Ferropericlase crystallization under upper mantle conditions

Ferropericlase is a common lower mantle phase that can occur as inclusions in “superdeep” diamonds. However, it has been proposed that some of these inclusions could be formed in the upper mantle (Brey et al. 2004). To explore this hypothesis, two types of experiments on ferropericlase crystallization were conducted at 5–12 GPa and 1500–1700 °C: (1) equilibrium experiments with Ca–Mg–Fe carbonate + MgO + olivine mixtures and (2) “sandwich” experiments containing a layer of metallic iron overlain by olivine, with a carbonate layer on top. The equilibrium experiments produced a series of carbonate–silicate melts saturated in olivine and ferropericlase. These melts contain 2–12 wt% SiO2, which correlates negatively with CaO and CO2 contents and positively with temperature. A comparison of the composition of the carbonate–silicate melts with literature data for melts saturated in olivine and low-Ca pyroxene indicate that melts saturated in ferropericlase cannot be produced by crystallization of melts derived by melting carbonated harzburgite or lherzolite at upper mantle conditions. On the other hand, the sandwich experiments reveal that ferropericlase and diamond (or metastable graphite) can crystallize simultaneously during the reduction of carbonate–silicate melt. The resulting ferropericlase crystallizing in equilibrium with olivine will be richer in Fe compared with lower mantle ferropericlase in equilibrium with bridgmanite. Thus, the considerable variation in Mg# values observed for some suites of ferropericlase inclusions in diamond could in part be attributable to ferropericlase formation in the upper mantle.