Amorphization in natural omphacite and its implications

Su, Wen; Zhang, Ming; Chen, Jing; Bromiley, Geoffrey; Yang, Kai; Redfern, Simon A. T.; Lin, Jian

doi:10.1016/j.jseaes.2010.10.001

Cited by 5 publications

(5 citation statements)

References 67 publications

(69 reference statements)

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“…We confirm that the chemical homogeneity of omphacite is probably due to reaching equilibrium at the peak PT conditions. Microtextural analysis reveals that the colonies of symplectites of the analyzed sample are morphologically similar to those described in previous studies (e.g., Mysen and Griffin, 1973;Boland and van Roermund, 1983;Anderson and Moecher, 2007;Su et al, 2011;Moulas et al, 2015). The symplectite formation is promoted in quartz-bearing eclogites unlike other contexts (quartz-free eclogites) in which this process may require intergranular transport of silica in a fluid phase or oxidation of omphacite (Mysen and Griffin, 1973).…”

Section: Omphacite Morphology and Compositionsupporting

confidence: 82%

“…Matsumoto and Banno (1970) first reported the P2/n space group for omphacite which is now the consensus among X-ray crystallographers (e.g., Cameron and Papike, 1981;Sasaki et al, 1981). Omphacite with space group P2/n is a cation-ordered pyroxene with two distinct M1 and two M2 sites, which can undergo a structural phase transition at high temperatures (Su et al, 2011). At temperatures above 800 °C, omphacite is disordered with respect to its cations in M1 and M2 sites and exhibits space group C2/c.…”

Section: Introductionmentioning

confidence: 99%

“…It is also clear that during exhumation, high-pressure rocks undergo, among other important processes, cooling, decompression and hydration of their main mineral phases, triggering retrogradation to amphibolite, epidote-amphibolite and green schist facies, with a notorious and easily differentiable symplectitic stage. Several authors discuss the occurrence of several stages of development of omphacite (e.g., Buatier et al, 1991;Su et al, 2011;Martin and Duchêne, 2015). Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite (Su et al, 2011).…”

Section: Introductionmentioning

confidence: 99%

“…Several authors discuss the occurrence of several stages of development of omphacite (e.g., Buatier et al, 1991;Su et al, 2011;Martin and Duchêne, 2015). Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite (Su et al, 2011). Previous investigations have already addressed the crystal chemistry of omphacites (e.g., Matsumoto et al, 1975;Rossi et al, 1983), its phase transitions (e.g., Carpenter, 1981;Pavese et al, 2000) and the solid solution properties along the augite-jadeite joint (Ballaran et al, 1998).…”

Section: Introductionmentioning

confidence: 99%

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Crystal-Chemical and Structural Characterization of Omphacite in High-Pressure Eclogites From the Arquía Complex on Southwestern Pijao, Central Cordillera (Colombian Andes)

et al. 2022

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Omphacite found in eclogites from the Arquía Complex (Colombia) was characterized using scanning electron microscopy, electron probe microanalysis and single-crystal X-ray diffraction. The sample is chemically homogeneous. Omphacite usually shows symplectitic intergrowths or a relictic character partially and totally included in amphibole. The transformation of omphacite to symplectites colonies characterizes the initiation of the eclogites retrograde metamorphism. The reaction history can be summarized as the decomposition of omphacite into symplectites as a product of decompression through the reaction: omphacite + quartz = plagioclase + clinopyroxene (low in Na), which can be considered as a discontinuous precipitation reaction. The structure of the studied omphacite single-crystal is successfully refined in the P2/c space group. This structure is interpreted as an intermediate towards omphacite with space group P2/n. The structure of omphacite shows silicate chains formed by two crystallographically different silicon tetrahedra (T1 and T2) with an O2-O3-O2 angle of 169.002 (2)°, indicating that the chain is slightly distorted. The Na and Ca cations occupy the octahedral and 8-coordination sites. Fe occurs as Fe2+ and Fe3+ and are distributed over the octahedral sites M1 and M11, respectively.

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Section: Omphacite Morphology and Compositionsupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Crystal-Chemical and Structural Characterization of Omphacite in High-Pressure Eclogites From the Arquía Complex on Southwestern Pijao, Central Cordillera (Colombian Andes)

et al. 2022

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“…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 ., ). These discoveries point to the possibility of transient solid‐state phase transformations, both polymorphic and non‐polymorphic, in the formation of natural high‐ P rocks.…”

Section: Discussionmentioning

confidence: 99%

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

Yang

Zhang

Anping

et al. 2016

Journal Metamorphic Geology

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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...

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