Petrofabric And Water Content Of Mantle Predotite Xenolith From Mingxi,Fujian Province

The study of upper mantle is an important content of the earth science. Peridotitexenoliths form Mingxi Fujian province are natural rocks of the upper mantle, thestudies of their petrography, microstructure, geochemistry and mineral water canprovide us some important information of the upper mantle, such as structure,composition, deformation mechanism, the stress and rheology state et.al. The researchobject of this paper is peridotite xenoliths from Mingxi, using electron backscatterdiffraction technique (EBSD) to study their characteristics of microstructures. We alsousing Fourier-transform infrared spectroscopy technique (FTIR) to measure watercontent in minerals of the xenoliths, in order to provide important basic data supportfor further study of water in the upper mantle. The present research results are mainlyfrom the following several aspects:(1) The minerals’ formation temperature condition of garnet peridotites from Mingxiarea is1079~1238degrees, the formation pressure is22~26kbar and the formationdepth is70~83km, equivalent to continental increase heat flow value of75mW/m2.(2) Forsterite (Fo) in spinel/garnet two pyroxene peridotite has obvious latticepreferred orientation, with water results measured by FTIR, can determine the olivinefabric types is A (010)[100]. Chrome diopside (Di) and enstatite (En) don’t haveobvious lattice preferred orientation.(3) Spinel face and garnet face peridotite xenoliths have the same fabriccharacteristics, suggesting that although their origin in different depth, formationtemperature and pressure conditions, but they all belong to the condition type as hightemperature, low strain rate and poor water. Because the upper mantle consistingmainly of olivine and olivine has obvious lattice preferred orientation, certainly willmake the anisotropy of upper mantle in Mingxi area.(4) In garnet two pyroxene peridotite from Mingxi area, the water content ofchromium diopside (Di) is107~323ppm, the average value is225ppm; the water content of enstatite (En) is in the range of22~119ppm, the average value is83ppm;the water content of forsterite (Fo) is3~24ppm, the average value is12ppm; pyrope(Pyr) revealed no absorption peak in infrared absorption spectroscopy, so it does notinclude water. In spinel two pyroxene peridotite, the water content of chromiumdiopside (Di) is70~227ppm, the average value is153ppm; the water content ofenstatite (En) is36~77ppm, the average value is59ppm; the water content offorsterite (Fo) is in the range of5~28ppm, the average value is16ppm.(5) Compared with forsterite(Fo), chrome diopside(Di) and enstatite(En) don’t haveobvious lattice preferred orientation, but their water contents are significantly higherthan that of olivine, so we can’t find positive correlations between water content andlattice preferred orientation. Maybe with the changes of temperature, pressure anddepth condition, water will diffuse to mineral grain boundaries and fracture, and theninfluence the petrofabric of minerals in peridotite xenolith.