P-T-ƒO2-X constraints on the partitioning behavior of trace elements between clinopyroxene and alkali melts: An experimental study

Abstract

Titanian Ca-clinopyroxene (Ti-Cpx) is one of the most characteristic phases and main carriers of trace elements in alkali ultrabasic to intermediate magmas, making it a key mineral for geochemical studies on the origin and evolution of this type of system. Starting from natural basanite and tephrite compositions, we conducted a series of experiments under one atmosphere pressure (1-atm) with oxygen fugacity (fO₂) controlled from QFM-2 to QFM+2 and moderate pressure (0.5–2.0 GPa) with fO₂ around the CCO buffer. The results allowed a better examination of the P, T, ƒO₂, and compositional controls on $D_{\mathrm{element}i}^{\mathrm{Cpx}/\mathrm{melt}}$, and indicated that: (1) the Lattice Strain Model (LSM) successfully fits the divalent and trivalent cations into the M2 site and trivalent cations into the M1 site; (2) divalent cations are strongly influence by clinopyroxene composition, with their compatibility decreasing at the M1 site and increasing at the M2 site as the DiHd component rises; (3) redox-sensitive elements, such as Cr, V, and Eu exhibit systematic variation with ƒO₂ and T; (4) high-field strength elements (HFSEs) replace Mg via the Tschermak reaction, resulting in a variation in $D_{\mathrm{HFSE}}^{\mathrm{Cpx}/\mathrm{melt}}$ along the clinopyroxene compositional trend from Mg-rich augite to ferroan diopside; (5) the crystal and melt compositions determine the compatibility of the rare earth elements (REEs), where the latter particularly substitute Na instead of Ca; (6) the thermodynamic model based on the activity of the Na_0.5(REE)_0.5MgSi₂O₆ component reproduces closely the observed partitioning of the REEs under 1-atm. Esseneite (CaFe³⁺AlSiO₆) component increases with the oxygen fugacity and facilitates the incorporation of REEs, meaning that Ti-Cpx crystallized under oxidizing conditions will be REE-rich (except Eu). ${\left(D_{\mathrm{Ta}}/D_{\mathrm{Nb}}\right)}^{\mathrm{Cpx}/\mathrm{melt}}$ and ${\left(D_{\mathrm{Zr}}/D_{\mathrm{Hf}}\right)}^{\mathrm{Cpx}/\mathrm{melt}}$ remains almost constant, while ${\left(D_U/D_{\mathrm{Th}}\right)}^{\mathrm{Cpx}/\mathrm{melt}}$ correlates positively with the EnFs component, generating fractionation degrees variable of U/Th and constant of Ta/Nb and Zr/Hf during the evolution of crystal-melt systems. Trace element ratios such as V/Sc, V/Yb, and Sr/Y can be used as proxies of oxygen fugacity and pressure in Ocean Island Basalts (OIBs). This new experimental dataset, analyzed under controlled and extensive settings, particularly fO₂, offers a valuable tool for studying magmatic processes in alkali ultrabasic to intermediate systems through Ti-Cpx under crustal and upper mantle conditions.