Iron in monticellite as an oxygen barometer for kimberlite magmas

Abstract

Monticellite is a common magmatic mineral in the groundmass of kimberlites. A new oxygen barometer for kimberlite magmas is calibrated based on the Fe content of monticellite, CaMgSiO₄, in equilibrium with kimberlite liquids in experiments at 100 kPa from 1,230 to 1,350°C and at logfO₂ from NNO-4.1 to NNO+5.3 (where NNO is the nickel–nickel oxide buffer). The XFe_Mtc/XFe_liq was found to decrease with increasing fO₂, consistent with only Fe²⁺ entering the monticellite structure. Although the XFe-in-monticellite varies with temperature and composition, these dependencies are small compared to that with fO₂. The experimental data were fitted by weighted least square regression to the following relationship: $\Updelta {\text{NNO}} = \frac{{\left\{ {\log \left[ {0.858( \pm 0.021)\frac{{X_{\text{Fe}}^{\text{Liq}} }}{{X_{\text{Fe}}^{\text{Mtc}} }}} \right] - 0.139( \pm 0.022)} \right\}}}{0.193( \pm 0.004)}$ where ΔNNO is the fO₂ relative to that of the Nickel-bunsenite (NNO) buffer and XFe_liq/XFe_Mtc is the ratio of mole fraction of Fe in liquid and Fe-in-monticellite (uncertainties at 2σ). The application of this oxygen barometer to natural kimberlites from both the literature and our own investigations, assuming the bulk rock FeO is that of their liquid FeO, revealed a range in fO₂ from NNO-3.5 to NNO+1.7. A range of Mg/(Mg + Fe²⁺) (Mg#) for kimberlite melts of 0.46–0.88 was derived from the application of the experimentally determined monticellite-liquid Kd Fe²⁺–Mg to natural monticellites. The range in Mg# is broader and less ultramafic than previous estimates of kimberlites, suggesting an evolution under a wide range of petrologic conditions.