Common Patterns of Rare-Earth-Element Distribution in Garnet

William Carlson Group

Stephanie J. Moore – UT Austin

Understanding of rare earth element (REE) uptake and distribution in metamorphic garnets is essential to sound interpretation of petrologic and geochronologic analyses relying on REEs.  This study examines garnets in pelitic rocks from the Picuris Mountains, New Mexico and Passo del Sole, Switzerland and in a blueschist block from the Franciscan Complex, California.  Individual garnet crystals were sectioned to expose their morphological centers.  For each garnet, concentrations for Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu were collected by LA-ICPMS continuous line scans with a 20 μm spot size at 5 μm/sec,.

The data show centrally located peaks for heavy REEs, progressively more distal peaks for middle REEs, and rimward peaks for light REEs are common to the garnets.  Secondary peaks for HREEs, located rimward of the central peaks, occur in garnets from Picuris Mountains data, which are attributed to changes in the garnet-forming reaction.The commonalities of REE distribution seen in garnets from all localities — in both pelitic and mafic rocks, and spanning a range of crustal P-T conditions — appear to illustrate a pattern that may be ubiquitous, or nearly so, in regional metamorphic garnet.