Sub-millimeter Heterogeneity of Yttrium and Chromium during Growth of Semi-pelitic Garnet

Abstract

X-ray maps of yttrium and chromium in two garnets from a semipelitic schist from central Nepal reveal parallel, spiraling bands of high and low concentration � 10^30 m wide. This zoning mimics similar concentration bands defined by foliation in the matrix outside the garnets. The garnets apparently incorporated the matrix zoning intact during growth at conditions up to lower amphibolite facies.These observations and interpretations indicate that Yand Cr concentrations remained heterogeneous at the scale of tens of micrometers during breakdown of matrix minerals and growth of garnet. This conclusion extends results of previous work, which showed that Y and Cr do not homogenize at the centimeter to millimeter scale during metamorphism to amphibolite facies. The width of the concentration bands in the garnets places a constraint on the maximum effective intracrystalline diffusion coefficient for both Y and Cr in garnet of about 10 ^21 cm 2 /s for peak metamorphic conditions of about 9 kbar at 5708C. Different concentrations of Yand Cr at different positions around the rim of each garnet indicate disequilibrium for these elements between garnet and matrix phases. The presence of an internal foliation defined by the Yand Cr concentration bands rules out pre-kinematic garnet growth. Most useful in conjunction with other indicators, zoning of these elements in garnet is thus a tool for deciphering the relative timing of deformation and garnet growth.