Trace element partioning between coexisting metamorphic minerals and trace element zoning in metamorphic minerals from Gagnon Terrane, Western Labrador

Yang, Panseok (2002) Trace element partioning between coexisting metamorphic minerals and trace element zoning in metamorphic minerals from Gagnon Terrane, Western Labrador. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Because of their wide-range in chemical properties, trace element partitioning and zoning in metamorphic systems provides valuable information on various metamorphic processes which are not recorded or preserved by major elements. Trace element partitioning between coexisting minerals and major and trace element zoning patterns were investigated using an electron microprobe and a laser ablation microprobe - inductively coupled plasma - mass spectrometry for pelites in Gagnon terrane, western Labrador. -- Large variations in the partitioning of Sc and Cr between coexisting garnet and biotite indicate lost or modification of equilibrium compositions by garnet resorption and disquilibrium, respectively. Crystal lattice strain of garnet controls trace element partitioning, so that the distribution of elements occupying each cation site in the garnet structure shows parabolic relations with ionic radius. Sc and Ti partitioning are controlled by Al contents in biotite and Ca in garnet, respectively. Weak to moderate thermal dependence of Co, Zn and Zr partitioning maybe related to increases in thermal expansivities with metamorphic grade. -- Similar to garnet and biotite pairs, trace element partitioning between biotite and muscovite shows controls of crystal lattice strain. The distributions of Li, Sc, Sr and Ba show dependence on both temperature and major element compositions. V and Zr partitioning is moderately dependent on temperature. -- For garnets with spiral internal fabrics, growth zoning for some major and trace elements exhibits a spiral pattern (spiral zoning), implying that incorporation of these elements was mainly restricted to discrete growth regions of the crystal at any one time. Low amphibolite-facies garnets show Cr zoning parallel to foliation, indicating that the garnet overprinted a matrix fabric with a heterogeneous Cr distribution during growth (overprint zoning). In contrast, in mid/upper amphibolite-facies garnet porphyroblasts, Cr zoning becomes concentric, indicating an enhanced diffusion rate. -- Various Mn concentrations in the same annulus from garnet porphyroblasts in a thin-section suggests that Mn did not achieve thin-section scale equilibrium during the annulus formation, raising questions concerning the use of Mn concentration as a time marker. The origins of Y-annuli are suggested in this study: (1) Y-annuli produced by garnet resorption and regrowth characterized by an irregular and discontinuous annulus with a steeper slope on the inner side of the annulus, (2) Y-annuli produced by discontinuous breakdown of Y- and REE-rich minerals characterized by euhedral annuli and (3) Y-annuli produced by changes in garnet growth speed. Trace minerals responsible for the formation of Y-annuli are identified using REE ratios on small intervals on the annuli. -- Phosphorus and Y concentrations in garnet and apatite coexisting with apatite and xenotime, respectively, vary systematically with metamorphic grade. This study demonstrate that trace element zoning combined with partitioning data provides valuable information on P-T, reaction histories, scale of equilibrium and kinetics.

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