Geochemistry Of Boron And Chlorine Isotopes In Sedimentary Environment

Boron and chlorine has two stable isotopes respectively, 11B and 10B, 3 7 Cl and 35 Cl. In this thesis the geochemistry of the stable isotopes of boron and chlorine in sedimentary environment have been investigated by thermal ionization mass spectrometry (TIMS), including ocean, salt lake and loess deposit. A further discussion is also given for the determination of boron isotopes based on PTIMS or NTIMS.History of boron and chlorine isotopic geochemistry(chapter 1) Since 1980s, with some high precise methods of determining boron and chlorine isotopes, the variations of boron and chlorine isotopes in nature have been evidenced by high precise determination and a lot of investigations.Many significant works about boron and chlorine isotopic geochemistry have been done in oceans, salt lakes, groundwater, evaporated sediment and hydrothermal fluids. This chapter gives a brief summary of including analytical methods and application in earth science.Further investigation of boron isotopic measurementThe positive thermal ionization mass spectrometry has been widely used to determine isotopic composition of the boron and chlorine in geological sample. However, it is very important that the pure boron and chlorine is separated from natural samples for isotopic measurement of boron and chlorine by PTIMS. Further, it is more difficulty to separate boron from halite, coral and carbonate of loess, with low boron concentration. A group of experimental technology is described for separating the boron or chlorine sedimentary minerals and natural water. In addition, a further discussion of the boron and chlorine isotopic measurement is given (chapter 2).An isobaric interference for boron isotopic measurement by negative thermal ionization mass spectrometry (NTIMS) has been studied. The result shows that the CNO- is not only from the organic material, but also is from nitrate in loading reagent in NTIMS. Monitoring the mass 43 ion intensity and 43/42 ratio of blank are necessary for the boron isotopic measurement in NTIMS, other than is only boron content (chapter 3). Geochemistry of boron and chlorine in salt lakesThe isotopic compositions of chlorine in salt lake brine coexisting with halite, oil-field water and river water from the Qaidam Basin (Qinghai, China) have been examined using High-precision measurement of chlorine isotopes based on thermal ionization mass spectrometry of the Cs2Cl+ ion. The variation of theδ37Cl value in these salt lakes is mainly associated with brine evaporation and halite precipitation and with the hydrochemistry of input water. The chlorine isotopic composition may reflect the degree of evaporation of the salt lake brine (chapter 4).Theδ37 Cl value of salt deposits in the Kunteyi are reduced with the upward and represent eight cycle. This variation is controlled by paleoclimate condition. The systematic variation inδ37Cl of sale deposits are useful tool for the study of paleoclimate of salt deposits in drought and stronger evaporated condition (chapter 5).The mechanism of incorporating boron into halite during evaporation of salt lake brines is subject to dispute, and there have few studies of the boron concentrations and isotopic compositions during this process due to low boron concentration in halite. A group of evaporation experiments from artificial solutions and salt lakes brines have been analyzed in this study. The results of boron concentration and isotopic composition analyses demonstrate that the boron in halite comes mainly from fluid inclusions. These values are controlled by the boron isotopic composition of the boron sources, pH values and Na/Ca ratios in the salt lake brines. The variation of boron isotopes in halite may be used to trace the hydrochemical evolution and paeleoevaporation environment in salt lakes (chapter 6).A relation of the boron and chlorine isotopes to the hydrochemistry of the salt lake brine has been investigated. The results show that the variations of the boron and chlorine isotopic composition are associated with evolution of salt lake brines. The boron isotopic composition of brine is under control of source water and Ca concentration of the brine. The chlorine isotopic composition is controlled by brine evaporated. A negative relation between the boron and the chlorine isotopic composition is given (chapter 7).Composition of boron isotopes in coral from South China Sea Boron isotopic composition and concentration in the corals from South China Sea have been investigated by using thermal ionization mass spectrometry. A discussion is given for the relation among boron isotopic composition, pH value, boron concentration and age .The results indicate that boron isotopic composition of the corals varies in a range of 22.7 to 24.8%, and appears in the positive relation with the boron concentration. The pH values of ancient seawater in last 7000 years are calculated based on the correlative equation between boron isotopic composition and pH value. Preliminary study shows that boron isotopic composition of coral may become a tracer of the sea level change.Composition of boron isotopes of carbonate nodule in loess Boron isotopic composition of nodule has been determined by NTIMS. The result show that there is a positive relationship betweenδ17B and 8 7 Sr/ 86 Sr. Boron isotopes of nodule in loess may trace the weathering process.