Trace element partitioning between orthopyroxene and anhydrous silicate melt on the lherzolite solidus from 1.1 to 3.2 GPa and 1,230 to 1,535°C in the model system Na₂O–CaO–MgO–Al₂O₃–SiO₂

Abstract

We determined experimentally the Nernst distribution coefficient $D_{i}^{{{\text{opx}} - {\text{melt}}}} = \frac{{c_{i}^{\text{orthopyroxene}} }}{{c_{i}^{\text{melt}} }}$ between orthopyroxene and anhydrous silicate melt for trace elements i in the system Na₂O–CaO–MgO–Al₂O₃–SiO₂ (NCMAS) along the dry model lherzolite solidus from 1.1 GPa/1,230°C up to 3.2 GPa/1,535°C in a piston cylinder apparatus. Major and trace element composition of melt and orthopyroxene were determined with a combination of electron microprobe and ion probe analyses. We provide partitioning data for trace elements Li, Be, B, K, Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Sm, Nd, Yb, Lu, Hf, Ta, Pb, U, and Th. The melts were chosen to be boninitic at 1.1 and 2.0 GPa, picritic at 2.3 GPa and komatiitic at 2.7 and 3.2 GPa. Orthopyroxene is Tschermakitic with 8 mol% Mg-Tschermaks MgAl[AlSiO₆] at 1.1 GPa while at higher pressure it has 18–20 mol%. The rare earth elements show a continuous, significant increase in compatibility with decreasing ionic radius from D ^opx−melt_La  ∼ 0.0008 to D ^opx−melt_Lu  ∼ 0.15. For the high-field-strength elements compatibility increases from D ^opx−melt_Th  ∼ 0.001 through D ^opx−melt_Nb  ∼ 0.0015, D ^opx−melt_U  ∼ 0.002, D ^opx−melt_Ta  ∼ 0.005, D ^opx−melt_Zr  ∼ 0.02 and D ^opx−melt_Hf  ∼ 0.04 to D ^opx−melt_Ti  ∼ 0.14. From mathematical and graphical fits we determined best-fit values for D ^M1₀ , D ^M2₀ , r ^M1₀ , r ^M2₀ , E ^M1₀ , and E ^M2₀ for the two different M sites in orthopyroxene according to the lattice strain model and calculated the intracrystalline distribution between M1 and M2. Our data indicate extreme intracrystalline fractionation for most elements in orthopyroxene; for the divalent cations D ^M2−M1 _i varies by three orders of magnitude between D ^M2−M1_Co  = 0.00098–0.00919 and D ^M2−M1_Ba  = 2.3–28. Trivalent cations Al and Cr almost exclusively substitute on M1 while the other trivalent cations substitute on M2; D ^M2−M1_La reaches extreme values between 6.5 × 10⁷ and 1.4 × 10¹⁶. Tetravalent cations Ti, Hf, and Zr almost exclusively substitute on M1 while U and Th exclusively substitute on M2. Our new comprehensive data set can be used for polybaric-polythermal melting models along the Earth’s mantle solidus.