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

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