Toward a coherent model for the melting behavior of the deep Earth’s mantle (original) (raw)

2017, Physics of the Earth and Planetary Interiors

At higher pressures, the slope of the (Fe,Al)-bearing Bg melting curve remains fairly constant up to the CMB pressure of 135 GPa, which contrasts with the flattening reported for MgSiO 3. This makes the different available results to converge to a temperature of 5100-5500 K for the melting of Bg at the CMB. We remind that the melting curve of MgSiO 3-Bg should always plot at higher temperature than that of the (Fe,Al)-bearing composition, due to the incompatible character of Fe and Al. Thus, our experimental determination of the (Fe,Al)-bearing Bg melting curve implies that the melting temperature of MgSiO 3-Bg at very high mantle pressures was underestimated by a couple hundred degrees in some previous studies (de Koker and Stixrude, 2009; Mosenfelder et al., 2009). Thus, for further discussions, we will choose the higher melting curve of MgSiO 3 Bg as determined from ab initio calculations (Stixrude and Karki, 2005). It can be modeled using a modified Simon and Glatzel equation [T=T 0 (P/a + 1) 1 / c ] with parameters T 0 =91 K, a=0.00125 GPa and c=2.83 (Simon and Glatzel, 1929). We note that the liquidus temperature profile reported previously for peridotite (Fiquet et al., 2010) appears superimposed with the melting curve of MgSiO 3-Bg derived from ab initio calculations and shock experiments (Figure 1). This is unexpected because (i) the more complex chemical composition of the Bg phase itself, compared to MgSiO 3 , and (ii) additional Fp and CaSiO 3-perovskite (CaPv) phases should both lower the peridotite liquidus below the melting temperature of pure MgSiO 3. In their study, (Fiquet et al., 2010) observed large crystals of Bg at the center of the heated spot, where the melting behavior was monitored using in situ X-ray diffraction. Chemical composition of Bg grains measured at different locations on a sample synthesized at ~61 GPa show Fe/(Mg+Fe) and Al/Si ratios of ~2.5% and ~8%, respectively. It is possible that their reported measurements of the peridotite liquidus correspond in fact to the melting curve of these large grains of MgSiO 3-bearing Bg. 3.4. VOLUME OF BRIDGMANITE MELTING