地球化学5-1.ppt

Chapter 5：Isotope Geochemistry Applications of Isotope geochemistry For example §51 The reason for variation of isotopic composition in nature I. Property of nuclide核素的性质 Isotope nuclide’s stability is relative Radioactive isotope 3）the degree of departure to standard sample in parts in 103 5）同量异位衰变和分支衰变 衰变结果质量数不变，即衰变子体与母体是同量异位素—同量异位衰变 一些放射性原子可部分地衰变为一种原子，同时又部分地衰变为另一种原子，例如40K部分衰变为40Ar、部分衰变40Ca， —分支衰变。 Third reason? First decay Second fractionation Third ? Mixing different sources §52 isotope geochronology I. radioactive decay law： I. radioactive decay law： I. radioactive decay law： 3．The rate of radioactive decay is independent of all external influences, such as T, P, etc. Two isotopes of the same element are chemical identical and therefore that chemical processes cannot change, or fractionate, the ratio of two isotopes of the same elements. I. radioactive decay law： 4. The atom number of a nuclide before the decaying and after the decaying is the function of time. decay前nuclide核素和decay后nuclide核素的原子数，只是时间的函数。 Precondition 5) closed system, the radioactive isotope and radiogenic isotope no in exchange with other system 自然界Sr有四个isotope：88Sr 82.56%87Sr 7.02%（部分是由87Rb decay而成，部分是矿物岩石形成时固有的） 86Sr 9.86%（是非放射成因，自地球形成后为一常数） 84Sr 0.56% 由87Rb decay而产生的87Sr，可根据radioactive isotope dating 公式（1）： 3. Rb-Sr isochron method 假设：①a group of samples, are 　from a single source and forming time ②The isotopic composition of the daughter must have been homogeneous at the time of the event we wish to date.母源中Sr在样品形成时，isotope已均一化，为此，样品中的初始值(87Sr/86Sr)0各处相同； ⑤ 作Rb、Sr定量分析，算出每个样Rb/Sr比值； ⑥结合Rb/Sr比值和镜下观察结果选择无后期作用叠加的5个样品，送Rb—Sr isotope Laboratory ⑦Rb—Sr isochron method样品用全岩样，为了拉开比值亦可挑选合适的单矿物与全岩样共同成isochron. 可以通过将样品用酸溶解并用阳离子交换色谱法分离和提纯出Sr在质谱仪上直接测定 example 87Sr/86Sr=0.708961, 86Sr/88Sr=0.1194, 84Sr/88Sr=0.00675, for a sample. Sedimentary Rock K2O6%的海绿石不适合于定年 K2O7%的海绿石则能较可靠地用于定年(K-Ar and Rb-Sr) Glauconite-A greenish clay mineral, a hyd