Actual Microcosmic Trace of Mantle Fluid in Deep Geological Process: Experimental Evidence with Petrography, SEM-EDS and EPMA

摘要

Alkali-rich intrusive rocks with various mantle xenoliths along the Jinshajiang-Ailaoshan deep fault, formed in the Himalaya period, are closely related to the polymetallic deposits in western Yunnan, China. They may be regarded as an indicator for deep geological process and mantle fluid action which played an important role in the petrogenesis and mineralization. Iron (Fe) rich microcrystalline glasses filling among crystal particles, cracks, and cleavage lines with reticulated veins or irregular clumps associated with siliconizing, carbonating and sulfidizing alteration, and Fe-rich melt xenoliths were found in alkali-rich intrusive rocks. The polished thin sections of Fe-rich microcrystalline glasses and Fe-rich melt xenoliths do not display optical characters, which are appearing to be black opaque and can not reflect light on polarized light microscope through transmitted polarized light and reflected polarized light. Based on analysis of petrography, petro-chemistry, electron probe microanalysis (EPMA), scanning electron microscopy analysis (SEM), energy dispersive spectrometer (EDS), and X-ray powder diffraction (XRD) we discover that the Fe-rich melt xenolith and Fe-rich microcrystalline glasses are of nano-micron grade crystalline texture and unmixing texture composed of nano-micron grade minerals, which are mainly quartz, apatite and silicates, especially ilmenite, Ti-bearing specularite and some amorphous crystals. More importantly, we also detect typomorphic minerals of mantle such as moissanite, ferrite and Cr-bearing ferrite according to ratios calculation. Studies show that all the above-mentioned microcrystals are ultra-microcosmic substance trace suggesting the composition and nature of mantle fluid with the similarities of the melt. Studying these two kinds of nano-micron grade substance traces are of great significance to reveal mantle fluid action in deep geologic process with more profound understanding of the mineralizing mechanism in the earth's crust.