Effect of Carbonates on Crystallization and Composition of Potassium-Bearing Clinopyroxene at High Pressures

摘要

In paper devoted to the distribution of potassium between Cpx and carbonate melt, it was stated for the first time that clinopyroxene containing > 1 wt % K_2O may crystallize at 1300-1700℃ and 5-11 GPa from carbonate liquid containing 14-33 wt % K_2O. KCpx with as much as 5.75 wt % K_2O was obtained in experiments as a product of melt crystallization in the Di-Grs-K_2CO_3 and Di-Grs-K_2Mg(CO_3)_2 systems at 7 GPa and 1500-1600℃. Such high K_2O concentrations in Cpx in carbonate-silicate systems suggest a substantial influence of carbonates on incorporation of potassium into clinopyroxenes at high pressures. However, a similar ultrapotassic clinopyroxene (up to 5.6 wt % K_2O) was synthesized later in the Di-Lc system at the same pressure, and this result stimulated a special experimental study focused on the influence of carbonates on crystallization of high-K clinopyroxene. With this fact in mind, we have conducted a series of runs in the Di_(40)Lc_(60)-Dol_(40)Lc_(60) join at 7 GPa. The choice of the system and experimental conditions were dictated by the experimental results in the basic CaMgSi_2O_6-KAlSi_2O_6 join at the same pressure.