Microstructure and Mechanism of Quartz Exsolution in Clinopyroxene

Xu Haijun; Zhao Sutao; Wu Yun

doi:10.3799/dqkx.2016.080

Abstract：Exsolution lamellae-bearing minerals preserve information on the physicochemical conditions of the precursor homoge-neous 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 pyroxe-nite from high pressure and ultrahigh pressure metamorphic terranes around the world.Most of such silica precipitates are iden-tified asα-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[112-0].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 cli-nopyroxene:(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 [112-0]Qz//[001]Cpx;and (3)11% quartz precipitates share the same orientation for the s-planes with (112-1)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 quantita-tive 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 ofα-quartz rather than coesite.Integrated an-alyses of published high pressure and high temperature experiments show that the cation vacancy and Ca-Eskola (CaEs)compo-nent in clinopyroxene are affected by bulk chemistry,pressure and temperature.The solubility of SiO2 in clinopyroxene are sen-sitive 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 for-mation mechanism of the oreiented silica in clinopyroxene is more complicated than we might initially assume.The oriented pre-cipitates ofα-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 reac-tions,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.