THE SYSTEMATICS OF TRACE-ELEMENT PARTITIONING BETWEEN COEXISTING MUSCOVITE AND BIOTITE IN METAMORPHIC ROCKS FROM THE BLACK-HILLS, SOUTH-DAKOTA, USA

Coexisting muscovite and biotite in forty-nine staurolite- and sillimanite-zone schists from the southern Black Hills, South Dakota, USA, have been analyzed by ICP spectrometry for major and selected trace elements. This study represents the first comprehensive effort to document and explain trace-element partitioning behavior between coexisting micas in metamorphic rocks. Overall, the data reveal systematic clement distributions across a wide thermal-compositional range. Mean Henry's Law partition coefficients [K(D)* (bio/mus)] are as follows: Mn (14 +/- 6 (1sigma)), Ni (14 +/- 6), Zn (13 +/- 8), Li (4.0 +/- 1.1) Ti (2.9 +/- 0.7), Co (2.6 +/- 1.6), Yb (2.0 +/- 1.7), Cu (1.9 +/- 1.2), Y (1.5 +/- 1.5), Be (1.1 +/- 1.5), Cr (1.0 +/- 0.2), La (0.8 +/- 0.7), V (0.7 +/- 0.1), Zr (0.6 +/- 0.2), Ba (0.5 +/- 0.3), Sc (0.4 +/- 0.1), Sr (0.4 +/- 0.4), and Na (0.2 +/- 0.1). This sequence is governed largely by the crystal structure of the micas and their major-element compositions. Several structural effects on K(D)* have been identified. First, the observed K(D)* sequences Cr3+ > V3+ > Sc3+ and Ni2+ > Co2+ > Cu2+ are just as predicted from relative octahedral site preference energies, indicating that crystal-field effects influence the partitioning behavior of transition-metal cations. Second, the presence of smaller (i.e., more collapsed) interlayer sites in muscovite (relative to biotite) favors substitution in muscovite of cations smaller than K+, namely, Na+, Sr2+, Ba2+, and La3+. Likewise, interlayer cations larger than K+ (e.g., H3O+, Rb+, and Cs+) are predicted to substitute preferentially into biotite. Third, tetrahedral Fe3+ is predicted to favor biotite over muscovite because of larger tetrahedral sheets in biotite (due, in turn, to more Al and less Si). Fourth, the occurrence of heterovalent interlayer cations in micas suggests that their partitioning behavior is partly governed by charge-balance reactions. As a general compositional effect on K(D)*, the preferences exhibited by biotite for Li+ and divalent cations of first-row transition elements reflect its high abundance of comparably sized vi(Mg, Fe2+) relative to coexisting muscovite. Likewise, relatively strong affinities of muscovite for Cr3+, V3+, and Sc3+ reflect its high stoichiometric vi(Al, Fe3+) abundance. Element-specific compositional effects on K(D)* are less evident; but there are indications that Mg2+ partitioning affects that of Li+, Ni2+, and Mn2+. Temperatures inferred from Mg-Tschermak (MgSiAl-2) exchange between coexisting muscovite and biotite (HOISCH, 1989) provide a convenient datum by which to evaluate the thermal behavior of analogous vector components involving trace elements. Several of these components appear to possess exchange potentials (DELTAG(rxn)) sufficiently large so that respective equilibrium constants approach unity with temperature increase. These components include the following: NiSiAl-2, MgSiCr-1Al-1, MgSiV-1Al-1, MgSiSc-1Al-1, CrAl-1, VAl-1, and Li2Si viAl-1 (vi)square-1 ivAl-1. In contrast, any thermal sensitivity of other such components is masked by analytical scatter.