Pressure‐induced incipient amorphization of α‐quartz and transition to coesite in an eclogite from Antarctica: a first record and some consequences

摘要: Monocrystalline quartz inclusions in garnet and omphacite from various eclogite samples the Lanterman Range (Northern Victoria Land, Antarctica) have been investigated by cathodoluminescence (CL), Raman spectroscopy imaging, situ X-ray (XR) microdiffraction using synchrotron. A few inclusions, with a clear-to-opalescent lustre, show ‘anomalous’ spectra characterized weak α-quartz modes, broadening of main peak at 465 cm−1, additional vibrations 480–485, 520–523 608 cm−1. CL imaging indicate that this ‘anomalous’α-quartz occurs as relicts within ordinary α-quartz, it was preserved internal parts small inclusions. XR diffraction circular patterns display irregular broad spots, some which an anomalous d-spacing tightening ∼2%. They also very weak, hazy clouds compatible coesite but not α-quartz. spectrometry characterize anomalies respect to (i) pressure-induced disordering incipient amorphization, mainly revealed 480–485 608-cm−1 bands, together (ii) lattice densification, evidenced tightening; (iii) cryptic development coesite, 520–523 cm−1 being (iv) Brazil micro-twinning. This represents first example amorphization metastable phase crustal rock. issue is really surprising because observed impactites known occur between 15 32 GPa during ultrahigh-pressure (UHP) experiments room temperature, principle irrelevant under normal geological P–T conditions. shock (due seism?) or local overpressure inclusion scale expansion mismatch its host mineral) seem only mechanisms can produce such rocks. discovery throws new light on modality quartz-coesite transition pressure regimes (non-lithostatic v. lithostatic) high-pressure/UHP metamorphism. In particular, could favour transition, allow growth already experimentally observed.