Partitioning of manganese between forsterite and silicate liquid

Abstract

Partition coefficients for Mn between forsterite and liquid in the system MgO-CaO-Na 2O-Al 2O 3-SiO 2 (+ about 0.2% Mn) were measured by electron microprobe for a variety of melt compositions over the temperature range 1250–1450°C at one atm pressure. The forsterite-liquid partition coefficient of Mn (mole ratio, MnO in Fo/MnO in liquid, designated D mn fo−Liq) depends on liquid composition as well as temperature: at 1350°C, D Mn Fo−Liqranges from 0.60 (basic melt, SiO 2 = 47wt%) to 1.24 (acidic melt, SiO 2 = 65wt%). At lower temperatures, the partition coefficient is more strongly dependent on melt composition. The effects of melt composition and temperature on D Mn fo−Liq can be separately evaluated by use of the Si:O atomic ratio of the melts. A plot of D mn Fo−Liq measured at various temperatures vs melt Si:O for numerous liquid compositions reveals discrete, constant-temperature curves that are not well defined by plotting D Mn Fo−Liq against other melt composition parameters such as melt basicity or MgO content. For constant Si:O in the melt, In D Mn Fo−Liq vs reciprocal absolute temperature is linear; however, the slope of the plot becomes more positive for higher values of Si:O, indicating a higher energy state for Mn 2+ ions in acidic melts than in basic melts. Comparison of Mn partitioning data for the iron-free system used in this study with data of other workers on iron-bearing compositions suggests that the effect of iron on Mn partitioning between olivine and melt is small over the range of basalt liquidus temperatures.