Transmission electron microscopy of omphacite and other minerals in eclogites from the CCSD borehole, China: indications for their deformation and temperature history

作者：Mueller, Wolfgang Friedrich; Xu, Zhiqin; Brenker, Frank E.

来源：European Journal of Mineralogy, 2011, 23(4): 645-659.

DOI：10.1127/0935-1221/2011/0023-2133

• crystal defects
• TEM
• CCSD
• eclogite
• omphacite
• garnet
• rutile
• amphibole
• analcime

Seven eclogite samples from 223 to 584 m depths in the Chinese Continental Scientific Drilling main borehole (CCSD-H) in the Sulu ultrahigh-pressure metamorphic (UHPM) terrane, Eastern China, were studied by transmission electron microscopy (TEM), in order to characterise their microstructures. We observed, among others, omphacite, jadeitic diopside, garnet, amphibole, rutile, Na-rich plagioclase, quartz, K-feldspar, analcime, and diamond (contamination). Omphacite shows a well-known diffusion-ontrolled disorder-order phase transition which gives rise to antiphase domains (APDs). In our samples we observed a variation of the APDs' size between similar to 5 nm and 2 mu m which is correlated with the content in jadeite (Jd) component. The broad maximum of similar to 1-2 mm APDs' size is centred on Jd50. This size drops to similar to 5-10 nm for Jd37 and Jd66. The size variation can be explained with the T-path of the respective omphacites. While the large APDs of the omphacites formed and coarsened during subduction and exhumation, the smallest APDs formed and grew during exhumation. In contrast to other eclogite occurrences, deformational defects in omphacites from the CCSD-eclogites are mostly very rare. Occasionally, stacking faults parallel to (010), free dislocations with the Burgers vector [001] and those with 1/2[110] connected with antiphase domain boundaries (APBs), deformation twins on (100) and small-angle grain boundaries were observed. The explanation is that most CCSD-omphacites were strongly affected by recrystallisation, which took place above 600 degrees C during subduction up to its maximum temperature of similar to 750 degrees C and continued during exhumation above 600 degrees C into the regime of amphibolite facies for several tens of Ma. Garnet shows very rarely crystal defects, while amphiboles usually displayed dislocations and chain multiplicity faults. Rutile of one sample, which contained a few percent Fe, showed fully coherent, nano-sized platelets (Guinier-Preston zones) parallel to (100) and (010). The existence of fluids during retrogression is documented by K-feldspar and analcime in quartz.

• 出版日期2011-7
• 单位中国地质科学院