DIFFUSION-CONTROLLED SYNKINEMATIC GROWTH OF GARNET FROM A HETEROGENEOUS PRECURSOR AT PASSO DEL SOLE, SWITZERLAND

Abstract

Synkinematic porphyroblasts of garnet from Passo del Sole, Switzerland, display textural and chemical characteristics that record crystallization mechanisms governed by intergranular diffusion and influenced by compositional heterogeneity. Quantita-tive textural analysis yields evidence for size–isolation correlations, reflecting diffusional suppression of growth due to compe-tition for nutrients among neighboring crystals. Compositional zoning, including extraordinary oscillatory zoning of Ca, reveals a previously undocumented relationship: a direct proportionality between the radii of crystals and their rates of radial growth. That relationship arises in diffusion-controlled growth from a heterogeneous precursor: slow intergranular diffusion preserves local heterogeneities in the distribution of nutrients, causing crystals to grow at a rate dependent upon the local supply of nutri-ents; thus faster-growing crystals are larger at all stages of the process. These results therefore provide no support for the hypoth-esis that deformation during metamorphism eliminates diffusional controls on porphyroblast growth by accelerating transporta-tion of nutrients. Variable extents of chemical equilibration during growth are reflected in variable degrees of uniformity in chemical zoning among crystals of garnet. In concert with prior findings, Fe and Mg appear to have equilibrated rock-wide throughout the crystallization interval, whereas low Mn concentrations in garnet cores, for crystals in textural settings that would restrict diffusional supply, indicate disequilibrium for Mn during early, low-temperature growth. In contrast with prior findings, intricate yet identical patterns of oscillatory Ca zoning in all crystals indicate equilibration rock-wide for Ca during most, if not all, of the crystallization interval.