Compositional zoning of calcite in a high-pressure metamorphic calc-schist: clues to heterogeneous grain-scale fluid distribution during exhumation

Calcite in former aragonite-dolomite-bearing calc-schists from the ultrahig h-pressure metamorphic (UHPM) oceanic complex at Lago di Cignana, Valtourna nche, Italy, preserved different kinds of zoning patterns at calcite grain and phase boundaries. These patterns are interpreted in terms of lattice di ffusion and interfacial mass transport linked with a heterogeneous distribu tion of fluid and its response to a changing state of stress. The successio n of events that occurred during exhumation is as follows: As the rocks ent ered the calcite stability field at T = 530-550 degrees C, P ca. 1.2 GPa, a ragonite occurring in the matrix and as inclusions in poikilitic garnet was completely transformed to calcite. Combined evidence from microstructures and digital element distribution maps (Mn-, Mg-, Fe- and Ca-K alpha radiati on intensity patterns) indicates that transformation rates have been much h igher than rates of compositional equilibration of calcite (involving resor ption of dolomite and grain boundary transport of Mg, Fe and Ca). This rend ered the phase transformation an isochemical process. During subsequent coo ling to T ca. 490 degrees C (where lattice diffusion effectively closed), g rains of matrix calcite have developed diffusion-zoned rims, a few hundred micrometres thick, with Mg and Fe increasing and Ca decreasing towards the phase boundary. Composition profiles across concentrically zoned, large gra ins in geometrically simple surroundings can be successfully modelled with an error function describing diffusion into a semi-infinite medium from a s ource of constant composition. The diffusion rims in matrix calcite are con tinuous with quartz, phengite, paragonite and dolomite in the matrix. This points to an effective mass transport on phase boundaries over a distance o f several hundred micrometres. if matrix dolomite has supplied the Mg and F e needed for incorporation in calcite. In contrast, diffusion rims are lack ing at calcite-calcite and most calcite-garnet boundaries, implying that on ly very minor mass transport has occurred on these interfaces over the same T-t interval. From available grain boundary diffusion data and experimenta lly determined fluid-solid grain boundary structures, inferred large differ ences in transport rates can be best explained by the discontinuous distrib ution of aqueous fluid along grain/phase boundaries. Observed patterns of d iffusion zoning indicate that fluid was distributed not only along grain-ed ge channels, but spread out along most calcite-white mica and calcite-quart z two-grain junctions. On the other hand, the inferred non-wetting of calci te grain boundaries in carbonate-rich domains is compatible with fluid-calc ite-calcite dihedral angles > 60 degrees determined by Holness and Graham ( 1995) for a wide range of fluid compositions under the P-T conditions of in terest. Whereas differential stress has been very low at the stage of diffu sion zoning (T > 490 degrees C), it increased as the rocks were cooling bel ow 440 degrees C (at 0.3-0.5 GPa). Dislocation creep and the concomitant in crease of strain energy in matrix calcite induced migration recrystallisati on of high-angle grain boundaries. For that stage, the compositional micros tructure of recrystallised calcite grain boundary domains indicates signifi cant mass transport along calcite two-grain junctions, which at the establi shed low temperatures is likely to have been accomplished by ionic diffusio n within a hydrous grain boundary fluid film ("dynamic wetting" of migratin g grain boundaries).