HRTEM CHARACTERIZATION OF SILICEOUS PHASES IN UHP CLINOPYROXENE FROM THE BOHEMIAN MASSIF-CLUES TO ECLOGITE EXHUMATION PATHWAYS AND WATER CONTENT IN MANTLE-DERIVED PYROXENE

This study utilizes high resolution transmission electron microscopy (HRTEM) to characterize the composition and crystallography of micro- and nano-phases of SiO₂ and SiO₂-rich minerals previously thought to be quartz from UHP Bohemian Massif Nové Dvory eclogites (studies show peak metamorphic P-T of 34-43 kbar and 840-950°C). They are found isolated in cores of UHP clinopyroxenes (cpx) known to be anhydrous, occurring with previously-unseen hydrous minerals. The cpx likely contains a small amount of the Ca-Eskola endmember (Ca_0.5□_0.5AlSi₂O₆)—supersilicic, non-stoichiometric, vacancy-containing, and reported to be pressure sensitive and highly unstable at lower pressure. Vacancies allow substitution of excess silica in its structure at high P and T. Precise P-T conditions of the decomposition reaction (2Ca_0.5□_0.5AlSi₂O₆ → CaAl₂SiO₆ + 3SiO₂) proposed as the siliceous exsolution mechanism in this pyroxene is still debated. Characterization of these minerals will help constrain this reaction and retrograde P-T conditions of the eclogite; it may also show solubility of SiO₂ in cpx as a viable geobarometer for UHP metamorphic rocks. In addition, mantle-derived cpx has been shown to contain large amounts of water—some studies show up to 3000 ppm weight H₂O in omphacite and amount may be heavily dependent upon geochemical environment; the observed phenomenon may indicate important reservoirs for water in mantle pyroxene.

Our HRTEM micrographs, electron diffraction patterns, and electron dispersive spectrometry show nano-scale amorphous silica-rich phases and crystallographically-oriented α-cristobalite inside cpx, with (111) of cristobalite oriented approximately parallel to (010) of cpx. We also find an assemblage of hydrous minerals—epitaxial high Mg/Al alkali amphiboles and Na-Ca trioctahedral micas in contact with SiO₂. Presence of α-cristobalite and hydrous minerals is unexplained at documented peak metamorphic P-T from current bulk phase diagrams, but numerous studies have found experimental evidence for changes in phase stability fields with small particle size. Local micro-environments and particle size-determined surface energy may shift phase stability inside the enclosing crystal.