantle material undergoes pressure‐release melting at a depth z= 0 (localized melting). Compaction of the matrix occurs in a boundary layer whose thickness (reduced compaction length δR) is proportional to the square root of the matrix viscosity. In the Earth's mantle, δR∼ 10–100 m, indicating that compaction cannot be important over large distances. Model II examines the case in which melting occurs over a depth range of order h (distributed melting). In the limit h≪δR, the solution is the same as for the case of localized melting, except in a ‘melting layer’ of thickness ∼h near z= 0. In the more realistic limit h≫δR, compaction makes a negligible contribution to the balance of forces associated with melt segregation. This result is also valid for the more general case of two‐dimensional flow. Compaction is therefore likely to be of negligible importance in the Earth's mantle, with the consequence that melt segregation can be accurately described by Darcy's law. Copyright © 1985, Wiley Blackwell. All rights reserved