Zinc partitioning between mantle minerals and basaltic melts: Application to revisit the Zn/FeT redox proxy

Abstract

Zn/Fe^T (Fe^T=Fe²⁺ + Fe³⁺) ratios in primitive melts have been proposed as a redox proxy to assess the redox states of the upper mantle. However, to effectively use the melt Zn/Fe^T ratio as a redox proxy, it is necessary to compare variations of melt Zn/Fe^T ratios induced by changes in oxygen fugacity (fO₂) with variations due to changes in Zn-Fe contents and mineralogy of the sources. Here we show that the melt Zn/Fe^T ratio variation caused by fO₂ change can be expressed as Δ(Zn_m/${\text{Fe}}_{\text{m}}^{\text{T}}$) = (Zn_per/${\text{Fe}}_{\text{per}}^{\text{T}}$)* $\Delta ({Fe}_m^{2+}/{Fe}_m^T)$/(${\text{D}}_{\text{Zn}}^{\text{per/m}}$/${\text{D}}_{{\text{Fe}}^{\text{2+}}}^{\text{per/m}}$). Zn/Fe^T ratios in most arc and MORB peridotites (Zn_per/${\text{Fe}}_{\text{per}}^{\text{T}}$) are 9.0 ± 1.0*10⁻⁴, and melt Fe²⁺/Fe^T ratio variation resulted from fO₂ change [$\Delta ({Fe}_m^{2+}/{Fe}_m^T)$] can be easily obtained by the existing model. Hence, if the Zn and Fe²⁺ partition coefficients (${\text{D}}_{\text{Zn}}^{\text{per/m}}$ and ${\text{D}}_{{\text{Fe}}^{\text{2+}}}^{\text{per/m}}$) between melt and peridotite are known, the melt Zn/Fe^T ratio variation resulting from fO₂ change can be estimated. In this study, we determined D_Zn between olivine, orthopyroxene, clinopyroxene and basaltic melts at 0.75–2.5 GPa and 1250–1450 ℃. Our data show that melt composition (expressed as MgO content) dominantly controls the mineral-melt Zn partitioning under peridotite melting conditions. These data, along with published mineral-melt ${\text{D}}_{{\text{Fe}}^{\text{2+}}}$ data, enable us to calculate appropriate ${\text{D}}_{\text{Zn}}^{\text{per/m}}$/${\text{D}}_{{\text{Fe}}^{\text{2+}}}^{\text{per/m}}$, which is 0.78 ± 0.02 under arc and MORB spinel peridotite melting conditions. Based on these parameters, the calculated average melt Zn/Fe^T ratio variation caused by per log unity fO₂ change in typical fO₂ span of arc and MORB mantles is only ∼ 0.69 ± 0.20*10⁻⁴. Alternatively, melt Zn/Fe^T ratio variations caused by changes in Zn-Fe contents and mineralogy of peridotites are ±1.10*10⁻⁴. These findings indicate that melt Zn/Fe^T ratio variation induced by one log units fO₂ change is on the same order of magnitude as those caused by changes in Zn-Fe contents and mineralogy of peridotites. Therefore, unless the precise chemical and mineralogical compositions of the mantle sources can be determined independently, the melt Zn/Fe^T redox proxy is unsuitable for tracing the mantle fO₂. Our study, combined with recent works on melt Cu and V/Sc redox proxies, suggests that these redox proxies, previously considered as the strongest evidence for a reduced arc mantle, might not support the idea of a reduced arc mantle.