IMPACT OF TEMPERATURE-TIME PATHS ON GROWTH ZONING OF RARE EARTH ELEMENTS IN METAMORPHIC GARNET

Pelitic garnets from the Picuris Mountains, New Mexico and from Passo del Sole, Switzerland display patterns of rare-earth-element (REE) distribution consistent with diffusion-limited uptake of REEs, but these patterns are strikingly dissimilar for garnets that experienced different T-t histories during growth.

On centrally sectioned crystals, concentrations of HREEs (Lu, Yb, Tm, Er) and MREEs (Ho, Dy, Tb, Gd) were collected by LA-ICPMS as line scans; concentrations of LREEs (Eu, Sm, Nd) were acquired by SIMS as spot analyses. In the Picuris Mountains garnets, diffusion controls on REE patterns are dominant, but are modified by changes in garnet-forming reactions. In large garnets, which nucleated earlier in the T-t history, HREEs are mainly present as central peaks, MREEs as central peaks plus annular maxima, and LREEs only as annular maxima. Within the MREEs, the annular maxima are located progressively rimward with decreasing atomic number. A change in the garnet-forming reaction produced spikes in the concentrations of HREEs and MREEs that are found progressively rimward with decreasing atomic number; no spikes are distinguishable for the LREEs. In the small Picuris garnets, which nucleated later in the T-t history, all REEs have central peaks and lack annular maxima. In contrast to the size-dependence of REE distribution in the Picuris garnets, crystals of all sizes from Passo del Sole have all REEs present as central peaks.

Numerical models of REE uptake limited by rates of intergranular diffusion demonstrate that the highly variable patterns seen in natural samples result from different ranges of temperature during garnet growth. For REE distributions in Picuris garnets, variations with crystal size are explained by larger temperature ranges for early-nucleated garnets and smaller temperature ranges for late-nucleated garnets. At Passo del Sole, where all nucleation was nearly simultaneous, all garnets experienced relatively small temperature ranges during growth, yielding patterns like those of the small Picuris garnets. In the Picuris garnets, the effects of diffusion are also apparent in the positions of the HREE and MREE spikes: their progressively rimward locations with decreasing atomic number indicate similarly decreasing intergranular diffusivities.