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

Abstract

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 ÔanomalousÕ Raman spectra characterized by weak a-quartz modes, the broadening of the main a-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 ÔanomalousÕ a-quartz occurs as relicts within ordinary a-quartz, and that it was preserved in the internal parts of small quartz inclusions. XR diffraction circular patterns display irregular and broad a-quartz spots, some of which show an anomalous d-spacing tightening of 2%. They also show some very weak, hazy clouds that have d-spacing compatible with coesite but not with a-quartz. Raman spectrometry and XR microdiffraction characterize the anomalies with respect to a-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 ÔanomalousÕ a-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 a-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 highpressure ⁄ 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.

Effect of grain size distribution on Raman analyses and the consequences for in situ planetary missions

“…However, the Raman spectrum of coesite is normally characterised by additional peaks at 204, 270, 355 and 426 cm −1 , which were not present in our sample . A similar situation was previously noted in the quartz spectrum in an eclogite from Antartica . In the latter case, it was proposed that the coesite crystalline structure was attained at the nanometric scale within anomalous α ‐quartz (called cryptic coesite), which developed under high pressure.…”

Indirect measurement of stress distribution in quartz particles embedded in a glass matrix by using confocal Raman microscopy

“…The two possible directions of tetrahedral tilt are equally likely, and they give rise to two distinct twin orientations, commonly denoted as Dauphin twins [37]. It was previously proposed that the widening of A 1 Raman mode is due to the appearance non-visible E Raman mode [38]. But in Figure 5a it is observed that the intensity for E Raman modes remains constant and the intensity of the signal at 468 cm -1 varies.…”

Significance of an amorphous SiO2 phase in a pseudomorph after coesite enclosed in garnet from ultrahigh‐pressure eclogite, Su–Lu Belt, eastern China

“…The Raman band of 520–523 cm −1 corresponds to the four‐membered rings of a corner‐sharing SiO 4 tetrahedron of the coesite (Kingma & Hemley, ). The incipient APSI phase characteristically shows high intensity ratios I 265 / I 465 and I 402 / I 465 , where I ν indicates the intensity of the Raman band ν (Godard, Frezzotti, Palmeri, & Smith, ; Palmeri et al., ). In contrast to the incipient APSI phase, the APSI phase of the Yangzhuang sample described in this study does not show any Raman bands assigned to those of α‐quartz and coesite (spots 1–2 in Figure ).…”

“…The weak band at 602 cm −1 can be assigned to the symmetric stretching vibration of Si‐O in a planar three‐membered SiO 4 ring (D 2 band) of amorphous SiO 2 glass (Awazu & Kawazoe, ; Ivanda et al., ). Raman spectra, TEM observation, and SAED analysis (Figure d,e) suggest that the APSI phase in the sample is almost entirely in an amorphous state, unlike the incipient APSI phase reported from the Antarctica (Palmeri et al., ) and the western Alps (Frezzotti et al., ) samples.…”

“…Our interpretation might suggest that the APSI phase did not form due to the pressure‐induced amorphization process proposed by the studies on the Antarctica (Palmeri et al., ) and western Alps (Frezzotti et al., ) samples. The Yangzhuang sample suggests that metamorphic fluid might have strongly controlled the transition of coesite and the formation of various types of polyphase pseudomorphs during a later stage of exhumation.…”