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

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

Trace element partitioning between coexisting biotite and muscovite from metamorphic rocks, Western Labrador: structural, compositional and thermal controls

Coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western Labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to ICP-MS (LAM-ICP-MS) for selected trace elements—Li, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf and Ta. The samples have experienced a single prograde Grenvillian metamorphism ranging from 490 to 680°C and from 7 to 12 kbar. The trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure, major element composition and temperature on the partitioning of each element between biotite and muscovite. Overall, trace element distributions are systematic across the range of metamorphic grade and bulk composition, suggesting that chemical equilibrium was approached. Most distribution coefficients (biotite/muscovite) show good agreement with published data. However, distribution coefficients for Co and Sr are significantly different from previous determinations, probably because of contamination associated with older data obtained by bulk analysis techniques. The sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas. In particular, distribution coefficients exhibit the sequence Cr3+ (0.615 Å) > V3+ (0.64 Å) > Sc3+ (0.745 Å) in VI-sites, and Ba2+ (1.61 Å) > Sr2+ (1.44 Å) and Cs+ (1.88 Å) > K+ (1.64 Å) > Rb+ (1.72 Å) > Na+ (1.39 Å) in XII-sites. The distributions of Li, Sc, Sr and Ba appear to be thermally sensitive but are also controlled by major element compositions of micas. V and Zr partitioning is dependent on T and may be used to cross-check thermometry calculations where the latter suffer from retrograde re-equilibration and/or high concentrations of Fe3+. The ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

Zircon macrocrysts in (sub)volcanic silica-undersaturated rocks are an important source of information about mantle processes and their relative timing with respect to magmatism. The present work describes variations in trace element (Sc, Ti, Y, Nb, lanthanides, Hf, Ta, Pb, Th, and U) and isotopic (U-Pb) composition of zircon from the Drybones Bay kimberlite, Northwest Territories, Canada. These data were acquired at a spatial resolution of ≤100 µm and correlated to the internal characteristics of macrocrysts (imaged using cathodoluminescence, CL). Six types of zircon were distinguished on the basis of its luminescence characteristics, with the majority of grains exhibiting more than one type of CL response. The oscillatory-zoned core and growth sectors of Drybones Bay zircon show consistent variations in rare-earth elements (REE), Hf, Th, and U. Their chondrite-normalized REE patterns are typical of macrocrystic zircon and exhibit extreme enrichment in heavy lanthanides and a positive Ce anomaly. Their Ti content decreases slightly from the core into growth sectors, but the Ti-in-zircon thermometry gives overlapping average crystallization temperatures (820 ± 26 °C to 781 ± 19 °C, respectively). There is no trace element or CL evidence for Pb loss or other forms of chemical re-equilibration. All distinct zircon types are concordant and give a U-Pb age of 445.6 ± 0.8 Ma. We interpret the examined macrocrysts as products of interaction between a shallow (<100 km) mantle source and transient kimberlitic melt

Growth characteristics and age at first migration of anadromous and resident Arctic char (Salvelinus alpinus) and lake trout (Salvelinus namaycush) in the Canadian Arctic (2006-2008)

In the family Salmonidae, lake trout (Salvelinus namaycush) are considered the least tolerant of salt water. There are, however, sporadic reports of lake trout in coastal, brackish habitats in the Canadian Arctic. Otolith microchemistry analyses conducted on lake trout and Arctic char (Salvelinus alpinus) from four Arctic lakes in the West Kitikmeot region of Nunavut, Canada, revealed that 37 of 135 (27%) lake trout made annual marine migrations. Anadromous lake trout were in significantly better condition (K = 1.17) and had significantly higher C:N ratios (3.71) than resident lake trout (K = 1.05 and C:N = 3.34). Anadromous lake trout also had significantly higher d15N (mean = 16.4 per mil), d13C (mean = -22.3 per mil), and d34S (mean = 13.43 per mil) isotope ratios than resident lake trout (means = 12.84 per mil, -26.21 per mil, and 1.93 per mil for d15N, d13C, and d34S, respectively); results were similar for Arctic char and agree with results from previous studies. Mean age of first migration for lake trout was 13 years, which was significantly older than that for Arctic char (5 years). This could be a reflection of size-dependent salinity tolerance in lake trout, but further research is required. These are the first detailed scientific data documenting anadromy in lake trout