Microstructure and Mechanism of Quartz Exsolution in Clinopyroxene

Exsolution lamellae-bearing minerals preserve information on the physicochemical conditions of the precursor homogeneous host and are helpful in understanding the subduction depth as well as the processes of decompression recorded in the host rocks during exhumation. Oriented silica precipitates in clinopyroxene have been reported widely in eclogite and garnet pyroxenite from high pressure and ultrahigh pressure metamorphic terranes around the world.Most of such silica precipitates are identified as α-quartz which in part coexist with hydrous minerals such as calcic amphibole.Such oriented precipitates are elongated parallel to the c-axis of host clinopyroxene, while the long axes of quartz being either c[0001]or a[1120]. Electron backscatter diffraction (EBSD) analyses demonstrate that the majority (96%) of quartz precipitates have topotactic relationships with their host clinopyroxenes.Three types of crystallographic topotactic relationships have been identified between quartz and host clinopyroxene: (1) 50% quartz precipitates share the same orientation for the c-axes with [0001]_Qz//[001]_Cpx; (2) 35% quartz precipitates share the same orientation for the a-axes with [1120]_Qz//[001]_Cpx; and (3) 11% quartz precipitates share the same orientation for the s-planes with (1121)_Qz//(100)_Cpx.Other quartz axes and planes disperse in large or small girdles around the shared axes or planes.Calcic amphibole precipitates also have a strong crystallographic relationship with host clinopyroxene, i.e., (100)_Amp//(100)_Cpx, [010]_Amp//[010]_Cpx, [001]_Amp//[001]_Cpx, [100]_Amp∧[100]_Cpx≈32°. The results provide quantitative microstructural evidence supporting an exsolution origin for oriented quartz needles/rods in clinopyroxene and demonstrate that the exsolution of quartz from clinopyroxene occurred within the stability field of α-quartz rather than coesite.Integrated analyses of published high pressure and high temperature experiments show that the cation vacancy and Ca-Eskola (CaEs) component in clinopyroxene are affected by bulk chemistry, pressure and temperature. The solubility of SiO₂ in clinopyroxene are sensitive to bulk chemistry, and the CaEs content in clinopyroxene at high pressure conditions is buffered by free silica phase and kyanite. The CaEs contents in clinopyroxene depend strongly on pressure, which shows rapid increase with pressure up to 6 GPa and then decrease with pressure. By contrast, the CaEs contents in clinopyroxene decrease slightly with temperature which indicates that the effect of temperature is relatively week.On the basis of the above integrated analyses on high pressure experiments and observations on mineral association and microstructural results from natural samples, we suggest that the formation mechanism of the oreiented silica in clinopyroxene is more complicated than we might initially assume. The oriented precipitates of α-quartz and calcic amphiboles in host clinopyroxene are probably formed during multi-stage mechanism involving exsolution, diffusion and exchange of multiple substances, nucleation and growth, recrystallization and some retrograded reactions, which are probably promoted by supercritical fluid or partial melting during exhumation. This study suggests that the texture of oriented quartz precipitates in clinopyroxene is neither necessary nor sufficient for UHP rocks, i. e., it cannot be used as an indisputable UHP-indicator.