Paper No. 83-1
Presentation Time: 1:00 PM
BA AS A PROXY FOR MONAZITE DISSOLUTION/RE-PRECIPITATION IN METAMORPHIC ROCKS
Monazite [(Ce,Th)PO4] is a key geochronometer used to time events in metamorphic rocks. The mineral often appears at the garnet isograd, contains large amounts of radiogenic elements and low amounts of common Pb, and thus is ideal for U-Th-Pb geochronology. Unlike zircon, the isotopic systematics of monazite can reset during subsequent metamorphic events via dissolution/re-precipitation reactions. Differences in the chemistry of specific zones in monazite sometimes have unexpected internal age relationships, suggesting its major elements are unreliable proxies for resetting reactions. To understand if dissolution/re-precipitation reactions can be monitored by trace elements, we analyzed monazite from a pegmatite in the Amelia mining district of Virginia and those in garnet-bearing rocks collected across a large-scale shear zone in western Turkey. These grains were chosen because they have textures consistent with fluid interactions, have large and variable amounts of bulk abundance of common Pb, and yield a range of ages. The Amelia monazite was analyzed using an electron microprobe, laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS). It has numerous microcracks adjacent to compositional discontinuities with respect to primary zoning, consistent with recrystallization. Using both SIMS and LA-ICP-MS, the amount of 204Pb, a proxy for common Pb, in the Amelia monazite linearly correlates with Ba, suggesting the presence of a stoichiometric phase or possible substitution mechanism. Analyses of Turkish monazite inclusions in garnet and matrix show the same result. We suspect that Ba enters sites in the monazite structure or forms trace amounts of a secondary Ba-rich phase (e.g., gorceixite, Ba-bearing plumbogummite) during dissolution/re-precipitation. Analyses of dated monazite for Ba may prove a reliable indicator to gauge if the grains experienced resetting fluid-rock interactions.