Rare Earth Element Mapping of Garnet by Laser Ablation ICP-MS

The introduction of the electron microprobe brought tremendous advancement to our understanding of mineral forming reactions, thermobarometry and the patterns and controls of major element zoning. Today, the application of laser ablation ICP-MS (LA-ICP-MS) is providing the same advancements, but at the trace element scale. While considerable attention has been paid to the behavior of trace elements in a crystal-melt system, the number of studies addressing trace elements in metamorphic minerals is few. We have developed methods using LA-ICP-MS to rapidly construct quantitative trace element maps of geologic (minerals, corals, coal, etc.) and non-geologic (tree rings, sheep horns, fish scales, metals, etc.) materials. Experimental determination of trace element partitioning and the parameters that control it are determined almost exclusively on experimental melt products and natural glasses. With this study we demonstrate that insight into the behavior of the trace elements during metamorphic mineral growth is now readily obtainable. Garnets from the Nason terrane, North Cascades, WA, USA have been analyzed in order to study trace element heterogeneity. In the Nason terrane, almandine-rich garnets in amphibolite facies rocks all possess distinct enhanced HREE compatibility in the cores with varying degrees of complex zoning towards the rim. A distinct trace-element-enriched annulus is present in nearly all garnets studied. Annuli enriched in the HREEs and Y are common, but annuli enriched only in the LREEs are also present. We suggest the annuli are the result of the breakdown of REE-rich minerals such as monazite, apatite and/or xenotime. The HREE zoning patterns from rim to core can be used to model the changing mechanism for growth zoning. While it is clear that various mechanisms operate during the entire growth history, it appears that intercrystalline diffusion is the dominant mechanism producing HREE zoning in the early stages of garnet growth.