ELECTRON AND X-RAY IMAGING OF MINERAL ZONATION AND SYMPLECTITE IN MANTLE XENOLITHS FROM FANGSHAN, EASTERN CHINA

Studies of mantle-derived xenoliths are important for understanding the composition of the subcontinental lithospheric mantle, mantle evolution and continental dynamics. The spinel peridotite xenoliths from Fangshan, eastern China are mainly composed of olivine, orthopyroxene, clinopyroxene and spinel with some interstitial phases. A previous study reported that compositional analysis by electron probe microanalysis (EPMA) of these mantle xenoliths showed that olivine is zoned with the rime rich in Fe. With many interstitial fine-grained phases or aggregates, quantitative EPMA may not be a suitable technique to characterize these phases as EPMA generally requires a phase with size larger than at least 1 µm.

In this study various imaging techniques, such as backscattered electron (BSE) imaging and X-ray mapping with WDS and EDS, were employed to study the compositional variations and zonations in the mantle minerals. Our work by BSE imaging and X-ray mapping reveals that the spinel is often significantly zoned or shows ubiquitous symplectitic rim and that feldspar occurs as interstitial phase between spinel and clinopyroxene. The formation of symplectitic spinel and interstitial feldspar is apparently related to the reaction spinel + orthopyroxene + clinopyroxene + fluid = feldspar + olivine, further indicating that the metasomatic process was related to infiltration of an alkali-rich fluid into the peridotites.