Melting of a chromite-bearing harzburgite and generation of boninitic melts at low pressures under controlled oxygen fugacity

Abstract

Compositions of melts and coexisting olivine, chromite and Ca-poor pyroxene have been determined in the temperature range 1242 °–1405 °C at 1 atm and at 10 −10–10 −5 atm oxygen fugacity (fO 2). Experiments were also conducted at 5 kbar to examine the effect of pressure on the composition of the melts. Starting materials are mixtures of olivine, enstatite, diopside and chromite separated from peridotites and a chromitite. The compositions of melts coexisting with olivine, Ca-poor pyroxene and chromite are relatively silica- and MgO-rich (SiO 2, 53–57; MgO, 11–20 wt.%) and are similar to those of some boninites. Melts formed at 5 kbar from the same starting material are also boninitic, although the silica contents are slightly lower than those formed at 1 atm. The effect of fO 2 on the composition of melts is small; the total iron content in the melts slightly increases with increasing fO 2. The Cr 2O 3 contents in the melts saturated with chromite range from 0.34–1.82 wt.%. The experimental results suggest that boni nitic magmas can be formed even under anhydrous conditions, if magmas are equilibrated with harzburgite at shallow levels. Some boninitic lavas reported in ophiolite complexes may have been formed under such conditions. The composition of chromite coexisting with olivine and Ca-poor pyroxene becomes more iron-rich with increasing fO 2. Magnetite component in chromite increases from 0.4–2.2 mole% at 10 −10 atm fO 2 to 8–10% at 10 −5 atm. fO 2 at temperatures between 1240 and 1350 °C. The Cr (Cr + Al) ratio of chromite varies from 0.52 at about 1250 °C to 0.79 at about 1400 °C. The partition coefficient for the Mg-Fe +2 distribution between coexisting chromite and olivine [( Fe 2+ Mg ) chr ( Fe 2+ Mg ) ol] has a range of 3.25–4.03 in the temperature range of the present experiments and varies as a function of temperature and Cr (Cr + Al) ratio of chromite. The isotherms for 1250 °, 1300 °, 1350 ° and 1400 °C are drawn from the present experimental results. These results combined with those of the previous experiments are applicable to the estimation of temperature and oxygen fugacity of the crystallization of chromite in mafic magmas.