Rare-Earth-Element (REE) Uptake in Garnet: Testing Competing Models
Steven Arauza
My undergraduate research project involved testing competing models for the process of rare earth element (REE) uptake in garnet. Ratios of REEs in garnet are often used in the literature as a means of dating metamorphic rocks and, by using field relationships, constraining ages for tectonic models. These studies are often conducted under the assumption that REE ratios in garnet represent a state of whole-rock equilibrium, but it has been documented in previous literature that this is not always the case. Only recently has the process by which REEs are incorporated into garnet begun to appear in the literature.
Current models of REE uptake in garnet invoke either diffusion-limited (Skora et al. 2006) or reaction-controlled processes (Konrad-Schmolke et al. 2008) but are based on similar observed chemical zoning patterns from similar samples. However, in some circumstances these models should result in different chemical zoning patterns. My study measured patterns of rare-earth-element concentrations in samples with bulk compositions and P-T conditions different from previous studies, in order to test the applicability of both models and to expand our understanding of this process. It documents previously undescribed patterns of REE zoning in garnet that testify to an unrecognized diversity and complexity in the process of REE uptake in garnet.
Seven garnets were analyzed from three localities representing different metamorphic facies and bulk compositions. Two large garnets from the Picuris Mountains, New Mexico, featured a pattern of HREE maxima and MREE minima in their cores. MREEs in these samples were localized in secondary peaks toward the rims. The smallest garnet from this locality featured maxima of all measured REE in the core of the crystal, with progressively wider peaks for lighter REEs. Two garnets from Passo del Sole, Switzerland featured similar patterns of single maxima in their cores for all REEs. Two garnets from the Franciscan Complex, California, showed irregular REE concentration patterns that were non-concentric.
Samples from the Picuris Mountains and Passo del Sole are interpreted as recording diffusion-controlled REE uptake in garnet. The consistent trend of wider peaks for MREEs in these samples implies slower rates of intergranular diffusion for MREEs than HREEs. In larger Picuris samples, spikes for all REEs in garnet interiors represent shifts in the garnet- forming reaction as had been identified in work by Crawford (2008). At these spikes, HREEs peak at the same point in the traverse while MREEs peak progressively further toward the rim due to slower rates of diffusion, recording the superposition of diffusional processes on a reaction event. Irregular patterns of REE concentration in Franciscan samples are representative of a completely different process of REE uptake from previous studies. The diversity of patterns uncovered in this study emphasizes the need for thorough characterization of REE patterns in garnet as a basis for proper geochronologic interpretation of Lu-Hf and Sm-Nd ages.
After a summer internship at the Lunar and Planetary Institute this summer, I will be attending the University of California, Santa Barbara to continue studying metamorphic rocks as Dr. Brad Hacker’s PhD student. My undergraduate research experience has made me realize that I would like to continue my education at the graduate level.