EXPERIMENTAL AND THEORETICAL CONSTRAINTS ON MELT DISTRIBUTION IN CRUSTAL SOURCES - THE EFFECT OF CRYSTALLINE ANISOTROPY ON MELT INTERCONNECTIVITY

In partially molten systems, the equilibrium distribution of melt at t he grain scale is governed by the principle of interfacial energy mini mization. In ideal sources (i.e. partially molten rocks that are monom ineralic, have single-valued solid-liquid and solid-solid interfacial energies, and are subject to hydrostatic stress) the wetting angle The ta is known to be a unique characteristic which specifies the melt con figuration for a given melt fraction. Crustal rocks cannot be modelled as ideal sources because of their polymineralic nature, the moderate to high anisotropy of interfacial energies which characterizes common refractory minerals, and the possible presence of a crystallographic p referred orientation. That partially molten crustal rocks depart from ideal sources is documented by a series of high-P, high-T experiments illustrating the textural relationships of biotite and amphibole with silicic melts. The melt distributions observed in these experiments di ffer significantly from those expected in ideal sources: (1) crystal-m elt interfaces are commonly planar, rational faces rather than smoothl y curved, irrational surfaces; and (2) the concept of a unique wetting angle does not hold as shown in the biotite-silicic melt system. Thes e textural features demonstrate that anisotropy of crystal-melt interf acial energy is a factor of primary importance in modelling the grain- scale distribution of partial melts. The petrological implications of our study are the following: (1) At high degrees of anisotropy and low melt fractions, melt is predicted to form isolated, plane-faced pocke ts at grain corners. The overall shape of these pockets, and therefore the value of the connectivity threshold phi(c) are expected to be ver y sensitive to the ratio of solid-solid to solid-liquid interfacial en ergies, gamma(ss)/gamma(sl) (phi(c) is the melt fraction at which melt interconnectivity is established). Melt pockets with low volume-to-su rface ratio, and low (but non-constant) wetting angles should prevail at high gamma(ss)/gamma(sl), resulting in very low values of phi(c) ( less than or equal to 1 to a few vol%). Higher values of phi(c) a high volume-to-surface ratio of melt pockets, and high wetting angles are expected at low gamma(ss)/gamma(sl). (2) The wetting angle at hornblen de-hornblende-melt junctions, at 1200 MPa-975 degrees C, is 25 degrees . A review of existing data indicates that quartz-melt and feldspar-me lt wetting angles are also low to moderate (12-60 degrees). A very low value of phi(c) should, therefore, be the general rule during crustal anatexis. In particular, a connectivity threshold lower than 3-4 vol% is predicted for partially molten amphibolite. (3) In biotite-rich ro ck-types, such as melanosomes in migmatites, the combination of a pron ounced crystalline anisotropy and a marked preferred orientation of mi ca flakes leads to a very low permeability (normal to layering). Bioti te-rich melanosomes should therefore impede chemical interactions betw een neighbouring leucosomes and mesosomes.