Z-CONTRAST IMAGING OF CATIONS AND CATION ORDERING IN OMPHACITE FROM SULU TERRANE, EASTERN CHINA

Coupled ordering between Ca-Mg and Na-Al in omphacite can provide information about thermal history of the mineral. Both ordering states and other non-period features in omphacite from ultra-high pressure (UHP) metamorphic rocks can provide information about the history of subduction and exhumation of their host rocks. Previous results indicate that the omphacite crystals in UHP eclogite contain needle-like ß-cristobalite (instead of quartz) precipitates in the central part of the omphacite. The precipitates keep crystallographic orientation relationship with their host omphacite. The host omphacite indicate that silicate chain in pyroxene also serves as substrate for heterogeneous nucleation of the ß-cristobalite.

The omphacite also contains anti-phase domain boundaries (APB) and mixture of C- and P-lattice domains with several tens of nano-meters in size. With a spherical aberration scanning transmission electron microscope (STEM), both positions and occupancies of atoms can be imaged directly at sub-Å resolution. The intensity is directly related to atomic numbers of the atoms (I ~ Z²). Local ordering between Ca-Mg and Na-Al and structural defects in the omphacite can be obtained based on their occupancies. The omphacite is not fully ordered. There are about 30% of Na in Ca sites, and about 30% of Ca in Na sites. Isolated double chains were also observed in the omphacite. Ordering sequences of Ca and Na are different across the double chain, and they are in APB relationship. The phenomenon indicates that ordering happened after the double chain formation. The double chain is not a product from late stage alteration reaction. The double chain with Na on M4 sites serves as “nucleation” sites and anti-phase domain boundary (APB) for the coupled ordering in the omphacite domains across it. The isolated double chains in omphacite could be hydrogen carries through subduction into deep mantle. This project is supported by National Science Foundation.