中国地质大学-2012春地球化学课件-第5章5-K-Ar

1、K-Ar定年法40Ar-39Ar定年法Re-Os定年法14C定年法,K-Ar定年法基本原理,K-Ar定年法适合于富K的岩石和矿物； 钾是地壳中最丰富的8个元素之一，并且是许多造岩矿物的主要组分。例如：云母(micas)、钾长石(feldspars)、似长石(feldspathoids)、粘土矿物(clay minerals)和蒸发盐。,K-Ar定年法基本原理,K同位素的丰度： 39K=93.2581%；40K=0.01167%; 41K=6.7302%； 大气圈的Ar同位素组成： 40Ar=99.60%；38Ar0.063%；36Ar0.037%; 40Ar/36Ar=295.5 10.48%

2、的40K衰变为40Ar；89.52%的40K衰变为40Ca； 放射成因40Ca丰度变化在多数岩石中非常有限， 非放射成因的40Ca在总钙中占97%，因此，KCa方法作为测年工具其应用受到限制。,K-Ar定年法基本原理,某一富K系统中放射性成因的40Ar和40Ca的增加可以表示为：,其中：= e+ e 0.58110-10y-1； = 4.96210-10y-1 = e+ = (0.581+4.962)10-10y-1 =5.543 10-10y-1,富K系统中放射性成因40Ar：,K-Ar定年法基本原理,K-Ar定年的基本公式：,由于Ar属于惰性气体且在硅酸盐熔体中的溶解度很低，因此假定矿物或

3、岩石形成时40Ar0为0。,在40Ar的实验室分析过程中，一般易受到大气Ar的影响，故需从测定的40Ar中扣除大气成因的Ar：,K-Ar定年法基本原理,假定测定的36Ar均为大气Ar的同位素组成：,K-Ar定年法基本假设,No radiogenic 40Ar produced by decay of 40K in the mineral during its lifetime has escaped; The mineral became closed to 40Ar soon after its formation, which means that it must have cooled

4、rapidly after crystallization, unless its formed at a low temperature; No 40Ar was incorporated into the mineral either at the time of its formation or during a later metamorphic event; An appropriate correction is made for the presence of atmospheric 40Ar;,K-Ar定年法基本假设,the mineral was closed to K th

5、roughout its lifetime; the isotopic composition of K in the mineral is normal and was not changed by fractionation or other processes except by decay of 40K; The decay constants of 40K are known accurately and have not been affected by the physical or chemical conditions of the environment in which

6、the K existed since it was incorporated into the Earth; The concentrations of 40Ar and K were determined accurately;,K-Ar定年法基本假设,Argon loss from minerals may occur because Ar is a noble gas and therefore does not form bonds with other atoms in a crystal lattice. Ar 丢失的原因如下：1. Inability of a mineral

7、lattice to retain Ar even at low temperature and atmospheric pressure;2. Either partial or complete melting of rocks followed by crystallization of new minerals from the resulting melt;,K-Ar定年法基本假设,3. Metamorphism at elevated temperatures and pressures, resulting in complete or partial Ar loss by di

8、ffusion depending on the temperature and duration of the event;4. Increase in temperature due to deep burial or contact metamorphism, causing Ar loss from most minerals without producing any other physical and chemical changes in the rock;,K-Ar定年法基本假设,5. Chemical weathering and alteration by aqueous

9、 fluids, leading not only to Ar loss but also to changes in the K concentration of minerals;6. Solution and redeposition of water-solute minerals such as sylvite(钾盐);7. Mechanical breakdown of minerals, radiation damage, and shock metamorphism. Even excessive grinding during preparation of samples f

10、or dating by the K-Ar method may cause partial Ar loss.,过剩Ar的问题,其中：40Ar0代表了岩石或矿物中非放射性成因的Ar同位素，其来源有：,Ar that was dissolved in the magma and that may have originated from the mantle of the Earth or by outgassing of old K-bearing minerals of the crust; Ar that was evolved during later thermal metamorph

11、ism of the rocks and that diffused into the minerals during that event; Atmospheric Ar adsorbed on grain boundaries and microfractures while the rock was exposed to the atmosphere in the field and in the laboratory;,等时线的斜率：,K-Ar定年的基本公式,K-Ar法难以用来进行沉积岩的定年，原因在于：1. The most abundant kinds of sedimentary

12、 rocks (sandstone, siltstone, and shale) are either entirely composed of or contain mineral particles derived from them may be used to study their provenance, but they generally do not indicate the time of deposition;2. Sedimentary rocks composed primarily of authigenic minerals either do not contai

13、n sufficient K for dating or do not quantitatively retain the radiogenic 40Ar that forms by decay of 40K after deposition;,沉积岩的K-Ar定年,40Ar-39Ar定年法,用热中子照射富K样品，易形成39Ar不稳定，该核素不稳定，易通过衰变形成39K。 39K半衰期为269a。因此，人为地假定在样品分析的过程中39Ar是稳定同位素。,在热中子照射富K样品时易形成39Ar：,40Ar-39Ar定年基本原理,The duration of the irradiation,The

14、 neutron flux density at energy ,The capture cross section of 39K for neutrons having energy ,40Ar-39Ar定年基本原理,40Ar-39Ar定年基本原理,由于监测样品的年龄已知（tm)，其40Ar*/39Ar可以测出，故J可以求出：,则样品的年龄(t)可以求出：,40Ar-39Ar定年基本原理,理想情况：It is assumed that all of the 40Ar in the irradiated sample is either radiogenic or atmospheric, a

15、ll of the 36Ar is atmospheric, and 39Ar is produced only by 39K. In this case, the measured values of the 40Ar/39Ar and 36Ar/39Ar ratios can be used to calculate the desired ratios radiogenic 40Ar to 39Ar:,大气的40Ar/36Ar,Ca衰变也可以形成36Ar和37Ar ，故也需要校正,Dalrymple and Lanphere (1971)给出了通用的40Ar*/39Ar校正参数：,A=

16、measured value of the 40Ar/39Ar ratio; B=measured value of the 36Ar/39Ar ratio; C1= 40Ar/39Ar ratio in the atmosphere,=295.5; C2= 36Ar/37Ar ratio produced by interfering neutron reactions with Ca, 2.720.01410-4; C3= 40Ar/39Ar ratio produced by interfering neutron reactions with K, 5.90.4210-3; C4= 3

17、9Ar/37Ar ratio produced by interfering neutron reactions with Ca, 6.330.04310-4; D= 37Ar/39Ar ratio in sample after correcting for decay of 37Ar,Incremental Heating Technique,If the sample has been closed to Ar and K since the time of initial cooling, the 40Ar*/39Ar ratios and thus the dates calcula

18、ted at each step should be constant. If radiogenic Ar was lost from some crystallographic sites but not from others some time after initial cooling, the 40Ar*/39Ar ratios of the gas released at different temperatures may vary, and a spectrum of dates will then result from which the time elapsed sinc

19、e initial cooling may be inferred.,Turner (1968,1969)提出的模型,40Ar-39Ar定年基本原理,The first gas to be released at the lowest temperature originates from the surfaces of the grains and from sites that lose Ar readily. This gas has a low 40Ar*/39Ar ratio, corresponding to time elapsed after accumulation of r

20、adiogenic 40Ar resumed after metamorphism(变质年龄). Gas fractions released at higher temperatures have higher 40Ar*/39Ar ratio because this Ar is removed from retentive sites that lost smaller fraction of 40Ar* during metamorphism. Ultimately, the 40Ar*/39Ar ratios may reach a plateau corresponding to

21、a date that approaches the time elapsed since original cooling of the mineral without subsequent loss of 40Ar（冷却年龄或形成年龄）.,40Ar-39Ar定年（过剩Ar）,过剩Ar的来源： The excess 40Ar enters the minerals of igneous rocks at the time of their crystallization from the magma; The 40Ar collects in grain boundaries and fra

22、ctures in minerals of igneous and metamorphic rocks or diffuses into the crystal lattice; 标志：“saddle-shaped” spectrum of dates; 若excess 40Ar均匀地分布在矿物或岩石中，则：the incremental heating technique does not distinguish between the radiogenic 40Ar that formed by decay of 40K in the rock and the excess 40Ar th

23、at was added after or during crystallization.,Ar isotope correlation diagram,K-Ar和40Ar*/39Ar定年过程中，均需对扣除大气成因的40Ar。其校正方法假定所分析样品中的36Ar都来自大气，但是由于当岩石或矿物形成时，进入其中的40Ar/36Ar与大气中相应比值并不相同；,If the 40Ar/36Ar ratio of the inherited Ar is great than 295.5; an apparent excess of radiogenic 40Ar will result; If the

24、 40Ar/36Ar ratio of the inherited Ar is less than 295.5;there will be an apparent deficiency of radiogenic 40Ar;,Ar isotope correlation diagram,斜率：,截距：,例如：北美Portage Lake Volcanics 斜长石的坪年龄:1048Ma (1047.4Ma),40Ar*/39Ar激光剥蚀法,优点： Single grains of minerals can be dated without interference from inclusion

25、s of other minerals; The small volume of melt greatly reduces the volume of the Ar blank and thereby reduces analytical errors; Different minerals or several grains of the same mineral in a polished specimen of rock can be dated; Mineral separations, which are time consuming and not always effective

26、, are unnecessary;,Re-Os定年法,Re的衰变常数仍有不确定性； 常见的造岩矿物中Re-Os的含量很低；,Re与Mo的化学性质相似，因此以与Mo发生类质同象（例如：辉钼矿）； Re还易与Cu发生类质同象，故也可以出现在Cu的硫化物中； 碳质球粒陨石的Re和Os有很好的相关性： Re=0.0646Os+8.10 普通球粒陨石：Re=0.0823Os+3.83 铁陨石：Re=0.0725Os+89.57,铼是一种具两个同位素(185Re与187Re)的难熔金属。 187Re占总铼的62%，经过衰变成为187Os(具7个天然同位素的铂族元素)。象Pb一样，锇也是亲硫-亲铜元素，

27、与U-Pb系统不同的是，Re与Os在地幔与地壳系统间表现出显著的分馏。铼是中等不相容元素，相当易于从地幔分配进入到岩浆液相中，而锇是高度相容元素强烈趋向于保持在地幔中。 这些独特的地球化学性质表明了Re-Os法在矿床形成、岩浆成因和地幔演化的地质年代学与地球化学研究中具有大的潜力。,根据矿物中Re,Os含量分类，有以下三种情况： 低Re，低Os：常见的造岩矿物； 高Re，低Os：辉钼矿，Cu-Ni-PGE硫化物； 低Re，高Os：铱锇矿、自然金、铬铁矿、辉砷镍矿； 磁铁矿、钛铁矿和石榴石也可以用来Re-Os同位素体系定年，目前的研究仍比较薄弱。,其中衰变常数有许多推荐值： (1.640.05)

28、10-11y-1 (Lindner et al., 1989); 1.6610-11y-1 (Shen et al., 1998); (1.6660.06)10-11y-1 (Lugmair et al., 1989); (1.6660.017)10-11y-1 (Faure, 2005),基本原理,或,辉钼矿(MoS2)是具有高的Re含量(0.2451690 ppm)，其形成时基本不含Os，因此辉钼矿中Os均是Re放射性衰变的产物187Os：,因此，只要测定辉钼矿中Re，Os含量就可以计算出辉钼矿的形成年龄。,14C法 天然14C是在大气层上部，宇宙射线产生的中子（n)与大气中的氮核（14N）发生核反应的产物，其反应式为： 14Nn 14C1H 新产生的14C在大气层中很快被氧化变成14CO2，并与大气层中原有的CO2充分混合后扩散到整个大气层中。大气层中CO2通过与溶解于海水中的CO2的交换，以及通过植物的光合作用，动物对食物中碳的吸收等，使水圈与生物圈中都存在着宇宙辐射成因的14C。,氮是大气中比较丰富的元素，14C的产生速度主要取决于由宇宙射线产生的中子数