Melting of enstatite (MgSiO₃) from 10 to 16.5 GPa and the forsterite (Mg₂SiO₄)‐majorite (MgSiO₃) eutectic at 16.5 GPa: Implications for the origin of the mantle

A split sphere multianvil apparatus (USSA 2000) has been used to determine the melting curve of enstatite (MgSiO 3 ) from 10 to 16.5 GPa and the forsterite (Mg 2 SiO 4 )‐majorite (MgSiO 3 ) eutectic at 16.5 GPa. Use of a new capsule material, rhenium, allowed long runs of up to 2 hours at temperatures up to 2380°C with no observed contamination of the sample and with stable temperature control. Melting temperatures of enstatite are up to 160°C higher than those of Kato and Kumazawa (1986a). Two invariant points have been located, one for orthoenstatite+clinoenstatite+liquid at 2230°C, 11.9 GPa, and one for clinoenstatite+majorite+liquid at 2370°C, 16.4 GPa. Forsterite melts incongruently at 2350°C, 16.5 GPa to liquid and AnhB (Mg 14 Si 5 O 24 ). In the system Mg 2 SiO 4 ‐MgSiO 3 at 16.5 GPa, the forsterite‐majorite eutectic has been located at 34 wt % Mg 2 SiO 4 , a composition similar to that determined by Kato and Kumazawa (1986a) at 20 GPa but much lower in Mg 2 SiO 4 than several previous estimates based on melting experiments on mantle peridotite compositions. This forsterite‐poor location for the eutectic is in apparent conflict with the suggestion that the peridotitic upper mantle is a crystallized eutectic‐like liquid formed at about 16 GPa.