Calculated mineral reactions in the Earth's mantle

Abstract

Thermochemical and compressibility data are used to calculate the approximate pressures and temperatures required for the reaction of clinoenstatite to Mg_2SiO_4 (spinel or olivine) or to periclase and stishovite, the reaction of forsterite to Mg_2SiO_4 (spinel) or periclase and stishovite, and the breakdown of Mg_2SiO_4 (spinel) to periclase and stishovite. The calculated reaction lines for Mg_2SiO_4 (olivine) → 2 MgO + SiO_2 and MgSiO_3 → MgO + SiO_2 are p = 199±17 + 0.021T and p = 182±17 + 0.024T (p in kilobars and T in degrees Celsius). These reaction lines lie at higher pressures than previously estimated. Because of the formation of Mg_2SiO_4 (spinel) these two reactions probably do not take place in the earth's mantle. It appears, rather, that the reactions Mg_2SiO_4 (olivine) → Mg_2SiO_4 (spinel), Mg_2SiO_4 (spinel) → 2 MgO + SiO_2, and 2 MgSiO_3 → Mg_2SiO_4 (spinel) + SiO_2 take place along p = 110±11 + 0.024T, p = 321±38 − 0.013T, and p = 133±19 + 0.039 T, respectively.