Ca-Eskola component in clinopyroxene: Experimental studies at high pressures and high temperatures in multianvil apparatus

In order to investigate the pressure implications of silica exsolution from clinopyroxene in ultrahigh-pressure metamorphic rocks, we have measured systematically the values of the Calcium-Eskola (Ca-Esk) component of clinopyroxenes over a considerable range of pressure and temperature. A series of anhydrous experiments using powdered glass of composition (wt.%): SiO2 = 59.29, Al2O3 = 23.12, MgO = 5.44, CaO = 10.37, and Na2O = 1.78 were conducted in a Walker-style multianvil apparatus in the P–T range of 6–12 GPa and 900–1200 °C. We find that all clinopyroxenes synthesized in equilibrium with garnet, kyanite and SiO2 (coesite or stishovite) are cation deficient. The highest value of the Ca-Esk component (32–38 mol%) is reached at ~ 6 GPa followed by slight decrease beginning at 8 GPa as the pyroxene progressively dissolves into garnet and more rapid decrease at 10–12 GPa when stishovite becomes stable in the assemblage. Comparison of our data with those available in the literature, which, all together, covers a wider range of P and T, shows that the Ca-Esk activity depends strongly on bulk chemistry, especially the silica content. In general, integrated analyses of published experiments on similar compositions show that the Ca-Esk component rises rapidly to its highest values from 1 to 4–6 GPa, then slightly decreases to ~ 9 GPa followed by remarkable decrease to 15 GPa, culminating in replacement of pyroxene by garnet in equilibrium with stishovite. We conclude that the SiO2 exsolution lamellae observed in clinopyroxenes from many ultrahigh-pressure terranes can be explained by destabilization of the Ca-Esk component during decompression. However, because silica solubility in clinopyroxene reaches its maximum at 6–8 GPa, the practical upper pressure limit that can be inferred from this system is 200–250 km.

Research highlights
► Ca-Esk in CPx is maximal at 4–8 GPa decreasing at 15 GPa when Grt+Stish are stable.
► Ca-Esk instability in CPx during uplift leads to SiO2 exsolution lamellae formation.
► SiO2 exsolution lamellae in CPx indicate rocks exhumation from depths ~200–250 km.