SIZE-PROPORTIONAL GARNET GROWTH: AN EFFECT OF PRECURSOR HETEROGENEITY

Syn-deformational garnets in kyanite-grade pelitic schist from the northern margin of the Lucomagno nappe, near Passo del Sole, Switzerland, exhibit compositional zoning — including extraordinary Ca oscillations — that indicates the garnets grew at rates proportional to their sizes. This relationship has not been previously documented for porphyroblasts but is consistent with diffusion-controlled growth from a heterogeneous precursor.

Fine-scale Ca oscillations form an intricate yet identical pattern in garnets of all sizes. Correlated oscillations originated contemporaneously in all garnets, as evidenced by their identical compositions and uniform relationship to sigmoidal inclusion trails. Five growth zones were identified on the basis of correlated Ca oscillations. The distance between zone boundaries is indicative of the rate of radial growth of the crystal integrated over a specific time interval. Measurements of corresponding growth zones on 20 central sections reveal a positive correlation between zone width (growth rate) and garnet radius. Likewise, smoothly varying Mn profiles for 8 garnets were divided into 16 growth intervals defined by identical Mn contents; these Mn growth zones also yield a positive correlation between radial growth rate and radius.

This correlation is unexpected for either diffusion-controlled growth from a homogeneous matrix (a negative correlation) or interface-controlled growth (a slope of zero). A positive correlation, however, is consistent with diffusion-controlled growth from a heterogeneous precursor, because sluggish intergranular diffusion of a heterogeneously distributed critical garnet nutrient in the precursor would cause the garnets to grow at rates dependent upon the local availability of that nutrient. Although one might hypothesize that active deformation in these rocks would eliminate diffusional controls on garnet growth by accelerating transportation of nutrients, these results negate that possibility.