Liquidus temperatures of komatiites and the effect of cooling rate on element partitioning between olivine and komatiitic melt

Abstract

Archean komatiites are the hottest magmas preserved on Earth and are thus unique probes of its thermal evolution. Estimating their eruption temperatures remains problematic, however, because the uppermost (A1, A2) zones of komatiite flows contain randomly oriented spinifex-textured olivines, indicative of rapid cooling and growth. Fe–Mg partitioning between olivine and assumed komatiitic liquid typically shows departures from equilibrium, extending towards higher \(K_{\text{D}}^{{{\text{Fe}}^{2 + } - {\text{Mg}}}}\). If these higher values are a disequilibrium effect, using them to calculate parental magma composition would lead to errors in estimated liquidus temperatures. In order to investigate this possibility, we have performed experiments on two komatiite compositions, the classic Barberton Aluminium Undepleted Komatiite (AUK) sample 49J (32.2 % MgO) and Munro AUK sample 422/95 (23 % MgO). Isothermal experiments to constrain phase equilibria on 49J at atmospheric pressure, between 1360 and 1600 °C at 1.7 log units below and 1.1 above the fayalite–magnetite–quartz (FMQ) buffer reveal a liquidus temperature (T liq) of 1616 °C, ~40 °C lower than a previous estimate. The \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) ranges between 0.320 and 0.295 at FMQ − 1.7, with a slight negative dependence on temperature. To replicate the conditions that prevailed during the quenching of komatiites in their upper chill zones, experiments with a constant cooling rate at FMQ − 1.7 were performed on 422/95 (T liq = 1450 °C) at 0.5, 1.5, 2.5, 6.5 and 16 °C/min. Olivine morphology changes from euhedral to tabular at low cooling rates, hopper at intermediate, and skeletal and chain structures at high rates. Concurrently, the \(K_{\text{D}}^{{\Sigma {\text{Fe}}{-}{\text{Mg}}}}\) increases monotonically from an equilibrium value of 0.305 to 0.376 at 16 °C/min, reflecting the inability of unwanted cations to diffuse away from the growing olivine. The high \(K_{\text{D}}^{{\varSigma {\text{Fe}}{-}{\text{Mg}}}}\) between olivine and komatiitic liquid caused by rapid cooling accounts for the systematically lower Mg#s in spinifex olivines with respect to their euhedral counterparts in natural komatiites. The maximum MgO content of komatiite liquids in the Archean is revised upwards to 32 wt%, implying temperature excesses in Archean plumes relative to ambient mantle were greater than in their contemporary equivalents.