Crystal-melt equilibria involving potassium-bearing clinopyroxene as indicator of mantle-derived ultrahigh-potassic liquids: an analytical review

Crystal–liquid equilibria, including phase relationships of minerals with silicate and/or carbonate melts, are reviewed in order to understand the occurrence of clinopyroxene with up to 2 wt.% K2O (KCpx). This mineral occurs as inclusions in diamond from kimberlite pipes and in garnet from garnet–clinopyroxene potassium-poor silicate rocks intercalated with diamondiferous silicate–carbonate rocks of the Kokchetav Complex, northern Kazakhstan. The analysis of the available experimental data allowed estimation of the effect of P, T and compositional parameters on the equilibrium of KAlSi2O6 (in Cpx)=1/4K4Si2O6+3/4Al4/3Si2O6 (in melt) in various silicate systems. A strong dependence of the K2O partition coefficient both on pressure and SiO2 and Al2O3 contents in the melt was identified and thermodynamically described. The resulting thermodynamic equation allows the calculation of pressure of the KCpx formation within interval 50–100 kbar for known melt compositions. A model for the formation of KCpx in deep-mantle potassium-rich carbonate–silicate magmas was derived using the available experimental and petrologic data. The formation of KCpx from any potassium-poor (K2O70 kbar. This chemical zoning of potassium-bearing clinopyroxene suggests its crystallization together with feldspar from silicate melt during rapid ascent of melt toward the Earthʹs surface from depth about 200 km. Since Tschermak-type substitution has not been observed in either pyroxene, we suggested that feldspar resulted from the peritectic reaction KAlSi2O6+[SiO2]L/fl=KAlSi3O8 at relatively shallow mantle levels. A hypothetical phase diagram for the system Ca(Mg,Fe)Si2O6–KAlSi2O6 is presented.