Effects of redox state on chromium isotope fractionation in the mantle: Insights from spinel peridotite xenoliths

Abstract

Chromium (Cr) isotope fractionation in the mantle is generally interpreted as induced by mantle processes like partial melting and metasomatism, but the influence of oxygen fugacity remains poorly understood. Here, we report whole-rock and minerals elemental concentrations, minerals Fe³⁺/ΣFe ratios and whole-rock Cr isotope compositions for Late Cretaceous-Cenozoic basalt-borne spinel peridotite xenoliths from the North China Craton (NCC). The results show that the northwest and eastern NCC is underlain by a newly fertile lithospheric mantle (olivine Mg# = 0.89–0.91), while the central NCC retains shallow relics of ancient refractory mantle (olivine Mg# = ∼0.92). The two mantle types show some overlapping in the range of oxygen fugacity (fertile: FMQ-1.61 to FMQ-0.19; refractory: FMQ-0.92 to FMQ + 1.58), but the fertile one (mean FMQ-0.75) tends to be more reduced than the refractory one (mean FMQ + 0.20). Furthermore, the former shows a relatively wide range of Cr isotopic values from −0.27 ± 0.03 ‰ to −0.03 ± 0.04 ‰ than the latter (ẟ⁵³Cr = −0.20 ± 0.06 ‰ to −0.13 ± 0.06 ‰), but the ẟ⁵³Cr values of these peridotites are not correlated with the indices for partial melting (e.g., whole-rock Mg# and Al₂O₃ contents) and metasomatism (e.g., (La/Yb)_N). These lines of observation jointly suggest that the Cr isotope variations may be influenced by the redox state of the lithospheric mantle, and isotope fractionation tends to be restricted under oxidized conditions.