The Chemical Compositions Of The Basalt Glasses Synthesized Under High Pressure And High Temperature

The destruction and lithosphere thinning of the eastern North China Craton hasbeen widely recognized between the Geoscientists. However, the time, mechanismand the Geodynamic factors of the destruction and lithosphere thinning remainscontroversial. As to the mechanism of it, at least three views have been proposed:delamination, lithosphere hydration and thermal-mechanical-chemical erosion. Theseviews show that the destruction of the North China Craton is closely related to water.So it is necessary to research the content of water in the asthenosphere under NorthChina Craton. But we can’t determine the water content of the mantle lithospheredirectly as the rock specimens in the asthenosphere can’t be obtained directly. It isproposed that the speciation and content of water and the compositions in the glassessynthesized by rapid quenching from the melt at high temperature and high pressurecan reflect that in the melt.Basalt is one of the most widely distributed igneous rocks, which has greatsignificance in the inversion of mantle material composition, the analysis of structuralenvironment and the dynamics of the deep lithosphere. Therefore, we chose the veryrepresentive rocks that produced by the North China Craton, which respectively werethe Feixian Early Cretaceous basalt, Sihetun Early Cretaceous high-Mg basalt,Tietonggou high-Mg Early Cretaceous high-Mg diorites and Fuxin Late Cretaceousbasalt as start materials, which were ground into powder. Then we added anappropriate amount of water into the powder. Finally we made them melt under highpressure and high temperature, by rapid quenching from which we obtained16water-bearing glass specimens.We are going to simulate the Earth’s interior condition to study the electricproperties of these glasses specimens under high pressure and high temperature. Thenthe measurement results are going to be compared with the results from geomagneticdetection. If the electric properties are similar, it indicates that the water content andchemical compositions in the asthenosphere are closed to that in our synthesized glasses. Therefore, before the study of the electric properties under high pressure andhigh temperature, it is necessary to study the chemical compositions and water contentin these glass specimens.We analyzed the main elements in all of the synthesized glass specimens, andfound that most of the specimens were of great homogeneity. Fourier transforminfrared spectrometer was carried out on these specimens. It was found that there areabsorption peaks at～4500cm^-1on the infrared spectrum of each specimen, whichindicated that there was OH formed water in all of our specimens. Apart from thosespecimens synthesized without water such as the Feixian basalt glass FX-1,theTietonggou high-Mg dioritic glasses TTG-1and the Fuxin basalt glass FW-1, therewere obvious peaks at both～5200cm^-1and～1630cm^-1,which indicated that waterexisted in these glasses as both OH and molecular H₂O.We calculated the OH andmolecular H₂O content in our specimens on the basis of the peaks at～4500cm^-1and～1630cm^-1respectively, associated with the Lambert-Beer Law. The seriousoverlap of peaks in the typical OH absorption area prevents us to calculate the totalwater content in the specimens by the peaks at～3550cm^-1. As OH and molecularH₂O are the main speciations of the water in the minerals, we assumed water exists inthese synthesized glasses as nothing but OH and molecular H₂O. So in this paper wetook the sum of the content of OH and molecular H₂O as the total water content.Furthermore, we determined the atomic percentages of different elements in ourspecimens. However the main elements were net fully detected, which may be as aresult of the rough surface of our specimens. The XPS results of some specimens arefar from ideal, but there also are results of great satisfactory, such as the FX-1, FX-2,TTG-2, TTG-3, TTG-4and FW-2.We assumed their chemical formula on the basis ofthe atomic percentages reasonably and finally determined the chemical formula,associated the water content obtained from FTIR.