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

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