We have studied the influence of Ca-Tschermaks (Calcium Tschermaks or CaTs) content of clinopyroxene on the partitioning of trace elements between this phase and silicate melt at fixed temperature and pressure. Ion probe analyses of experiments carried out in the system Na2O-CaO-MgO-Al2O3-SiO2, at 0.1 MPa and 1218°C, produced crystal-melt partition coefficients (D) of 36 trace elements (Li, Cl, Sc, Ti, V, Cr, Fe, Co, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, In, Sn, Sb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta and W), for clinopyroxene compositions between 10 and 32 mol% CaTs. Partition coefficients for 2+ to 5+ cations show, for each charge, a near parabolic dependence of log D on ionic radius of the substituting cation, for partitioning into both the M1 and M2 sites of clinopyroxene. Fitting the results to the elastic strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal-melt partition coefficients from elastic moduli. Nature 372, 452-454] we obtain results for the strain-free partition coefficients of theoretical cations (D0), with site radius r0, and for the site's Young's Modulus (E). In agreement with earlier data our results show that increasing (iv)Al concentration in cpx is matched by increasing D, E(M1), E(M2) and D0 for tri-, tetra- and pentavalent cations. The degree of fractionation between chemically similar elements (i.e. Ta/Nb, Zr/Hf) also increases. In contrast, D values for mono-, di- and hexavalent cations decrease with increasing (iv)Al in the cpx. The large suite of trace elements used has allowed us to study the effects of cation charge on D0, r0 and E. We have found that D0 and r0 decrease with increasing cation charge, e.g. r0 = 0.66 Å for 4+ cations and 0.59 Å for 5+ cations substituting into M1. Values of E(M1) and E(M2) increase with cation charge as well as with increasing (iv)Al content. The increase in E(M2) is linear and close to the trend set by Hazen and Finger [Hazen, R.M., Finger, L.W., 1979. Bulk modulus-volume relationship for cation-anion polyhedra. J. Geophys. Res. 84 (10) 6723-6728] for oxides. E(M1) values are much higher and do not fit the trend predicted by the Hazen and Finger relationship. (C) 2000 Elsevier Science B.V. All rights reserved.
- Experimental petrology
- Trace element partitioning