Crystal-chemical and thermal controls on trace-element partitioning between coexisting garnet and biotite in metamorphic rocks from western Labrador

Trace-element concentrations of coexisting garnet and biotite in thirteen metamorphic rocks from western Labrador, ranging from lower-greenschist to upper-amphibolite facies, were determined by laser-ablation microprobe - inductively coupled plasma - mass spectrometry (LAM-ICP-MS). Systematic trace-element distributions across wide compositional and thermal ranges suggest that equilibrium was approached during metamorphic crystallization. Zn depletion in garnet and biotite from staurolite-bearing assemblages demonstrates the control of mineral assemblage on the concentrations of trace elements. Mean molar distribution coefficients [(D(i)(*)(garnet/biotite)] from greenschist-facies assemblages have the following values: Sc 5.03 ± 1.27 (1σ), Eu 1.99 ± 1.69, Cr 0.83 ± 0.53, V 0.39 ± 0.13, Co 0.27 ± 0.07 and Zn 0.12 ± 0.02. For upper-amphibolite-facies assemblages, the D(i)(*) values are: Sc 8.98 ± 7.08, Eu 64.77 ± 41.66, Cr 0.88 ± 0.56, V 0.17 ± 0.06, Co 0.46 ± 0.08 and Zn 0.15 ± 0.03. Other trace elements have concentrations near or below detection limits in either garnet or biotite. The partition of Sc and Ti between garnet and biotite is controlled by the contents of (IV)Al in biotite and Ca in garnet, respectively, indicating that crystal chemistry exerts an influence on trace-element partitioning. In contrast, the partition of Co and Zn shows a thermal dependence and is less sensitive to compositional changes in the host minerals. Distribution coefficients for Zn, Sm, Eu and Gd between garnet and biotite vary systematically with both the Mg/(Mg + Fe) values of garnet and biotite as well as with metamorphic temperature; it was not possible to isolate these two competing factors. The partition of elements between coexisting garnet and biotite is strongly controlled by crystal structure, so that the distribution of elements occupying each cation site in the garnet structure is characterized by a parabola-shaped peak in a diagram where distribution coefficients are plotted against ionic radius. Our study indicates that some instances of irregular partitioning of trace elements between garnet and biotite, found in previous work utilizing bulk analyses of mineral separates, may have been due to inclusions, impurities, and zoning involving trace elements.