Origin of High-Pressure Disordered Metastable Phases (Lonsdaleite and Incipiently Amorphized Quartz) in Metamorphic Rocks

Raman spectral point analysis and Raman mapping have been applied to study the nature of carbonbearing microinclusions C3-1 and C5-1 within zircon in phengite-kyanite-coesite eclogite sample 62a from the Straumen Eclogite Pod in the Western Gneiss Region (WGR), S.W. Norway. Graphitic carbon is always present with a degree of structural disorder comparable to that found in other ultra high pressure metamorphic (UHPM) terranes; it is compatible with an origin by a partial retrogression of the coexisting diamond due to depressurization by exhumation. Hematite coexists with diamond in C3-1. In both microinclusions, the Raman wavenumber position of the diamond peak varies from point to point, from the ideal value of 1332 cm )1 , frequently down to 1328 cm )1 and sometimes down to 1322 cm )1 in C3-1. The bandwidth also increases proportionally. Some spectra reveal doublets with peaks at 1328 and 1322 cm )1 and even a triplet. These Raman data are discussed in terms of one or other kind of structural disorder of sp 3 -carbon, for example metamictization, overtemperature or overpressure, nitrogen impurities and especially stacking disorder, that is, with the possible existence or coexistence of 3C, 2nH and ⁄ or 3mR polytypes in a mixture in the same monocrystal. The question of a metamorphic or a contamination origin of the diamond and the disordered sp 3 -carbon in slide 62a is evaluated; numerous arguments favour the UHPM origin. These new data contribute to the ongoing debate on the various petrological and geodynamical models (including possible deviatoric stress) for the creation of diamond in UHPM rocks in general. This new locality of diamond at Straumen, the fourth in the WGR but the first in the southwestern part of the UHPM area of the WGR, provides a minimum pressure estimate for UHP metamorphism of 3.5 GPa. Taken together with other published P-T estimates, 3.75 ± 0.75 GPa covers all of the estimates of the minimum pressure for coesite-, diamond-or orthopyroxene-bearing eclogite types in the WGR.

“…Also He et al. (2002) created ‘hexagonal diamond’ from cubic diamond by shear stress from a shock wave (see discussion on shock in Godard et al. , 2011).…”

Section: Discussionmentioning

confidence: 99%

“…The number gives the number of sheets in the unit cell (Bundy & Kasper, 1967; Frondel & Marvin, 1967; Spear et al. , 1990; see more recent references reviewed in Godard et al. , 2011; Smith et al.…”

Section: Discussionmentioning

confidence: 99%

A Raman spectroscopic study of diamond and disordered sp³‐carbon in the coesite‐bearing Straumen Eclogite Pod, Norway

Smith

Godard

2012

“…Furthermore, the spatial extents of up to several metres of the same types of the eclogitized gabbro and the pressure gradient in the wallrock to the eclogite breccia at Yangkou (Fig. c) are at an outcrop scale and are inconsistent with the tectonic overpressure model arguing for grain‐scale pressure variations (Jamieson, ; Godard et al ., ; Tajčmanová et al ., ). The rocks are essentially unfoliated and are thus unlikely related to tectonic overpressure caused by crust‐scale shear deformation (Schmalholz & Podladchikov, ).…”

Section: Discussionmentioning

confidence: 97%

“…In recent years, amorphization of quartz has increasingly been found in high‐ P rocks from Antarctica and the Western Alps (Palmeri et al ., ; Frezzotti et al ., ). Lonsdaleite, a hexagonal disordered diamond synthesized at >13 GPa and >1000 °C and found earlier only in meteorite shocked rocks, has been recently reported to be associated with a fault in the Kokchetav Massif in Kazakhstan (Dubinchuk et al ., ; Godard et al ., ). Amorphization of omphacite in a coesite‐bearing eclogite from the Dabieshan in China was also reported (Su et al ., ).…”

Section: Discussionmentioning

confidence: 97%

Petrological evidence for shock‐induced high‐P metamorphism in a gabbro

Yang

Zhang

Anping

et al. 2016

Microlites (minute spherulitic, dendritic, skeletal, acicular and poikilitic crystals) diagnostic of crystallization in quenched melt or glass in fault rocks have been used to infer fossil earthquakes. High‐P microlites and crystallites are described here in a variably eclogitized gabbro, the wallrock to the coesite‐bearing eclogite breccia at Yangkou in the Chinese Su‐Lu high‐P metamorphic belt. The studied hand specimens are free of discernible shear deformation, although microfractures are not uncommon under the microscope. In the least eclogitized gabbro, the metagabbro, stellate growths of high‐P minerals on the relict igneous minerals are common. Dendritic garnet crystals (<1−5 μm) grew around rutile and/or phengite replacing ilmenite and biotite, respectively. Skeletal garnet also rims broken flakes of igneous biotite and mechanically twinned augite. Radial intergrowths of omphacite and quartz developed around relict igneous orthopyroxene and are rimmed by skeletal or poikilitic garnet where a Ti‐bearing mineral relict is present. Acicular epidote, kyanite and phengite crystallites are randomly distributed in a matrix of Na‐rich plagioclase, forming the pseudomorphs after igneous plagioclase. In the more eclogitized gabbro, the coronitic eclogite located closer to the eclogite breccia, all the igneous minerals broke down into high‐P assemblages. Thick coronas of poikilitic garnet grew between the pseudomorphs after igneous plagioclase and ferromagnesian minerals. The igneous plagioclase is replaced by omphacite crystallites, with minor amounts of phengite and kyanite. Thermodynamic modelling of the plagioclase pseudomorphs shows an increase in P–T in the wallrock from the metagabbro to the coronitic eclogite, and the P–T variation is unrelated to H2O content. The fluid‐poor pressure overstepping scenario is unsupported both by phase diagram modelling and by whole‐rock chemical data, which show that the various types of eclogitized gabbro are all fairly dry. A large pressure difference of >2 GPa between the metagabbro and the coesite‐bearing eclogites ~20 m apart cannot be explained by the subduction hypothesis because this would require a depth difference of >60 km. The microlites and crystallites are evidence for dynamic crystallization due to rapid cooling because constitutional supercooling was unlikely for the plagioclase pseudomorphs. The lack of annealing of the broken biotite and augite overgrown by strain free skeletal garnet is consistent with a transient high‐P–T event at a low ambient temperature (<300 °C), probably in the crust. Therefore, the eclogitization of the wallrock to the eclogite breccia was also coseismic, as proposed earlier for the eclogite facies fault rocks. The outcrop‐scale P–T variation and the transient nature of the high‐P–T event are inconsistent with the other existing tectonic models for high‐P metamorphism. The fact that the less refractory but denser biotite is largely preserved while the more refractory but less dense plagioclase broke down completely into hig...

“…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 ).…”

Section: Discussionmentioning

confidence: 99%

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

Taguchi

Miyake

Enami

et al. 2018

Here, we report the first discovery of an amorphous SiO2 phase (APSI phase) in a pseudomorph after coesite included in garnet from an ultrahigh‐pressure (UHP) eclogite from the Su–Lu metamorphic belt, eastern China. Using transmission electron microscopy, Raman spectroscopy and selected area electron diffraction, we show that the internal structure of the pseudomorph consists of an APSI phase with nano/submicrocrystalline particles of quartz and a polycrystalline K‐bearing fibrous sheet‐silicate phase (KFSS phase). The APSI phase‐bearing aggregates included in the garnet might have formed by reactions involving a supercritical fluid during exhumation by the following processes: (1) the development of radial cracks within the host garnet by the phase transition of coesite to quartz; (2) the decomposition of a part of the pseudomorph following infiltration of supercritical fluid; (3) the precipitation of the KFSS phase from the fluid phase during subsequent exhumation and cooling, which was likely promoted by a change in the metamorphic fluid from supercritical and/or subcritical to aqueous fluid; and (4) the rapid precipitation of the APSI phase under a metastable (non‐equilibrium) state, such as quenching, during a later stage of the exhumation. Whether the APSI phase generally formed during exhumation and survived widely throughout the Su‐Lu terrane is unknown. However, the presence of the APSI phase in a UHP eclogite provides new insight into the geodynamic phenomena occurring at continental collision zones.