Present-day ultrahigh-pressure conditions of coesite inclusions in zircon and garnet: Evidence from laser Raman microspectroscopy

Coesite (the dense, high-pressure polymorph of quartz) occurs as inclusions in mechanically strong minerals in deeply subducted, metamorphosed crustal rocks in a number of Eurasian collisional orogens. it is the primary indic ator mineral of ultrahigh-pressure (P) metamorphism. Whereas some coesite i nclusions are untransformed, most exhibit partial transformation to palisad e quartz and a concomitant increase in volume (resulting in rupture and rad ial fracturing of the host grain). Coesite can be identified by its diagnos tic Raman spectrum; the strongest band (at atmospheric pressure, room tempe rature) is at 521 cm(-1). Laser Raman microspectroscopic analyses of coesit e inclusions within garnet and zircon in ultrahigh-P metamorphic rocks from Kazakhstan, Indonesia, and China reveal consistent differences in the Rama n spectra of (1) partially transformed coesite + quartz (main Raman band at 521 cm(-1)) and (2) untransformed monomineralic coesite grains (main band at 525-526 cm(-1)). Applying the room-temperature calibration of pressure d ependence of the coesite Raman spectrum, we conclude that the latter coesit e inclusions are subject to a remarkable pressure differential of 19-23 kba r with the host grains, and are still undergoing pressure-temperature condi tions on or close to the quartz-coesite equilibrium boundary.