Temperature and pressure effects on the partitioning of V and Sc between clinopyroxene and silicate melt: Implications for mantle oxygen fugacity

The partition coefficients of V and Sc between clinopyroxene and silicate melt (D_V^Cpx/SM and D_Sc^Cpx/SM) have been determined experimentally at 1200–1400 °C and 0.8–2.3 GPa, using a hornblende- and clinopyroxene-rich mantle rock in graphite-lined Pt₉₅Rh₀₅ capsules. The results show that the D_V^Cpx/SM and D_Sc^Cpx/SM values decrease from 3.8 to 2.3 and from 2.6 to 1.1, respectively, as the experimental temperature and pressure vary from 1200 °C and 0.8 GPa to 1400 °C and 2.3 GPa. The presence of water in silicate melts may also reduce D_V^Cpx/SM and D_Sc^Cpx/SM. These results imply that the effects of temperature, pressure, and melt water content on D_V^Cpx/SM should be considered when using V systematics in cratonic mantle peridotites to constrain cratonic mantle oxygen fugacity (f_O2). However, although the dominant V in the present silicate melt is mixed V³⁺ and V⁴⁺, the D_V^Cpx/SM/D_Sc^Cpx/SM together with literature data obtained at similar f_O2 shows a nearly constant value of 1.68 ± 0.26, regardless of temperature, pressure, melt composition, and melt water content, indicating that these factors cannot cause fractionation of Sc³⁺ from mixed V³⁺ and V⁴⁺ in mantle melts through clinopyroxene/silicate melt partitioning. Therefore, in combination with V/Sc systematics in primitive MORBs and arc basalts, using D_V^Cpx/SM and D_Sc^Cpx/SM obtained at 1 bar and dry conditions should be valid to constrain mantle f_O2, except for the case that the D^Cpx/SM for Sc³⁺ can be demonstrated to be fractionated from the D^Cpx/SM for mixed V⁴⁺ and V⁵⁺, which are present in oxidized basalts.