The Improvements Of Calcium And Magesum Isotope Analytical Methods And Their Implications For Tracing The Magma Source Of Miocene Magmatic Rocks In The Lhasa Terrane, South Tibet

As major rock-forming elements,calcium and magnesium isotopes are widely distributed in terrestrial reservoirs and play an important role in various geochemical processes.Both elements have large relative mass differences between their isotopes.This characteristic can potentially produce significant isotopic fractionations in many geological processes and makes them excellent geochemical tracers.Since the development of isotopic analytical techniques,high-precision measurements of Ca and Mg isotopes have been achieved and the studies of Ca and Mg isotopes have become one of the research hotspots and frontier fields in isotope geochemistry.Although the analytical methods for Ca and Mg isotopes have been established in many laboratories,there still exist some problems that may affect the analysis results.Therefore,it is necessary to improve and perfect the existing Ca and Mg analysis methods.On the other hand,the Tibetan plateau,known as a natural laboratory of continental dynamics,always has been a research focus for geologists.During the Miocene period,a series of post-collision magmatism widely occured in Lhasa Terrane,south Tibet and formed a set of potassic-ultra-potassic rocks and adakites.Studies on these rocks can not only help us to understand the uplift process and mechanism of the Tibetan Plateau,but also provide an important way to study the material compositions and their evolution and the geodynamics mechanism of the deep part of the Tibetan Plataeu.In this thesis,we focus on the improvements of Ca and Mg analysis methods and the geological applications of Ca and Mg isotopes.Based on the reported Ca and Mg isotope analysis methods,we firstly improve the chemical separation procedures for Ca and Mg isotopes,then measure the Ca and Mg isotopic compositions of Miocene potassic,ultra-potassic rocks and adakites in Lhasa terrane,South Tibet to investigate their source and petrogenesis.Some progessess are gained as follows:1.A chemical separation procedure for Ca isotope is reestablished based on the condition experiments of factors that influence the effect of element chemical separation,such as ion-exchange column types,resin volumes,acid types and molarities,sample loading amounts,sample lithology and blank,etc.This procedure achieves a good separation for calcium from matrix elements like K,Sr and obtains nearly full recovery.The total procedure blank is controlled in less than 2‰?relative to sample loading amount?,which has reached the international level.Meanwhile,some international standards went through this procedure show good consistency with documented results,indicating our procedure ensures the possibility of high precise and accurate measurement and can be applied to study the Ca isotopic compositions of geological samples.2.Combined with the previous procedures provided in literatures,an improved Mg isotope chemical separation procedure is established through detailed evaluations on leaching acid concentrations,ion-exchange column types and blank under our exisiting laboratory condition.This procedure achieves a better separation effect between Mg and matrix elements like K,Ti and obtains high Mg recovery.By comparing the difference of sample data obtained from different chemical separation procedures in two laboratories,it is certain that our chemical separation procedure for Mg isotope has feasibility.The long-term total procedure blank is also controlled in less than 2‰,indicating no significant interference for measurement.In addition,this procedure can separate Ca and Mg isotopes at the same time,which can further improve the efficiency of chemical separation.3.The Ca and Mg isotope studies of ultra-potassic rocks from Mibale and Maiga areas in midwestern Lhasa Terrane,South Tibet suggest that there is no influence from weathering and magma evolution,and the metasomatic agents in the source of these rocks are derived from marine carbonate sediments of subduction neo-Tethys oceanic crust during the subduction process.These ultra-potassic rocks have relatively homogenous Ca isotopic compositions with?^44/40Ca values vary from 0.59 to 0.78 and an average of 0.69±0.02,which is obviously lower than that of the upper mantle(?^44/40Ca=1.05±0.04),bulk silicate earth(?^44/40Ca=0.94±0.05)and igneous rocks(?^44/40Ca=0.80±0.10).This indicates that some end-member materials with low?⁴⁴Ca value had join into the mantle source.The?²⁶Mg values of ultra-potassic rocks display limited varication from-0.38 to-0.19 with an average of-0.28±0.09,which are similar to the upper mantle value(?²⁶Mg=-0.25±0.07)within the analytical uncertainty.Meanwhile,the ultra-potassic rocks show significantly positive correlation between Ca and Mg isotopic compositions,suggesting that the metasomatic agents in their enriched mantle source should be characterized by low?⁴⁴Ca and?²⁶Mg values,and the metasomatic agents maight be marine carbonate sediments from subduction neo-Tethys oceanic crust.According to the two-endmember mixing modeling of Ca-Mg isotopes in the mantle source,the metasomatic agents in mantle source consist of both calcite and dolomite carbonates,and the calcite carbonates account for the major proportion.4.The Ca and Mg isotopic compositions of potassic rocks and adakites from middle-southern Lhasa terrane,south Tibet prove that these two types of rocks have different magma sources and also have different genetic relationship with contemporaneous ultra-potassic rocks.Potassic rocks from Chazi area and adakites from southern Lhasa terrane both have inhomogenous Mg isotopic compositions.The?²⁶Mg values of potassic rocks vary from-0.31 to 0.44 and the?²⁶Mg values of adakites range from-0.21 to 0.14,indicating that these rocks have higher?²⁶Mg values than the upper mantle and they should be derived from the crust.Besides,the Ca isotopic compositions of most postassic rocks are lower than the upper mantle and bulk silicate earth,with a range from 0.61 to 0.69.Only two potassic samples have the same?^44/40Ca values to bulk silicate earth.The?^44/40Ca values of adakites range from 0.30 to 0.76 with an average of 0.50,which are not only significantly lower than that of the upper mantle and bulk silicate earth,but also lower than the potassic rock values.This suggests that potassic rocks and adakites were derived from different crust source regions.In addition,compared with the Ca and Mg isotopic compositions of contemporaneous ultra-potassic rocks,it is found that the potassic rocks have genetic relations with ultra-potassic rocks and their source possibly had been influeced by the ultra-potassic magma.However,there is no direct relationship between adakites and ultra-potassic rocks and the ultra-potassic magma didn't provide materials for the formation of adakites.