Phase equilibria and liquid lines of descent in alkali basalts at one atmosphere and the role of oxygen fugacity

Abstract

This study presents new experimental data on the effect of oxygen fugacity (fO₂) and bulk composition on phase equilibria and liquid lines of descent in alkali basalts. Crystallisation experiments were performed at one atmosphere, in the temperature range 1220–1050 °C and at fO₂ conditions corresponding to fayalite–magnetite–quartz (FMQ) equilibrium and the following log-unit deviations from this equilibrium: FMQ-1, FMQ + 1.5 and FMQ + 3. Experiments were grouped into two series, those on primitive compositions (9.9–11.9 wt% MgO; total alkali 3.1–5.3 wt%), and those on evolved compositions (5.1–5.8 wt% MgO; total alkali 5.7–8.6 wt%). Experiments on primitive compositions produce assemblages of olivine, clinopyroxene and spinel, with plagioclase present in less silica-undersaturated compositions. Experiments on evolved compositions produce similar assemblages, though olivine is often absent at higher fO₂. Whitlockite is present at low temperature, plus occasional Fe-Ti oxides. Nepheline is only observed in the most silica-undersaturated composition at low temperature. Mineral compositions and stability are strongly controlled by bulk composition and redox conditions. High MgO bulk contents favour the early appearance of olivine, while the appearance of clinopyroxene is dictated by both MgO and CaO contents. Increasing silica-undersaturation produces assemblages dominated by clinopyroxene and a high melt fraction at any given temperature, promoting alkali enrichment during melt evolution. Oxidising conditions enhance the crystallisation of spinel and other Fe-Ti oxides, in which both silica and alkalis are incompatible, through increased melt Fe³⁺/ΣFe. This promotes more modest enrichments in Na₂O and K₂O with respect to SiO₂ as crystallisation proceeds, with the effect strongest where bulk concentrations of FeO_tot are high and P₂O₅ are low. It follows that fO₂ exerts a stronger control over the liquid lines of descent of more primitive liquids than evolved ones. We combine our results with those from the literature to calibrate new models for both \({K}_{{{\rm D}}_{{{\rm Fe}}^{2+}-\text{Mg}}}^{{\rm Ol}-{\rm Melt}}\) and \({K}_{{{\rm D}}_{{{\rm Fe}}^{T}-\text{Mg}}}^{{\rm Ol}-{\rm Melt}}\) in alkaline systems, with the latter formulated as:

$${K}_{{{\rm D}}_{{{\rm Fe}}^{T}-\text{Mg}}}^{{\rm Ol}-{\rm Melt}}(\pm 0.012, \text{a.a.d.})=0.442 \left(\pm 0.029\right)+0.287 \left(\pm 0.035\right)\left[{\text{X}}_{{\text{SiO}}_{2}}\right]-0.442\left(\pm 0.027\right)\left[\frac{{\text{X}}_{{\text{Na}}_{2}\text{O}}+{\text{X}}_{{{\rm K}}_{2}\text{O}}}{{\text{X}}_{{\text{SiO}}_{2}}}\right]- 0.168\left(\pm 0.022\right)\left[{\text{X}}_{\text{Fo}}\right]-0.138(\pm 0.008)\frac{1}{(1-\left[\frac{{\text{Fe}}^{3+}}{\Sigma \text{Fe}}\right])}$$

where \({\text{X}}_{i}\) is the mole fraction of \(i\) in the melt, \({\text{X}}_{\text{Fo}}\) is molar MgO/(MgO + FeO_tot) of olivine, Fe³⁺/ΣFe represents Fe valence in the melt and a.a.d. is the average absolute deviation. We also present a simple model for estimating \({K}_{{{\rm D}}_{{{\rm Fe}}^{T}-\text{Mg}}}^{\text{Cpx}-\text{Melt}}\):

$${K}_{{{\rm D}}_{{{\rm Fe}}^{T}-\text{Mg}}}^{\text{Cpx}-\text{Melt}}\left(\pm 0.028,\text{a.a.d}.\right)=-1.00 \left(\pm 0.064\right)+2.129 \left(\pm 0.063\right)\left[\frac{\text{T }\left(^\circ \text{C}\right)}{1000}\right]- 1.675\left(\pm 0.021\right)\left[\text{Mg}\#\right] +0.144(\pm 0.006)\frac{1}{1-\left[\frac{{\text{Fe}}^{3+}}{{\Sigma \text{Fe}}}\right]}$$

where Mg# is molar MgO/(MgO + FeO_tot) of clinopyroxene and Fe³⁺/ΣFe represents Fe valence in the melt. These new formulations allow equilibrium \({K}_{{{\rm D}}_{\text{Fe}-\text{Mg}}}^{\text{Mineral}-\text{Melt}}\) to be estimated more reliably in oxidising, alkaline systems where Fe³⁺/ΣFe is high.