Origin of High-Pressure Disordered Metastable Phases (Lonsdaleite and Incipiently Amorphized Quartz) in Metamorphic Rocks: Geodynamic Shock or Crystal-Scale Overpressure?

摘要: This article is a review on anomalous metastable phases observed in ultrahigh-pressure metamorphic (UHPM) rocks. Quartz incipiently amorphized under pressure and lonsdaleite, hexagonal polytype of diamond, cannot form at equilibrium rocks, as they have no P–T stability field. However, can metastably place coesite respectively, if crystallization the latter hampered by kinetics. Besides impactites shocked meteorites, these two been reported few UHPM Incipiently quartz has evidenced Raman spectrometry X-ray microdiffraction eclogites from Antarctica. could be more common, but it easily escapes notice, being apparently confined to micro-inclusion exhibiting an ordinary appearance. Lonsdaleite dozen among which diamond-bearing gneiss Kokchetav Massif (Kazakhstan), many occurrences are doubtful need re-evaluated. Indeed, lonsdaleite difficult distinguish identified definitively only or electron microdiffraction. UHP (typically, P >12.5 GPa 300 K for quartz) when unfavorable kinetics prevents formation coesite/stishovite respectively. occur either medium high T , during very short event (i.e., shock), longer (inclusion-scale overpressure?) low diffusion hence too slow enabling any solid–solid transitions. The occurrence such supports hypothetical role nonlithostatic metamorphism, least shocks minute scale.